Climate Change and the European Water Dimension - Agri ...

the lagoons of the Upper Adriatic, fish farms are separated from the open lagoon by
embankments that block them from the ebb and flow of the tide. They are shallow
basins of salt and brackish water and they make up special environments that have
been equipped for fish breeding and sometimes hunting. Within their confines are
water areas, artificial and natural channels, salt marshes and structures for regulation
of salt and fresh water flow.
The “open lagoon” and water flows
The currents and the sea level in the Venice Lagoon are driven mainly by two
different forcing functions. The first one is tidal forcing, which is strictly periodic and
deterministic, so that it can be predicted with certainty. The second kind of forcing are
non–periodic functions, such as wind and atmospheric pressure (Gacic et al., 2004).
The meteorological component is the main factor of the frequent flooding of Venice.
The average daily volume of water that enters the lagoon from the sea is about 400
million m3. The volume of water exchanged with each tidal cycle is about 350 million
m3 during spring tide and 175 million m3 during neap tide (http://www.salve.it). The
maximum water flow in one inlet can exceed 12000 m3/s, more than the volume of
the River Po in full flow (Bianchi et al., 2004).
Man-induced changes over the past century have increased the amounts of water
exchanged with the sea through the inlets (due to their reconfiguration and
construction of jetties) and reduced the lagoon’s resistance to incoming tides as a
result of loss of extensive areas of saltmarsh and the excavation of the principal
navigation channels. Consequently currents are stronger and erosion rates high.
The water contribution of the drainage basin
Moreover 900 million m3 of fresh water flow into the lagoon every year through the
2,515 km of the hydrographic network. The yearly quantity of rainwater from the
drainage basin is 1.4 km3 (588 mm) and directly into the lagoon, 0.3 km3. The
annual quantity of nutrients from the drainage basin is estimated to be 7000 tons of N
and 1500 tons of P. The average concentration of nutrients at the mouths of the
rivers is about 10 mg/l of N and 1 mg/l P (http://www.salve.it).
A number of ecosystems
The Lagoon of Venice is a fragile and complex environment in the transition between
fresh and sea water, composed of a continuous series of ecotones, in which the
anthropogenic impact increased over the course of many centuries. Moving from sea
to land, specific fauna and flora characterize different ecosystems. Along the coastal
dunes and the wooded areas behind the dunes live amphibians, reptiles, birds, and
mammals, which adapt well to medium to high daytime temperatures. The fish
species along the external areas of the lagoon are typical of a marine environment.
Areas of the littoral strips have different levels of plant colonisation: in the wet areas
behind the dunes, pond vegetation can be found. Patches of wind-breaking
vegetation can interrupt sand vegetation.
The Venice lagoon is one of only two tidal coastal wetlands in the Mediterranean
region; it is also characterised by a transitional Mediterranean/Atlantic climate
system. Consequently, the salt marshes (“barene”) represent some unique features,
exposed during low tides and flooded by high tides. Their characteristic vegetation is
composed of several halophytic species that survive within a narrow range relative to
water level. Together with the associated mudflats, they are also an essential habitat
for many bird species (breeding, feeding, over-wintering etc.).
168