Krill-copepod interactions at South Georgia, Antarctica, II. Euphausia superba as a major control on copepod abundance

Euphausia superba (hereafter 'krill') and copepods are major zooplankton taxa in the Southern Ocean, but there is little information on how they interact. This paper investigates their coincidence across a wide range of temporal and spatial scales to examine whether copepod distribution is related to that of krill. During 2 summers of high krill abundance near South Georgia (1996 and 1997) copepod abundance was <40% of that during an abnormally low krill year (1994). No such depletion was found north of the Polar Front, where krill were rare. Analysis of 2 mesoscale data sets showed that krill, rather than food or environmental factors, were most strongly implicated in copepod distribution. An area of persistently high krill abundance just north of South Georgia was characterised by exceptionally few copepods. Fine-scale relationships between patches of krill and copepods were studied with a Longhurst Hardy Plankton Recorder. Within krill swarms copepod abundance was low, but more dispersed krill associated with high concentrations of copepods. Copepods also appeared to live deeper and to make more extensive vertical migrations when krill were present. The inverse relationship between krill and copepod abundances thus occurred repeatedly and across a wide range of scales. The facts that krill swarms are mobile and were unrelated to hydrography further suggest that the inverse relationship was caused by krill. This could arise from competitive exclusion, direct predation or both. Evidence for competition is that South Georgia copepods rely on high phytoplankton biomass for recruitment and krill can remove this. Predation is suggested by the fact that crustaceans were found in krill guts in this region during both summer and winter. During the 1996 summer, experimentally derived predation rates on copepods, combined with krill biomass values, suggested a significant impact on small copepods. Therefore we suggest that copepod numbers can be controlled by a combination of competition and predation by krill.

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