Basin-scale oceanographic processes, zooplankton community structure, and diet and reproduction of a sentinel North Pacific seabird over a 22-year period

Climate-driven variation in the distribution, abundance and phenology of low trophic-level organisms can have cascading effects within marine food webs, and understanding these complex ecosystem dynamics is of vital importance in an age of dramatic environmental change. We tested the hypothesis that marine conditions associated with the state of the Pacific Decadal Oscillation (PDO) exert primary influence on the biogeographic affinities of the zooplankton community, and thus the composition of nestling diets and reproduction of a zooplanktivorous seabird, Cassin's Auklet (Ptychoramphus aleuticus). The 22-year study (1996-2017) took place on Triangle Island, British Columbia, Canada, located at the northern extreme of the California Current System (NCCS) and site of the world's largest breeding colony. Based on annual zooplankton tows in the vicinity of Triangle Island, the biomass of 'Subarctic' zooplankton species (copepods) was elevated in years in which the PDO was in its negative (cold-water) state. The biomass of 'Subarctic' zooplankton was unrelated to both the El Nino-Southern Oscillation (ENSO) and North Pacific Gyre Oscillation (NPGO) indices. Principal Components Analysis (PCA) showed that in those PDO-negative years, diets fed to nestling auklets included more copepod biomass and less euphausiid biomass. Virtually all (> 99%) of the copepod biomass consisted of just one species, Neocalanus cristatus, one of the North Pacific Ocean's diagnostic 'Subarctic' copepods. PCA also identified a secondary effect of the El Nino-Southern Oscillation (ENSO) on diets, in that auklet parents delivered more larval fish and less copepods-plus-euphausiids to nestlings in years in which ENSO was in a strongly positive state (i.e., El Nino). The growth rates of nestling auldets, indexed by their mean mass at 25 days of age, was higher in years in which they received more copepods-plus-euphausiids rather than larval fish, and in years in which they received more copepods rather than euphausiids. A previous study in the Central California Current System found that ENSO (to the mid-1990s) and then the NPGO (since the mid-1990s) exerted primary influence on productivity in Cassin's Auklets. In combination, the two studies illustrate how basin-scale climatic processes can non-homogeneously influence productivity in a single species across oceanic domains. For Cassin's Auklet, the contrast can be attributed to latitudinal differences in the prey base (more copepod-based in the north, more euphausiid-based in the south), and linked to ocean transport, temperature, and phenology. This correlational study provides mechanistic insight into ecological functioning in the offshore waters of the NCCS, with application to marine systems elsewhere. We highlight the particular importance of the 'Subarctic' copepods to a zooplanktivorous predator in the NCCS, and the value of seabirds as indicators in this rich marine ecosystem.