Marine Ecology of Seabirds in Polar Oceans

Patterns of seabird species' distributions differ between theAntarctic and the Arctic. In the Antarctic, distributions areannular or latitudinal, with strong similarities in speciescomposition of seabird communities in all ocean basins at agiven latitude. In the Arctic, communities are arranged meridionally,and show strong differences between ocean basins and, at a givenlatitude, between sides of ocean basins. These differences betweenthe seabird communi ies in the Northern Hemisphere and the SouthernHemisphere reflect differences in the patterns of flow of majorocean current systems. At smaller spatial scales, in both hemispheresthe species composition of seabird communities is sensitiveto changes in watermass characteristics. The distribution of avian biomass is affected by both physicaland biological features of the ocean. In the Antarctic, muchseabird foraging is over deep water, and withinseason, small-scalepatchiness in prey abundance and availability in ice-free watersis likely to be controlledprimarily by the behavior of the prey,rather than by physical features. Thus, prey availability maybe unpredictable in time and space. In contrast, in the NorthernHemisphere, most seabirdforaging is concentrated over shallowcontinental shelves, where currents interact with bathymetryto produce predictable physical features capable of concentratingprey or making prey more easily harvested by seabirds. Ice cover appears to be the most important physical featurein the Antarctic. An entire community of birds is specializedto use prey taken near the ice edge. These prey consist of avariety of species, some of which are normally found much deeperin the water than the birds takingthem can dive. The open-waterportion of the marginal ice zone is also an important foraginghabitat for Antarctic marine birds. In the Arctic, a food webbased on underice algae is used by marine birds, but few ifany data exist on avian use of the open water segment of themarginal ice zone. Recent simultaneous surveys of birds and their prey indicatethat only rarely does the small-scale abundance of birds matchthat of their prey; correlations between predators and preyaregenerally stronger at larger scales. Evidence is accumulatingin the Antarctic that the largestaggregations of krill may bedisproportionately important to foraging seabirds.