Enclosure experiments were conducted during April. June/July and September in the eutrophc estuary Roskilde Fjord, Denmark, to reveal the effects of inorganic nutrients, suspensionfeedng bivalves Mytilus edulis and planktivorous fish (three-spined sticklebacks Gasterosteus aculeatus) on the zooplankton community > 45 pm. The additlon of inorganic nutrients did not increase zooplankton biomass although it did increase the chlorophyll level, indicating that zooplankton production was not food hmited. Filtration by M. edulis reduced the number of tintinnid ciliates and rotifers during all 3 experiments, but not the abundance of the larger zooplankton species. Additions of planktivorous fish reduced the densities of larger zooplankton species Acartia tonsa and Pleopis polyphernoides but not of smaller species. An immense increase in numbers of A. tonsa and P. polyphernoides was observed in enclosures without fish, indicating that the larger crustacean zooplankton is strongly predator controlled. Thus, the qualitative and quantitative development of the zooplankton community in the enclosures was controlled in 2 ways; from the top of the size spectrum by G. aculeatus and from the bottom of the size spectrum by M. edulis.

Biomasses of bacterioplankton and heterotrophic flagellates and bacterial net production rates were measured intensively during three periods of 16-25 d in estuarine experimental enclosures manipulated by additions of inorganic nutrients, fish, and mussels. Tightly coupled oscillations in biomasses of bacteria and flagellates were found, particularly during summer. The amplitude of the oscillations increased in response to artificial eutrophication with inorganic nutrients and decreased in response to predation control from added mussels. Losses of bacterioplankton by grazing were assessed indirectly with carbon balances for each enclosure and directly with procaryotic inhibitors and with 3H-labelcd bacteria. Estimates obtained by these three independent approaches showed clearance rates averaging 1.5-2.5 nl h-l flagellate-l.Comparison of grazing rates to increases in flagellate biomass revealed minimum estimates of flagellate carbon yield averaging 33%. High minimum yields were found in some enclosures during summer. Carbon sources other than bacteria may have been used by heterotrophic flagellates in these situations.

During three periods of 16 to 25 days, bacterioplankton production, bacterial cell volume, chlorophyll a, CO2 assimilation, and particulate organic carbon were measured in enclosures situated in the eutrophic estuary Roskilde Fjord, Denmark. The enclosures were manipulated with respect to sediment contact and contents of inorganic nutrients, planktivorous fish, and suspension-feeding bivalves. Nutrient enrichment, the presence of suspension feeders, and sediment contact induced pronounced changes in bacterial production, as well as minor changes in bacterial cell volume; however, these effects seemed to be indirect, transmitted via phytoplankton. Bacterial production, measured as [3H]thymidine incorporation, closely followed changes in phytoplankton biomass and production, with time lags of 5 to 10 days. Good correlations of mean bacterioplankton production to chlorophyll a concentration and CO2 assimilation suggested phytoplankton to be the dominating source of bacterial substrate, apparently independent of nutrient stress. Zooplankton >140 jum, bivalves, and sediment seemed to provide insignificant, if any, substrate for bacterioplankton, and benthic suspension feeders seemed not to act as direct competitors for dissolved organic carbon. The bacterioplankton mean cell volume, measured by image analysis, changed seasonally, with the smallest cells during the summer. Within each period, the bacterial cell volume correlated positively to growth rate and negatively to temperature.