The sessile benthic community patterns on hard bottoms in response to continental vs. marine influence in northern Adriatic lagoons

Abstract

The sessile hard bottoms zoobenthos community was studied in three northern Adriatic lagoons (Venice, Caorle and Grado-Marano). On the basis of previous researches (1993–1994 in Venice; 1997 in Grado-Marano), these lagoons had been subdivided into ecological sectors. The time consistency of such a subdivision has been assessed. The largest number of species was found in the lagoon of Venice, showing a high heterogeneity of environments and an intense shipping traffic, coupled with aquaculture activities that favoured the arrival of exotic species. A very poor community of obligate oligohaline species was found in Caorle. The natural and stable environment of Grado-Marano lagoon, with its large area of freshwater influence, hosted an intermediate number of species. The multi-dimensional scaling ordination plot of 27 representative stations from the three lagoons (samples of July 2000) clearly distinguished a cluster of “estuarine” sites from the remaining ones, that could be classified as “inner lagoon” sites, “vivified lagoon” sites and “lagoon mouth” sites. When compared with past results by Anosim test and Simper analysis, a lesser degree of discrimination between clusters of sites could be observed. In the following year (samples of July 2001), a larger number of sampling stations was considered in Venice and Grado-Marano. The multi-dimensional scaling plot for the community data of Grado-Marano showed fairly recognisable clusters, that substantially repeated those defined in 1997. As far as Venice is concerned, about 25% of the examined stations had shifted towards a different group, compared with those identified in 1993–1994; in particular, the “urban”, “inner lagoon” and “estuarine” sectors and their peculiar assemblages survived only in small areas with unique features of harsh environmental conditions. The progressively enhanced marine influx in all the lagoon sectors (but the ones directly affected by the outflow of the only river still discharging into the lagoon) has, on the one hand, reduced the most evident effects of eutrophication and urban pollution, and on the other, led to a banalisation of the lagoon communities.