PHYSICAL CONTROL OF POPULATION-DYNAMICS IN THE SOUTHERN-OCEAN

Horizontal advection is the single most important mechanism by which physics of the Southern Ocean exerts control on population dynamics of its resident zooplankton. Advection determines whether or not zooplankton will reside in food-poor areas (e.g. Antarctic Circumpolar Current) or in food-rich areas (e.g. coastal shelves and frontal zones); the shape and structure of advective features further determine the residence time of zooplankton in a given food regime. Advection per se thus bears directly on rates of growth, mortality, and reproduction. The intensity of advection on characteristic spatial scales in the Southern Ocean redistributes zooplankton to such a great degree that it utterly destroys the demographic integrity of populations. The spatial scale of advective features, determined by the internal Rossby radius, is much smaller in the Southern Ocean (order 5 km) than at lower latitudes. At the same time, the life cycle of Antarctic zooplankton is relatively long; copepods typically have a one-year life cycle, and that of krill is longer. A group of zooplankton that begins its life cycle as an identifiable population is subjected to the small-scale dispersive forces of advection for such a long time that by the time individual members reach maturity they will have been dispersed to such a great degree that they are no longer recognizable as a single population. We conclude that the nature of physical control of zooplankton population dynamics in the Southern Ocean calls into question the very concept of a population. (C) 1995 International Council for the Exploration of the Sea