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

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Abstract
In lakes, fish and invertebrate predation are recognized as strong structuring forces on zooplankton communities. The object of this investigation was to study whether predation has a similar potential in a coastal area of the brackish Baltic Sea and if it could explain observed differences in zooplankton community structure between a reference area and an eutrophied area. Species composition and daily vertical migration of zooplankton and zooplanktivores, and the diets of the latter, were studied in July and August at two 30 m deep stations differing in primary productivity. The biomass of zooplankton >35 μm was dominated by copepods, but cladocerans and rotifers also occurred in significant numbers. The dominating zooplanktivores were herring (Clupea harengus) and the mysid shrimp Mysis mixta. They fed almost exclusively on zooplankton, mainly copepods, and their estimated food consumption equalled or exceeded the summer copepodite production. The structure of the zooplankton community cannot be explained by effects of predation or by feeding conditions alone. Increases in rotifer and cladoceran abundances with increased primary production suggests effects of food supply. However, a generally rapid decline in the annual summer peak of cladocerans may be caused by predation. The total abundance of copepods did not increase with improved feeding conditions, but there was a shift in species dominance. The copepod Ewytemora affinis hirundoides, which was intensively preyed upon, increased with increased phytoplankton production, while Acartia bifilosa and/or A.longiremis, which was less eaten, decreased. Predation may explain a pronounced daily vertical migration of the most predated copepods. They occurred in deeper water during the day, when the visually feeding herring were active, and moved closer to the surface at night when M.mixta left the bottom, to forage in the water column.