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e-Journal

 

Venice and the Environmental Hazards of Coastal Cities
(Released December 2006)

 
  by Carolyn Scearce  

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The lagoon habitat

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Lagoons exist as transitional environments between land and sea. The Venice lagoon, Italy's largest lagoon, covers 550 km2. Sand bars partially isolate the lagoon from the Adriatic, with direct exchange with the sea only occurring at three inlets that individually connect the lagoon's northern, central, and southern portions. The lagoon consists of a number of interrelated habitats. The terrestrial portion includes the lagoon's interior islands and sandbar and the surrounding coastal strip. The aquatic portions, which account for more than 90% of the lagoon's surface area, include channels, shallows, mud flats and salt marshes. These varying habitats host a number of organisms and diverse communities (VWA, 2006).

photo of coast
Channels, mud flats, and salt marsh in Venice lagoon

The average depth of the lagoon is approximately one meter, with the majority of the surface area composed of shallows, mud flats, and saltwater marshes. Channel depths range from 1-15 meters deep. Most of the circulation within the lagoon takes place through the network of channels and tidal creeks. Natural and artificial islands within the lagoon range in size from a few hundred square meters to several square km. The islands of the central portion of the lagoon that comprise the historic site of Venice are densely inhabited. Over 30,000 people inhabit islands in the lagoon outside of the boundaries of the historic city, with other islands remaining uninhabited or abandoned.

The salt marshes and sea grass beds within Venice lagoon serve important functions, both in maintaining the morphology and ecological integrity of the lagoon environment, and in serving as important habitat for organisms living within the system. The salt marsh area within the lagoon has decreased from 115 km2 to 33.5 km2 over the course of the last two centuries. Factors contributing to this decline include land reclamation for human use, erosion, pollution, subsidence, and decreased accretion of sediments (Cecconi, 2005). Sea grass meadows have also been declining over the last several decades (Acri et al., 2005). A study comparing sea grass populations from 1990 and 2002 showed a continued decline; also, the distribution of sea grass species has altered. The northern and southern sections of the lagoon show the greatest overall losses of sea grass; Zostera noltii, which used to be the primary species has decreased most dramatically. Only the central lagoon has shown increased sea grass coverage, mostly by Zostera marina, the species of sea grass that is becoming dominant in the lagoon as Zostera noltii declines (Rismondo et al., 2005). The structure provided by salt marsh and sea grass habitat serves as important substrates, food sources, and refuge for organisms living within the lagoon. As these habitats continue to decline the populations of organism that use these habitats are likely to decline as well.

marsh photo
Salt marsh vegetation in Venice lagoon

Venice lagoon hosts the largest population of water birds in Italy. Over 60 species of aquatic birds have been observed in the lagoon. These bird populations make use of salt marsh and island habitats for nesting, over wintering, or as stopover points. In January as many as 130,000 birds make use of the lagoon. 80-90% of Italy's Redshank and Sandwich Terns nest within the lagoon (Scarton, 2005). The lagoon also serves as a productive fish habitat, hosting many euryhaline species, fish that migrate between lagoon and sea environments to complete their lifecycles (Ravera, 2000). Other organisms that make their home in lagoon include bivalve mollusks and gastropods, cephalopods, and crustaceans.

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