Contrasting Food Web Dynamics Across Oceanographic Boundaries in Subarctic Pelagic Ecosystems

ABSTRACT

Understanding the structure of pelagic food webs across multiple trophic levels is critical for elucidating species interactions, energy flow, and anthropogenic impacts on production. This study presents a comprehensive analysis of the eastern North Pacific pelagic food webs, focusing on the Gulf of Alaska (GoA) during late winter of 2019. We demonstrate that oceanographic boundaries within the GoA delineate distinct ecological provinces, with potential implications for fisheries management and climate change responses. Analyzing stable isotopes in plankton, micronekton, and nekton—including five Pacific salmon species, other pelagic fishes, and cephalopods—we found pronounced differences in food web structure between the northwest (NW‐GoA) and southeast (SE‐GoA) regions. In the low‐nutrient, picoplankton‐dominated SE‐GoA, there was greater trophic niche overlap among gelatinous organisms, micronekton, and nekton, suggesting a more interconnected and trophically diverse food web. Temperature and nutrients emerged as important predictors of small zooplankton biomass, highlighting the role of bottom‐up processes in shaping community structure. Additionally, the high nekton/micronekton biomass contrasted with the low macrozooplankton biomass, consistent with top‐down control by upper trophic levels. In contrast, the nutrient‐rich, diatom‐dominated NW‐GoA exhibited higher macrozooplankton biomass, particularly euphausiids, supporting a “wasp‐waist” food web structure with lower trophic diversity. The high biomass and distinct isotope niches of gelatinous organisms in this region suggested a separate trophic pathway, potentially sustained by a prominent microbial food web. These regional differences suggest that climate‐driven changes in oceanographic conditions may reshape species interactions and energy transfer pathways, with important consequences for ecosystem functioning and the productivity of higher trophic levels, including Pacific salmon.