Plankton communities and nutrients in the Venice Lagoon. Comparison between current and old data.

A comparison between recent data (1997–2002) and older data (1975–1980) related to the Venice lagoon is presented here. These data have been focused on plankton and dissolved nutrient distribution obtained within the framework of differing research projects. Investigations were carried out in the central and northern areas of the Lagoon of Venice. Comparing the two data sets, a general increase in orthosilicates and in dissolved inorganic nitrogen (DIN) was observed. Conversely, phosphates showed a decrease in polyhaline waters (salinity between 18 and 30 PSU) because of the reduced amounts of phosphorous compounds in detergents, following Italian laws enforced in the 1980s. In the recent data set (1997–2002), phytoplankton abundance was higher than in the years 1975–1980 because of more frequent diatom blooms. The increased resuspension processes induced by dredging for molluscs probably favour a mobilisation of benthic epipsamic diatoms from sediments, resulting in a more pelagic life style. Regarding zooplankton, a decrease in the standing stock, more marked in winter, was revealed. These results indicate that the trophic conditions of the Lagoon of Venice are subjected to natural and anthropogenic temporal changes that can be understood only through the analysis of uninterrupted time series.