Interannual variability of the South Georgia marine ecosystem: biological and physical sources of variation in the abundance of krill E. J. MURPHY, 1,* J. L. WATKINS, 1 K. REID, 1 P. N. TRATHAN, 1 I. EVERSON, 1 J. P. CROXALL, 1 J. PRIDDLE, 1 M. A. BRANDON, 1 A. S. BRIERLEY 1 AND E. HOFMANN 2 1 British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 OET, UK 2 Center for Coastal Physical Oceanography, Old Dominion University, Norfolk, VA 23529, USA ABSTRACT Interannual variability is a characteristic feature of the Southern Ocean ecosystem, yet the relative roles of biological and physical processes in generating these ¯uctuations are unknown. There is now extensive evidence that there are years when there is a very low abundance of Antarctic krill (Euphausia superba) in the South Georgia area, and that this variation affects much of the ecosystem, with the most obvious impacts on survival and breeding success of some of the major predators on krill. The open nature of the South Georgia ecosystem means this variability has large- scale relevance, but even though there are unique time series of data available, information on some key processes is limited. Fluctuations in year-class success in parts, or all, of the krill population across the Scotia Sea can generate large changes in the available bio- mass. The ocean transport pathways maintain the large-scale ecosystem structure by moving krill over large distances to areas where they are available to predator colonies. This large-scale physical system shows strong spatial and temporal coherence in the patterns of the interannual and subdecadal variability. This physical variability affects both the population dynamics of krill and the transport pathways, empha- sizing that both the causes and the consequences of events at South Georgia are part of much larger-scale processes. Key words: ecosystem, interannual variability, krill, models, Southern Ocean, transport INTRODUCTION Marked ¯uctuations in the distribution and abundance of Antarctic krill, Euphausia superba Dana, are a striking manifestation of large-scale interannual vari- ations in the operation of the Southern Ocean eco- system (Mackintosh, 1972, 1973; Croxall et al., 1988; Makarov et al., 1988; Maslennikov and Solyankin, 1988; Priddle et al., 1988; Loeb et al., 1997). Although the ®rst observations of this variation were made in the early part of the century, it is only in recent years that speci®c research programmes have been targeted at gaining an understanding of the processes generat- ing the ¯uctuations. One of the main areas where these observations have been made is around South Georgia. The ®rst observations of variation in the South Georgia ecosystem were made in association with the whaling industry during the 1930s (Harmer, 1931; Kemp and Bennett, 1932). These observations sug- gested that there were years in which different whale species dominated and that these were associated with `warm' or `cold' periods as de®ned by air temperature around the island. The suggestion was made at that time, that the patterns were linked to the ice-edge position and re¯ected the availability of prey items around South Georgia. It was not until the 1970s that Mackintosh (1972, 1973) analysed the krill data for the period from 1926 to 1931 and 1951, and investi- gated the availability and population biology of krill in the area. The suggestion that the krill population in the South Georgia area was not self-sustaining was also highlighted (Marr, 1962; Mackintosh, 1972). Mack- intosh was able to link the occurrence of krill around the island to warm and cold periods which were as- sociated with changes in the concentration and extent of sea-ice and the regional oceanography in the Scotia Sea (see also Baker, 1977). Analyses of the data were, however, complicated by the dif®culties at that time with the interpretation of the data on the longevity and growth of krill. *Correspondence. Fax: +44 1223 221259; e-mail: e.murphy@bas.ac.uk Received for publication 20 May 1998 Accepted for publication 27 July 1998 FISHERIES OCEANOGRAPHY Fish. Oceanogr. 7:3/4, 381±390, 1998 Ó 1998 Blackwell Science Ltd. 381