The toxic dinoflagellate Alexandrium ostenfeldii promotes incapacitation of the calanoid copepods Eurytemora affinis and Acartia bifilosa from the northern Baltic Sea

Dense localized blooms of the toxic dinoflagellate Alexandrium ostenfeldii are a recent phenomenon in the low salinity waters of the Baltic Sea. This study reports results from laboratory experiments investigating the interaction between PSP toxin producing strains of A. ostenfeldii and two copepod species, Eurytemora affinis and Acartia bifilosa, from the northern Baltic Sea. Copepod grazing rates were studied by incubating the copepods with mixed cultures of A. ostenfeldii isolated from a bloom area. The effect of A. ostenfeldii on copepod condition and behavior was studied by exposing copepods to the mixed cultures and cell-free filtrates. Grazing on A. ostenfeldii cells was negligible for both copepod species. The 24 h incapacitation incubations indicated that the condition of both copepod species was negatively affected by the presence of A. ostenfeldii. The behavioral changes observed took place during the first hours of exposure to A. ostenfeldii cells, and little or no change was observed in the condition of the copepods after 6 h compared to 24 h. Similar observations were made for E. affinis incubated in cell-free filtrates of A. ostenfeldii, while the response of A. bifilosa was less pronounced. Our study shows that the copepods do not graze on A. ostenfeldii due to rapid behavioral disturbance and incapacitation by the algal cells and their exudates. This represents the first observation of a negative effect of the A. ostenfeldii on co-occurring biota in the northern Baltic Sea.