ROLE OF PREDATION IN CONTROLLING BACTERIAL AND HETEROTROPHIC NANOFLAGELLATE STANDING STOCKS IN THE COASTAL ADRIATIC SEA - SEASONAL PATTERNS

The role of bacteria and heterotrophic nanoflagellates (HNF) in the microbial food web was studied over 1992/93 in Kastela Bay (Adriatic Sea), The average production of bacteria (12. 3 mu g C d(-1)) and HNF (4.8 pg C d(-1)) suggests that these organisms fix a significant amount of organic carbon. Small HNF (<8 mu m) were the most important bacterial grazers, accounting, on average, for 80 % of the total grazing on bacteria, whereas abundance and production of HNF were controlled by ciliate grazing, Bacterivorous protozoa stimulated bacterial growth and contributed to an enhanced turnover of bacterial biomass. About 20% of annual bacterial production was channelled through the microbial loop. However, bacterial carbon flux through the microbial loop showed marked seasonal oscillations with considerably higher values recorded during the warmer part of the year (June to November). Thus, in that period the microbial loop could be an important Link between primary production and higher trophic levels. Grazing rates obtained by an RLB (radioactive-labelled bacteria) method were regularly higher than values obtained by the size fractionation method.