Energetics and feeding dynamics of Euphausia superba in the South Georgia region during the summer of 1994

Measurements of adult Antarctic krill (Euphausia superba) gut contents, evacuation and egestion rates, as well as digestive efficiency, were carried out during February-March 1994 in the vicin ity of South Georgia to estimate in situ daily ration. These were combined with acoustically derived biomass data to calculate the grazing impact of Antarctic krill and its contribution to the carbon flux in the region. Individual levels of gut pigment concentrations and evacuation rates ranged from 27 to 1831 ng chlorophyll a-eq. ind.−1 and from 0.133 to 0.424 h−1, respectively. Losses of pigment fluor escence during digestion were very high, ranging from 58 to 98% of the total pigment digested. Daily carbon consumption estimated using the gut fluorescence method varied from 0.234 to 0.931 mg C ind.−1 day−1 (or 0.4–1.7% of body carbon), compared to ˜2.73 mg C ind.−1 day−1 (or ˜5% of body carbon) using the faecal pellet production data. The 3-fold higher daily ration estimated using egestion rate data may be explained by predation on micro-and mesozooplankton. Maximum krill grazing impact ranged from 0.4 to 1.9% of the total phytoplankton stock or from 10 to 59% of the total daily primary production. However, grazing impact on the microphytoplankton (>20 μm) was substantially higher, at times exceeding 100% of the daily microphytoplankton production. It is suggested that to meet its energetic demands, kriil must consume a substantial proportion of heterotrophic carbon.