Dynamics of phytoplankton guilds under dystrophic pressures in Lesina lagoon

1. We analyzed the dynamic of phytoplankton guild during an dystrophic crisis observed in an area of Lesina Lagoon from July to September 2008. 2. A two-weekly scale, patterns of variation of phytoplankton biomass (as Chla), numerical abundance, species richness, diversity, taxonomic and size structure were compared in two stations: one within the area affected by the crisis (AT2 station) and one outside, as control (AT3 station). 3. Phytoplankton biomass and cell abundance varied significantly between sampling stations and sampling times. Biomass was on average about 100 time higher in AT2 than AT3 station, with peaks of 180,67±22,12 mgChla/m3 (8 July) and 134,05±47,26 mgChla/m3 (5, September). Cell density was on average about 30 times higher in AT2 station, with values always higher than 20*106 cell/l. 4. Species richness and diversity did not varied significantly between stations and not show a trend with the evolution of anoxic crisis. 5. The taxonomic composition is similar in the two stations (taxonomic similarity among stations was 76,04%), even if the relative abundance of species was significantly different. 6. As regard size structure, in the AT2 station, microphytoplankton dominates at beginning of biomass bloom (dominant taxa: Thalassiosiraceae, average weight (AW) 163,51 pgC), whereas from the end of July until September and in the AT3 station, nanophytoplankton was the dominant fraction (dominant taxa: until the middle of August: Algae indet AW 3,14 pgC and Navicula sp. AW 2,50 pgC).