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

Measurements of adult Antarctic trill (Euphausia superba) gut contents, evacuation and egestion rates, as well as digestive efficiency, were carried out during February-March 1994 in the vicinity 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. ~' and from 0.133 to 0.424 h", respectively. Losses of pigment fluorescence 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 indr" day (or 0.4-1.7% of body carbon), compared to -2.73 mg C indr day" (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 microand mesozooplankton. Maximum krill grazing impact ranged from 0.4 to 1.9% of the total phytoplaniton stock or from 10 to 59% of the total daily primary production. However, grazing impact on the microphytoplankton (>20 urn) was substantially higher, at times exceeding 100% of the daily micTophytoplankton production. It is suggested that to meet its energetic demands, krill must consume a substantial proportion of heterotrophic carbon. Introduction The pelagic ecosystem of South Georgia exhibits enhanced primary and secondary productivity relative to the surrounding open waters (Hardy and Gunther, 1935; Hart, 1942). Copepods and Antarctic krill are responsible for two major pathways in the vertical transport of organic carbon to the benthic subsystem: sedimentation of faecal material and vertical migrations. While copepods are very abundant in waters of the Antarctic Circumpolar Current surrounding South Georgia (Atkinson, 1989; Voronina et ai, 1994), the presence of Antarctic krill in the waters surrounding the island is dependant mainly upon passive transport of animals into the area via surface currents (Priddle et al, 1988). This feature is very important as South Georgia supports some of the largest seabird and Antarctic fur seal colonies in the Southern Ocean. These predators are believed to consume ~7 million tonnes of krill per year (Croxall et al, 1985). Extensive investigations on the feeding ecology and behaviour of copepods (Atkinson et al, 1992a, b) and krill (Morris et al, 1983,1984; Morris and Priddle, 1984; Morris and Ricketts, 1984) have been carried out in the region around South Georgia. However, the food consumption of the local krill population has generally been estimated indirectly (Garke and Morris, 1983). The gut-fluorescence method is at present one of the most widely employed in the measurement of in situ feeding rates of zooplankton. Although this method has been recently © Oxford University Press 399 Downloaded from https://academic.oup. com/plankt/article-abstract/19/4/399/1480141 by guest on 29 July 2018