The Venetian Lagoon and its Ecosystem.

The Venetian Lagoon and its Ecosystem. It is the most important survivor of a system of estuarine lagoons, that in Roman times extended from Ravenna north to Trieste. During the 5th to 6th century, the lagoon gave security to people under Roman rule; fleeing Hun and Lombard invaders.

1. LOCATION

2. DEVELOPMENT

3. HISTORICAL PERSPECTIVES

4. METROPOLITAN CITY OF VENICE

5. THE ISLANDS

6. THE LAGOON ECOSYSTEM

1. LOCATION

The Venetian Lagoon stretches from the River Sile in the north to the Brenta in the south and has a surface area of around 550 square kilometres (212 square miles). It is comprised of around 8% land, including Venice itself and the many other islands both natural and artificially made.

The lagoon is the largest wetland in the Mediterranean Basin, with about 420 square kilometres open to the tides of the Upper Adriatic. About 11% is permanently covered by open water or “canal”, as the network of dredged channels are called; while around 80% consists of mud flats, tidal shallows and salt marshes. For comparison, the nearby Marano-Grado Lagoon, is the northernmost lagoon in the Adriatic Sea; having a surface area of around 160 square kilometres (62 square miles).

The Venetian lagoon is connected to the Adriatic Sea by three inlets: Lido, Malamocco and Chioggia. The lagoon is subject to significant variations in water level, due to twice daily tidal flows. The most extreme rises, known as “acqua alta” (high water), occur when spring tides combined with high winds (Scirocco and Bora), regularly flood Venice; most frequently during the autumn / winter period. The lowest tides are known as “neap tides” and when combined with winds from the north-west; smaller canals in Venice have so little depth of water, they are made unnavigable. (See link to my post on “Acqua alta and the MOSE Barrier Project” and “The Great Venetian Flood of 1966“, at bottom of page).

– The Venetian Lagoon and its Ecosystem,

2. DEVELOPMENT

The Lagoon was formed about six thousand years ago, when the Holocene marine transgression following the Ice Age, flooded the upper Adriatic coastal plain; creating a system of lagoons that extends along the entire northwestern Adriatic coast. Later, around 2000 years ago, this system was segmented into separate subsystems, including the Venice Lagoon.

The Lagoon of Venice is the most important survivor of a system of estuarine lagoons, that in Roman times extended from Ravenna north to Trieste. The lagoon was sparsely inhabited from ancient times, most probably by itinerant fishermen, hunters and salt collectors. It was only during the 5th to 6th centuries, after the fall of the Western Roman Empire, that many people fleeing the Venetian mainland from Hun and Lombard invaders; settled in a number large enough numbers to found the city of Torcello and later the Rialto area of Venice.

It also provided a valuable and plentiful food source, but unfortunately no fresh drinking water from bore-holes, due to the brackish lagoon environment. Fresh water was provided from the development of rain water collection and well-head systems. (Please see the link to my post on this subject below). Later, it provided natural protection for the growth of the Venetian Republic and its maritime empire.

It still provides a base for a seaport, the Venetian Arsenal and for traditional fishing, clam dredging, a limited amount of hunting and the newer industry of fish farming. Deposition of river sediments compensated for the sinking coastal plain and coastwise drift from the mouth of the Po tended to close tidal inlets with sand bars.

The present aspect of the Lagoon is due to human intervention. Between the 13th and 16th centuries, Venetian hydraulic projects to prevent the lagoon from turning into a marsh; reversed the natural evolution of the Lagoon. Pumping of aquifers since the 19th century, increased subsidence. Originally many of the Lagoon’s islands were marshy, but a gradual programme of drainage rendered them habitable. Many of the smaller islands are entirely artificial, while some areas around the seaport of Marghera are also reclaimed islands. The remaining southern islands are essentially dunes, including those of the coastal strip (Lido, Pellestrina and Treporti).

3. HISTORICAL PERSPECTIVES

Human impacts on the lagoon environment also increased through time, due to land use changes, pollution, fishing activities and dredging of channels. It has always been an important concern of the Venetian Republic in maintaining the lagoon in a healthy state and preserving the natural resources it provides. Over the centuries, a number of studies and monitoring programs were conducted to assess conditions and technical alternatives were also considered. Moreover, practical solutions were considered provisional and had to be tested on site.

Above: View over the lagoon with Porto di Lido-San Nicolò (Image taken before barriers yet in place)

In 1173, fishing was strictly controlled by the “Magistratura della Giustizia” , forbidding fishing activities considered damaging to the natural resources and enforcing regulations on fishing gear and net sizes. It also restricted the sale of fish solely to fish markets to prevent escalating prices and the sale of unhealthy seafood products.

In 1465, industrial activities such as glass, leather, fur and dye works were regulated and “Provveditori alla Sanità” (health superintendents), enforced strict rules against air and water pollution and fire risk; with relocations to islands or the mainland. Starting from the late 15th century, senators were appointed to control the misuse of the lagoon. In 1501, a special water authority called the “Magistrato alle Acque” was set up, with the specific task of governance of the lagoon ecosystem, especially its morphology.

Venice Lagoon is a system that has undergone constant natural and artificially made changes and extensive efforts have been made to stabilise and preserve its morphological and ecological features. When the Venetians settled in the area, the natural tendency was for sediment infilling of the lagoon, which was enhanced by deforestation of the mainland. To slow down this trend, between the 13th and the 16th centuries major rivers such as the Brenta and the Sile, were diverted away from the lagoon. Concurrently, the number of inlets was reduced and the sand bar reinforced. The Republic’s interventions even extended beyond the natural boundaries of the Venetian Lagoon. For example, in 1604 the flow of the Po River was diverted southward, through an artificial delta mouth; to prevent sediment infilling of areas close to the Venetian Lagoon.

A further sign of the great attention paid to planning was that fishermen and members of the general public were invited by law to monthly conferences on the lagoon status, since these people were recognised for their valuable experiences, insights, and views on the local environment.

In the 18th century, the combined effects of coastal subsidence and eustatic rise in sea level, increased the flooding frequency in the lagoon and the need to protect the city of Venice from the invading sea. Sea-defences were constructed along the coastal strip, by the end of the century. Under Austrian rule and until 1934, the shape of the Lido, Malamocco, and Chioggia inlets were altered and outer dikes were built along the sea.

The late 19th and beginning of the 20th centuries, marked the onset of industrialisation; a new era of major anthropogenic changes of the ecosystem. During the period from 1924 to 1960, increased urbanisation of the mainland and land reclamation for agriculture, aquaculture and industry; reduced the total surface of the lagoon by 3280 hectares. A major industrial area, the Mestre-Maghera complex, including chemical and oil industries was established; leading to the dredging of new deep navigation channels. Due to growing environmental concerns, a third industrial area was never built; but a large area of the lagoon had already been converted to solid land. Today, it is one of Italy’s most important industrial centres.

Subsequent to this massive development period, both industrial and agricultural pollution (inorganic and organic) and deep channel dredging; had a major deleterious impact on the lagoon ecosystem. In the 1980’s the build up of nutrient , especially the level of phosphorus in the lagoon, resulted in a massive micro-algal growth, covering almost all shallow areas, where water movement was low and residence time high. As the algae decayed, the anoxic conditions severely affected both the bottom sediments and water. The release of hydrogen sulfide even adversely affected human activities in Venice and other surrounding land areas. Large areas of the bloom, amounting to about 60,000 tons; had to be physically cleared and regulations introduced in 1989, to enforce a total ban on phosphorus in detergents.

Throughout the 20th century, groundwater withdrawals for industrial purposes, natural subsidence and sea level rise, led to the lowering of Venice and part of its lagoon; which further increased the frequency of flooding. Over this period, the number of people working in the lagoon decreased sharply and the number of city of Venice’s inhabitants declined to the present figure of around 53,000; due to a variety of causes, such as the cost of living, property rents, inconvenience of the car-free environment and more limited work opportunities. Environmental concerns have also been expressed, at the sheer number of tourists (especially day-trippers), said to be around 20million per annum and the damage caused by so many large cruise ships visiting the city. Unfortunately, the city needs the revenue.

Today, climate change is another major threat to the Venice Lagoon. Atmospheric temperature and sea level rise are predicted to increase significantly over the next century, which will lead to greater frequency of flooding. According to some research projections; extensive areas of the western part of the northern Adriatic Sea coastline, could be permanently lost by 2100. In order to mitigate the impact of flooding on the city of Venice, the MOSE Barrier Project has now been completed and tested; together with a range of associated environmental and educational projects and sea-defence improvements. Its operation will be to temporarily close lagoon inlets, concurrent with extreme high tide events. However, the consequence of this intervention has been long debated, because of possible adverse effects on the lagoon ecosystem; especially if the barriers have to be frequently used.

4. METROPOLITAN CITY OF VENICE.

Today, the Venice Lagoon is mostly included in the Metropolitan City of Venice, while the south-western area is part of the Province of Padua. The Metropolitan City of Venice (Municipality or Comune) is also subdivided into six administrative boroughs (municipalità). Each borough is governed by a council (Consiglio) and a president, elected every five years. (Note: the relative populations below are as of 2009, for comparison.

The historic city of Venice itself, has a current population of around 53,000. Most of the workforce now commutes daily from the mainland). Lagoon: 1. Venezia (Historic city)-Murano-Burano 69,136. 2. Lido-Pellestrina 21,664 Mainland (terraferma): 3. Favaro Veneto 23,615 4. Mestre – Carpenedo 88,592 5. Chirignago – Zelarino 38,179 6. Marghera 28,466

The main cities inside the lagoon are Venice and Chioggia (at the southern inlet). Lido di Venezia and Pellestrina However, the majority of inhabitants of the Metropolitan City, as well as its economic core, its airport and its harbour; stand on the western border of the lagoon, around the former towns of Mestre and Marghera. At the northern end of the lagoon, there is the town of Jesolo, the famous seaside resort; and the town of Cavallino-Treporti.

5. THE ISLANDS

The largest islands or archipelagos by area in descending size, excluding coastal reclaimed land and the coastal barrier beaches are: Venice 5.17 km2 Sant’Erasmo 3.26 km2 Murano 1.17 km2 Chioggia 0.67 km2 Giudecca 0.59 km2 Mazzorbo 0.52 km2 Torcello 0.44 km2 Sant’Elena 0.34 km2 La Certosa 0.24 km2 Burano 0.21 km2 Tronchetto 0.18 km2 Sacca Fisola 0.18 km2 San Michele 0.16 km2 Sacca Sessola 0.16 km2 Santa Cristina 0.13 km2

Other inhabited islands include: Cavallino Lazzaretto Nuovo Lazzaretto Vecchio Lido Pellestrina Poveglia San Clemente San Francesco del Deserto San Giorgio in Alga San Giorgio Maggiore San Lazzaro degli Armeni Santa Maria della Grazia San Pietro di Castello San Servolo Santo Spirito Sottomarina Vignole

6. THE LAGOON ECOSYSTEM

WHAT IS THE VENETIAN LAGOON?

The lagoon is a large wetland coastal area in a continual state of instability, which communicates with the sea through openings or inlets; in such a way that the movement of water inside it is governed by the tide. In this way, lagoon morphology depends on the relationship and balance between the amounts of solid material brought by the sea, the fresh-water river system and the erosive forces of waves and seas.

Communication between the lagoon and the sea ensures, among other factors, the survival of the lagoon and its unique brackish water environment. The physical shape of the lagoon is modified and formed through the twice daily entrance and exit of the sea through the lagoon inlets. The sea can also be considered one of the main risk factors involved in the evolution of the lagoon basin, especially if the erosive actions of wave motion and coastal currents, predominate over the build-up of sediment accumulation.

About 78% of the lagoon surface is covered by vast expanses of water which are cut by a dense network of channels (natural and dredged) of varying depth. They vary in depth between 15 m for the Malamocco-Marghera Channel and 1-2 m. Of the 550 square kilometres of lagoon, 420 square kilometres are open to the tides of the Upper Adriatic, which is the highest in all of the Mediterranean. At every tidal cycle, around a third of the lagoon water is replaced. The movement of residual water tends to move in a clock-wise direction. Variations of maximum and minimum levels in the tide are determined by astronomical and meteorological factors: low pressure and the Scirocco and the Bora winds accentuate the high tides, causing the Upper Adriatic to swell up. Conversely, high pressure and winds from the north-west can cause the water in the lagoon to lower to such an extent; as to leave some of the smaller canals of Venice unnavigable.

(Note:The Moon, the Sun, and all the astronomical bodies exert an attraction force on the Earth and everything on it. In particular, oceans surfaces are in a state of equilibrium between the centrifugal force of the Earth and the attraction force of the astronomical bodies. Supposing our planet were covered entirely by water and influenced by the actions of a single satellite, the surface of the sea would act not as a sphere, but as a slight ellipsoid. This ellipsoid does not stand still but turns, “following” the body which attracts it. For an observer at a fixed point on the planet; the phenomenon appears as a cyclical variation of water levels).

Lowering the level of the lagoon with respect to the sea increased the frequency of flooding events and facilitated sediment erosion the loss of finer sediment. As a result of the diversion of major rivers from the lagoon, riverine input of particulate matter has not balanced the sinking of the lagoon, which was higher from 1930 to 1970; when groundwater and natural gas were extracted from the area. Combined with a continuous eustatic rise in sea level, subsidence has further increased relative to sea level rise, by about 1.5 mm every year, between 1972 to 2002.

(Note: Civil engineer Pietro Marcon in 1878, drew up a report entitled “Cenni cronologici delle principali vicende” (Brief Chronological History of the Main Events); which included a map titled “Historical Map of the Main Events and Hydraulic Works Affecting the Rivers, Lagoons, Forts and Coasts of Venice from the Beginning of the 14th Century to Today”. The map provides a historical summary of the morphological changes in the lagoon and man’s interventions down the centuries). .

MAIN ENVIRONMENTAL DRIVERS AND PRESSURES

Venice Lagoon is the largest lagoon systems in Italy and one of the largest in the Mediterranean Sea. It surrounds the city of Venice, with c 53,000 inhabitants and c 20,000,000 visitors per year. Along the lagoon boundary is Chioggia, with c 50,000+ inhabitants and Mestre, c 200,000+ inhabitants and is one of the most important industrial areas of Italy. Many different stressors act on the lagoon ecosystem, causing multiple environmental impacts. Among the major drivers of change there are:

land-based activities that deliver nutrients, heavy metals, and other pollutants (inorganic and organic).
fishing (in particular clam harvesting/dredging) and aquaculture activities (including water reclamation).
groundwater extraction, subsidence, eustasy and transportation (including deeper channel dredging), that affect physical and morphological features; forcing driving exchanges with the sea.
climatic conditions, including global warming.
lagoon governance

With regard to atmospheric forcing, the dominant winds are the Sirocco (warm and humid, summer wind from the southeast) and the Bora (in the autumn and winter, from the northeast). Flooding events, known worldwide as “acqua alta” (high water phenomena), mainly result from a combination of tide, seiches, and easterly winds. (Def: Seiches a temporary disturbance or oscillation in the water level of a lake or partially enclosed body of water, especially one caused by changes in atmospheric pressure).

Spring and fall are the seasons with the most rainfall, while June, September, and October the months with the least rainfall. Winter is the driest season. Water temperatures closely follow air temperatures, with a distinct seasonal cycle; minimal values occur in January, and maximal values in July. Average monthly temperatures generally range from 3°C to 24°C, but can reach 30°C and fall to 0°C. The lagoon covers an area of about 550 km2, of which about 150 km2 are used as extensive aquaculture farms and are closed to natural water exchanges. Islands cover about 45 km2.

A network of deep-water channels (65 km2), rooted at the three inlets of Lido, Malamocco and Chioggia; facilitates navigation and water exchange with the sea. Islands, wetlands (generally located just above the mean sea level), and tidal mud flats (that have a lower elevation and are frequently exposed to air during low tides); are connected by channels. The average depth is only about 1 m so there is a strong relationship between pelagic and benthic environments. Here, decomposition processes are substantial, and biogeochemical processes accelerated. (Def: The pelagic zone refers to the water column, where swimming and floating organisms live. The benthic zone refers to the bottom, or in the bottom sediments and organisms living on and in the bottom are known as the benthos).

The lagoon is characterised by a twice daily tidal regime with a range of about ±0.7 m. Tidal exchanges with the sea, typically amounts to about one-third of the total lagoon volume per tidal cycle. Adriatic waters entering the lagoon are typically oligotrophic, or at most mesotrophic. (Def: Oligotrophic – characterised by a low accumulation of dissolved nutrient salts, supporting but a sparse growth of algae and other organisms and having a high oxygen content owing to the low organic content. The opposite condition is Eutrophic – rich in nutrients and so supporting a dense plant population, the decomposition of which kills animal life by depriving it of oxygen). The Po Delta Plume, which sustains eutrophic conditions in the Adriatic, generally flows toward the centre of the Adriatic Basin or veers southward and only marginally affects the coastal area close to the Venetian Lagoon.

Subsidence and Eustacy The sediments of a lagoon are made up of organic or inorganic particles originating from the land, the atmosphere or the sea. The transportation of sediments within a lagoon can occur in the following ways: through river tributaries, through wave motion, dredging, or through coastal currents coming in from the lagoon inlets.

A lagoon survives only if the correct balance exists between accumulation (tendency to silt- up and turn to land and erosion (tendency to become a marine environment). Subsidence, or the lowering of ground level, is the result on the surface of processes occurring underground. Subsidence can occur because of natural reasons (for the most part tectonic movement beneath the earth) or the progressive consolidation caused by the geostatic loads of fine alluvial deposits, (such as silt and clay). Or because of unnatural causes; commonly the intense extraction of underground water supplies or to a lesser degree, natural gas. Eustasy is the variation which occurs in sea level. During the coldest periods of geological time, precipitation is held back in the form of ice and consequently, the level of the sea lowers. The contrary happens during the hottest periods. Eustacy, is a phenomenon independent of subsidence.

A lagoon therefore has three possible destinies:

if erosion and sedimentation compensate for each other, the lagoon environment survives, even if its equilibrium is precarious and unstable;
if the solid materials brought by the rivers and the sea prevail, a lagoon tends to silt- up and become land, (as is the case of the Po delta).
if on the other hand, the erosive forces of waves and tides prevail, a lagoon is transformed into part of the sea. This is the current tendency of the lagoon of Venice.

Freshwater Discharges The lagoon receives freshwater discharges from 11 major tributaries plus several minor rivers and a number of regulated channels used primarily for agriculture. Freshwater discharges derive partly from rain and snow melt-water in the drainage basin and partly from anthropogenic control associated with agricultural use. Maximal discharges occur from October to April and are larger in the northern part of the lagoon than in the central and southern sectors The drainage basin covers 2000 sq km, mainly in the Veneto region, one of the most intensively cultivated areas in Italy; hence, rivers carry a substantial nutrient load, today roughly equivalent to 4000 tons per year of nitrogen and 200 tons per year of phosphorus.

Nutrient Loads Nutrient loads to the lagoon are generated mainly by agriculture, industrial activity and sewage inputs. They have a positive effect on fishing and biodiversity when delivered in moderate amounts, insofar as they sustain primary production and food web processes. However, an excess of nutrients may also fuel eutrophication, which can trigger secondary effects such as hypoxia, mortality of benthic organisms and the loss of biodiversity.

Long-term research into trends of nutrient loads derived from primary sources, show that from the 1950’s to the 1980’s, the increased use of fertilisers in agriculture and the industrialisation of the drainage basin; caused a threefold rise in both phosphorus and nitrogen loads. The construction of wastewater treatment plants and the total ban of phosphorus from detergents in 1989 led to a marked reversal in the trend of phosphorus load, which has been reduced to less than half of the amount recorded in 1950. Nitrogen loads have decreased only since the 1990s, and current loads are now comparable to those of the mid-1960s due to the introduction of nitrification/denitrification processes at the major treatment plants, to the closure of fertilizer factories, and to improved fertilization practices in agriculture.

Pollutants Pollutants from industrial waste and other sources, such as from drainage basins, urban areas, waste incineration and boat engines have been discharged to the lagoon, with a peak input during the 1960s and 1970’s and have been accumulating in the lagoon sediments. Up to recently, attention was mainly focused on heavy metals (Hg, Pb, As, Cd, Zn, and Ni), but lately great concern has been directed toward all classes of persistent organic pollutants (POPs), such as dioxins, polychlorinated biphenyls (PCBs), hexachlorobenzene (HCB), and polycyclic aromatic hydrocarbons (PAHs). Industries are the major sources of these pollutants, which are delivered to the lagoon through point and non-point pollution pathways, as well as atmospheric fallout.

Fishing Activities Fishing can have significant effects on ecosystem structure, changing communities; decreasing biomass of targeted species and increasing mortality of non-targeted species. The Venetian lagoon has long been exploited by small-scale fishing activities using traditional methods. Aquaculture has been carried out in the Venice Lagoon for many centuries in restricted areas, mainly fishing ponds termed “valli da pesca”. This is an extensive practice that has had little impact on system trophodynamics. Significantly large variations in catch have occurred during the last century. This was probably be due to a combination of changes in fish abundances, fishing activity (over-exploitation, more powerful engines and fishing devices), changes in those resources targeted and greater regulation.

Semi-industrial exploitation developed in the early 1990’s, that targeted the Manila clam; introduced in 1983 to increase lagoon production. Fishing vessels targeting the Manila clam, use heavy-impact fishing gear (dredging) and this activity is considered to be a major cause of ecological imbalance in the lagoon; since it strongly interacts with the benthic compartment. It also contributes to the process of sediment erosion and loss from the lagoon. Regulations have been introduced, to significantly limit the area of the lagoon allocated to this activity.

Climate Change Climate change is considered a major threat to the survival of Venice Lagoon, mainly because of the projected increase in sea level. However, other possible factors which could modify system trophodynamics, include projections of increased warming in the summer up to 5°C by the end of this century, increased precipitation in the autumn and winter and decreased precipitation in the spring and summer. Indeed, an increase in temperature and a decrease in annual rainfall have been recorded over the last 70 years. Other direct and indirect impacts due to climate change can be hypothesised; including modification of habitats and species distribution.

Lagoon Governance Lagoon governance is an additional driver of change. For example, the issuance of regulations on land use or fishing, has had far reaching effects on biota and habitats in the lagoon. The effects are mediated by both bottom-up and top-down processes. The harvesting of macroalgae standing crop during the 1980’s is another example.

WATER AND LAND – FACTS, FIGURES AND SATELLITE IMAGERY

The Water and Land System The lagoon of Venice is the largest Italian lagoon, having a surface area of about 550 square km, of which about 420 square km are open to the tides of the Upper Adriatic. It has the highest tides in the Mediterranean.

The sea and the lagoon are connected through the three inlets of Lido (to the north), Malamocco and Chioggia, (to the south). Within the lagoon perimeter exist: islands, reclaimed areas, coastal strips, shallows, mud flats, salt marshes and fish farms. The land system of the lagoon territory is made up of all of dry land, natural or artificial (islands, reclaimed areas, banks and coastal strips) and represents about 8% of the overall surface area of the lagoon. The remaining 92% is made up of the water system, which includes channels (12%) and shallows, mud flats and salt marshes (80%).

The salt marshes are considered part of the water system even if in some cases they are higher than some areas of the historic centre which is considered dry land. The criteria for assigning salt marshes to the water system is in fact based on their function, which includes regulating lagoon hydrodynamics without opposing tidal expansion. The surface area of the water system is 503 square km, whereas that of the land system is 37 square km. The lagoon can also be divided up as follows:

The open lagoon, or the water area subjected to the tide, including mud flats and salt marshes and excluding islands and reclaimed areas, covers a surface area of about 418 square km;
The closed lagoon, not subjected to the tide, which includes all of the fish farms with their internal islands, along with mud flats and salt marshes, excluding, however, the banks which define their borders, for a surface area of about 85 square km.
Banks which cover a surface area of about 7.5 square km;
Islands, excluding Lido, Pellestrina and Treporti and including reclaimed areas, for a surface area of about 29 square km.

The lagoon ecosystem territory is also part of the drainage basin, whose influence is important not only as far as the volumes of fresh water coming into the lagoon are concerned, but also because of its role in bringing sediments and pollutants.

Fresh and salt water volume changes

The average daily volume of water that enters the lagoon from the sea is about 400 million cubic m. The volumes of water exchanged with each tidal cycle are equal to about 350 million cubic m during spring tide and 175 cubic m. during neap tide. The maximum total volume at the three inlets is about 20,000 cubic m of water per second. The maximum tidal volume at the Malamocco inlet, the inlet in which the greatest volume of water moves, is 8,000 cubic m per second (during spring tide cycles), an amount almost equal to the volume of the river Po in full flow. 900 million cubic m of fresh water flow into the lagoon every year through the 2,515 km of the hydrographic river network.

Salt Marshes, Mud Flats and Shallows

Mud flats are soft land areas, with no vegetation which are normally underwater; emerging only during particular tidal conditions (low tides during spring tide cycles). Salt marshes are important because of their role in regulating lagoon hydrodynamics. They are consistent ground areas which are almost always above water and are only sometimes submerged. They enhance water exchange, lessen the action of wave motion and provide a home for both a wide variety of vegetation, typical of brackish water areas and for a wide variety of bird life.

Channels and Water Areas

Above: Satellite image of Venice and Lido area showing deeper water channels. Note how deeper channels correspond to the the smaller southern islands close to the Lido peninsula.

The exchange between lagoon and sea occurs for the most part through the lagoon channels, which branch off from the three inlets. The main morphological structures of the lagoon are channels (principal, secondary and tidal creeks) with a surface area of around 67 square km and lagoon beds (including mud flats and salt marshes) with a surface area of around 436 square km. They vary in depth between 15 m for the Malamocco-Marghera Channel and 1-2 m. Smaller channels branch off main channels to become smaller and smaller and ever more winding; these are the so-called tidal creeks which cross salt marshes; to often finish in brackish rainwater ponds. Other than the natural channels which have a winding path, one can recognise artificial straight channels; that have been dug over the years.

Below: NASA satellite images centred on Venice and the Lido Inlet. Top year 2000. Bottom year 2013. Focus on the Lido Inlet and you can see in 2013, the earlier stage of the Mose Barrier Project: the extra external barrier across inlet, the Mose Island construction and the Port facilities mostly on the right side of the inlet. Now in 2021, the barriers themselves have been completed and tested. Note also from north to south on the mainland, the Airport and runways, town of Mestre and the Port of Marghera. You can also see the 2013 image, that the sediments are stirred up more in the northern lagoon, mostly due to heavy boat traffic and some fresh-water incursion

FAUNA AND VEGETATION – A BRIEF GUIDE

Fauna of coastal environment There are two typical habitats for fauna that live along the coastal strips: the dunes and the wooded areas behind the dunes. The species that live in these areas are generally thermophilous, that is they adapt well to environments with high insolation (exposure to sun’s rays) and medium to high daytime temperatures. Most of the fauna is made up of amphibians, reptiles, birds, and mammals. The fish along the external areas of the lagoon (inlets, jetties, and deep channels) is made up of species typical of a marine environment.

Fauna of salt marshes and brackish water wetlands

The salt marshes, environments flooded by the tide, make up a unique habitat for some animal species. Vast wetlands which are exposed during low tides provide many bird species with an excellent feeding ground. More stable areas are preferred nesting grounds for the Black-winged Stilt, the Redshank, the Common Tern, and the Sandwich Tern.

Fauna of clay quarries

The artificial outline of the lagoon was created through reclaiming some interesting transitional environments like swamps, wetland meadows and woods, and backwaters. These environments were once part of the lagoon. The clay quarries in the inland territory behind the lagoon have been spontaneously re-colonised by the fauna which once lived in these original transitional environments: little molluscs, insects, reptiles, and amphibians. During the migratory season, birds find a needed temporary habitat and feeding grounds in these areas.

Fauna of fish farms and lagoon edge areas

These environments are the only areas where fresh fluvial waters and brackish waters meet in the lagoon. Almost all of the coot and the ducks in the whole lagoon territory are concentrated in fish farm areas during the winter months and during the migratory season. The reed bed environments offer nesting areas for numerous species: Ardeidae, like the Purple Heron, the Little Egret, the Night Heron, the rare Marsh Harrier, and the numerous passeriformes. Among the small mammals, the Harvest Mouse and the Miller’s Water Shrew are the most interesting species, but Badgers and Polecats have also survived. Reptiles like the Water Snake and the Swamp Turtle Palustrine also find their homes here. (Note: The Passeriformes is the largest order of bird classification and includes more than half the world’s different bird species, with more than 5,000 unique species classified as passerines. With more than half the world’s birds classified as passerines, these birds are familiar to all bird watchers).

Vegetation of coastlines

Areas of the littoral strips have different levels of plant colonisation which correspond to different zones of the ecosystem. These zones move from the first sand, through the strip of dunes, until the area further back where dunes begin to consolidate. In the wet areas behind the dunes, pond vegetation can be found. Sand vegetation can be interrupted by patches of wind breaking vegetation.

Vegetation of salt marshes

The salt marshes, land areas which are only submerged during the highest tides, are covered by a thick growth of plants. They are inhospitable for most plant types, but are ideal for halophytic species, that is plants which need salty soils. Salt marshes, however, differ from one another. Rushes and reeds, for example, grow in the ones nearest to the edge of the lagoon, where there is a greater quantity of fresh water.

Vegetation of lagoon beds

There are four species of eelgrass native to the lagoon: Zostera marina and Cymodocea nodosa, that colonise beds of more than a metre in depth, and Zostera noltii and Ruppia maritima, that usually colonise around salt marshes and mud flats. Up until recently, eelgrass was an important element of lagoon bed vegetation. Vast meadows of eelgrass covered lagoon beds, consolidating them with its complex system of roots. The growth of this plant has now greatly diminished and algae species (in some cases rare ones) have spread throughout the lagoon.

Vegetation of reed beds

Plants adapted for fresh water environments grow around waterways and swamps. Typical plants that can be found in these areas are the common reed and the cattail. Following the waterways, the reeds make their way into the lagoon along the channels. In the northern part of the lagoon, fresh water marsh vegetation thrive in brackish water environments, which are rich in species that can adapt and live in brackish habitat.

Links (internal–external)

Acqua alta and the MOSE Barrier Project

The Great Venetian Flood of 1966

Venetian Wells – Drinking Water

Rick Steve’s Europe: Venice and Its Lagoon (youtube.com)

The Venetian Lagoon and its Ecosystem The Venetian Lagoon and its Ecosystem The Venetian Lagoon and its Ecosystem

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