Spatial structure of the Southern Ocean ecosystem: predator-prey linkages in Southern Ocean food webs

1. The food chain of the Southern Ocean has often been characterized as simple and homogeneous. However, the population processes of a key prey organism, krill Euphausia superba, operate over ocean basin scales and are strongly influenced by large scale abiotic factors. 2. A model was developed in which the local prey abundance was regulated by a continuous, hydrodynamically mediated, supply rate and the concentrating effects of abiotic-biotic interactions. This model was used with estimates of the annual predator demands and the prey concentration for the South Georgia area to investigate the relationship between flow rate and depletion in prey concentration as a function of distance from a predator colony. 3. The model results indicated that concentrating factors need to be large to produce the build-up of krill densities of the order estimated to occur in the South Georgia area, with the peak retention rates required some distance offshore. It was, however, found that, for the estimated supply rates, the region does not need to be an area of particularly high prey concentration to support the estimated predator impact. 4. Differential predator foraging ranges produced a more complex response to the reduction of the abundance of particular predators by harvesting, than in the situation where foraging ranges overlapped completely. In such a system the more inshore foraging predators encountered the greatest changes in prey abundance. 5. Random fluctuations in the interannual prey availability were introduced into the simulation of the flow system. This could lead to apparent population cycling in predator and prey abundance due to the interactive form of the system, although the prey population dynamics were not involved. The system enhanced variability such that inshore foraging predators encountered greater variation in prey supply. 6. The model results emphasize the importance of investigating the magnitude and timing of the horizontal fluxes of secondary production in this spatially distributed ecosystem.

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