Grazing by Antarctic krill Euphausia superba on Phaeocystis antarctica: an immunochemical approach

An enzyme-linked immunosorbent assay (ELISA) for Phaeocystis antarctica was developed and used to determine the extent of grazing by Euphausia superba on this prymnesiophyte. First, a specific antiserum to P. antarctica was produced and tested on phytoplankton cells and extracts. Then, the antiserum was tested on extracts of stomachs from krill fed diets of either Thalassiosira antarctica or P. antarctica, and also on mixtures of these extracts. The antiserum response was a logarithmic function of the percentage of the extract mixture that came from P. antarctica-fed krill. In the field, the antiserum was tested on krill collected during December 1996 to February 1997 in the Palmer Long-Term Ecological Research study region west of the Antarctic Peninsula. The antiserum response suggested that krill ingested P. antarctica at 20% of the stations tested, most of them in nearshore areas. Krill grazed P. antarctica only at stations where concentrations of the marker pigment for P. antarctica 19'-hexanoyloxyfucoxanthin, were above 0.150 mu g I super(-1). At these stations, the percentage of P. antarctica in the krill stomach extracts was positively correlated with chlorophyll a concentrations, but negatively correlated with the proportion of P. antarctica within the phytoplankton community. Overall, P. antarctica appeared to comprise a relatively small proportion of the phytoplankton utilized by krill, both spatially and temporally.