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Seasonal variations in abundance and carbon biomass of ciliated protozoa and micrometazoa were studied from

This paper addresses the dynamics of the prokaryotic picoplankton community in the coastal and open sea areas of the central Adriatic and in the coastal area of the southern Adriatic. This involved the study, from January to December 2005, of bacteria (total number of non-pigmented bacteria; high nucleic acid content (HNA) bacteria; low nucleic acid content (LNA) bacteria), cyanobacteria (Synechococcus and Prochlorococcus) and heterotrophic nanoflagellates. During the warmer seasons, in the mainly oligotrophic area under investigation into the Adriatic Sea, bacterial densities and bacterial production have shown an increase in values and domination of the LNA group of the bacterial population. In contrast, in those areas influenced by karstic rivers, the domination of HNA bacteria in total abundance of non-pigmented bacteria and high values of bacterial production was estimated throughout the investigated period. Our results show the importance of both HNA and LNA bacterial groups in the total bacterial activity throughout the investigated area. The biomass of bacteria was mostly predominant in the prokaryotic community, while within the autotrophic community Synechococcus biomass mostly predominated. During the warmer seasons, an increase in autotrophic biomass was observed in relation to non-pigmented biomass. The importance of predation in controlling bacteria by heterotrophic nanoflagellates was pronounced during the warmer period and in the coastal areas.

Bacterial and heterotrophic ndnoflagellates (HNF) abundance, as well as bacterial production and chlorophyll a levels, were measured at live sites extending from the coastal zone toward the open Adriatic in the period from March to October "i995. The investigated areas were grouped into trophic categories according to concentrations of chlorophyll u. All the biotic-parrsmeters increased along the trophic gradient, ledding to eutrophy, but they did not increase at the same rate. The bacterial biomass : phytoplankton biomass {BB : chl u) ratio decreased from about 10 in the very oligntrophic area to 0.8 at the eutrophw site. In contrast, the bacterial abundance : HNF abundance ratio iB : HNF) increased from 1000 bacteria per 1 tlagellate in the oligotrophlc system to .1700 bacteria flagellate' in the eutrophic area. Decreasing BB : chl a and :ncreasing B : HNF ratios along the trophic gradient might rellect the dflterent structures of the microbial food web. Relationships between bacterial abur~danc:e and production, and chl a and t-{NF showed that bacterial abundance along the trophic grddient was regulated by the interplay between nutrient supply and grazing pressure. But in the oligotrophic system, bacterial abundance was more closely related to bacteriaL production and chl u than m the eutrophic system, suggesting stronger control of bacteridl abundance by substrate supply C)n the other hand, the coupling between bacteria and HNF, and uncoupling between bacterial dbundance and production in the eutrophic system, showed that the importance ot bactenovorv increased in richer systems.

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