I TA L I A N H A B I TAT S
Lagoons, estuaries and deltas
23
Italian habitats
Italian Ministry of the Environment and Territorial Protection / Ministero dell’Ambiente e della Tutela del
Territorio e del Mare
Friuli Museum of Natural History / Museo Friulano di Storia Naturale · Comune di Udine
I TA L I A N H A B I TAT S
Scientific coordinators
Alessandro Minelli · Sandro Ruffo · Fabio Stoch
Editorial committee
Aldo Cosentino · Alessandro La Posta · Carlo Morandini · Giuseppe Muscio
“Lagoons, estuaries and deltas · Boundaries between the sea and rivers”
edited by Alessandro Minelli
Texts
Annelore Bezzi · Mauro Bon · Francesco Bracco · Daniele Curiel · Francesca Delli Quadri ·
Giorgio Fontolan · Gilberto Gandolfi · Alessandro Minelli · Andrea Rismondo · Francesco Scarton ·
Margherita Solari · Davide Tagliapietra · Marco Ulliana · Mariacristina Villani
In collaboration with
Maria Manuela Giovannelli · Antonella Miola · Marco Sigovini
English translation
Alison Garside · Gabriel Walton
Illustrations
Roberto Zanella
Lagoons, estuaries and deltas
Boundaries between the sea and rivers
Graphic design
Furio Colman
Photographs
Nevio Agostini 7, 15, 21, 82, 87, 88, 143, 145 · Archive Museo Friulano di Storia Naturale 81, 84, 85,
90, 107/2, 148 · Archive Naturmedia 86, 137 · Mauro Bon 66, 132 · Massimo Buccheri 80 ·
Marco Cantonati 32 · Compagnia Generale Ripreseaeree 12, 13 · Ettore Contarini 101/2, 105 ·
Giuseppe Corriero 58 · Daniele Curiel 24, 25, 26, 27, 28, 29, 30, 31 · Ulderica Da Pozzo 9, 11, 16, 19,
23, 135, 144 · Vitantonio Dell’Orto 6, 20 ,41, 74, 75, 77, 112, 115, 116, 118, 119, 120, 121, 123, 124,
126, 127, 128, 129, 130, 131, 133, 134 · Dario Ersetti 22, 39, 79, 147 · Giorgio Fontolan 10, 18 ·
Francesco Scarton 117, 122, 142 · Paolo Glerean 138 · Giuseppe Muscio 76 · Nicola Novarini 113, 114 ·
Arnaldo Piccinini 65, 68, 69, 70, 67 · Andrea Rismondo 33, 34, 35, 36, 37 · Lorenzo Seitz 40, 46, 47, 48, 56 ·
Marco Sigovini 50, 51 · Fabio Stoch 14, 59, 60, 61, 72, 140 · Egidio Trainito 42, 44, 45, 52, 53, 54, 62, 63,
64, 71, 73 · Marco Uliana 83, 91, 92, 93, 94, 95, 96,97, 98, 99, 100, 101/1, 102, 103, 104, 106, 107/1,
108, 109, 110, 111, 139 · Roberto Valle 125 · Augusto Vigna Taglianti 57 · Francesco Zaramella 49, 141
© 2009 Museo Friulano di Storia Naturale, Udine, Italy
All rights reserved.
No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or
by any means, without the prior permission in writing of the publishers.
ISBN 88 88192 46 8
ISSN 1724-6539
Cover photo: Lagoon of Grado and Marano (Friuli, photo: U. Da Pozzo)
M I N I S T E R O D E L L’ A M B I E N T E E D E L L A T U T E L A D E L T E R R I T O R I O E D E L M A R E
M U S E O F R I U L A N O D I S T O R I A N AT U R A L E · C O M U N E D I U D I N E
Contents
Italian habitats
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Annelore Bezzi · Francesca Delli Quadri · Giorgio Fontolan
Submerged vegetation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Daniele Curiel · Andrea Rismondo
1
Caves and
karstic
phenomena
2
Springs and
spring
watercourses
3
Woodlands
of the Po
Plain
4
Sand dunes
and beaches
5
Mountain
streams
6
The
Mediterranean
maquis
Aquatic invertebrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
Davide Tagliapietra · Alessandro Minelli
Fishes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
Gilberto Gandolfi
Terrestrial vegetation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
Francesco Bracco · Mariacristina Villani
7
Sea cliffs and
rocky
coastlines
8
Brackish
coastal lakes
9
Mountain
peat-bogs
10
Realms of
snow and ice
11
Pools, ponds
and
marshland
12
Arid
meadows
Terrestrial invertebrates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
Marco Uliana · Alessandro Minelli
Terrestrial vertebrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
Mauro Bon · Francesco Scarton
13
Rocky slopes
and screes
14
High-altitude
lakes
15
16
Beech forests The pelagic
of the
domain
Apennines
17
Volcanic
lakes
18
Mountain
conifer forests
Conservation and management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
Annelore Bezzi · Mauro Bon · Francesco Bracco · Francesca Delli Quadri ·
Gilberto Gandolfi · Mariacristina Villani
Teaching suggestions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145
Margherita Solari
Select bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
19
Seagrass
meadows
20
21
Subterranean Rivers and
waters
riverine
woodlands
22
23
Marine bioLagoons,
constructions estuaries
and deltas
24
Italian
habitats
Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
List of species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
Introduction
ANNELORE BEZZI · FRANCESCA DELLI QUADRI · GIORGIO FONTOLAN
■ Foreword
Italy is a country with more than 7500
km of various types of coastline. From
the geomorphological viewpoint, one
of the most interesting aspects of these
coasts is the relationship between the
sea and the rivers that discharge into it,
modelling as they do the coastline by
Mouth of the river Bevano (Emilia Romagna)
distributing freshwater and sediments.
The sea acts with tides, waves and
long-term fluctuations in level (eustatism). This volume of “Italian Habitats”
illustrates three environments which result from interactions between rivers
and the sea: deltas, lagoons and estuaries. The vegetation and fauna of
submerged environments are the subject of the first chapters, followed by
descriptions of above-water environments.
■ Deltas
When a river flows into the sea, the speed of its current is drastically reduced,
causing the sediment it carries to be deposited. If the energy of the waves is not
sufficient to remove the deposited material completely, an accumulation forms,
partly emerging and partly submerged, which is called a delta. This term derives
from the triangular shape of many sedimentary bodies of this type, being similar
to the capital letter of the same name in the Greek alphabet. In reality, the
morphology and dimensions of a delta may vary widely, and in most cases differ
significantly from the ideal model of the Nile Delta, which was the original
inspiration for the name. Each delta is subdivided into a delta plain, delta front
and pro-delta. The plain is a flat area, situated just above sea level, crossed by
branching river channels (distributaries) edged by natural banks. Extremely
heterogeneous morphological elements meet here that result in the development
of diversified habitats. Fluvial processes are prevalent in the upper part; during
Po Delta (between Emilia Romagna and Veneto)
7
spates, large amounts of fine sediment
escape from the channel beds and
accumulate on the delta plain; the
RIVERbanks of the channels rise, but are also
DOMINATED
DELTAS
frequently breached, with the formation
of breach fans and the consequent
WAVETIDEdeposition of sand outside the channel.
DOMINATED DOMINATED
DELTAS
DELTAS
Processes due to marine dynamics act
WAVES
TIDES
in the lower part of the delta plain and
Classification diagram of different delta types
the morphologies of coastal types meet,
with frequent mixing of fresh and sea
waters: lagoons, bays, sandbanks, tidal flats, abandoned strips of beach, and
sand dunes. Sediments may be very heterogeneous in this zone, depending on
the environment and how they were deposited.
The delta front is the shallower part of the submerged delta, mainly composed
of sandy sediment. This is the most active part of the deltaic structure, and the
most interesting from the morphodynamic point of view. It is here that river
water loses speed as it is discharged, and is consequently less capable of
maintaining transported sediments in suspension. These then accumulate in
structures called mouth bars (loss of competence).
The mouth bars assume different shapes according to the hydrodynamic
behaviour of the river at the point where it enters the receiving basin, i.e, the sea.
The way in which river water expands into sea water, i.e., through turbulent mixing
or as a floating body of water, depends not only on the speed of the current and
slope of the seabed, but mainly on the density distribution of the water column.
This distribution is generally influenced by the seasons, with alternating high and
low regimes. The density distribution of the water column may be:
● homopycnal: when the waters are well mixed, with similar densities both at
the surface and on the bottom;
● hypopycnal: when river water is less dense than that of the sea, because of
a low sediment load. It tends to form a surface layer until some distance from
the mouth, gradually losing its load of suspended sediments, which settle. This
condition is common when there is little water in the river;
● hyperpycnal: typical of full spate regimes, when river water is denser than
that of the sea and tends to move in contact with the bottom.
The terminal part of the deltaic structure that develops towards the sea is called
the pro-delta. This is the deepest part, originating from finer materials
transported out to sea and deposited in layers on the seabed. In cross-section,
this sediment accumulation is typically wedge-shaped and represents a
ry
tua
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orm
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ate
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El o
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RIVER
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8
significant historical and geological archive. The sediments that form it contain a
record of the environmental and anthropic changes that have taken place in the
river basin from which the sediments originated.
According to a classical scheme, deltas are classified on the basis of shape,
which is the result of the combined action of the sediment load carried by the
river, the energy of wave motion, and the influence of tides. The Mississippi, in the
Gulf of Mexico, is a typical example of a river-dominated delta, in which the river’s
contribution dominates the physical modelling operated by the sea. This type of
delta develops in conditions of shallow water, where currents, waves and tides
are very weak, and therefore in a sheltered gulf or sea into which the river flows
carrying large quantities of water and sediments. In these conditions, the delta
tends to advance rapidly towards the sea (progradation). The result is a delta plain
with a very uneven coastline, with tongues of emerging land with the typical
elongated (digitate) shape jutting out into the sea, like a bird’s foot. These are
produced by the rapid seaward advance of the distributaries, confined by natural
banks, which erode the sandbars previously deposited at the mouth. In Italy, the
most obvious examples of this type of delta are the mouths of the Po and Isonzo.
Instead, the São Francisco, in Brazil, is the prototype of the wave-dominated
delta, with its classic cuspidate shape. The morphology is characterised by a
small number of distributary channels, from the mouth of which a very regular
coastline departs, forming two lateral wings. The action of waves and currents
Mouth of the river Isonzo (Friuli Venezia Giulia)
9
10
along the coast prevail over the other components, carrying fine sediments
out to sea and redistributing sand along the shore to form the typical beaches.
The Italian examples are extremely varied, consisting of almost all the deltas
on the Tyrrhenian coast (rivers Arno, Ombrone, Tiber and Volturno) and the
Tagliamento on the Adriatic. In points where the discharges are very limited,
the river mouth may become obstructed by accumulated sediments, which
are only removed on the occasion of full spates. Examples are the mouths of
the seasonal streams which discharge into the Ionian Sea in Basilicata (rivers
Cavone, Bradano, Basento) and Calabria (Torbido, Precariti, Barruca, Novito),
and, in Sicily, the Simeto in the Gulf of Catania.
The growth phases of cuspidate deltas are marked by belts of sand or coastal
dunes separated by depressions which sometimes contain marshes or pools. In
some cases, these survive over long periods of time, and may then be clearly
identified at the sides of the active delta as testimony to its evolutionary history.
An example is the delta of the Tagliamento, which formed about 2000 years
ago, in which eight belts of dunes can be identified, separated by depressions.
The Arno and Ombrone share a similar evolutionary history: initially (between
about 6000 and 25,000 years ago), they formed their own plains by filling wide
lagoon inlets, into which they flowed with internal deltas. These lobes are still
visible today on satellite images as drier areas. Once they had filled in their
lagoons, they began to discharge directly into the sea only during Etruscan-
Roman times. Their growth was subsequently very rapid (around 7 km in 2500
years), due to sediments arriving from the drainage basins which were rapidly
being deforested by man. On the delta of the Ombrone in particular, there are
between 12 and 16 main sand belts, some of which are still separated by
pools, known as “chiari”.
The history of the evolution of the Po is extremely complex, as the river has
been considerably influenced by human interventions aimed at regulation of its
distributor channels. According to descriptions by Pliny and Polybius, there
were three main distributaries during the 2nd-5th centuries A.D., forming the
same number of cuspidate deltas. In more recent times, with a hydrographical
network completely modified by man, the so-called modern delta has been
formed, now extending for about 730 km2 and more similar in shape to that of
digitate deltas. This evolution was caused by the increased sediment transport
and raising of the banks of the distributor channels by the hand of man, which
has caused their rapid progress towards the sea.
The general reduction in river sediment loads during the latter half of the last
century has recently led to general erosion of the Italian deltas and adjacent
coasts. However, in this respect, it should be emphasised that the shoreline on
both sides of a river mouth is, by its very nature, extremely ephemeral and
subject to sudden variations both seawards and landwards, in response to high
or low discharges of river water and high tides.
Mouth of the Allaro, a seasonal stream in Calabria
Mouth of the Piave (Veneto)
11
12
Types of deltas and estuaries
Annelore Bezzi · Francesca Delli Quadri · Giorgio Fontolan
Isonzo
Long sandbars
edge the sea
outlet of the only
distributor channel
of the river Isonzo,
characterised by a
weak regimen of
waves and shallow
bottoms
(scale 1:100,000).
Tagliamento
The strong currents
along the shoreline
have caused the
cuspidate delta of
the Tagliamento to
become asymmetric.
The dune cordons that
reveal the formation of
the nearby land over
the last 2000 years,
have been obliterated
by farming
(scale 1:200,000).
Po
The modern delta
of the Po, which
extends for
approximately 730
km2, has a similar
shape to that of
digitate deltas, with
various distributor
channels flowing
out to sea with their
loads of sediment
(scale 1:500,000).
Cavone
The currents that
transport sand
along the shore
partially close the
mouth of this small
river on the Ionian
coast of Basilicata.
The emerging
sandbars are only
eroded when the
river is in full spate
(scale 1:25,000).
Ombrone
Owing to the lack of
urbanisation in the
area, the sandy belts
of the cuspidate
delta of the Ombrone
are still clearly
visible, equally
aligned on either
side of the river
mouth as testimony
to the different
phases of growth
(scale 1:100,000).
Tacina
A typical image of
the Ionian coast of
Calabria: the
meagre discharge of
the Tacina seasonal
stream is not
sufficient to remove
the sediment
completely, which
tends to obstruct
the mouth
(scale 1:25,000).
13
14
■ Estuaries
When tides are dominant, river mouths assume a typical longitudinal funnelshaped configuration, often with banks elongated in the direction of the river.
The sea may penetrate hundreds of kilometres upstream, and the main physical
process is mixing of fresh and salt waters to create estuaries.
In the oceanographic literature, the term estuary has a more general meaning.
UNESCO uses Pritchard’s definition: “a semi-closed coastal body of water that
has free communication with the open sea and within which the seawater
dilutes significantly with the freshwater arriving from the surface hydrographical
network”. In other words, it is the mixing of masses of fresh and salt water that
gives rise to estuarine conditions within semi-closed basins. It is thus common
to apply the name estuary to internal lagoon areas (such as the Lagoon of
Venice and its estuary), where the arrival of a sufficient number of watercourses
dilutes the marine characteristics of the basin and generates a marked salinity
gradient between the edges of the lagoon and tidal inlets.
Following this principle, the salt wedge may periodically intrude into the rivers
discharging into the lagoon, extending the estuary environment to the final
stretches of these watercourses.
The above definition is too generic for any more precise distinction of the
different geomorphological contexts, especially in the distinction between
lagoons and estuaries. An estuary might be more properly indicated as “the
lowest part of a river valley flooded by the sea, subject to tidal fluctuations and
the meeting of fresh and salt waters”.
Seawater, therefore, may enter watercourses or correspond to an area of
mixing, depending on how well tidal currents contrast river ones. According
to this definition, given the limited tidal ranges of the Mediterranean, it is not
easy to identify typical estuaries on a large scale in Italy. The Italian coasts are
classified in the micro-tidal category (i.e., a tidal range of less than 2 m),
ranges generally being less than 30 cm. This means that, compared with the
Atlantic coasts of France and Northern Spain, which have tidal ranges of
more than 4 m, the phenomena caused by tides are much less obvious along
Italian coasts.
There are no large rivers with estuaries in Italy. Smaller watercourses with low
sediment loads may form a confined but significant estuary environment in
semi-closed basins, examples being the lagoons of Grado-Marano, Caorle and
Venice, and the smaller inlets of Scardovari, Canarin and Goro in the Po Delta.
An example of a river mouth on the coast of the open sea with estuarine
features is the river Magra, which flows into the Ligurian Sea: hydrographical
studies of its last stretch have identified stratification, water with salinity
comparable to that of seawater being found permanently on the bottom,
which reaches as much as 5-7 km from the mouth when the river is low.
Mouth of the Mignone (Latium): when the river is low, the water stagnates and does not reach the sea
Area of the Goro inlet, Po Delta (Emilia Romagna)
15
■ Lagoons
16
17
In Italy, the northern Adriatic is characterised by abnormal tidal excursions, with
average spring tides of 1.1-1.2 m which, occurring as they do along a
prevalently low-lying coast and affected by limited wave motion, gives rise to
typical morphologies and habitats.
These conditions, together with the significant sediment loads of the major
Alpine rivers, led to the formation of the large lagoons of Grado-Marano and
Venice.
Those in the northern Adriatic are therefore the only Italian cases of true
lagoons, meaning areas of semi-enclosed sea due to the presence of
peninsulas or systems of barrier islands and regulated by the ebb and flow of
tides through one or more channels of communication with the sea (tidal inlets).
In fact, no stretch of water should be called a “lagoon” which is sited in
environments with practically no tide, such as the Ligurian, Tyrrhenian and
Ionian coasts.
The northern Adriatic lagoons originated 5000-6000 years ago, when the sea
flooded vast alluvial plains, and created large bays bordered by rapidly forming
deltas. The sediments from the rivers Isonzo, Tagliamento, Piave and Adige
were progressively transported along the shoreline, until they were distributed
in the form of lidos, thus separating the large bays from the sea (then regulated
by the tidal ebb and flow) and forming the lagoons.
In greater detail, according to studies of deep sediments, the central part of
the Lagoon of Venice began to form around 6000 years ago, whereas in the
northern sector of the lagoon and the delta of the river Adige, the first lagoonal
deposits appear to date back to slightly earlier times. The first dune belts of
the coastal system of the Piave, further to the north, date back to just over
High tide level
Sand bank
Channel
Tidal level
Shoulder
Mudflat
Tidal creek
Levels
High tide, maximum
High tide, average
Mean sea level
Low tide, average
Secondary
channel
Thatched fishermen’s huts in the Lagoon of Grado-Marano
Cross-section of a lagoon
Low tide, maximum
Tides
Annelore Bezzi · Francesca Delli Quadri · Giorgio Fontolan
Tides are caused by the force of
attraction that the Moon and, to a
lesser extent the Sun, exerts on the
Earth. For this reason, the phases of
the moon during a lunar month (29.5
days) give rise to significant
differences in tidal ranges.
During the phase of the new moon, let
us imagine that the Moon lies on a line
linking the Sun and the Earth and is
thus not visible. At full moon, it is
entirely visible, because it is situated
on the same alignment but on the side
opposite the Sun.
During these two lunar phases, called
syzygy, the attractions exerted by both
Sun and Moon are combined, and so
the differences between high and low
tides are at their maximum (“spring
tides”).
Vice versa, when the Moon, Earth and
Sun are at right angles to one another,
the neap phase occurs (only half the
Moon is visible), and the tides have a
small range (“neap tides”).
The Lagoon of Grado-Marano (Friuli Venezia
Giulia) during low (top) and high (bottom) tide.
Moon
SPRING TIDE
NEAP TIDE
Earth
lunar
solar
Sun
combined
TIDAL DATA IN RELATION TO PHASES OF THE MOON
days
0
2
4
6
8
10
12
14
16
18
20
2
new
livello dell’acqua (m)
18
quarter
1
0
SPRING TIDE
NEAP TIDE
full
5000 years ago. Lagoon sediments
found at depths of 6 metres are about
6000 years old.
In the sector between the Piave and
the Tagliamento, lagoon sediments
deposited on the Pleistocene plain are
dated to around 5000 B.C. Further
east, the marine ingression creating the
Lagoon of Marano goes back to
Torcello in the Lagoon of Venice (Veneto)
between 5500 and 4200 years ago.
The Lagoon of Grado appears to have
been formed much more recently, as testified by the presence of the Romans in
the area, and can be traced to the process of easterly migration of the mouth of
the Isonzo.
The natural factors contributing towards the formation and maintenance of
lagoons are, in the first place, those linked to the hydrodynamic system, which
is closely regulated by the tidal action: the hydrographical network of a lagoon
is composed of a dense criss-crossing of channels of different widths and
depths, which allow the masses of water that enter through the tidal inlets to
expand within the basin. Each individual inlet serves a lagoon basin, separated
from the adjacent one by a watershed, an imaginary line representing the point
of separation between the masses of water entering the lagoon from two
neighbouring inlets. Between the watershed of two adjacent basins, the
masses of water only mix in exceptional cases (e.g., strong winds), so the
waters that flow in through an inlet as the tide rises, drain out through the same
inlet as it falls.
The network of lagoon channels can be subdivided into the larger main
channels, which develop from the area of the tidal inlet towards the interior.
They are often the relicts of the ancient fluvial system preceding the marine
ingression, and are in some cases connected with the tributary watercourses of
the lagoon itself. Secondary channels are smaller and shallower, they branch
out from the main ones, and distribute and receive the waters from the
surrounding land.
During the period of spring tides, when the tidal range is at its peak, the water
that flows through the lagoon inlets reaches its highest speeds, even faster
than 1 m/s, and rapidly spreads through the main channels and then through
the secondary ones, until it reaches the tidal flats. As the system of channels
gradually multiplies, water speed falls to a few cm/sec. When the water reaches
the tidal flats, the speed of the current again sharply diminishes, due to lateral
19
20
Water exchange in a lagoon occurs through small channels (tidal creeks), mainly during high tides
expansion of water masses. This
process is repeated in the opposite
direction during the ebb tide: the main
channel acts as a collector, all the
secondary channels distribute on the
tidal flats, and the lagoon waters flow
out towards the sea, reaching their
maximum speed at the tidal inlets.
This is the hydrodynamic model which
is responsible for the arrival, transport
and deposition of sediments inside the
lagoon, giving rise to the various
morphologies found in the basin.
The typical morphologies that
characterise the northern Adriatic
coastal lagoons are grouped in three
morphological zones, according to
Sandbanks may be stabilised by vegetation
(“Zangheri” sandbank meadow, Emilia
their position with respect to sea level:
Romagna)
● the shoals, islands, beaches and
internal coast of the lagoon are above the average level of high tides;
● the main channels and tidal inlets are below the average level of low tides;
● the intertidal areas, i.e., those lying between the average levels of low tides
and high tides, which are periodically submerged. Typical examples are the
tidal flats, i.e., the shallow lagoon beds and secondary channels, which emerge
when low tides discharge water towards the sea.
In lagons, islands are portions of land never submerged by high tides. They may
constitute relicts of ancient beaches, or have originated from the accumulation
of sediment transported by rivers.
Shoals are the most characteristic landforms of lagoon environments. They are
tabular raised areas, mainly composed of silty-clay sediments, which are only
submerged by water in exceptional cases. Shoals have irregular topography
and may contain saltpans, which receive an exchange of water through small,
tortuous channels (tidal creeks), especially during high tides.
Because of the complexity of the processes causing the formation of lagoons
(distribution of freshwater and materials from rivers, marine ingressions, and
later drops in water level), shoals are classified on the basis of their different
origins:
● shoals of ancient river edges, or ones located at the sides of watercourses
which still discharge into the lagoon, with a characteristic long, narrow shape;
21
22
shoals composed of the still emerging part of an ancient coastal plain invaded by brackish waters: they are located on the edge of the lagoon towards dry
land, and their sediments contain a thick layer of deposits of continental origin,
overlain by a thin layer of lagoonal type;
● residual shoals of a marshy environment, with layers of peat in the sediments: they were formed by river alluvial deposits in the lagoon basin which,
following emersion, became covered by marsh vegetation. After river waters
had been diverted they no longer received sufficient amounts of sediment to
impede their rapid sinking, mainly due to compaction of the peat;
● channel shoals: these are in equilibrium, with sufficient sedimentation to
compensate for slow sinking. Their shape is typically concave, with a lowerlying central area; the side bordering the channel is generally steep, whereas
the other slopes gently towards a marsh or tidal flat. Channel shoals form at the
points of convergence of lagoonal flows and in the zones of watershed where,
in ideal conditions, the resulting water velocity is almost zero. These sandbanks
grow in height because the waters that flow over them during high spring tides,
slowed by vegetation, release part of their solid load, and expand due to the
slowing down that sea currents of different intensity and direction exert reciprocally.
For some authors, this last type constitutes a shoal in the strict sense - the
result of the deposition of sediments arriving exclusively from the sea.
●
Mouth of the Agri (Basilicata)
Tidal flats are lagoon beds that only
emerge at low tide, especially spring
tides, and are not therefore suitable for
colonisation by pioneer plants. They
slope very gently (with gradients of only
a few dozen centimetres per kilometre)
and have a system of secondary
channels discharging towards the main
channel.
These channels are meandering and a
few metres wide; their minimum level
coincides with the average of the low
spring tides, so their maximum depth
(slightly more than a metre) depends on
the tidal range and coincides with the
point of confluence with the main
channel.
Sandbanks, mudflats and channels in the
Lagoon of Grado-Marano
Lastly, some of the most typical
environments in Italian lagoons are the
fish-farm basins, exploited since early times for rearing and capturing fish.
These often original basins are generally surrounded by embankments that
separate them from the main lagoon, in order to protect them from the effects
of tides and to regulate the entry of brackish and freshwaters from the
hinterland. They currently occupy vast portions of the Lagoon of Venice and
that of Grado-Marano and, to a lesser extent, those of Caorle and Caleri.
As regards sediments, there is generally a progressive reduction in particle size
from the tidal inlets towards more internal areas. These variations are usually
associated with the energy level of the environment: lower where the basin is
deeper, more sheltered and further from the sea; higher close to the tidal inlets,
in the immediate vicinity of the channels, and in areas where the water is
shallower and the bed is affected by the wind, waves and sea currents. Sandy
sediments are deposited near the tidal inlets, often forming characteristic
morphologies known as flow-tide delta (on the lagoon side) and ebb-tide delta
(on the seaward side).
Sand becomes progressively finer along the main channels and further in,
especially on the tidal flats; fine sand is progressively replaced by mud, which
becomes dominant towards the interior of the lagoon, to become exclusive, in
its finest fraction, closest to the internal lagoon coast, as well as to the
watershed.
23
Submerged vegetation
DANIELE CURIEL · ANDREA RISMONDO
■ Foreword
In optimal environmental conditions, the
distribution of brackish water vegetation
differentiates between that part of the
body of water directly influenced by the
sea and the more landward one, which
is affected by confinement of waters or
sometimes by freshwater. With distance
Chaetomorpha
from the points of exchange with the
sea, submerged vegetation becomes
simplified, because, as the type of sediments alters, from prevalently sandy to
silty and becomes more subject to re-suspension, the morphological and
physico-chemical characteristics of the water also change.
Most of the phytobenthos of lagoons is typically of marine origin, with species
that are capable of tolerating marked reductions in salinity. This dominance of
marine species is also due to the fact that the typical organisms of internal
waters are rarely capable of adapting to salinity higher than 5‰.
When the arrival of freshwater is limited, salinity becomes less important for
regulating the presence and abundance of submerged vegetation, whereas
water transparency, sedimentation, hydrodynamics and the availability of
nutrients become more so. As these environments are shallow (sometimes less
than 40-50 cm), water turbulence due to weather - or human activities, such as
mollusc-farming - may easily trigger events of re-suspension that reduce water
transparency and limit compaction of sediments.
When dystrophic conditions change with season, few variations occur in the
structure of the communities, and the resilience of the plant communities allows
typical conditions to be restored within a reasonable time. If dystrophic conditions
persist for more than one year, submerged vegetation substantially modifies, with
the gradual substitution of species with long life-cycles and high ecological value
(such as seagrasses, brown algae like Cystoseira barbata and Fucus virsoides) by
species with short life-cycles (such as the Ulvaceae or Cyanophyceae).
Sargassum in the Lagoon of Venice
25
26
■ The macroalgae: green algae
Compared with the marine environment, where red algae prevail, green algae
are the best-represented plants in the transition zone, especially in terms of
abundance. These algae are generally euryecious and find optimal conditions
in the shallows of internal and confined areas where water exchange is limited
and nutrients are plentiful. In the transition environments of the northern
Adriatic, the most abundant and best-known alga is Ulva laetevirens, with its
widened leaf thallus, commonly known as sea lettuce. Other species of the
genus Ulva (U. rotunda, U. curvata) or Gayralia oxysperma (morphologically
similar but with a single layer of cells in the lamina) never reach the distribution
and biomass levels of U. laetevirens.
Another very widespread algal component is that of the Enteromorpha species,
a group of green algae recently unified with the genus Ulva. These green algae,
with tubular simple or branched thalli that may measure from a few centimetres
to 50-60 cm, live in eutrophic environments. The most common species include
U. intestinalis, U. clathrata and U. flexuosa. In summer months, the gases
present in the cavities and products of photosynthesis raise the thalli from the
bed, forming drifts floating on the surface of the water.
During the summer, especially in the internal waters of fish-farm basins or in
association with seagrasses, uniform carpets of the green filamentous algae
Ulva laetevirens
Chaetomorpha aerea and C. linum
(perhaps conspecifics) are very
frequent. In the summer months they
are often mixed with green algae of the
genus Cladophora (e.g., C. sericea, C.
albida, C. rupestris, etc.) In the 1980s
and early 1990s, all the lagoons in the
northern Adriatic were afflicted by
blooms of these green macroalgae, the
The green alga Valonia aegagropila
prevalent species in terms of extent
and biomass being Ulva laetevirens.
Recorded since the 1940s-50s in muddy areas of the Po Delta, in the Valli di
Comacchio and Lagoon of Venice, but today much rarer and limited only to
hollows, are the subspherical-shaped thalli, 3-8 cm long, of the green alga
Valonia aegagropila. These macroalgae may form extensive, uniform,
continuous coverings up to 30-40 cm thick, and with a wet biomass of more
than 10 kg/m2. In fish-farm basins where salinity is maintained constant at
values slightly above 10‰, partly in order to favour hunting, green algae
belonging to the genera Chara and Lamprothamnion (L. papulosum) are found
on the bottoms, mixed with the seagrasses Nanozostera noltii or Ruppia (R.
maritima and R. spiralis) or with the green alga Valonia. These macroalgae are
often not taken into consideration by marine algologists because they are only
found in transition environments and adjacent flowing waters.
Coherent substrates in these brackish environments host a few macroalgae
which would have difficulty in living on loose substrates because of their need to
anchor themselves to something solid. Both in vivified areas and in internal areas
close to the transition to brackish waters, on the highest levels which are only
wet during high tides (at the limit between the intertidal and supralittoral areas),
are green algae of the genus Blidingia (B. minima, B. ramifera), which are
resistant to emersion. The height of the tides and wave motion caused by
shipping may raise the colonised level by up to 20-30 cm. Below this, starting
from average sea level, in the intertidal zone, a filamentous distichous green alga
(Bryopsis plumosa) may be observed, followed, but only in vivified environments
like the Lagoon of Venice, by the green alga Codium fragile subsp.
tomentosoides, with its cylindrical dichotomous branching thallus. This alga, a
native of the Pacific Ocean and Asiatic seas, has been present in the northern
Adriatic since the 1970s-80s. Although viewed as one of the most invasive
algae, because of its ability to adapt to differing conditions of salinity and
temperature, only isolated thalli are found, at least in the Lagoon of Venice.
27
28
Alien species
For more than a decade now, several
species endemic to other seas have been
found on both soft and firm substrates
in transition environments in the
Mediterranean. They have arrived thanks
to the voluntary and involuntary actions of
man. Until the Suez Canal was opened in
1869, the introduction of new species into
the Mediterranean occurred accidentally
through the Strait of Gibraltar. With the
opening of the Suez Canal, the Red Sea
and the Mediterranean, having remained
separated for more than 10 million years,
were put into communication again.
For hydrological and ecological reasons,
migrations are prevalently towards the
Mediterranean. In the last 20-30 years,
this slow but continual introduction
of species has become insignificant
compared with the much faster pace with
which man has favoured the introduction
of new species along the coasts of the
Mediterranean. Transition environments,
historically the sites of important
economic activities (fishing, shipping and
port structures, etc.), are consequently
the main points at which new species
arrive. If they adapt to the new
environmental conditions, they can spread
until they eventually reach an equilibrium
with native species. The spread of others,
which do not find optimal conditions in
the new environment is limited, or they
disappear. In the Mediterranean, the
two most striking examples of transition
environments affected by many alien
species, are the Lagoon of Thau in
Southern France and the Lagoon of
Venice. In the latter, more than 20 species
have been recorded on the substrates
off Chioggia and Venice since the early
1990s, and, because they are found
outside their potential distribution area,
they are correctly defined as alien species
Daniele Curiel · Andrea Rismondo
or NIS - non-indigenous species. There
have only been a few records for some of
them (e.g., Sorocarpus sp., Prasiola sp.),
probably because they have not adapted
to the new and complex lagoon
conditions. Instead, others have spread
over the years and become integrated
with native species (e.g., Antithamnion
nipponicum, Grateloupia turuturu,
Codium fragile subsp. tomentosoides).
Yet others have spread rapidly and
become dominant in the macroalgal
community, with widespread diffusion and
high abundance (Sargassum muticum,
Undaria pinnatifida, Polysiphonia
morrowii). A very unusual case, which
now involves various lagoons of the
Mediterranean, the Atlantic coasts of
France and the northern seas, is that of
the two brown algae, Sargassum and
Undaria. Although having different lifecycles - Sargassum being perennial and
Undaria annual - in the climate of the
Northern Adriatic these two species have
thalli observable from October and
November, which disappear between
June and July. Attempts have been made
to eradicate them in various locations, so
far with little success. Both species have
extremely efficient dispersal systems for
their millions of spores, which can be
taken great distances by tides - and even
fragments of thalli have high viability.
Undaria
■ Red algae
As in the marine environment, red algae
are a numerically conspicuous part of
the aquatic vegetation in transition
environments, especially in those more
highly vivified, like the lagoons of
Venice and Grado-Marano.
Being ecologically sensitive, especially
to any reduction in salinity and increase
in turbidity (natural or induced), red
algae are limited in the fish-farm ponds,
the inlets in the piallasse (marshy areas)
between Romagna and the Veneto or
towards the interior shores of the large
lagoons of Venice and Grado-Marano.
Only more tolerant species, which can
Gracilaria
adapt to waters rich in nutrients and
sediments arriving from the rivers, can survive. On loose beds, the species
typical of brackish water include those of the genus Gracilaria (G.
bursapastoris, G. armata and G. gracilis). Gracilariopsis longissima (previously
known as Gracilaria verrucosa) formed extensive macroalgae covers in the
1980s, alone or in association with Ulva.
Today, the phenomenon of algal bloom appears to be more limited in the two
large lagoons of the northern Adriatic, but reappears almost annually in the small
to medium-sized transition environments in the Po Delta, although less intensely
than in the past. With currents and winds, the thalli of Gracilaria / Gracilariopsis
roll around on the bottom and accumulate in sheltered areas, where they may
proliferate.
In summer months, in the Lagoons of Venice and Grado-Marano, with their
reduced hydrodynamics, more or less extensive covers of Spyridia filamentosa
are to be found.
Mainly in the spring and summer months, on loose bottoms without macroalgal
cover, natural coherent substrates such as the valves of molluscs (Tapes,
Cerastoderma, Cassostrea or serpulid tubes), or artificial substrates (stones,
shingle) favour the presence of red algae with pulvini (cushion-like
enlargements), 5-10 cm tall.
Among the most eye-catching are the branched filamentous forms of
Ceramium (the dark-red C. virgatum, or C. diaphanum, with alternating light
29
30
and dark bands) and the Antithamnion (A. cruciatum, and, in the Lagoon of
Venice, A. nipponicum, a native of Oriental seas). Among the branched
laminar forms, Nitophyllum punctatum and Radicilingua thysanorhizans are
especially striking.
Conspicuous numbers of red algae are also to be found on the leaves of
seagrasses, especially Cymodocea nodosa. On the oldest (external) leaves, a
rich community of medium-small species develops, including the filamentous
forms Ceramium, Antithamnion, Chondria capillaris or Chondria dasyphylla, and
various species of the genus Polysiphonia. The red encrusting coralline algae of
the genus Hydrolithon and Pneophyllum fragile may also be abundant,
especially in the months of scarce or no growth of leaves.
In the Lagoon of Grado-Marano, but to an even greater extent in that of Venice,
there are many artificial coherent substrates (stones, shores of the islands,
signalling devices for shipping, etc.), where species typical of these substrates
may be observed. Above the average tidal level, Gelidium pusillum and
Gigartina acicularis are frequent, with their branched filamentous thalli firmly
anchored to the substrate and able to resist emersion for some hours. Below
the average tidal level are species of the genus Polysiphonia (P. morrowii, a
species native to the Oriental seas, P. harvey and P. denudata) and, lower down,
Rhodymenia ardissonei, a sciophilous (shade-loving) alga, indicative of the lack
of light in these environments caused by water turbidity.
Polysiphonia morrowii
■ Brown algae
Brown algae do not find ideal conditions
for growth in transition environments.
On the coherent substrates of the two
largest lagoons, species of ecological
value can only be found in areas more
affected by seawater distribution, with
Fucus virsoides (a species of boreal
affinity, present in the Mediterranean
only in the northern Adriatic), Cystoseira
barbata or Petalonia fascia.
Dictyota dichotoma var. dichotoma and
Scytosiphon lomentaria may also be
observed in more internal areas.
The microscopic species of the genera
Myrionema, Leptonematella and the
Fucus virsoides
filamentous forms Ectocarpus and
Hinksia are typically found on the leaves of the seagrasses. The latter are also
the only brown macroalgae which, in the presence of nutrient availability and
reduced hydrodynamics, may also flourish on loose substrates. In the Lagoon
of Venice, this occurs mainly amongst the seagrasses when, following their
summer proliferation, the filamentous thalli of Ectocarpus become entangled
with the seagrass leaves, forming larger and larger clumps.
Two brown algae, Undaria pinnatifida and Sargassum muticum, which are
endemic to Far Eastern seas, have been found on artificial solid substrates in
the Lagoon of Venice since the early 1990s. The large size of the thallus and
the abundances these species are able to reach render them highly
competitive with native species.
Mention should also be made of the algae of the genus Vaucheria. In the deep
beds of the lagoons of Venice and Grado-Marano, there has been a
proliferation of these macroalgae in the past decade; unlike all the other
species, they do not need a firm substrate to anchor themselves because they
grow partly immersed in sandy-muddy sediments. Vaucheria dichotoma var.
marina, today identified as V. submarina, is more common and has been
recorded since 1800, but V. piloboloides has also recently been found in the
Lagoon of Venice. These macroalgae play an important role in compacting the
sediment layer and greatly reducing re-suspension of sediments during
periods of water turbulence.
31
32
Phythoplankton
The highly variable environmental
conditions of brackish-water
environments, in terms of salinity level
(10-40‰), temperature (sometimes
exceeding 30 °C), turbidity (Secchi disk
values up to 10-30 cm) and dissolved
oxygen (from anoxic to super-saturated
conditions), mean that only
phytoplankton with a wide ecological
range and high quali-quantitative
variability during the course of the year
can survive. As well as abiotic factors,
an important role is also played by biotic
factors, such as the presence or
absence of herbivorous predators.
For clearer understanding of variability
over the year, in the Lagoon of Venice
in the early years of the 21st century
there have been minima of 1x105
cells/litre in winter and peaks of 19x106
cells/litre in spring-summer.
The most abundant taxa in all transition
environments are the Bacillariophyceae
and Pyrrophyta, more commonly known
as diatoms and dinoflagellates. Diatoms
have a silicified cell wall (frustule)
formed of two interlocking valves.
In the Lagoon of Venice, the most
common and abundant diatoms belong
Nitzschia (approx. 2000x)
Daniele Curiel · Andrea Rismondo
to the genera Chaetoceros, Cocconeis,
Nitzschia, Thalassiosira and
Skeletonema and, among the forms
which constitute mucilaginous tubules
and are also epiphytes of seagrasses,
Navicula and Melosira. Species of the
genera Prorocentrum and Gyrodinium
are frequent among the dinoflagellates.
In the Lagoon of Grado-Marano,
the diatoms of the genera Synedra,
Navicula and Rhizosolenia are frequent
and abundant. Among the
dinoflagellates are Exuviella,
Gymnodinium and Prorocentrum.
In the brackish-water lagoons of Emilia
Romagna, diatoms of the genera
Nitzschia, Thalassiosira, Skeletonema
and Melosira are common.
Of the dinoflagellates, those of the
genera Prorocentrum, Peridinium or
Glenodinium have often produced
large-scale algal blooms.
The microalgae belonging to the
chlorophytes (green algae) are less
abundant, but no less important.
They include species belonging to the
Volvocales (Dunaniella being typical
of hyper-saline basins) and
Chlorococcales (with Chlorella).
■ Seagrasses
33
Lagoon beds may be covered by
varying densities of meadows of
seagrass, a small but important group
of flowering plants which are fully
described in the Italian Habitats volume
“Seagrass Meadows”.
Posidonia oceanica, Zostera marina,
Nanozostera noltii, Cymodocea nodosa
and two species of the genus Ruppia
are present in the Adriatic.
The success of these macrophytes in
the underwater environment of lagoons
is due to the various morphological and
physiological adaptations which they
have had to develop to survive so well.
Posidonia oceanica
These include adaptation to the salty
environment and to submersion (hydrophytic habitus), the capacity to resist
the action of waves and tidal currents, and hydrophilic pollination and
dissemination.
SPECIES
GEOGRAPHICAL DISTRIBUTION
Posidonia oceanica
Gulf of Trieste, including Grado coast
Western Istrian coast
Venetian coast (dead mattes)
Zostera marina
Western Istrian coast
Gulf of Trieste
Lagoons of Venice and Grado-Marano
Lagoon areas of Po Delta
Nanozostera noltii
Istrian bays and sheltered areas
Western Istrian coast
Gulf of Trieste
Lagoons of Venice and Grado-Marano
Lagoon areas of Po Delta
Cymodocea nodosa
Istrian bays and sheltered areas
Lagoons of Venice and Grado-Marano
Scattered along lidos of Jesolo, Eraclea and Caorle
Ruppia sp.pl.
Shoal and drainage areas of northern Adriatic lagoons in general
Cocconeis (approx. 2000x)
Seagrass distribution in the Adriatic Sea
34
Nanozostera noltii
The main factors regulating the
presence and distribution patterns of
seagrasses on the beds of coastal
lagoons are, in order of importance:
particle-size distribution of sediments,
water turbidity, temperature patterns,
local hydrodynamics and resulting water
exchange, eutrophication and the
consequent presence of macroalgae.
The quality of waters and sediments
and the hydro-morphological factors
of basins tend to favour one or other of
the existing species.
In the Adriatic coastal lagoons, Ruppia
is present in a variety of microenvironments. These cannot be
precisely characterised in terms of
Ruppia
salinity, because the genus is to be
found not only in the fish-farm ponds of Venice and Grado-Marano and in
some habitats with very low salinity, but also in pools nearer the sea, which
are subjected to high salt concentrations.
In the northern Adriatic lagoons, Nanozostera noltii is a typical species of
marshes and tidal flats, the more internal and confined areas subjected to
cycles of emersion and with lower salinity than that of the sea.
However, in the larger and morphologically more complicated basins, such as
the Lagoon of Venice, the species may have more than one phenotype,
sometimes colonising tidal flats with low, dense carpets and thus ensuring the
morphological protection of these systems.
At other times it forms extensive meadows on the deeper beds, where it
grows much larger and with longer leaves. In the northern Adriatic, it is
substantially homogeneous, being found both in the Lagoon of Venice and in
that of Grado-Marano, on the same morphological systems and therefore on
tidal flats and on the edges of deeper channels, on clearly silty beds. Very
rare and extremely localised in the other, smaller, Adriatic basins, it has been
reported close to the sluices or entry channels of waters such as those at
Comacchio.
Since the late 1990s, N. noltii has been undergoing a highly generalised
process of retreat from the areas of sandbanks - particularly marked in the
Lagoon of Venice. This has many causes - such as the general increase in
35
36
turbidity and prolonged periods of emersion during very hot sunny periods but the intensified erosion processes, especially in the lagoons suffering from
severe human pressures, should not be underestimated.
The larger Zostera marina, with leaves 6-7 mm wide and up to a metre long,
lives almost always submerged in more turbulent and less confined waters,
preferring bottoms with silty sand.
Its distribution area covers truly lagoonal waters, as in the lagoons of Venice
and Grado-Marano, where it forms extensive meadows. In the Po Delta and
the brackish lagoons along the Romagna coast, its populations are rarer and
more scattered, mainly in the channels linked with the sea.
However, even truly marine waters cannot be excluded, because along the
coast east of Grado, near the mouth of the river Primero, which has no
artificial protection and is characterised by entirely natural beds, there is a
whole alternation of sandbars and depressions which is widely colonised by
Z. marina. Alternating with Cymodocea nodosa, it most probably takes
advantage of the mixed sandy-silty sediments and the presence of
discharging freshwater.
Zostera marina is the most euryhaline of the species described here,
tolerating salinity of between 5‰ and that of seawater; for this reason,
paradoxically, it may spread towards the coastline, and in any case in the
directions of flows of freshwater, more than Nanozostera noltii can.
In the lagoons of Venice and Grado-Marano, N. noltii forms large vegetated
beds that extend from vivified sites to others which are partly confined. It
avoids areas subject to water stagnation, as the upper limit of the favourable
temperature range for this macrophyte is around 25-27°C.
Cymodocea nodosa, with a dense, robust root system, is found close to inlets
where sediments are predominantly sandy and water exchange is medium to
high. A stenohaline species, it cannot resist conditions of high turbidity and,
although it grows at depths of up to 10 metres along the Istrian coast, it is not
found deeper than 4 metres near the lagoon inlets, on occasional delta
sandbars and in the other Adriatic coastal lakes like those in Apulia. C.
nodosa is a species of sub-tropical origin and is highly seasonal, especially in
the northern Adriatic lagoons, where the temperature may drop below zero in
winter and rise above 30°C in summer.
In winter, the macrophyte uses a strategy of conservation of the root system
and hypogeal (buried) stalks, where sugar is stored, and this may lead to the
loss of all its leaves. Only with the marked rise in temperature, usually in May,
is there rapid and intense re-growth which, in the space of a few days, gives
rise to dense new meadows of brilliant green.
At the inlets, on coastal sandbars, and in the brackish coastal lakes of the
southern Adriatic, C. nodosa retains its leaf apparatus in the winter months,
although it may be reduced, degraded and covered by epiphytes.
Foglie di Zostera marina
Cymodocea nodosa
37
39
Aquatic invertebrates
DAVIDE TAGLIAPIETRA · ALESSANDRO MINELLI
Deltas, estuaries and lagoons are
transitional
coastal
ecosystems
characterised by gradients of many
important environmental factors, such
as water exchange, salinity, sediment
structure, turbidity and nutrient load.
The transition gradient is generally
perpendicular to the coastline, from the
sea landwards, with consequently
progressively changing habitats and
biological communities.
■ Variations in biodiversity
Mouth of the Idume near Lecce (Apulia)
Gradually penetrating a transitional ecosystem, there is a progressive decline in
the number of species. This phenomenon occurs in both directions - from the
sea towards the land, and vice versa - and biodiversity reaches its minimum in
the area where river and sea meet.
The accumulation of organic matter in areas subject to slow exchange and
hydrodynamics plays a key role in modulating the availability of oxygen, which
becomes a strongly limiting factor that increases the vulnerability of
biocoenoses. In Mediterranean lagoons, where the contribution of freshwater is
lower and the seawater is less diluted, the reduction in the number of species
along the sea-land axis is mainly attributed to the hydrology of the basins and
sediment properties, and only secondarily to salinity.
Tides greatly accelerate these exchanges, and their cyclical nature generates
characteristic models of time-space distribution. The periods of emersion and
submersion dictated by the tidal regime are other important factors affecting
biological communities in transition environments.
Most of the species which live in these areas are of marine origin, so that the
degree of connection with the sea greatly influences the recruitment of
species which require a larval stage involving marine dispersal, and this has
Cerastoderma shells
40
Detail of the shell of Acanthocardia spinosa,
a bivalve
repercussions on the biodiversity and
structure of the communities. It is
therefore to be expected that, from
the sea landwards, the progressive
divergence from marine environmental
conditions are tolerated by fewer and
fewer species.
The opposite situation occurs for
species of river origin. Towards the river
mouth, there is a progressive departure
from typical river environments and the
number of species tolerating the new
environmental conditions gradually
falls. This transition from a continental
to a marine environment in deltas,
estuaries and lagoons combines these
two-way changes in biodiversity from
the sea towards rivers and vice versa.
■ Classification zones and their fauna
A salinity-based empirical method to classify coastal and transitional waters
was proposed in 1958 after the IUCN Venice Symposium, an event at which the
major international experts met to agree on a system of classification. The
following zones were identified: hyperhaline, with salinity >40‰; euhaline 4030‰; mixohaline 30-0.5‰; and limnetic 5-0.5‰.
As most transitional environments fall within the wide mixohaline zone, the
latter was further subdivided into: mixo-euhaline with salinity >30‰ but lower
than that of the adjacent sea; (mixo-)polyhaline, 30-18‰; (mixo-)mesohaline,
18-5‰; and (mixo-)oligohaline, 5-0.5‰.
However, zoning based on salinity is extremely variable over time and is only
based on one of the factors of the transition gradient. Instead,
physiographical zoning is based on the most obvious morphological and
structural characteristics of the environment, is less variable over time, and
may be integrated, where needed, with the “Venice System”. The broad
physiographical zoning of a typical transition environment such as a coastal
lagoon is as follows.
Moving from the sea towards the river, a lagoon basin is divided into three
large sections. The first is the zone of the actual inlet in which the sea’s
energy is increased. Here, both inside and outside the inlet, the sediments,
composed generally of sand or silty sand, fan out to form tidal flats.
Channels then lead to a second zone, the central basin, which is generally
rather shallow. This is where the energy from both sea and river is lowest and
abundant deposits of fine sediments are found. As the central basin forms
the most extensive part of the lagoon, it is not homogeneous and slopes
gradually towards more internal shallows, with sandbanks along the edges
of the channels.
The complex interplay of tidal energy, river energy and hydrodynamics in
general decides how close the sandbanks approach the sea, reducing areas
of open water in the central basin.
Marshland and sandbanks succeed one another towards the lagoon edge as
far as the third large zone, composed of the mouth of the river. Here we find
sandbanks, first mesohaline and then oligohaline, dominated by reed-beds,
which are progressively replaced by less salty environments, until the true river
environment is reached. It should be noted that drainage operations in almost
all Italian lagoons have greatly reduced the zones of lagoon inlets and
oligohaline marshes.
In a delta dominated by tides, river energy is such that the central basin
disappears and, with it, the capacity to trap fine sediments, which are thus
deposited further inland, past the line of the mouth.
Lagoon of Venice, with the typical bricola, signalling deep-water channel for shipping
41
42
The crab
The common crab or green crab
Carcinus aestuarii (= C. mediterraneus) is
a decapod crustacean characteristic of
the transition and coastal environments
of the Mediterranean and the Black Sea.
However, its status as a species is under
discussion - according to some authors,
it is merely a subspecies of Carcinus
maenas, the affine and vicariant form
(i.e., occupying the same ecological
niche) which lives along the Atlantic
shores of Europe.
The species is well-adapted to the high
spatial and temporal variability of
environmental conditions in lagoons,
estuaries and deltas. It is able to
tolerate salinity values higher than those
typical of the sea or ranging as low as
an oligohaline state, and temperatures
of between approximately 0°C and
30°C. The wide ecological range of the
crab is confirmed by its omnivorous
feeding habits, although predatory and
Carcinus aestuarii
Marco Sigovini
detrivorous behaviour prevails. In
conditions of high density, the common
crab can regulate the abundances of its
prey species (bivalves, polychaetes,
and other invertebrates), significantly
influencing the community structure.
In turn, it constitutes an important food
resource for limicolous birds and
estuarine and sea fish such as eels,
gobies and sea bass.
Like all arthropods, the crab must moult
several times during its lifespan,
replacing the outer shell or exoskeleton,
to allow for growth (see drawing). This
process is divided into four stages: premoult; true moult, which only lasts for a
few hours; post-moult, and inter-moult,
the stage of “resting” and preparing for
the next cycle. In general, adult males (m)
moult in spring and autumn, whereas
females (f) undergo only the first moult,
later than the males. Mating takes place
during this latter stage (a). The females
A
I
II
B
XII
f
III
XI
m
X
IV
a
VI
D
IX
V
VIII
VII
C
The life-cycle of the crab. On the left, a diagram showing moult phases - dashed line: periods of the
moult; continuous line: inter-moult (for abbreviations, see text)
lay their eggs between autumn and
winter, depending on location. In many
Mediterranean estuaries and lagoons,
female crabs migrate towards the sea
in winter, where the eggs then hatch.
This is followed by a planktonic larval
stage, during which stages called the
protozoea, zoea (A) and megalops (B),
follow one another. At the end of the last
stage, the larvae, having returned to the
estuarine or lagoon environment, settle
down as benthic organisms, preferably
in protected habitats like seagrass
meadows, where they find shelter during
their juvenile stage (C). From this
moment onwards, they will moult several
times a year, with lessening frequency,
until reaching sexual maturity (D).
Exploitation of crab meat as a human
food resource is only of importance in
the northern Adriatic, where a traditional
type of fishing exists, principally at
Burano and Chioggia, of high
ethnological and economic interest.
Its targets are the moulting crabs,
called moleche, or moeche, which are
immediately removed from the water to
stop re-calcification of the exoskeleton.
Detailed knowledge of the crab’s lifecycle is extremely important in the
process of selection, and the molecanti,
as these fishermen are called, have a
whole series of names for the crabs
during their various stages: gransio bon,
an individual that still has to moult;
gransio mato or duro, specimens which
will not moult again during the fishing
season; spiàntano, close to moulting;
capelùo, harvested at the moment when
the shell detaches; and mastrùzzo or
struzzo, a few hours after the moult.
Females with mature ovaries (masanéte)
are also considered as delicacies and
are gathered in autumn for the market.
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44
■ Benthic fauna
In analogy to the succession of
different types of bed, which from sand
become progressively silty sand, silt
and then clay, populations of benthic
macroinvertebrates evolve - from the
communities classically attributed to
the biocoenoses of fine sand in tidal
Nassarius mutabilis
deltas, to fine surface sand and muddy
sand in environments sheltered from
the more open areas of the central basin, until we reach the euryhaline and
eurythermal lagoon biocoenoses of the more internal parts. We will therefore
start from an imaginary inlet in a tidal delta, first on the seaward and then on the
landward side.
In the area facing the inlet, we find species typically connected with substrates
with a high sand content and more turbulent waters. Some species of bivalves
are well-known in fish markets, like the clam (Chamelea gallina) and the razor
shell (Ensis minor), whose elongated shape allows it to bury itself rapidly in the
case of danger. Tellins (Tellina pulchella, T. nitida, T. tabula, Donax semistriatus
and D. trunculus) are also extremely common.
Frequenters of beaches are certainly familiar with Acanthocardia tuberculata
and A. echinata, bivalves with a lovely heart-shaped shell, the surface of which
is characterised by radial longitudinal ribs with more or less pronounced
tubercles, marked by deep inter-rib furrows. These species are accompanied
by the miniscule Lentidium mediterraneum, millions of which often remain
stranded at low tide, rapidly destined to enrich the fine shell detritus on the
shore, the white Spisula subtruncata and the larger Mactra stultorum.
It is also easy to identify some of the gastropods, such as the murex (Bolinus
brandaris), together with its cousin Hexaplex trunculus, an active predator of
bivalves whose spawn, like small white sponges, are often beached along the
shore, and Nassarius mutabilis, a small ventricose snail which acts as a
scavenger, feeding mostly on dead animals.
Some families of molluscs arouse particular interest, in that species belonging to
them succeed one another in a characteristic manner along the transition
gradient. This is the case of venerid bivalves, mainly represented here by
Chamelea gallina, and nassariid gastropods, represented by Nassarius mutabilis.
Polychaetes include Owenia fusifromis, a tubicolous filtering worm, which lives
in the sediment in a tube it creates by cementing grains of sand, and Arenicola
marina, which is responsible for the mounds of sand with the appearance of
thick spaghetti that may be seen on sandy silty beds at low tide. This worm is
extensively used as bait by fishermen and can tolerate sometimes considerable
reductions in salinity. Another interesting polychaete is Sabellaria spinulosa,
whose colonies have recently begun to cover hard substrates with a compact
layer of tubes constructed with cemented sand, giving the overall appearance
of a beehive. These polychaetes are important in that they create new
substrates for other organisms.
Among the crustaceans are some species of portunid crabs with moderate
swimming ability belonging to the genus Liocarcinus, like the sand crab
(Liocarcinus vernalis). Close to the shore lives the crab which might be defined
as the king of the lagoon, Carcinus aestuarii (see box). Among the nooks and
crannies of breakwaters and outer sea walls lurks what is probably the most
robust of the Mediterranean crabs, the yellow or warty crab (Eriphia spinifrons),
accompanied in the intertidal zone by the very fast-moving Mediterranean
shore crab (Pachygrapsus marmoratus).
The echinoderms include Echinocardium cordatum and Schizaster canaliferus,
two heart-shaped sea urchins which live buried in sediment. Rocky substrates
are home to Paracentrotus lividus, the classical sea urchin that causes so
much pain to careless bathers descending the cement blocks often used to
create breakwaters.
Sand crab (Liocarcinus vernalis)
45
46
Echinocardium cordatum
The isopod Tylos latreillei passes
daytime hours buried in dry sand,
usually a long way from the foreshore;
at sunset it comes out into the open on
to the wet beach, where it feeds on
various organic materials: beached
marine animals, algae, etc. Before
dawn, it is once again resting in the dry
sand. Regarding this species, worthy of
mention here is the mass migration
observed on Volano beach on the
evening of 10 August 1968 by Antonio
Mussels (Mytilus galloprovincialis)
Giordani Soika, an attentive and
untiring scholar of the natural history of
the Veneto lagoons and the Po Delta. He saw millions of individuals, composing
an almost compact column a dozen metres wide, marching on the damp sand
along the shore, in a southerly direction.
Many of these species enter the lagoon and settle in internal areas in tidal flats.
Here the razor shell (Solen marginatus) replaces Ensis minor, while Chamelea
gallina is less frequent, being flanked by other venerids like Dosinia lupinus and
Paphia aurea. The Philippine clam (Tapes philippinarum) starts to put in an
appearance here, but is much more common in the next zone. This species,
introduced into the northern Adriatic in the last twenty years, has the typical
characteristics of invasive species, such as a high reproduction rate and great
tolerance to estuarine and lagoon environments. Unfortunately, very destructive
systems are used to harvest it. As well as the Philippine clam, other exotic
species introduced by man have become established in these environments in particular, the bivalve Scapharca inaequivalvis and the gastropod Rapana
venosa. The spread of Scapharca is easily explained by the fact that it is
resistant to anoxia, being able to rely on haemoglobin as a respiratory pigment.
The large gastropod Rapana venosa, of Indo-Pacific origin, causes problems,
since it is a predator which grazes on mussel and oyster beds. Its flesh is
edible, but not greatly appreciated.
The extremely common mussels (Mytilus galloprovincialis) find their ideal
lagoon habitat in this zone. Indeed, this is where artificial mussel-beds are set
up, prior to being moved some miles offshore, where the contribution of
brackish water stimulates the growth of phytoplankton and where
environmental conditions during the critical summer and winter periods are
more stable, guaranteeing a longer growing period.
47
48
Nassarius mutabilis declines in abundance, being replaced by other nassariids
such as Nassarius corniculus, N. nitidus and, to a lesser extent, Cyclope
neritea. Nassariids pass their time buried in the sediment, leaving only their
siphons protruding, and with these they monitor the water, waiting to detect the
smell of the dead or injured animals on which they feed. Their sense of smell is
particularly acute, as they have a chemical sensor inside their siphons enabling
them to locate odours dozens of metres away with great precision.
The zone adjacent to lagoon inlets often contains luxuriant seagrass meadows,
with a prevalence of Cymodocea nodosa and Zostera marina. This is a speciesrich environment, where organisms which benefit from organic matter in the
sediment that accumulates at the base of the plants are associated with
organisms which choose the fronds as their preferred habitat. Among the
former, both oligochaetes and sedentary polychaetes are to be found in
abundance, such as the capitellid Notomastus latericeus, which is tubicolous
as a juvenile but lives in tunnels in sediment as an adult.
The invertebrates populating the seagrass fronds include isopods like Idotea
baltica, which look like small fragments of leaves. They may sometimes be seen
“navigating”, clinging to pieces of leaf that they use as boats, pushed by the
beating of their paddle-shaped pleopods. Tanaidacea like Apseudes latreillei
are also frequent, living in small tubes on the surface of the seagrasses, and
easily distinguished from isopods by the pincers on their first pair of limbs. A
great many amphipods live among the seagrass fronds, like the gammarid
Gammarus aequicauda, and graceful shrimps like Palaemon elegans, with their
slender legs ringed in yellow and blue, and P. adspersus, which nibble at any
particles they are able to grasp. Browsers on the microalgal film that grows on
the leaf blades include various species of gastropods, among which Gibbula
are particularly widespread, for example, G. adriatica, and the small cerithiid
gastropods Bittium reticulatum and B. scabrum.
In this environment rich in submerged vegetation, Asterina gibbosa also make
their appearance - delicate starfish a couple of centimetres in diameter, and the
small and fascinating anthozoan Anemonia viridis, which anyone who has
ventured into underwater meadows without adequate protection knows, to
their cost, because these anemones can sting with painful results.
In the meadows closer to the inlets Pinna nobilis is also present. This is a pretty
pinnate-shaped bivalve of large size, whose fan may rise about thirty
centimetres from the bottom and whose byssus used to be spun and woven in
some Italian localities (the island of Sant’Antioco off the south-western coast of
Sardinia was once famous for this). P. nobilis is accompanied by the dark cockle
(Chlamys varia) and the lucinid Loripes lacteus. The latter is an unusual bivalve,
in that it plays host to bacterial symbionts in its gills. These fix carbon dioxide
present in the water, using energy obtained from oxidation of sulphides, which
are abundant in the sediments at the base of seagrasses. Some of the organic
Palaemon elegans
Pinna nobilis
49
50
Xilophagous organisms
Among the rigid substrates available to
benthic communities, wood has the
double function of acting as a support
for encrusting populations, and food
for other organisms, mainly bacteria,
fungi and xylophagous invertebrates
(“wood-eaters”).
As the activity of bacteria and fungi is
very slow, most of the macroscopic
degradation of wood is the work of
invertebrates.
These include bivalve molluscs
belonging to the family of the teredines,
among which the most common are
Bankia carinata, Lyrodus pedicellatus,
Nototeredo norvegica and Teredo
navalis, which excavate large tunnels
as wide as a finger, and crustaceans the isopods Limnoria lignorum and
Limnoria tripunctata and the amphipod
Chelura terebrans, which are responsible
The teredine Lyrodus pedicellatus
Marco Sigovini
for a denser network of galleries.
The evolutionary history of these
organisms has mostly taken place within
environments like lagoons, estuaries and
deltas, where there are large quantities
of wood in contact with the water.
In heavily anthropised environments,
the majority are breakwaters or other
wooden structures, placed there by
man, and their degradation involves
high replacement costs.
The teredines have a wormlike body
with a small shell at the front end.
Evolutionary processes have
transformed this from a protection for
the animal into a digging implement,
rendering it similar to the head of a
drill bit, which these molluscs rotate
with movements of their robust foot.
These animals colonise a site when
planktonic larvae arrive and
subsequently metamorphose and grow
inside the wood, building tunnels along
the entire submerged section.
The dispersal dynamics of individual
species are influenced by the duration
of their larval stage, which lasts for two
or three weeks in T. navalis but only a
few hours in L. pedicellatus. The tunnels,
lined by a calcareous layer, have
practically imperceptible dimensions in
the first stretch, but increase markedly
in diameter and length as the organism
grows. The wood is pulverised and
partly used as food by means of
digestion assisted by symbiont bacteria.
Salinity and temperature are the
principal environmental factors
determining spatial distribution:
T. navalis, euryhaline (tolerating salinity
of 10‰), is the species best adapted
to estuary environments; Lyrodus
pedicellatus finds its optimum in less
desalinated conditions, and other
species require water more like that of
the sea.
Xylophagous crustaceans are also
widely distributed in estuaries, deltas
and lagoons. Although they lack a
planktonic larval stage, their diffusion is
guaranteed by currents which transport
individuals to new sites.
These organisms only attack wood
within the mid-littoral belt. In synergy
with the waves, the outer layers
become detached, conferring the
typical “hour-glass” shape on highly
degraded wooden piles at the point of
mean sea level.
Although the system of tunnels is often
highly unstable, it represents a new
habitat that can be colonised by many
other species of invertebrates.
The “hour-glass effect” caused by xylophagous organisms eating away the wood
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52
compounds obtained from carbon
dioxide fixation are then transferred to
the bivalve.
The central basin of a lagoon may be
divided into two large bands. The first,
more external, facing towards the sea,
has a succession of facies of classical
biocoenoses of the muddy sands found
in sheltered environments; the second,
more internal, towards the river and dry
land, contains some facies of euryhaline
and eurythermal lagoon biocoenoses. In
Upogebia pusilla
the external band of the central basin,
characteristic venerid bivalves are
Paphia aurea and the carpet shell clam (Tapes decussatus). As already
mentioned, Tapes philippinarum is very common, if not completely dominant.
Other species of bivalves characteristic of this zone are Corbula gibba, Gastrana
fragilis and Loripes lacteus.
The rich phytoplankton and abundance of suspended organic matter explain
the frequency of other efficient filterers, such as Mytilaster minimus and the
oyster Crassostrea gigas. The latter species is extremely common, both here
and in the more internal area. Oysters were introduced into Italian lagoons and
estuaries around the middle of the last century and spread rapidly, creating true
miniature reefs in some sites which form a habitat of hard substrate particularly important for the promotion of biodiversity in environments
otherwise characterised by muddy substrates.
The gastropods Nassarius reticulatus and Cyclope neritea abound on lagoon
beds, together with high numbers of Cerithium vulgatum and C. alucaster, with
its elongated shell which may reach eight centimetres in length. Bittium and
Gibbula are also abundant on beds covered by macrophytes.
Roving polychaetes include some predators of considerable size, sometimes a
few dozen centimetres long, such as Nephthys hombergii, Marphysa and
Glycera. In this band, the numbers of nereidid polychaetes like Alitta succinea
(=Neanthes succinea) and the ragworms Perinereis cultrifera, P. rullieri and
Hediste diversicolor begin to be significant, although they reach their peak in
the next area. The mouth apparatuses of these polychaetes, even when due
account is taken of scale, are extraordinary and formidable: among the
predators, eunicids like Marphysa possess many pairs of mandibles of varying
shape, whereas Glycera has a characteristic evertible pharynx with four hooked
mandibles. Although they cannot be
defined purely as predators, the
nereidids possess two powerful,
falciform, serrated mandibles, with a
set of conical teeth. Among the
sedentary polychaetes, some are
tubicolous like Amage adspersa and
the maldanids, also known as bamboo
worms, because of the unusual
segmented structure of their body,
which is similar to a bamboo cane.
The cirratulids of the genus Cirriformia
Hermit crab (Diogenes pugilator)
are sedentary polychaetes of a reddishorange colour, with long filaments like
tentacles, which give them the appearance of jellyfish. They live buried just
beneath the surface of the sediment, from which their tentacular filaments
emerge to capture the organic matter on which they feed.
The high eutrophication of lagoons, estuaries and deltas, especially in the
central basin, also explains the abundance, among the polychaetes, of species
typical of environments with high organic loads, such as Capitella capitata,
Heteromastus filiformis and Polydora spp.
Polychaetes of the genus Polydora are not only found in areas rich in organic
matter. When we eat an oyster or observe the inside of the valves, we may
note blackish blisters containing mud and covered by a thin layer of motherof-pearl. In them live specimens of Polydora cornuta (=P. ligni) and P. ciliata.
These polychaetes can perforate the shell of their host the oyster, aided by
specialised bristles and, probably, acid secretions, until they penetrate the
internal cavity. Sensing a foreign body, the oyster secretes a layer of motherof-pearl around the hole made by the polychaete, forming a chamber within
which the worm then lives.
Along the edge of channels, many holes about as wide as a finger may be seen:
these are the ventilation shafts of the burrows of a crustacean prized as bait by
fishermen: Upogebia pusilla. This thalassinid decapod, pale greenish-beige in
colour, vaguely similar to scampi, reaches a length of around ten centimetres
and lives in Y-shaped tunnels that reach down into the seabed to a depth of
about fifty centimetres.
The shells of gastropods, especially those of Nassarius nitidus, Cyclope
neritea and Cerithium, are often inhabited by the hermit crab Diogenes
pugilator, which is very abundant in this lagoon environment, especially
53
54
Cereus pedunculatus
along the edges of the largest tidal creeks between sandbanks and near
seagrass meadows.
A characteristic anthozoan of this belt is Cereus pedunculatus, which
attaches itself by its basal disc to shells lying just below the surface of the
sediment, from which its crown of tentacles protrudes.
Some echinoderms are to be found In the most highly vivified areas.
Examples are the holothurians Trachythyone tergestina and T. elongata, small
sea cucumbers a few centimetres in length, and the starfish Asterina gibbosa
in seagrass meadows.
Many crustaceans can be found on hard substrates such as channel banks
built in bricks, stone or cement, the poles and brìcole (the typical triangular
groups of thick oak pites defining deep-water channels), including the
amphipods Corophium acherusicum, C. acutum, C. sextonae, Stenothoe
tergestina and Jassa marmorata, and the cirripede Balanus amphitrite, easily
recognised by its vertical purple stripes on a white background.
We are now entering the innermost part of the central basin, an area of high
sedimentation, distinguished by many sandbanks; the seabed is silty-clayey
and the water is often desalinated. All the inhabitants of this area generally
tolerate both seawater dilution and the resulting decrease in salinity, and
relatively stagnant waters.
The bivalves which characterise this zone are Cerastoderma glaucum, Abra
segmentum (=A. ovata) and, in the more brackish areas also containing
groundwater springs, Scrobicularia plana. Cerastoderma glaucum is very
abundant in this band. Its rounded, slightly heart-shaped form prevents it
from sinking very deep into the substrate, from which it often protrudes,
becoming easy prey for crabs.
Scrobicularia plana is a bivalve reaching more than five centimetres in length,
whitish in colour and flattened in shape, which facilitates its burial in muddy
sediments. Its long siphons brush the surface of the sediment, producing
star-shaped traces around the holes from which they protrude.
The flattened shape and long siphons of Abra segmentum are very similar to
those of Scrobicularia, but are much smaller, being only about one centimetre
long. This bivalve also alternates a detrivorous diet with food, which it obtains
by filtering the water, and may be found in dense populations, sometimes
subject to great fluctuations in numbers.
It is in this belt that Nassarius nitidus progressively gives way to Cyclope
neritea. More than a general tolerance to environmental factors, it is likely that
their methods of reproduction determine the distribution of these two
nassariids. N. nitidus lays rows of pointed capsules containing around a
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56
hundred eggs, preferably on seagrass
leaves; the resulting larvae, before
settling, pass slightly less than a month
as planktonic forms carried by
currents. Conversely, Cyclope neritea
does not go through a larval dispersal
stage: it lays single eggs on hard
substrates, such as shells, from which
hatch miniature adults that can
immediately cope with a less stable
environment.
A sedentary polychaete typical of this
zone is the spionid Streblospio
shrubsolii. This small annelid, about
half a centimetre in length, loves
environments close to estuaries. It has
a very high growth rate and its colonies
Nassarius sp.
are very large, so it represents a good
food source for the fry of many species of fish.
Other polychaetes are associated with these species, such as Alitta succinea
and the predator Nephthys hombergii. The behaviour of the nereidid A.
succinea is particularly interesting, as it passes most of its existence on the
seabed but, when it reaches sexual maturity, abandons it at night. The gametes
of this worm are thus freed in the water close to the surface, which explains
why, at certain times of the year, towards midnight, up to 4-5000 Alitta eggs per
litre of water may be counted.
The most common amphipods are Microdeutopus gryllotalpa, which feeds on
fragments of algae, the detrivore Melita palmata, and the filter-feeder
Ericthonius punctatus. The hard substrates of this belt are encrusted with
barnacles (Balanus eburneus and B. improvisus), which also enter the next
zone, often in large numbers.
Our curiosity will undoubtedly be aroused by mounds of mud that rise from the
bottom of pools between sandbanks: these mark the exit holes of the complex
tunnels of another thalassinid crustacean, the ghost shrimp Callianassa
tyrrhena, which may extend for more than half a metre beneath the surface of
the sediment. The name of this species comes from its white translucent
appearance, which does make it appear as if it were the ghost of a prawn or
scampi. Another characteristic of the species is the larger size of one of the two
chelipeds (pincers), which is used for digging the tunnels.
Two very common amphibious gastropods, Truncatella subcylindrica and
Myosotella myosotis, are to be found on sandbanks.
Lastly, this small world between water and land is home to the amphipods
preferring sandy shores belonging to the genera Talitrus, Talorchestia and
Orchestia. These are extremely lively animals of marine origin, which hop
about swiftly on damp sand or amongst the beached detritus where they find
their food.
We are now nearing the estuary zone, where the entry of river water greatly
reduces the salinity of lagoon waters and enriches sediments with organic
matter.
On deep bottoms, Cerastoderma makes way for Scrobicularia, while the tiny
gastropods of the genus Hydrobia accompany the amphipod crustaceans
Corophium orientale and C. insidiosum in pools between sandbanks.
Corophium looks like a sort of mechanical excavator in miniature - only 1 or 2
centimetres in length, it has two very robust arm-like antennae about half the
length of its body, with which it displaces sediments, raking up the particulate
on which it feeds. This is the zone where the roving polychaete Hediste
diversicolor, which fisherman often use as bait, finds its most favourable
environment. Despite being a highly resistant species, it does need a certain
amount of oxygen, and so is often found in the intertidal zone. This polychaete
reproduces only once during its lifetime, which lasts at most a few years.
Amphipod of the genus Talitrus
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58
Gametes are released more or less in synchrony in the entire population. Eggs
and sperm leave the animal through an opening in the body wall, almost always
close to the head. However, not all animals of the same age reproduce in the
same year; some late developers mature during the following year, thus
constituting a valuable reserve if any unforeseen events compromise the
breeding success of their siblings.
Victorellid bryozoans are to be found on hard substrates. The hydrozoan
Cordylophora caspia and the tubicolous polychaete Ficopomatus enigmaticus
are introduced species capable of creating reefs of great size.
The amphipods Gammarus aequicauda, Leptocherirus pilosus and the
tanaidacean Heterotanais oerstedi form large populations in the first stretches
of this sector, but they are also found in other areas of the central basin,
because of their tolerance to environmental factors.
Shallows particularly rich in organic matter, in which there is very little available
oxygen, are inhabited by capitellid polychaetes, including Capitella capitata
and Heteromastus filiformis. Such is the affinity of Capitella for this type of
sediment that its larvae appear to be attracted by the hydrogen sulphide that
rises from it. The very abundant and tiny members of some species of tubifid
oligochaetes are often to be found together with Capitella. Capitellids and
oligochaetes are also abundant in environments rich in organic matter and not
desalinated, as in the deposits forming at the base of seagrasses.
Tubicules of polychaetes Ficopomatus enigmaticus
■ Planktonic fauna
Mesozooplankton (planktonic animals
0.2-2 mm long) play a very important
role as food source for the larvae of fish
and other filter-feeders. Their seasonal
variations in density follow those of
phytoplankton with a slight delay,
peaking between the end of spring and
summer. The importance of plankton is
recognised for all filtering organisms of
the benthos, but less expert naturalists
tend to under-estimate the role of
plankton in the diet of vertebrates.
Tangible proof of this comes from an
examination of the stomach contents
of the most common fish which live in
The harpacticoid copepod Euterpina acutifrons
these basins. The planktophagous
dietary regime of the young of five species of fish of commercial interest is welldocumented, such as the sea bass (Dicentrarchus labrax), the gilthead
seabream (Sparus auratus) and three species of mullet (Liza ramada, L. saliens
and L. aurata). The young of all these species, until they reach a length of five
centimetres, have a planktophagous diet, their preferred prey being calanids,
polychaete larvae and cirripede nauplii. Whitebait (Atherina boyeri) are also
active consumers of plankton. Small individuals feed mainly on the more
minute fraction of zooplankton, composed of the earliest larval stages (nauplii)
of copepod and cirripede crustaceans, as well as the larvae of polychaetes and
molluscs. As the fish grow, their attention turns more to the larger components
of the plankton, such as mysidacean crustaceans and decapod larvae, as well
as benthic organisms, especially small polychaetes and amphipods.
In proximity to the sea inlets, where fluctuations in salinity are more
pronounced, the biocoenosis is rich in species, with cladocerans, cyclopoid
copepods and neritic harpacticoids (for example, Oithona spp., Oncaea spp.,
Euterpina acutifrons) and tunicates (appendicularians).
In lagoons, zooplankton is mainly marine (neritic: i.e., typical of the open sea)
around the inlets and in the more external belt of the central basin. In internal
parts it also includes a native component, composed of organisms that can
pass their entire life-cycle in brackish waters, and also types of freshwater
origin. An important component of meroplankton are the larvae of various
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60
species of annelid polychaetes, gastropod and bivalve molluscs, and cirripede
and decapod crustaceans.
Within the marine component of plankton, the important group of the copepods
is represented by many species: the calanids, typical of neritic plankton,
include Acartia clausi and Paracalanus parvus; the cyclopoids include Oithona
nana, O. similis, O. plumifera, the pecilostomatoids include Oncaea sp.,
Corycaeus sp. and, lastly, the harpacticoids include Euterpina acutifrons and
Microstella norvegica. Among the cladocerans, which are more abundant in the
summer months, the most frequent species are Podon polyphemoides and
Penilia avirostris. Other zoological groups, present at least seasonally, are the
tunicates (appendicularians), hydromedusae, turbellarians, nematodes,
ostracods, amphipods, mysids, chaetognaths (Sagitta sp.), and also the eggs
and larvae of fish.
A very curious although irregular presence is Noctiluca miliaris, a spherical
unicellular organism almost a millimetre in diameter - an exceptional size for
an organism composed of a single cell. It is capable of emitting light if
stimulated by water movement - a phenomenon easily observed on the
occasions between spring and summer when Noctiluca reproduces in infinite
numbers in coastal waters. The components of the central basin, especially in
the internal part, include the larvae of cirripede crustaceans, some rotifers (two
species of Synchaeta and Brachionus plicatilis) and many copepods:
The copepod Canuella perplexa
Calanipeda aquaedulcis, Canuella
perplexa, Halicyclops sp., Harpacticus
sp., Microarthridium fallax, Acartia
margalefi and A. tonsa.
In the Lagoon of Venice, the seasonal
abundance of zooplankton is at its
minimum in January, with dominant
species Acartia clausi, Paracalanus
parvus, Centropages spp. and Oithona
spp. There is a slight increase in April,
and then a period of growth dominated
by Acartia tonsa until the peak in July.
The copepod Calanipeda aquaedulcis
This is followed by a rapid, progressive
decline in the following months.
Absolute maxima and minima, in summer and winter respectively, are recorded
in the most internal areas of the lagoon, mainly due to the thermophily (heatloving characteristics) of the dominant species. The larvae of polychaetes,
molluscs, crustaceans, decapods and fish peak during late spring. Again in the
Lagoon of Venice, in the last fifty years there have been marked changes in the
specific composition of the lagoon zooplankton, especially within the genus
Acartia. The most typically marine species, A. clausi, is still abundant near the
inlets, while the brackish-water species A. latisetosa has become increasingly
rare. A. margalefi typically occupies areas with intermediate characteristics, but
since the 1980s it has progressively declined in favour of A. tonsa which,
recorded for the first time in the Lagoon of Venice in 1992, has now become the
dominant species of the community.
In the oligohaline part, where a mass of denser saltier seawater penetrates
upriver near the bottom beneath the river waters, there is a mixing of brackish
and river water components of the plankton. Freshwater plankton, composed
mainly of rotifers, is joined by marine copepods and a few larvae of another
typically marine group, the polychaetes.
True plankton, naturally, exists in the sea and in still internal waters, especially
in lakes. Identification of true plankton in a river environment may be debatable.
In the flowing waters of a river, as well as swimming organisms (essentially fish),
organisms typical of lake beds may also be found, snatched from their natural
environment and carried away downstream. However, it is difficult to imagine a
true population of small river organisms, incapable of any particularly active
movement, which can live perennially in the water without being inexorably
carried off down towards the sea.
61
Fishes
GILBERTO GANDOLFI
The fish fauna in the brackish waters of
lagoons, deltas and estuaries includes
species that are best classified
according to the characteristics of their
biological cycles. Some are constantly
present throughout the year, like
anadromous species, the adults of
which migrate to fresh water to spawn,
and juveniles which follow the same but
Whitebait (Atherina boyeri)
inverse route to grow to maturity in the
sea; conversely, the adults of
catadromous species go to sea to breed, whereas juveniles ascend internal
waters. Then there are species present in brackish-water environments for
various periods during the year, with transfers to the sea to avoid unfavourable
environmental conditions and also for breeding - because they have floating
eggs which would not be able to develop in internal waters. Lastly, there are
both marine and freshwater species which are found in brackish waters only
occasionally.
■ Resident species
These are typically euryecious fishes, i.e., species able to tolerate wide and
rapid variations in the chemical and physical parameters of the environment in
which they live. Their capacities for adaptation allow them to pass their entire
life-cycle in lagoon environments. Lagoons are populated by a very diversified
resident fish fauna, due both to the different environmental conditions in
individual areas, and to influences determined by the times and ways in which
lagoons came into being in their particular geographical locations.
The populations of fish species in internal waters have been affected by several
events since the Messinian geological epoch, during a salinity crisis in the
Mediterranean about 5 million years ago. In the northern Adriatic, fish were also
influenced by preceding events in the middle Pliocene, when the Po Valley district
Black goby (Gobius niger jozo)
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64
was affected by links then existing
between the Adriatic and para-Tethys
seas, and by more recent events, during
the Pleistocene (approximately 2 million
years ago), caused by phenomena in the
Ice Ages. As regards the fish fauna of
internal waters in general, the Po Valley
district is richer in species and endemic
forms than peninsular Italy and the
Pipefish (Syngnathus acus)
islands, for the reasons mentioned
above and also because of the greater
quantities of internal waters and the consequent greater abundance and variety
of transition environments between fresh and sea waters.
The characteristic common to the resident fish species in lagoon waters is that
they are usually smaller than the marine or freshwater species belonging to the
same genera or families. Living in a highly unstable environment, with frequent
and rapid variations in temperature, availability of dissolved oxygen, salinity of
the water and availability of food, these populations anticipate sexual maturity
as much as possible, with the consequently reduced size of adults. In other
words, they adopt a strategy of population growth, defined as type r, in which
the adults produce a large number of descendants which quickly replace them.
In their turn, the new generations exploit favourable environmental conditions
and rapidly reach sexual maturity, managing to survive critical periods,
although in fewer numbers.
In lagoons, environmental crises generally occur in summer, because shallow
waters heat up rapidly and there is a consequent reduction in oxygen, which
dissolves in an amount inversely proportional to temperature. Conversely,
autumn spates in rivers carry large quantities of nutrients into the lagoons
which, after low winter temperatures, trigger a phase of high production in the
food chain in spring. This section describes resident fish species which are
common to all Italian lagoons.
The whitebait (Atherina boyeri) is a gregarious species, feeding mainly on
zooplankton and also adaptable to freshwater environments.
The tiny South European toothcarp (Aphanius fasciatus) is also a gregarious
species with a wide ecological range: it prefers lagoon shores rich in vegetation,
but may also live in fresh water and pools with high salt concentrations.
The pipefish (Syngnathus acus) and the black-striped pipefish (Syngnathus
abaster), with their slim threadlike bodies, live in lagoons on sandy or muddy
beds with abundant aquatic vegetation, sometimes migrating to coastal waters
to overwinter. They are gregarious species which feed on zooplankton. The
females release fertilised eggs into the incubating pouch of the males, which
feed the embryos through blood vessel connections until their development is
complete.
The peacock blenny (Salaria pavo) is a species with individual territorial
behaviour, linked to the benthic environment and found on beds on which hard
objects abound (pebbles, stones, submerged pieces of wood, etc.) suitable as
shelters. The females lay their eggs in the nests of the males, which protect
them until they hatch. This species has conspicuous sexual dimorphism.
The black goby (Gobius niger jozo), a typical inhabitant of the Mediterranean
basin, has similar biological characteristics to those of the peacock blenny:
sexual dimorphism accentuated in the breeding season, parental care by the
males, and a diet based on benthic invertebrates or small fish. Both the black
goby and peacock blenny have poor adaptive capacity to highly desalinated
waters.
The three-spined stickleback (Gasterosteus aculeatus) may be found in lagoon
areas scarcely influenced by salt waters and in channels adjacent to lagoons. In
Italy, the species is absent in southern regions and in Sicily; elsewhere there are
populations living in brackish waters and also freshwater ones, even far from
the coast. In the breeding season, the male assumes a conspicuous livery and
builds a nest using plant material. He then courts the female, inducing her to lay
Three-spined stickleback (Gasterosteus aculeatus)
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66
her eggs in the nest and, after fertilising
them, protects them by continually
fanning them with his pectoral fins, to
provide them with oxygen and keep
them clean.
In the lagoons of the northern Adriatic,
as well as the black goby, there are also
other species of the gobiid family.
The grass goby (Zosterisessor
Fish-farm ponds in the Lagoon of Venice
ophiocephalus) is the largest of the
gobiids which has adapted to lagoon
environments, sometimes reaching more than 20 centimetres in length. It lives
on muddy beds, in which the individuals of both sexes dig individual dens
among the seagrass roots. In the breeding season, which begins at the end of
February and lasts until May, the dens become more complex and are occupied
by one adult male, a few immature males, and two to four females.
Canestrini’s goby (Pomatoschistus canestrinii) is a species endemic to the
northern Adriatic, and frequents sandy beds. In the breeding season, the male
occupies a territory around some small submerged object (a mollusc shell,
submerged wood, seagrass rhizomes, etc.), beneath which the female lays her
eggs. The male looks after the eggs, which hatch within a few days. The
presence of Canestrini’s goby in a small lake at the mouth of the river Sinni in
the Gulf of Taranto is most probably due to accidental introduction into a fishfarm together with grey mullet fry.
The lagoon goby (Knipowitschia panizzae) is a very small species that rarely
exceeds four centimetres in length. It is practically endemic to Italian waters,
extending only as far as the lagoons of Dalmatia. Records relating to the
Lagoon of Lesina and lagoon environments of the Tyrrhenian coast very
probably regard accidentally introduced populations that have recently
become acclimatised. The lagoon goby lives on muddy beds, and the
territorial behaviour of the male during the breeding season is similar to that of
the previous species, but the hiding-place used as a nest is almost always a
shell of some small bivalve of the genus Cerastoderma. Beneath this, after
covering the half-shell with mud to conceal it, the male takes refuge to look
after the eggs.
Lastly, another representative of the gobiid family, Pomatoschistus tortonesei,
is found in the Stagnone di Marsala in Sicily. This is another tiny species, even
smaller than the previous one, which is typical of the lagoons on the African
coasts of the central Mediterranean; its biology is practically unknown.
■ Catadromous species
After feeding in internal waters, the
adults of these species move into
brackish waters prior to migrating to
the sea in order to breed, while the
juveniles make the reverse trip. Their
presence in brackish waters is thus
limited to specific periods of the year.
Eel (Anguilla anguilla)
Only one catadromous species, the eel
(Anguilla anguilla), passes through the
estuaries and deltas of Italian rivers in transit. It is known that the adults, having
migrated from European waters for thousands of kilometres to reach the
Sargasso Sea in the Atlantic Ocean, spawn there and die after releasing their
gametes. The larvae (leptocephali) are transparent and have a characteristically
flat shape. They are carried by the Gulf Stream towards the European coastline
over a period varying from one to more than two years and, once they reach the
continental shelf of Europe or enter the Mediterranean, by which time they are
about 7 cm long, they metamorphose and assume a cylindrical shape. At this
stage, they are called elvers, and begin to ascend rivers, especially at night
when there is a rising tide. In Italian waters, this happens from October in the
west until February in the Adriatic. The elvers distribute themselves over a wide
range of environments: coastal brackish waters, canals, ponds, rivers and
torrents, and lakes. They prefer soft bottoms and waters rich in vegetation,
where they act as predators, feeding on invertebrates and small fish,
prevalently at twilight and during the night. Their stay in fresh or brackish waters
lasts for a few years - three to four for those which stay in brackish waters or on
the plains, and up to seven or eight years for those living in colder waters. The
former appear to be destined to become males, the latter females. This
differentiation takes place after migration to the sea, as the adults which begin
their breeding migration in autumn have not yet developed gonads.
With maturity, their morphological characteristics modify, the colour on their
backs varying from brownish-yellow to brownish-black, that of the belly from
yellowish to silvery; the eyes become larger, the skin thickens, the pectoral fins
become more pointed and the intestine begins to degenerate because the
breeders do not feed during their long migration. They transform into what are
known as silver eels, with lengths and weights varying from less than 45 cm
and less than 200 g for those destined to differentiate as males, up to more
than 1 m and 2 kg for the females.
67
■ Anadromous species
68
Adriatic sturgeon (Acipenser naccarii)
The adults of these species, after
feeding at sea, move seasonally into
deltas, estuaries and lagoons, to
ascend rivers until they reach suitable
areas for spawning.
After a stay in internal waters, the
young individuals then take the inverse
route, to pass their growing stages in
marine waters until they reach sexual
maturity.
The fish species which make breeding
migrations in internal Italian waters
include the twaite shad and three
different species of sturgeon, plus two
species of lamprey – the latter
Beluga (Huso huso)
belonging to a separate zoological
class.
The breeding specimens of twaite shad (Alosa fallax) swim up Italian rivers in
spring, to spawn on gravel beds, sometimes very far from the sea. The young,
after an initial growth stage in the river, descend to the sea in autumn, where
they remain until they reach sexual maturity. Recently, differences in the
number of branchiospines have shown that the populations of the Tyrrhenian
are best considered as a separate sub-species (rhodanensis) from those of
the Adriatic (nilotica).
Of the three species of sturgeon that in the past ascended the major Italian
rivers in spring, the largest, the beluga (Huso huso), which is typical of the
Caspian Sea, Black Sea and the Adriatic, has only sporadically been recorded
in the Po for some decades, and appears to be close to extinction.
The common sturgeon (Acipenser sturio), which in the past had a wider
distribution - ascending to spawn in all the major rivers of the northern Adriatic
and also the Tiber and other rivers flowing into the Tyrrhenian Sea - seems to be
following the same destiny.
The third species, the Adriatic sturgeon (Acipenser naccarii), endemic to the
Adriatic Sea and smaller than the others (rarely reaching more than 150 cm
and 30 kg) appears to be able to pass its entire life-cycle in freshwater, as
demonstrated by the persistence of a small population in the middle
stretches of the Po and lower stretches of the Ticino, upstream from barriers
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70
that would be insurmountable during migration. In the last year, operations
to repopulate the Adriatic sturgeon have been under way, and specimens
from fish-farms have been introduced into the Po and the rivers of the
Veneto region.
Among the four species of lamprey present in Italian waters, two make
spawning migrations in rivers after passing their trophic stage in the sea as
ectoparasites on fish or marine mammals.
The sea lamprey (Petromyzon marinus) ascends rivers in spring or early
summer, sometimes attached to the flanks of migrating twaite shad or
sturgeon. It is found in all Italian rivers, whereas a second species, the river
lamprey (Lampetra fluviatilis), only goes up the estuaries of rivers flowing into
the Ligurian Sea and northern and central Tyrrhenian.
Overall, the populations of species which make obligatory breeding migrations
from the sea to internal waters, or vice versa, are those which have been most
badly affected by human acts of environmental degradation. As well as the
negative effects of water pollution on the development of eggs and initial
growing stages of the species that make potamodromous (river) migrations,
dams or other barriers built across rivers prevent adult individuals from
reaching the beds suitable for reproduction. Similarly, it is now often
impossible for eels to reach the stretches of rivers they inhabited in the past
during their growing stage.
Sea lamprey (Petromyzon marinus)
■ Seasonal species
A few marine species feed temporarily
in lagoons and estuaries, sometimes
swimming for long stretches upriver,
but they return to the sea both to
spawn and to overwinter.
Five species belonging to the mullet
family are frequent in all the brackish
waters of the Italian peninsula:
● flathead mullet (Mugil cephalus): fry
enter internal waters from August to
December, and adults migrate to the
sea from summer to early autumn;
● thinlip grey mullet (Liza ramada): fry
enter internal waters in autumn or at
the end of winter, and adults migrate to
Sea bass (Dicentrarchus labrax)
the sea during the last months of the
year;
● golden grey mullet (Liza aurata): fry enter internal waters in autumn or at the
end of winter, and adults migrate to the sea from September to November;
● leaping grey mullet (Liza saliens): fry enter internal waters in summer, while
adults migrate to the sea;
● thicklip grey mullet (Chelon labrosus): fry enter internal waters from April to
June and adults migrate to the sea from February to April.
These are all gregarious fishes with a diet based on plankton during the juvenile
stages. Adults feed on algae and detritus rich in organic matter.
These species are typical of lagoon and estuary environments with muddy and
sandy beds; some swim upriver for varying stretches, being able to tolerate
variations in salinity. In particular, the thinlip grey mullet has lengthened its
ascent of the Po in the last few decades, now reaching distances of more than
200 km from the delta in summer, probably as a consequence of recent
changes in the riverbed, which is much richer in organic matter than in the past.
Other species which utilise brackish water for growing-on of juvenile stages are
the sea bass (Dicentrarchus labrax) and the gilthead seabream (Sparus aurata).
In late winter and in spring, the fry of these two species enter lagoons to feed
on zooplankton.
Being less tolerant of desalinated waters and cold temperatures than the
mullets, they return to the sea during the winter months. Sea bass adults mostly
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■ Occasional species
prey on crustaceans and fish, and gilthead seabream mainly on bivalve
molluscs.
The five species of mullet, together with sea bass and gilthead seabream, are
traditionally farmed in sometimes artificially enclosed brackish ponds. As these
species cannot reproduce in brackish water, the fry are caught as they
instinctively try to move upstream to internal waters, or else, they are bred by
artificial techniques in which the males are induced to mature by treatment with
gonadotrophic hormones.
The plaice (Platichthys flesus luscus) lives in the lagoons and terminal stretches
of the rivers of the northern Adriatic. This sub-species spawns in the sea in late
autumn and winter.
The larvae, which still have bilateral symmetry, enter the internal waters where
they undergo metamorphosis when they are slightly more than 10 mm long: the
left eye migrates to the right flank next to the right eye; the small fish acquires
benthic habits, and its left flank gradually loses its pigmentation. The plaice,
now flat, feeds on small invertebrates.
Lastly, the sand goby (Pomatoschistus minutus elongatus) is found in lagoon
and estuary waters along Italian coasts. It is slightly larger than its congeners
which live permanently in lagoons. It spawns at sea and uses internal waters
for its trophic stage, returning to the sea in winter or when the rivers are in full
spate in early spring.
In certain conditions - for instance, the autumn spates of rivers, or the summer
penetration of the so-called “salt wedge”, which happens when waters are low some typically freshwater or marine species may occasionally be found in
brackish-water environments, thus demonstrating their capacity to live with
very different degrees of salinity to that to which they are normally exposed.
It is not rare, for example, to observe some species of cyprinids in estuary
waters, or the occasional pike (Esox lucius) in the most desalinated areas of
the lagoons. Sporadically, in the hottest summers, the trout (Salmo trutta), a
fish typical of mountain and hill waters, may even reach the sea in search of
cooler waters.
This sporadic behaviour mimics the regular anadromous migrations of the trout
which populate the tributary rivers of the Black Sea, northern Atlantic Ocean
and North Sea. The waters of the Mediterranean are too warm and salty for the
needs of this species.
Recordings of coastal marine species are more frequent - for example, the
sardine (Sardina pilchardus), sprat (Sprattus sprattus), corb (Umbrina cirrosa),
red mullet (Mullus barbatus) and sole (Solea solea). These are very probably
only episodic visits of species which are not resistant to significant variations
in salinity.
Fishing with large drop nets at the mouth of the Mignone (Latium)
Sole (Solea solea).
73
Terrestrial vegetation
FRANCESCO BRACCO · MARIACRISTINA VILLANI
Estuaries, deltas and lagoons - those
complex transition areas between fresh
and salt waters, between land and sea are environments where the incessant
interplay between the eroding action of
winds and tides and the arrival of new
material from rivers continually remodel
the lines of the landscape and,
following delicate equilibria, create a
set of complex ecosystems in continual
evolution.
The deposition of sediments creates
Vegetation on coastal dunes
dunes, beaches and tongues of sand
in the shape of sandbars which, over
time, will be demolished by the waves, the impetus of high river spates, or
subsidence.
Vegetation plays a key role in stabilising and consolidating submerged and
emerging sediments, which would otherwise be condemned to a state of
perennial movement. Marked geomorphological variability corresponds to a
variety of habitats, and consequently gives rise to plant communities which
differ in their response to the salt gradient or particle size and texture of the
substrate. Lagoon environments are differentiated into mudflats, sandbanks,
tidal creeks, saltpans, marshes and inlets, which are all dominated by
halophilous (salt-loving) vegetation, and the coasts and islands which host
psammophilous (sand-loving) vegetation.
■ Reed-beds
Reed-beds are one of the most representative and widespread types of
vegetation at river mouths and in coastal lagoons. In some situations - for
example, at the terminal stretches of the major Italian rivers (Po, Adige,
Tagliamento), favoured by the width of the riverbed and slow current - reed-
The Po Delta
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76
beds are so extensive that they confer
a characteristic physiognomy on the
delta landscape. Reed-beds are easily
identified by their typical appearance a dense and impenetrable grassland of
gigantic herbaceous plants that may
grow more than three metres above the
level of the water.
The undoubted main actor in this plant
community is the reed (Phragmites
australis), a tall grass which thrives in a
variety of environments: from coastal
areas to an altitude of 2000 metres,
from fresh to brackish waters, from
tranquil lakesides and ditches to
riparian strips along streams, to lagoon
sandbanks and marshes. Nor does the
Reed (Phragmites australis)
reed disdain environments without the
constant or frequent availability of water, like flood plains or riverbanks, even in
highly disturbed situations. Reeds have thin hollow stems, which may lengthen
to more than three metres, and grow straight and very close together, forming
dense stands of vegetation. They bear two rows of greyish linear-lanceolate
leaves with sharp edges, accompanied by fine tufts of hairs where they join the
stem. The long culms of reeds become more conspicuous in summer, when
greyish-purple inflorescences appear at the top. Each panicle, 20-30
centimetres in length, is formed of a large number of feather-like spikelets, each
bearing tiny flowers accompanied by soft and silky hairs. When the fruit have
ripened, the entire spikelet breaks off the panicle, leaving only the basal
elements (glumes) adhering. The hairs now become efficient organs of flight, to
aid dissemination of the spikelet by air currents.
The hypogeal (below-ground) apparatus of reeds is less obvious, but equally
unusual. Robust creeping rhizomes, buried in muddy beds, form a dense
subterranean tangle, a sort of net that constitutes the foundations of the reedbed. Each rhizome, covered in coriaceous scales, grows from year to year and
may reach several dozen metres in length, producing both anchoring roots and
new shoots at every node.
Reeds are widely known for their traditional uses: at one time, the stems were
cut during winter and used to insulate ceilings and roofs, to produce mats, stuff
the seats of chairs or make brushes. In the Polesine (part of the Po Delta), the
“arèle”, as reed mats are known locally, were placed vertically around the edges
of vegetable gardens as protection against the wind. Over time, these uses and
practices have progressively waned, but reeds are still exploited nowadays.
They are successfully used in operations of natural engineering, thanks to the
vigour of their vegetative reproduction. Portions of rhizomes or green cuttings
are collected in areas where reeds grow abundantly and transplanted to
wetland areas in need of environmental upgrading.
Reeds are also one species very often used in the creation of phyto-purification
systems, in which waste waters are channelled through a specially created
route to prolong contact times with the plants. This permits efficient uptake of
nitrogen, which is thus removed from the water and stored in plant tissues. The
capacity of reeds to survive conditions of prolonged submersion make them
ideal natural water purifiers.
Owing to their wide ecological range, reed-beds may be found in different
territorial ambits. In reality, the name reed-bed is used for different plant
communities, which share physiognomic uniformity but which have substantial
differences in composition, as the floral retinue changes according to
ecological conditions. The common denominator to all communities is the clear
quantitative dominance of reeds, a highly competitive species that creates a
dense canopy, but which is not very conducive to the growth of other plants. As
a consequence, few accompanying species survive.
Reed-bed in the Po flood plain
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Plants and coastlines
Mariacristina Villani · Antonella Miola
Humans have exploited deltas
and lagoons throughout history.
Since the earliest times, man and the
sea, just like man and rivers, have
contended for space in repeated
competition, first conquered by one,
then by the other, but more often with
no lasting results, because nature has
often re-appropriated surface areas
that man had previously succeeded
in wresting from it.
A significant example of this ongoing
battle is the Lagoon of Venice, which
has modified its geography
innumerable times over the course
of millennia. When, for various
reasons, the coastline has receded,
man has been able to exploit large
areas of grassland for crops or as
pasture for livestock, sometimes
also building roads.
An emblematic case of how plants
can help us to understand the past
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structure of the territory is that of the
ancient road, the Via Annia which,
traversing the plain between the
rivers Sile and Piave (NE of Venice),
linked the two large Roman
settlements of Altino and Concordia
Sagittaria.
Vegetation gives clues as to why its
route was moved further inland in
Roman times (1st century AD): finds
of fossilised plants, particularly the
pollen of halophilous species,
demonstrate the advance of the
lagoon, which at that time invaded
the area corresponding to the current
village of Ca’ Tron (near Roncade,
in the province of Treviso).
In practice, the sea had won back
what it had previously ceded
flooding the original route of the
road and making it necessary to
build an alternative one, further
from the coastline.
2 km
They are mostly heliophytes, i.e., with a subterranean portion that roots them to
the bed of the water-body, while the epigeal (above-ground) part emerges from
the surface and lengthens in the subaerial environment. These are highly
specialised plants. One of the main problems that plants in a reed-bed have to
solve is lack of oxygen, which is poorly soluble and diffuses slowly in water. The
roots, permanently buried in the asphyxial mud, with little oxygen, are
particularly affected. A mechanism that counterbalances this lack is the
translocation of oxygen from tissues that have plenty of it to others that do not
have enough. The key to this process is the existence of a particular
parenchymal tissue in the rhizome, called aerenchyma, which has many large
intercellular spaces that are interconnected forming a system of conduits
facilitating the movement of oxygen. In this way, oxygen can easily reach the
root apparatus from the green epigeal parts, where it is produced by
photosynthesis or taken from the atmosphere.
Reed-bed species also have to tolerate another negative characteristic of the
anoxic mud in which their roots are buried. The oxygen-poor environment
stimulates the blooming of an anaerobic bacterial flora whose metabolism
generates toxic products for the plants - although, by means of sophisticated
physiological strategies, the latter manage not to suffer the consequences.
Reed-beds are typical of wetlands and freshwater marshes, such as the small
lakes behind the dunes, but those which flourish best are found in the riparian
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LAGOON OF VENICE
The Roman road Via Annia (south of Treviso) in relation to the maximum expansion of the Lagoon
Marshy area at the mouth of the Agri (Basilicata)
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Heterotopy
Francesco Bracco · Mariacristina Villani
The proximity of the Eastern Alps to
the coast and the existence of efficient
transport systems towards the sea by
rivers has led to the presence of what
are more properly Alpine species
of plants in coastal areas and, in
particular, close to river mouths.
The survival of their plant populations
is probably closely linked to river
currents regularly carrying seeds
from higher land. This is an extreme
case of heterotopy, a biogeographical
term that describes the existence of
populations of species dislocated
outside their typical distribution
area, due to transport of dispersed
seeds. For example, the presence at
the northern Adriatic river mouths of
some species of shrubs which are
usually components of holm-oak
woodland is interpreted in this way:
rock buckthorn (Rhamnus saxatilis
ssp. saxatilis) and purple broom
(Cytisus purpureus) are examples.
One large tree also shares this
peculiar phyto-geographical
condition: the Austrian pine (Pinus
nigra) which, although it grows
spontaneously on the dunes
at the mouth of the Tagliamento,
is generally typical of the northeastern calcareous uplands.
The presence of spring heath (Erica
carnea) in this context has also been
interpreted in this way, but it may
also be a relict of the last Ice Age.
Austrian pine (Pinus nigra) at the mouth of the Tagliamento (Friuli Venezia Giulia)
belt along the terminal stretches of
rivers, in conditions of perennial or
prolonged submersion. Here, other
tall grasses are associated with reeds,
such as reed canary grass (Phalaris
arundinacea) and reed sweetgrass
(Glyceria maxima), and sedges like the
great pond sedge (Carex riparia) which,
due to their height, manage to be
competitive in capturing light.
Also frequently found are other species
of interest because they are rare and
Marsh spurge (Euphorbia palustris)
appear in the Red Lists of species at
risk of extinction, such as the marsh
spurge (Euphorbia palustris) or the fen ragwort (Senecio paludosus), with its
yellow flower clusters that stand out among the reeds during the summer.
In situations with shallow water and recurring, prolonged periods of drought,
species typical of water meadows are associated with reeds, such as wood clubrush (Scirpus sylvaticus), with its characteristic triangular stem and bracts like
large leaves that enfold the inflorescences; marsh woundwort (Stachys palustris)
with its purplish-red corolla, purple loosestrife (Lythrum salicaria) and yellow
loosestrife (Lysmachia vulgaris), with its bright yellow petals. Hedge bindweed
(Calystegia sepium), with white funnel-shaped flowers, is almost always present,
and succeeds in reaching the light by climbing up the reed stems. Another
recurrent, but less frequent climbing species is bittersweet (Solanum dulcamara),
which has clusters of dainty purple flowers with orange anthers.
Halophilic reed-beds. Salinity does not represent a grave problem for reedbeds: they grow both in moderately brackish waters and in more salty
conditions and, in response to the saline gradient, are accompanied by species
more or less tolerant of salinity. In littoral zones, at river mouths and in creeks,
where abundant freshwater mixed with seawater reduces the salt
concentration, indicator species typical of areas with a moderate level of
salinity appear next to reeds, such as the sea club-rush (Bolboschoenus
maritimus) with its inflorescence composed of reddish spikelets; soft-stem
bulrush (Schoenoplectus tabernaemontani), with its greyish-blue stems; spearleaved orache (Atriplex prostrata), the leaves of which are spear-like blades
covered by a mealy bloom; and sea aster (Tripolium pannonicum ssp.
tripolium), with its capitulum with tube-shaped flowers forming a central yellow
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disc surrounded by purple ligulate petals. In these situations, the saltmarsh
aster (Symphyotrichum squamatum) is also frequent: an alien species present
in disturbed environments and capable of living in soils with a modest salt
content. It is a plant with flowering branches equipped with very thin linear
leaflets bearing a large number of small heads of minute flowers which, at
dissemination, develop a delicate pink pappus.
Where the effects of seawater are more marked, salinity becomes a strongly
conditioning factor that has two effects: on one hand, it limits the vigour of
reeds, which grow shorter; on the other, it reduces floral richness, because only
a few species, with particular adaptation strategies, can tolerate high
concentrations of salt. In perennially haline environments, like sandbanks and
salt-marshes, some species are able to join the floral retinue of reed-beds,
although, in conditions of more accentuated salinity, they would form plant
communities and become dominant. The most eye-catching of these include
the sea-lavender (Limonium narbonense), belonging to the family of the
Plumbaginaceae. This plant is insignificant in the vegetative stage, when it has
only a basal rosette of spatula-shaped leaves, but becomes striking in late
summer and autumn, when the floral stem lengthens, bearing a corymbed
panicle dense with small pink-purplish flowers. These maintain their colour for
such a long time, even when the corollas dry, that they are often gathered and
sold in decorative bunches. The leaves and stem are sticky to the touch
because they are covered with cells that secrete the excess salts absorbed by
the plant - a sophisticated physiological specialisation that allows them to
survive high concentrations of toxic chlorides.
The composition of halo-hygrophilic reed-beds also includes the annual
seablite (Suaeda maritima), a plant species of the Chenopodiaceae that is
called roscan in the hinterland of the Veneto lagoons. This term is also
commonly used to designate other species belonging to the same family
which, like the annual seablite, are well-known because they are edible.
In halophilic reed-beds, another annual species plays a similar role. This is a
glasswort (Salicornia veneta), an annual succulent species with fleshy stems
and branches that appear to be leafless. The flowers, rudimentary and joined in
two groups of three, are also only just observable with the naked eye. As the
Latin name suggests, glasswort is a common and endemic plant of the Veneto
coast, where it forms the most external strips of vegetation of the sandbanks
on the lagoon edges, which are periodically submerged by seawater.
Another species composing halophilic reed-beds is shrubby glasswort
(Sarcocornia fruticosa), a perennial plant with woody stems at the base and
fleshy above; more robust than the previous Chenopodiaceae, it may
exceptionally reach one metre in height. With a similar morphology to that of
Salicornia veneta, with the exception of its size, shrubby glasswort used to be
eaten - although nowadays pollution in the lagoons makes this unadvisable.
Flowering of pink sea-lavender (Limonium narbonense)
Salicornia veneta
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■ Other marsh vegetation
The tall herbaceous plants that
accompany reeds in reed-beds may
take on a dominant role in favourable
conditions and characterise the
physiognomy of the vegetation, so that
reeds are relegated to a subordinate
role. An example are the tall grasslands
dominated by bulrushes (Typha latifolia
or T. angustifolia). Less extensive in
comparison to reed-beds, these plant
communities do not pass unobserved,
especially during their flowering and
fruiting seasons.
The species of the Typha genus have
an
unusual inflorescence, in which the
Typha latifolia
male and female flowers form two
separate clusters along the terminal axis. The male flowers, tiny and without
petals, constitute the upper portion of the inflorescence. The female flowers,
which are also tiny and without petals, form a kind of velvety sausage,
composed of many thousands of microscopic flowers set very close together.
Each female flower is composed of an ovary and supported by a peduncle,
surrounded by a whorl of long bracts. Not only the position, but also the time
the inflorescences remain on the axis are different: after the anthers have
opened and the pollen is freed, the male flowers have finished their biological
task, so they fall and leave a section of bare flowering axis above the sausageshaped spike. Instead, the female part turns brown after flowering, and
conserves the different parts of which it is formed. The ovary, transformed into
a small fruit, remains at the top of the peduncle. On this the bracts, which later
play an important role, are also preserved. During the winter, the inflorescence
dries and dissemination takes place: in response to hygroscopic stimuli, the
bracts spread out and cause the floral peduncles to break; the sausage-shaped
spike thus disintegrates and the seeds are transported by the wind in the form
of cottony wisps.
These communities are generally richer floristically than reed-beds, probably
because bulrushes form less dense populations than reeds and create less
severe shading conditions - and also because the long linear leaves
perpendicular to the ground do not develop dense canopies.
In areas where the influence of brackish
water is higher, halophilic reed-beds
give way to Puccinellio palustrisScirpetum compacti, we find the
typical formations of sea club-rush
(Bolboschoenus maritimus).
This robust member of the Cyperaceae
presents many affinities with reeds: a
robust hypogeal apparatus, dense
stems, and a wide ecological range
that allows it to live in both freshwater
and oligohaline environments.
Another species that can tolerate low
rates of salinity is round-headed clubrush (Scirpoides holoschoenus), which
forms communities in which it is
dominant, often in damp areas behind
Sea club-rush (Bolboschoenus maritimus)
dunes.
The bed of rushes is unmistakable when the inflorescences appear at the top of
the stems: they have the appearance of small balls subtended by a bract that
lengthens upwards almost as though it were an elongation of the stem.
If salinity increases, beds of maritime rushes appear, with sea rush (Juncus
maritimus), a species that is prickly to the touch, with its stems and rigid
pointed leaves borne by robust horizontal rhizomes. Vegetation with sea rush
flourishes best on sandbanks, where it usually grows in the more internal and
higher areas, where the effect of tides is felt less.
One of the interesting types of vegetation in wetter areas with weak saline
distribution is Mariscetum serratae, the term used for the community
dominated by sawgrass (Cladium mariscus), a vigorous species belonging to
the family of the Cyperaceae. The stem, cylindrical at the base and triangular at
the top, a few centimetres across, bears leaves with serrated, sharp margins
folded along the midrib. The inflorescence, which develops during the summer,
consists of an anthela composed of more overlapping and often interrupted
brownish anthelas. This species is more common in freshwater environments,
but its capacity to tolerate low levels of salinity means that it frequently appears
in delta areas. Although sawgrass is considered a sub-cosmopolitan species,
i.e., diffuse over most continents, the drainage operations that have taken place
in coastal areas in the past have drastically reduced the presence of these
communities throughout Italy.
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Low salinity is also tolerated by cordgrass (Spartina versicolor), a robust grass
that forms dense carpets on dunes in formations where it tends to substitute
marram-grass, although it is also found on the edges of lagoons. Less
halophilous than its congener small cordgrass (Spartina maritima), S. versicolor
mainly reproduces vegetatively, through subterranean rhizomes that give rise to
tussocks of leaves: because of this, its grasslands are very compact and
species-poor. When they do appear, the inflorescences are composed of three
or four sessile ears up to five centimetres in length.
Lastly, one peculiar aspect of the vegetational landscape of the deltas should
be mentioned - the belts of halo-nitrophilous vegetation. These communities
are dominated by annual species with the distinctive feature that they can
tolerate moderate salt contents and the physiological aridity that follows, but
also high nitrate contents. Although often covering small areas, they play an
important role as pioneer plants which anticipate the other more strictly
halophilous species. The best-represented families are the Chenopodiaceae,
with the oraches (Atriplex tartarica, A. prostrata) and the Polygonaceae, with
golden dock (Rumex maritimus) and sea knotgrass (Polygonum maritimum).
Their presence is often associated with disturbed environments. Natural plant
debris is often accompanied by quantities of waste transported and deposited
by water - a tacit but manifest accusation of modern man’s uncivilised
behaviour towards nature.
■ Consolidated dunes and damp hollows: a complex vegetational landscape
Sea knotgrass (Polygonum maritimum)
Mouth of the Reno (Emilia Romagna)
As well as the more halophilous vegetation, the combined morphogenetic
activity of the sea and rivers creates a complex landscape, which is also rich
in communities far removed from psammophilous, marshy, salty or brackish
ones.
A complex environmental picture is created that flanks forestry aspects, xeric
(heat-loving) vegetation, internal hollows containing freshwater, and coastal
water-bodies of brackish or salt water. This is possible because of the detailed
geomorphology of the landscape in both raised areas and low-lying ones. The
higher areas generally correspond to dunes stabilised by vegetation, and may
lie at some distance from the present-day coastline. Hollows are strongly
influenced by the presence of salty or brackish water if they are in contact with
the sea: conversely, if they are affected by the water table or surface water of
land origin, they will have typically freshwater characteristics.
In places where plants find it difficult to survive, as in the northern Adriatic
littoral arc, the complex environmental situation also provides a little space for
species of plants with different geographical distributions and very different
ecological requirements. In this area, and as far south as the Po Delta, on the
tops and flanks of consolidated dunes where the evolutionary process of the
soil is more advanced, the landscape at river mouths hosts woodlands of holm
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Mosaic of vegetation at the mouth of the Reno (Emilia Romagna)
oak (Quercus ilex), the evergreen oak more typical of the Italian Mediterranean
landscape. This does not appear to be an exceptional case, as this woodland
formation grows on the dune areas at the mouths of the Tagliamento and Adige
and in the Po Delta. The low impact of this woodland in today’s landscape is
due to the relatively small extent of the dune and palaeo-dune systems, but
also largely to the progressive elimination of this type of vegetation by man’s
exploitation of coastal areas.
In contrast to the image of instability and strong dynamism of the coastal
landscape, in these situations the soil - although essentially composed of sea
sand, and thus dry and well-drained, appears to be well-structured and has a
significant accumulation of organic matter in its shallower levels.
The holm oak can form a closed, continuous tree canopy that may reach a
height of 20 metres. Together with the holm oak, the most common tree
species is the flowering ash (Fraxinus ornus), which is always co-present, but
playing a decidedly subordinate role. Two generally Mediterranean tree species
but with different phyto-geographical characteristics are therefore associated:
the holm oak, typically distributed in the circum-Mediterranean areas with
marked summer aridity (steno-Mediterranean), and the flowering ash, with a
distribution area that covers not only the northern coasts of the Mediterranean,
but also gravitates around the Black Sea (euro-Mediterranean-northern Pontic).
The shrubs beneath the trees develop into a rather scattered cover, with a
mixture of species with differing phyto-geographical characteristics.
The tree vegetation of the dunes is associated in the landscape with shrub
communities, which represent structurally simpler evolutionary stages of the
vegetation cover, gradually preparing for colonisation by forest species.
The first stage of colonisation by woody vegetation is that of low-growing
shrubs, less than one metre high, defined as the precursor of the holm oak
woodland, characterised by climbing shrubs with a small herbaceous
contingent. The dominant species are wild asparagus, osyris and the heath
Erica carnea, accompanied, especially in the most northerly part of the Adriatic,
by purple broom (Cytisus purpureus) and the buckthorn Rhamnus saxatilis ssp.
saxatilis, a small spiny shrub with reddish bark and small brown flowers with
four petals. Very frequent, although not abundant, is the wall germander
(Teucrium chamaedrys), a prostrate dwarf shrub, with small coriacious lobate
leaves. The herbaceous species mainly derive from the neighbouring arid
grasslands and, among these, tor-grass (Brachypodium rupestre) and cypress
spurge (Euphorbia cyparissias) are frequent. It is within these ambits that the
holm oak makes a timid appearance together with the flowering ash, although
only at the seedling stage.
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Where the sea is close enough to
allow the salty air to reach the cordons
of stabilised dunes, the vegetation is
unable to form holm oak woodland,
but reaches its climax as compact
shrubland dominated by juniper
(Juniperus communis), identifiable by
its spiny-tipped leaves in whorls of
three and its bluish-black berries.
To this is associated, generally
subordinately, the sea buckthorn
(Hippophae fluviatilis), a densely
Rosemary-leaved willow (Salix rosmarinifolia)
branched shrub with silvery bark, linear
leaves that are white beneath, and
characteristic bright orange berries. This community is completed by many of
the shrubs already mentioned for holm oak woodland and scattered
herbaceous species including bladder campion (Silene vulgaris ssp. tenoreana)
and the sedge Carex liparocarpos.
River mouth environments sometimes contain systems of depressions that are
often affected by salinity, but others - for instance, along the coast of the Veneto
and Friuli plains - are influenced by the presence of fresh surface and groundwater,
with low nutrient loads and high concentrations of calcium carbonate. In these
cases, the depressions may contain accumulations of peaty sediments on which
unusual plant communities settle and which, at sea level and at a short distance
from the coast, sometimes include species that require cool environments.
Typical examples are the peaty grasslands of purple moor-grass (Molinia caerulea
ssp. caerulea) that are reminiscent of those in spring areas on the north-eastern
plain, of which they reproduce a good part of the floral retinue. This includes black
bogrush (Schoenus nigricans), narrowleaf plantain (Plantago altissima), fragrant
leek (Allium suaveolens) and marsh gentian (Gentiana pneumonanthe). In slowmoving or still surface waters within these grasslands, rare hydrophyte
communities of waters with low nutrient contents may also find refuge. An
example is the aquatic vegetation dominated by fen pondweed (Potamogeton
coloratus forma heterophyllus) with both floating and completely submerged
elliptical leaves, both types being translucent, veined, and often reddish.
The rosemary-leaved willow (Salix rosmarinifolia) also appears here as a woody
colonising species. This shrub, rarely exceeding a height of two metres, has linear
or linear-lanceolate leaves that are hairy and silvery beneath, and dark green and
shiny above. It is a plant with Euro-Asiatic distribution, which in northern Italy is to
be found only rarely on the peaty soils of
the high plain, montane and subalpine
belts. The delicate environment in which
it lives means that it is endangered
throughout Europe, and its presence in
coastal vegetation has also been
drastically reduced, so that it now only
survives in a few sites, such as those
close to the mouth of the Tagliamento.
More generally, where freshwater
conditions converge towards the general
mesotrophic or eutrophic state more
Marsh marigold (Caltha palustris)
typical of surface waters on the plains,
the marsh and aquatic vegetation found
in ditches and flooded hollows tends to coincide with that present in the waterbodies of the plains. In the slow-moving waters of the Po Delta, for example,
aquatic communities have been recorded with fennel pondweed (Potamogeton
pectinatus), a species which may also penetrate into brackish water. In the
bodies of freshwater, the typical communities are dominated by several
hydrophytes, all with large leaves growing on the surface of the water: the white
water-lily (Nymphaea alba), yellow water-lily (Nuphar lutea), fringed water-lily
(Nymphoides peltata) and water chestnut (Trapa natans). Only extremely rare
groupings of marestail (Hippuris vulgaris) have been recorded.
The hollows may also contain hygrophile or mesophile woodland vegetation,
dominated by species of deciduous broadleafs that are in stark contrast to the
evergreen woodlands on higher ground. This type of vegetation is even rarer
than that of the holm oak woodland. A prime example is the Bosco della
Mesola, which has been growing for the last thousand years on the complex of
dunes and depressions created by the sedimentation of the Po di Volano and
Po di Goro, within the Po Delta. The forest community that settles on the more
ancient dune systems with flatter morphology is, on one hand, reminiscent of
holm oak woodland and, on the other, is related to the mesophile forest
vegetation typical of the plains. The soil is moderately damp and only
sporadically water-logged, so a multilayered forest develops, with tall trees,
many shrubs, and sparse herbaceous undergrowth.
Lastly, the survival of marsh woodlands with alder (Alnus glutinosa) is very
infrequent on plains, but becomes exceedingly rare at river mouths. For example,
there are small areas of alder woods on soft black soil at the mouth of the Adige,
at times with marsh marigold (Caltha palustris) in the herbaceous undergrowth.
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