Role of environmental variables and heavy metal contamination in affecting diatom diversity in Adriatic transitional ecosystems.

In shallow ecosystems, in absence of macrophytes, benthic diatoms may play a key role for the food chain and in stabilizing surface sediments. To describe community diversity, dynamics and the interactions with planktonic diatoms, water and surface sediment samples (1 cm top-layer) were collected in almost 240 sites and cell abundance and taxonomic composition by means of conventional light microscopy were determined. Most of the observations were done in the Venice lagoon but surveys were also carried out in Grado-Marano, Goro and Lesina lagoons. Physico-chemical parameters and nutrient concentrations in water and sediments were measured in all sites; moreover, data on heavy metal concentrations were available for 25 sites. This study focuses particular attention to planktonic and benthic diatom diversity and to the factors affecting it. 128 and 133 taxonomic entities were identified in the water column and in surface sediments, respectively. The highest number of species was represented by pennate diatoms, whereas centric diatoms were often the most abundant. The number of taxa (2+23 in water; 4+25 in sediments) and the species richness (0.11+1.95 in water; 0.23+1.72 in sediments) varied significantly site by site. Seasonal trends were quite different if planktonic and benthic diatoms were compared: the former had significant seasonal fluctuations, whereas the latter tended to be more stable without a clear seasonal pattern. Planktonic contribute to the diversity of benthic community resulted negligible, but the contrary seemed more significant probably due to sediment resuspension, and a large taxonomic distincness was observed in the water. Both planktonic and benthic diatom spatial diversity was positively correlated with light availability and negatively with nutrient concentrations. Benthic diatom taxonomic composition depended also on sediment grain size and heavy metal concentrations. In particular, Cadmium, Cobalt, Chromium, Nickel and Zinc were found to have a significant negative effect on diversity.