Effects of currents and temperature on ecosystem productivity in Unimak Pass, Alaska, a premier seabird and biodiversity hotspot

Abstract

Understanding bio-physical relationships for areas of high biodiversity in marine systems can illuminate potential impacts of environmental change on concentrations of marine life. A biodiversity hotspot of singular significance in the North Pacific Ocean is Unimak Pass, Alaska, where millions of seabirds, hundreds of cetaceans, and thousands of pinnipeds concentrate each summer to forage on rich zooplankton and forage fish resources. We synthesized a large number of disparate datasets from different sources to examine the effect of current flows and temperature on productivity across trophic levels in Unimak Pass on the seasonal time scale. Our results verify the importance of these factors. This study is one of few to investigate how seabird reproductive success may vary relative to ocean currents, and corroborates other studies showing trait-mediated responses of seabirds to variation in oceanographic conditions, but at the local scale of an Aleutian pass. Somewhat surprisingly, seabird species with larger foraging ranges appeared to respond most to variability in ocean parameters at the seasonal scale. Our hypothesis that spring conditions promote summer productivity was not supported. Instead, contemporaneous conditions in summer had more and stronger effects on seabird production and foraging density. An important caveat of our study is that these relationships remain mechanistically poorly understood due to a lack of data on lower trophic levels and corresponding information on prey use and other aspects of trophic ecology across species. Nonetheless, we have shown that the breeding success of some seabird species on Aiktak Island, as well as seasonal average foraging densities of migratory shearwaters and other species, track currents and temperature variability in Unimak Pass, and may thus be useful as ecological indicators to the robustness of local pelagic food webs, and therefore ecosystem status assessments, in the region.