Trophic decoupling of mesozooplankton production and the pelagic planktivores sprat Sprattus sprattus and herring Clupea harengus in the Central Baltic Sea
Quantitative analysis of the predation impact of planktivores on zooplankton is essential for proper understanding of energy flow and trophic coupling in ecosystems. To quantify trophic dynamics between zooplankton and small pelagic fish in the Baltic Sea, we conducted a temporally resolved investigation on the diet, feeding and predation impact of the dominant planktivorous fish species sprat Sprattus sprattus L. and herring Clupea harengus L. in the Central Baltic Sea (Bornholm Basin). Bi-weekly to monthly coverage between April 2002 and November 2003 included net sampling of copepods, determination of predator stock size using hydro - acoustics and trawl surveys, and extensive stomach content sampling. Herring and sprat were mainly zooplanktivorous, feeding on the calanoid copepods Temora longicornis, Pseudocalanus acuspes and Acartia spp. and, in summer, on the cladocerans Bosmina spp. and Podon spp.. We observed temporally high predation impact of sprat and herring on T. longicornis and P. acuspes in April and May, when more than the production of these 2 copepod species (copepodite stages C1-6) was consumed (consumption/production [C/P] > 3.5). However, the impact on the population dynamics of both copepods was negligible. Likewise, the impact on Acartia spp. was considered negligible (C/P < 0.1). Integrated over the year, utilization of estimated copepod production by both clupeids was comparatively low, with only 18% of the annual production of T. longicornis and 36% of the annual production of P. acuspes consumed; the production of Acartia species (1.4%) and cladocerans (2.3%) was almost completely unused. Use of only 9% of the combined production of copepods and cladocerans indicates poor trophic coupling between mesozooplankton and pelagic planktivores in the Central Baltic Sea.
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