SO GLOBEC: Metabolism of winter krill

J.J. Torres , J. Donnelly, and J. Bellucci

College of Marine Science, USF, 140 Seventh Avenue South,
St. Petersburg, FL 33701

Overwintering Euphausia superba are faced with an energy shortfall due to the loss of their phytoplankton food source, a result of the vanishingly low chlorophyll biomass typical of the Antarctic water column during the fall and winter. Two strategies that are widely exploited to overcome food deprivation in overwintering species are the use of an energy depot as a source of fuel and a reduction in the need for fuel by reducing metabolic demand. The present study was designed to test the hypothesis that Euphausia superba reduces its metabolic demand as part of its overwintering strategy. Krill were captured in a Tucker trawl designed for gentle handling of specimens on two GLOBEC cruises to the Western Antarctic Peninsula (WAP) shelf, one in the austral fall (April-May) and the other in the austral winter (July-August). Specimens were rapidly sorted into cold (-1.0oC) filtered seawater and allowed to adjust to laboratory conditions for a minimum of 4 h. They were then placed in sealed, water-jacketed vessels filled with filtered (0.45µ) seawater and allowed to deplete the oxygen to intermediate levels of dissolved oxygen (80 mm Hg) at a temperature of -1.0oC . Oxygen consumption was continuously recorded with Clark polarographic electrodes. No differences were observed in oxygen consumption between the fall and winter seasons. The equation for the line describing metabolism vs mass in fall-winter WAP shelf animals was Y (µl O2 individual-1 h-1) = 0.310 X (mass (mg))0.772 , r2=0.754. WAP shelf fall-winter krill had a metabolic rate about 60 % of that of summer animals captured in the Weddell Sea (Y (µl O2 individual-1 h-1) = 0.514 X (mass (mg))0.792 , r2=0.9650) suggesting a profound drop in metabolic rate during the winter. Despite the observed dramatically reduced metabolism in fall-winter, WAP shelf krill showed a higher winter rate than E. superba captured in the Weddell Sea (Y= 0.213 X 0.807, r2=0.970).