Effects of diurnal feeding rhythms, species composition and vertical migration on the grazing impact of calanoid copepods in the Skagerrak and Kattegat

The grazing impact of copepods on phytoplankton was studied at 4 stations in the Skagerrak and Kattegat between May and October. Species composition ranged from a dominance of small ( ≈ 1 mm) herbivorous copepods close to the coast to a community dominated by Calanus finmarchicus and the carnivorous Paraeuchaeta norvegica offshore. Diurnal vertical migration was observed for Temora longicornis, C. finmarchicus and P. norvegica. The feeding activity measured as gut fluorescence was significantly higher at night for G. finmarchicus and T. longicornis whereas other species only occasionally fed more at night. The gut fluorescence was not different in copepods collected at 3 and 12 m depth. The total copepod community grazing rate was 2–5 times higher at night than at day and always higher at 3 than at 12 m. The coastal stations had a higher community grazing than offshore stations. The copepods grazed less than 4% of the phytoplankton standing stock per day but up to 48% of the daily primary production.