Grazing and food selection of the calanoid copepods Eurytemora affinis and Acartia bifilosa feeding on plankton assemblages containing Dinophysis spp

Grazing of Eurytemora affinis and Acartia bifilosa on plankton communities containing toxic dinoflagellates of the genus Dinophysis was experimentally studied in the northern Baltic Sea. The experiments were carried out in plankton communities with different concentrations and relative proportions of microplankton. In the experiments with high total cell concentrations, the dinoflagellate Heterocapsa triquetra predominated, and in these experiments they formed the main part of the copepod diet. When the total cell concentrations were lower and the relative Dinophysis spp. concentrations higher, the role of Dinophysis spp. as food became more important, although no marked change in the ingestion rates were found. Eurytemora affinis consumed up to 226 Dinophysis spp. cells (0.50 µg C) ind–1 day–1, and A. bifilosa up to 64 cells (0.1 µg C) ind–1 day–1. No positive selection for Dinophysis spp. was detected, and in fact, other species were preferred. Pectenotoxin-2 (PTX2) was found in two pooled samples of copepods after incubation (53 and 142 pg PTX2 ind–1, respectively). Although no positive selection for Dinophysis spp. was detected, these dinoflagellates formed an important part of the copepod's diet at low food concentrations in these experiments. It is thus possible that in environmental conditions characterized by low microplankton biomass, the copepods can have a minor role in toxin transfer in planktonic food webs of the northern Baltic Sea by accumulating toxins in their tissues or transferring them to faecal pellets.