Time and space diatom distribution in the surface sediment of the central part of the Venice lagoon

The aim of this paper was to improve the knowledge of the primary producers living in the Venice Lagoon by studying the composition and the distribution of the benthic diatom communities populating subtidal surface sediments. The upper 5 mm of sediment from four areas distributed between the Malamocco inlet and the mainland north of Venice were collected and analysed. Sampling was carried out monthly for one year (1998-99). Species composition, cell abundance and carbon content were determined by light microscopy after separation of cells from the sediment particles using hydrogen peroxide. A total of 147 taxa were identified, mostly pennate diatoms (Bacillariales) belonging to the genera Amphora, Cocconeis, Navicula, Nitzschia and Pleurosigma. Cell abundance ranged from 0.7 x 106 cells cm-3, near the Malamocco inlet (Station A), to 10.4 x 106 cells cm-3 near the mainland (Station C). Diatom carbon content was between 28 and 380 mg cm-3. Seasonal trends were slight, except at Stations D and C where the lowest abundance was found in October-November and the highest one in summer. Significant relationships between benthic diatoms and macrophyte biomass, sediment grain size, underwater light transmission, sediment fluxes and the salinity gradient were found.