IN-SITU GRAZING RATES AND DAILY RATION OF ANTARCTIC KRILL EUPHAUSIA-SUPERBA FEEDING ON PHYTOPLANKTON AT THE ANTARCTIC POLAR FRONT AND THE MARGINAL ICE-ZONE

Measurements of krill gut pigment content, evacuation rates and digest ive efficiency were obtained during January 1993 in the Atlantic secto r of the Southern Ocean, between the Antarctic Marginal Ice Zone (MIZ) and the Polar Front Zone (PFZ). These were combined with net and acou stically derived abundance and biomass data to estimate the in situ gr azing of Euphausia superba on the phytoplankton assemblages. Individua l ingestion rates of krill were 1.5 to 3 times higher than rates previ ously obtained with in vitro incubations. Gut pigment levels and evacu ation rates varied in the range of 0.01 to 10 mu g chlorophyll a equiv alents (chi a equiv.) ind(-1) and 0.10 to 0.31 h(-1), respectively. Pi gment losses to non-fluorescing products during digestion were very hi gh, in the range of 67 to 90% of the total pigment ingested, indicatin g that some of the gut pigment levels previously obtained without corr ection for digestive losses may have been underestimated by up to an o rder of magnitude. Krill population impact on the phytoplankton stock exhibited a large variability, in the range of 0.0014 to 2.68% of tota l integrated chlorophyll a and 0.023 to 50.8% of primary production co nsumed per day. The largest variations in impact levels were associate d with the method used to estimate krill abundance and biomass, with n et derived estimates being much lower (by as much as 2 orders of magni tude) than those obtained from acoustic data. Daily carbon rations obt ained from our measurements of pigment ingestion rates are among the l owest recorded for E. superba during the summer season and, with 1 exc eption, ranged between 0.15 and 1.68% of body carbon per day. A daily ration of similar to 13% body carbon was recorded only at 1 station in the MIZ which exhibited a dense phytoplankton bloom of similar to 3.5 pg chi I-1. On the basis of the energetic requirements of the summer krill population, it is suggested that throughout the PFZ and the MIZ E. superba must consume a much larger proportion of heterotrophic carb on than previously supposed. Gut content analysis suggests that this i s achieved by predation on meso- and microzooplankton.