Preliminary flow cytometric analyses of phototrophic pico- and nanoplankton communities in the northern Adriatic

Flow cytometry was used for the first time to determine the abundance of pico- and nanophototrophic plankton in the northern Adriatic during five cruises, from June to December 2006, along the west-east trophic gradient from the Po River delta (Italy) to Rovinj (Croatia). The first recorded occurrences of Prochlorococcus were obtained. Synechococcus were the most abundant phototrophs in the area, occasionally matched in number by previously rarely studied phototrophic picoeukaryotes. In terms of biomass the latter usually comprised more than 50% of the phototrophic pico-biomass, and sometimes up to 90%. Under-researched phototrophic picoeukaryotes thus seem to be much more important for this ecosystem than was previously thought, while preliminary results on the presence of Prochlorococcus indicate their minor importance in this hydrologically and biologically dynamic area. Estimations of the contributions of each size fraction to the total phototrophic biomass (pico-, nano- and micro-) reveal a higher portion of pico- and nano- fractions on average compared to the micro- fraction. Concluding, this study gives a more complete estimation of abundance and biomass of all major groups of pico- and nanophototrophs in this area than has been offered to date, thus allowing better insight into their potential relative importance both as producers and prey.