Regulation of zooplankton by suspension-feeding bivalves and fish in estuarine enclosures

Enclosure experiments were conducted during April, June/July and September in the eutrophic estuary Roskilde Fjord, Denmark, to reveal the effects of inorganic nutrients, suspension-feeding bivalves Mytilus edulis and planktivorous fish (three-spined sticklebacks Gasterosteus aculeatus) on the zooplankton community > 45 μm. The addition of inorganic nutrients did not increase zooplankton biomass although it did increase the chlorophyll level, indicating that zooplankton production was not food limited. 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 polyphemoides but not of smaller species. An immense increase in numbers of A. tonsa and P. polyphemoides 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.