Picophytoplankton contribution to phytoplankton community structure in the Gulf of Venice (NW Adriatic Sea)

The size distribution of the phytoplankton community has been analyzed on a monthly basis in the Gulf of Venice (NW Adriatic Sea), with the aim to assess the seasonal variation of the contribution of autotrophic picoplankton (APP) to total phytoplankton. The investigation was carried out in two stations characterized by different influences of fluvial inputs. APP was mainly made up of Synechococcus, the larger fraction (Utermohl Fraction Phytoplankton, UFP: > 2 mu m as maximum linear dimension) of diatoms and nanoflagellates. The average APP abundance (46 x 10(3) cells ml(-1)) and biomass (8.1 mu g C dm(-3)) indicate that APP was a persistent and significant component of the NW Adriatic phytoplankton. The highest APP abundance (up to 270 x 10(3) cells ml(-1)) and biomass (up to 49 mu g C dm(-3)) were recorded from summer to autumn, UFP peaked from late winter to late spring. The mean APP contribution to total phytoplankton abundance and biomass was 98% and 31% respectively. The seasonal variation of total phytoplankton biomass was mainly driven by the UFP fraction; however, a seasonal shift in the relative importance of UFP and APP occurred from spring to summer.