Omnivory and feeding selectivity in five copepod species during spring in the Bellingshausen Sea, Antarctica

Copepod grazing experiments were undertaken along a transect of five stations in the Bellingshausen Sea during austral spring. The stations spanned unproductive, nanoflagellate-dominated waters under pack ice to an open water bloom of large diatoms in the north. The feeding habits of five dominant species were compared by incubating them in natural sea water. Feeding rates on individual food taxa were calculated from optical cell counts. The larger species (Rhincalanus gigas, Calanus propinquus, and Calanoides acutus) tended to eat larger cells than did Oithona spp. and Metridia gerlachei, and cleared cells up to 100-200 $[$mu$]$m in length with increasing efficiency. Their clearance of longer cells remained high. Oithona spp. was the only species with high clearance rates on cells smaller than 10 $[$mu$]$m. Clearance rates on elongated cells were compared with those of shorter and fatter cells of similar volume. The longer cells were eaten at higher rates in 65% of comparisons, suggesting that cell elongation did not protect against ingestion by large copepods. Clearance rates of motile cells were compared with those of non-motile diatoms of similar size and shape. The copepods ranged widely, with Oithona spp. at one extreme, ingesting motile cells almost exclusively. M. gerlachei and C. propinquus showed a preference for motile taxa compared with similar sized diatoms. The fact that the motile fraction comprised an array of taxa (nanoflagellates, dinoflagellates, and ciliates), all with a tendency to be cleared more rapidly than diatoms by the above three species, suggests that they were using mechanoreception to detect prey. Oithona spp., M. gerlachei, and C. propinquus were actively feeding under the ice before the bloom, in low productivity waters characterized by a nanoflagellate community and a high inorganic detritus content. The selection for motile taxa persisted, by M. gerlachei at least, into the bloom, even though motile cells comprised only a small fraction of available carbon at that tim C. acutus and R. gigas by contrast, migrated to the surface layers to start feeding on the bloom. These two species appeared to have fed indiscriminately on both motile and non-motile taxa. The contribution of motile taxa to copepod diets depended on their contribution to the overall food source, the extent of motile/sessile selection, and also on their size and shape relative to the diatoms. Consequently the proportion of motile taxa in copepod diets ranged from 100% (Oithona spp. before the bloom) to 3% (R. gigas during the bloom)