Dynamics of the picoplankton community from coastal waters to the open Sea in the Central Adriatic

Abstract

Flow cytometry was used to describe seasonal cycles of Prochlorococcus (Prochl), Synechococcus (Syn), picoeukaryotes and heterotrophic bacteria in the central Adriatic Sea along the trophic gradient from January to December 2010. All picoplankton parameters decreased from eutrophic to oligotrophic areas, while the biomass ratio of bacterial to autotrophic picoplankton showed an increase along the trophic gradient. Bacterial biomass ranged from 5.28 to 21.20 μg C l-1. Increased values were present during warmer seasons with the domination of the low nucleic acid (LNA) group of bacteria. The high nucleic acid (HNA) bacterial group dominated during winter and spring. Bacterial production ranged from 0.09-0.45 × 104 cells ml-1 h-1. At coastal stations, increased production was present during the winter and spring and was more or less uniform at open sea stations. The biomass of Syn and Prochl ranged from 0.16 to 11.47 μg C-1l-1 and from 0.01 to 3.08 μg C l-1, respectively. They were elevated during the summer and the autumn at coastal stations and during late winter at the open sea. Syn biomass always dominated over Prochl, with 61.6-97.2% participation in the biomass of cyanobacteria. The biomass of picoeukaryotes ranged from 1.21 to 21.85 μg C l-1 and was the highest during the winter. Their biomass notably prevailedin autotrophic picoplankton (APP) biomass over that of picocyanobacteria during the whole year. Autotrophic components (Prochl, Syn and picoeukaryotes) made a greater contribution to picoplankton biomass in mesotrophic and eutrophic areas, while heterotrophic bacteria became more important under oligotrophic conditions.