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 mi crobial food web was studied over 1999/93 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 o f organic carbon. Small HNF (<8 mu m) were the most important bacteria l grazers, accounting, on average, for 80 % of the total grazing on ba cteria, whereas abundance and production of HNF were controlled by cil iate grazing, Bacterivorous protozoa stimulated bacterial growth and c ontributed 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 mark ed seasonal oscillations with considerably higher values recorded duri ng the warmer part of the year (June to November). Thus, in that perio d the microbial loop could be an important Link between primary produc tion and higher trophic levels. Grazing rates obtained by an RLB (radi oactive-labelled bacteria) method were regularly higher than values ob tained by the size fractionation method.