Stable isotope signatures in historic harbor seal bone link food web‐assimilated carbon and nitrogen resources to a century of environmental change

Anthropogenic climate change will impact nutrient cycles, primary production, and ecosystem structure in the world's oceans, although considerable uncertainty exists regarding the magnitude and spatial variability of these changes. Understanding how regional‐scale ocean conditions control nutrient availability and ultimately nutrient assimilation into food webs will inform how marine resources will change in response to climate. To evaluate how ocean conditions influence the assimilation of nitrogen and carbon into coastal marine food webs, we applied a novel dimension reduction analysis to a century of newly acquired molecular isotope data derived from historic harbor seal bone specimens. By measuring bulk δ13C and δ15N values of source amino acids of these top predators from 1928 to 2014, we derive indices of primary production and nitrogen resources that are assimilated into food webs. We determined coastal food webs responded to climate regimes, coastal upwelling, and freshwater discharge, yet the strength of responses to individual drivers varied across the northeast Pacific. Indices of primary production and nitrogen availability in the Gulf of Alaska were dependent on regional climate indices (i.e., North Pacific Gyre Oscillation) and upwelling. In contrast, the coastal Washington and Salish Sea food webs were associated with local indices of freshwater discharge. For some regions (eastern Bering Sea, northern Gulf of Alaska) food web‐assimilated production was coupled with nitrogen sources; however, other regions demonstrated no production‐nitrogen coupling (Salish Sea). Temporal patterns of environmental indices and isotopic data from Washington state varied about the long‐term mean with no directional trend. Data from the Gulf of Alaska, however, showed below average harbor seal δ13C values and above average ocean conditions since 1975, indicating a change in primary production in recent decades. Altogether, these findings demonstrate stable isotope data can provide useful indices of nitrogen resources and phytoplankton dynamics specific to what is assimilated by food webs. Environmental factors determine resource availability and primary production in marine ecosystems. Stable isotope data from consumer tissues can provide useful indices of nitrogen and carbon resources that are sequestered by primary producers, passed through the food web, and ultimately assimilated into top predators. Based on the stable isotope composition of harbor seal bone