The role of ciliates within the microbial food web in the entrophicated part of Kastela Bay (middle Adriatic Sea)

Interactions among phytoplankton, bacterioplankton, heterotrophic nanoflagellates (HNF), ciliated protozoa and copepod nauplii were studied in the eutrophicated part of Kastela Bay from May 1998 to November 1999. Special emphasis was placed on relationships between size categories of nonloricate ciliates (NLC) and other microbial food web components. Biomasses of phytoplankton and bacteria were primarily influenced by abiotic parameters. Temperature indirectly controlled variation in HNF biomass through the changes in biomass of bacteria and the smaller phytoplankton fraction. Besides FINF. bacterial biomass was affected by the NLC < 10(3) mu m(3) (Cell Length < 20 mu m). A small NLC size category (< 10(4) mu m(3) CL < 40 mu m) could limit the growth of HNF as they compete for bacteria and as a result of direct grazing. Nonloricate ciliates > 10(4) mu m(3) (CL > 40 mu m) had a strong mutual correlation and they seemed to be controlling the micro-phytoplankton fraction. During the colder part of the year, FINE abundance was regulated by ciliate grazing. The high impact of ciliates in summer 1998 Could have been influenced by the taxonomic composition of the phytoplankton community changing. as well as the higher eutrophication level in the study area. Predation by copepod nauplii on ciliates and carbon transfer-to higher trophic levels appear to be of relative importance only in the period when they are most abundant. This paper outlines the dominant relationships within the microbial food web and suggests that a significant amount of bacterial production, phytoplankton and HNF biomass could be transferred to higher trophic levels through the microbial food web.