KRILL-COPEPOD INTERACTIONS AT SOUTH GEORGIA, ANTARCTICA, I - OMNIVORYBY EUPHAUSIA-SUPERBA

Feeding by juvenile Antarctic krill Euphausia superba near South Georg ia was assessed during the austral summer of 1995/1996. Gut fluorescen ce results were compared with those from incubations in natural seawat er and seawater enriched with phytoplankton and zooplankton. In natura l seawater, with typically low food concentrations (median 56 mg C m(- 3)) the median ration was 0.68 % of krill carbon d(-1). Phytoplankton dominated carbon in the natural incubation water but dinoflagellates, ciliates and small calanoid copepods dominated the carbon intake of kr ill. In both natural and enriched water, maximum clearance rates were on 1 to 3 mm calanoid copepods. Copepods larger than this (e.g. late c opepodite stages of Calanoides acutus and Rhincalanus gigas) were clea red more slowly despite dominating the carbon in the enriched incubati ons. Oithona spp. were cleared more slowly than calanoids of similar s ize, despite their greater abundance and their similar contributions t o available carbon. These trends could reflect detection/escape intera ctions between krill and copepods. With enriched food, copepods domina ted krill diet, krill rations exceeded 10 % of body carbon d(-1) and r ations did not appear to reach a plateau even at food concentrations o f similar to 1 g C m(-3). This suggests that krill could feed rapidly during periodic encounters with layers or patches of zooplankton. Gut fluorescence revealed gut passage times of 3.7 to 6.3 h and an algal c arbon ration of 0.43 % d(-1), thus supporting the low algal carbon rat ions derived from the incubations. Published acoustic values of mean k rill biomass north of South Georgia that summer of 8.3 g dry mass m(-2 ) were combined with their clearance rates to give estimates of krill removing daily 0.2% of phytoplankton standing stocks, 0.6% of protozoa ns and 1.6% of small calanoid copepods. This impact on copepods is muc h higher than previous estimates from Antarctic amphipods and chaetogn aths. The long generation times of Antarctic copepods mean that krill were potentially important predators of small copepods during our stud y.