Feeding interactions of the copepods Eurytemora affinis and Acartia bifilosa with the cyanobacteria Nodularia sp.

We measured ingestion and clearance rates of two Baltic Sea calanoid copepods, Eurytemora affinis and Acartia bifilosa, on toxic and non-toxic cyanobacteria Nodularia sp. using the isotope technique. Eurytemora affinis fed actively on the non-toxic strain and moderately actively on the toxic strain, whereas A.bifilosa totally avoided feeding on both strains. This suggests that A.bifilosa rejected cyanobacterial filaments due to their nutritional inadequacy or difficult manageability. The different response of E.affinis to the non-toxic and toxic strains, in turn, shows that this copepod species was able to sense the presence of the toxin in cyanobacterial filaments and therefore fed less on the toxic strain. The interaction between A.bifilosa and Nodularia sp. was further examined (with the particle counting method) by measuring the clearance rates of A.bifilosa on edible green flagelIates in the presence of cyanobacteria. The presence or concentration of toxic Nodularia sp. did not affect grazing rates of A.bifilosa on Brachiomonas submarina. Since earlier studies have shown that ingestion of Nodularia sp. decreases egg production and increases mortality in E.affinis, we suggest that the occurrence of Nodularia sp. blooms in the Baltic Sea may favour individuals of copepod species capable of selective feeding, such as A.bifilosa.