Selective predation by herring and mysids, and zooplankton community structure in a Baltic Sea coastal area

In lakes, fish and invertebrate predation are recognized asstrong structuring forces on zooplankton communities. The objectof this investigation was to study whether predation has a similarpotential in a coastal area of the brackish Baltic Sea and ifit could explain observed differences in zooplankton communitystructure between a reference area and an eutrophied area. Speciescomposition and daily vertical migration of zooplankton andzooplanktivores, and the diets of the latter, were studied inJuly and August at two 30 m deep stations differing in primaryproductivity. The biomass of zooplankton >35 µm wasdominated by copepods, but cladocerans and rotifers also occurredin significant numbers. The dominating zooplanktivores wereherring (Clupea harengus) and the mysid shrimp Mysis mixta.They fed almost exclusively on zooplankton, mainly copepods,and their estimated food consumption equalled or exceeded thesummer copepodite production. The structure of the zooplanktoncommunity cannot be explained by effects of predation or byfeeding conditions alone. Increases in rotifer and cladoceranabundances with increased primary production suggests effectsof food supply. However, a generally rapid decline in the annualsummer peak of cladocerans may be caused by predation. The totalabundance of copepods did not increase with improved feedingconditions, but there was a shift in species dominance. Thecopepod Ewytemora affinis hirundoides, which was intensivelypreyed upon, increased with increased phytoplankton production,while Acartia bifilosa and/or A.longiremis, which was less eaten,decreased. Predation may explain a pronounced daily verticalmigration of the most predated copepods. They occurred in deeperwater during the day, when the visually feeding herring wereactive, and moved closer to the surface at night when M.mixtaleft the bottom, to forage in the water column.