Fall transition drives declines in densities and shifts in distribution of zooplankton and marine bird taxa in the San Juan Islands

Abstract

Numerous ocean and biological monitoring programs present in the San Juan Archipelago (SJA) allow for regional-scale correlations of ocean factors to near end-member trophic level representatives for an ecosystem perspective on seasonal climate shifts. This study investigates population density and density distribution variations of two marine bird and two zooplankton taxa as a function of several environmental drivers around the time period of the fall transition, an annual climatic shift of prevailing winds and accompanying physical and chemical oceanographic changes. Generalized additive models (GAM) were used to define possible nonlinear relationships and to determine the relative contributions of each environmental driver to taxon density. The GAM results were then used to predict density distributions over San Juan Archipelago. Common murre (Uria aalge), copepod, and amphipod high-density areas were predicted to be significantly reduced after the fall transition with 63%, 71%, and 71% reductions, respectively, relative to before the fall transition. Glaucous-winged gulls (Larus glaucescens) were not significantly impacted by this seasonal change and maintained their high-density areas throughout the fall transition. Sea surface temperature was found to be the most significant factor on all affected taxa, followed by mixing for zooplankton. Total abundance was not significantly different for common murres and copepods before and after the fall transition while amphipod abundance dropped after the fall transition, meaning that the decline in high-density areas for the murres and copepods were a result of a more even distribution of the population across SJA while amphipod densities decreased as a function of lower overall abundance. Identifying the tolerance of these taxa to environmental features gives insight on how their distribution might change in future climate scenarios and on conservation efforts of more economically important species that are directly related to these taxa.