Nano- and microplankton in Heraklion Bay, Crete: comparison of coastal waters and mesocosm enclosures assemblages <B>Nano- and microplankton in Heraklion Bay, Crete: comparison of coastal waters and mesocosm enclosures assemblages</B><P> Paraskevi Pitta(1), Pascal Divanach(1) & Maroudio Kentouri(1,2)</B><I><P> (1) Institute of Marine Biology of Crete, PO Box 2214, GR- 710 03 Heraklion, Crete, Greece<BR> (2) University of Crete, Department of Biology, PO Box 1470, GR- 71110, Crete, Greece</I><B><P> Abstract</B><P> Coastal nano- and microplankton assemblages of Heraklion Bay were used to study the response of plankton in oligotrophic areas to eutrophication. The impacts of different nutrient levels on the abundance of phytoplankton and zooplankton and on the structure and evolution of the East Mediterranean plankton assemblages were studied using a combination of a) <I>in situ</I> sampling at 1m depth from a coastal area, and b) the monitoring of three mesocosms (60 m3). In the first mesocosm sea water only was enclosed in order to assess the influence of enclosure conditions, while in the other two, varying quanitities of nutrients were added, forming an artificial eutrophication gradient.<P> In all cases samples were taken at four-day intervals, over a two-month period. All nano- and microplankton organisms (flagellates, diatoms, ciliates, rotifers) were then identified to species level and counted. Chlorophyll pigments and nutrient levels, as well as various other physical and chemical parameters, were measured.<P> The increase in nutrient concentration results in an acceleration of the successional stages in comparison with the natural environment. Enclosure conditions lead to increased phytoplankton blooms and dominance of the assemblages by a few species. Significant quantitative and qualitative differences were observed in both the chlorophyll concentrations and the structure and evolution of assemblages between those from the wild and those from the enclosures, irrespective of whether or not they had been fertilized with nutrients.<B><P> Keywords:</B> mesocosms, nanoplankton, microplankton, eutrophication, Aegean<P>