Phytoplankton changes and relationships with microphytobenthos and physico-chemical variables in the central part of the Venice lagoon

By comparison with measurements carried out in 1990–91, phytoplankton biomass in 1998–99 appears strongly reduced in the central basin of the Venice lagoon. Chlorophyll a concentrations,which in the past easily reached 100 _g dm_3,reached only 5 _g dm_3 during this investigation. Phaeopigment concentrations were close to the detection limit. The introduction of the bivalve Tapes philippinarum Adams and Reeve in the lagoon and its harvesting by hydraulic and mechanical dredges strongly increased the amount of sediment resuspension and settlement. This phenomenon caused a marked increase in water turbidity and the disruption of the benthic microlayer of the lagoon bottoms composed by bacteria, microphytobenthos and small macrophytes. Significant correlations between microphytobenthos abundance and the suspended and settled sediment, and with the underwater light availability were found. In the areas affected by the highest sedimentation fluxes significant negative correlations were also found between the abundance of the microphytobenthos and phytoplankton communities. Many benthic taxa such as Amphora, Cocconeis, Navicula, Nitzschia, Pleurosigma and Thalassiosira, were found in all the water column samples and they were more abundant than exclusively planktonic diatoms.