SEABIRD DISTRIBUTION AND OCEANIC FEATURES OF THE AMUNDSEN AND SOUTHERN BELLINGSHAUSEN SEAS

We describe the distribution and estimate the abundance of seabirds in the Amundsen and southern Bellingshausen seas, and attempt to identif y the mesoscale and larger ocean and ice features that explain the bir ds' discontinuous occurrence patterns. Our general objective was to as sess if ocean fronts, especially near the continental shelf break, enh ance feeding opportunities for top trophic-level predators. A variety of subsurface thermohaline fronts occurred on both sides of the shelf break, at shallower depths from west to east, and with warmer and salt ier water on their northern sides. Pack ice overlaid some of these fro nts, especially in the Amundsen Sea. Seabirds comprised either an ice group in pack ice or in polynyas, or an open-water group in waters nor th of the pack. In the Amundsen Sea, bird densities were near 0 birds km(-2) in waters overlying the continental shelf, an unexplained patte rn found previously in the Ross Sea but not repeated in the Bellingsha usen Sea (5 birds km(-2) over the shelf). Both groups were more abunda nt (densities 3-9 birds km(-2)) near the frontal zones, the ice edge a nd the shelf break. In the Amundsen Sea, the distribution of ice-group species was related statistically to proximity of fronts and the pack -ice edge, thermocline slope, and depth of the chlorophyll maximum; wa ter-group distribution was related to distance to the fronts and sea-s urface temperature. In the Bellingshausen Sea, distribution of both gr oups was related to distance to the fronts. Many thousands of petrels, found roosting during daylight on icebergs situated near the shelf br eak, escaped standard census techniques and, thus, are likely to have diluted positive correlations of avian occurrence patterns with physic al features of the environment. While the emperor penguin was seen in high numbers and can dive deep enough to forage within the subsurface fronts, those ocean and food-web processes that apparently affect incr eased food availability for surface and shallow-diving foragers in the frontal regions remain unclear.