On the trophic coupling between protists and copepods in arctic marine ecosystems

Abstract

Grazing experiments were conducted at different seasons with the large Calanus finmarchicus, C. glacialis and C. hyperboreus, and the small Acartia longiremis in Disko Bay, West Greenland and Young Sound, NE Greenland. Female copepods incubated in 200 μm screened natural water preferred large protists. Thus, particularly during the post-bloom period, the relatively large heterotrophic protists (ciliates and heterotrophic dinoflagellates) contributed substantially to the trophic coupling between protists and copepods. However, low grazing by C. glacialis and C. hyperboreus in mid-June suggests that large parts of the populations of these species had terminated feeding at this time, prior to overwintering. Clearance increased with ciliate and dinoflagellate size above 10 μm equivalent spherical diameter (ESD), equal to the size of the smallest heterotrophic protists. At a size of 30 to 40 μm ESD maximum clearance was observed. Grazing on Phaeocystis single cells of 5 μm by C. finmarchicus showed a lower size-limit for capture of this species <5 μm which contrasts with C. glacialis and C. hyperboreus, which had a lower size-limit near 10 μm. In addition to size and relative concentrations of phytoplankton and heterotrophic protists, prey and/or predator behavior is suggested to play an important role for copepod feeding.