TROPHIC INTERACTIONS AND CARBON FLOW BETWEEN PICOPLANKTON AND PROTOZOA IN PELAGIC ENCLOSURES MANIPULATED WITH NUTRIENTS AND A TOP PREDATOR

Experimental enclosures were used to follow responses of the planktoni c microbial food web to varying short-term (5 d) perturbations induced by adding inorganic nutrients (N and P) and a top predator (fish) dur ing a 21 d period in late summer, on the coastal area of the Baltic Se a. Biomass, production, growth and grazing of pico- and nanoplankton a ssemblages were estimated, and a carbon budget for the microbial loop during the experiment was constructed. The microbial food web was a hi ghly dynamic system. Varying perturbations due to nutrient loading and the top predator provoked eutrophication in the enclosures, but they affected the microbial loop only slightly. The amplitudes of oscillati on in abundance of coupled communities were amplified, but the frequen cies of oscillations in the microbial loop were not affected by the pe rturbations. Changes in the route of carbon flow through the microbial food web occurred in relatively short time scales. These changes seem ed to be dependent on the phasing of the coupled oscillations between the communities, and the structure within the different communities. C iliates were only loosely connected to the microbial loop: although ci liates and heterotrophic nanoflagellates (HNF) showed predator-prey-li ke coupled oscillations, the ciliates gained most of their carbon from other sources, and most of the HNF carbon loss was due to factors oth er than ciliates. HNF were the most important consumers of picoplankto n during the HNF maximum, but they were also dependent on other source s of nutrition.