Large-Scale Fluctuations in Distribution and Abundance of Krill — A Discussion of Possible Causes

Abstract

Unusually low abundance of krill may last for periods of several months in the Scotia Sea near South Georgia and in Bransfield Strait. Two longer data sets on krill predators suggest that such events may occur two or three times in a decade, and that the situation normally returns to normal in the following season. It seems most unlikely that these events can be ascribed to features of krill biology. Simple models of recruitment failure or mortality cannot explain the observed changes, and alteration in small-scale distribution is not indicated by the available data. More probable mechanisms must involve large-scale changes in distribution of krill brought about by ocean-atmosphere processes. Whilst natural variation in mesoscale features has an appropriate spatial scale, the likely duration is too short. However, a breakdown of hydrographic structure in the surface water over a large area would drastically decrease the residence time of krill and it would take a longer time to reestablish high krill biomass. A prolonged period of southwards airflow over the Scotia Sea is identified as the likely driving force in this model. Such an airflow has been identified from atmospheric pressure distribution in the winters of 1983 and 1986, and was associated with southwards displacement of both warm surface water and of pack ice in the northern Weddell Sea.