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

Euphausia superba thereafter 'krill') and copepods are major zooplankton ta xa in the Southern Ocean, but there is Little information on how they inter act. This paper investigates their coincidence across a wide range of tempo ral and spatial scales to examine whether copepod distribution is related t o that of krill. During 2 summers of high krill abundance near South Georgi a (1996 and 1993) copepod abundance was <40% of that during an abnormally l ow 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. Fin e-scale relationships between patches of krill and copepods were studied wi th a Longhurst Hardy Plankton Recorder. Within krill swarms copepod abundan ce 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 betw een 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 bo th. Evidence for competition is that South Georgia copepods rely on high ph ytoplankton biomass for recruitment and krill can remove this. Predation is suggested by the fact that crustaceans were found in krill guts in this re gion during both summer and winter. During the 1996 summer, experiment tall y 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 pred ation by krill.