Modeling Planktonic Food Web Interannual Variability of the Northern Gulf of Alaska Shelf

A 25-year (1996-2020) hindcast from a coupled physical-biogeochemical model is evaluated with nutrients, phytoplankton and zooplankton field data and is analyzed to identify mechanisms controlling seasonal and interannual variability of the northern Gulf of Alaska (NGA) planktonic food web. Characterized by a mosaic of processes, the NGA is a biologically complex and productive marine ecosystem. Empirical Orthogonal Function (EOF) analysis combining abiotic and biotic variables averaged over the continental shelf reveals that light intensity is a main driver for nanophytoplankton variability during spring, and that nitrate availability is a main driver for diatoms during spring and for both phytoplankton during summer. Zooplankton variability is a combination of carry-over effects from the previous year and bottom-up controls from the current year, with copepods and euphausiids responding to diatoms and microzooplankton responding to nanophytoplankton. The results also demonstrate the effect of nitrate availability and phytoplankton community structure on changes in biomass and energy transfers across the planktonic food web over the entire growing season. In particular, the biomass of large copepods and euphausiids increases more significantly during years of higher relative diatom abundance, as opposed to years with higher nitrate availability. Large microzooplankton was identified as the planktonic group most sensitive to perturbations, presumably due to its central position in the food web. By quantifying the combined variability of several key planktonic functional groups over a 25-year period, this work lays the foundation for an improved understanding of the long-term impacts of climate change on the NGA shelf. The northern Gulf of Alaska planktonic food web supports a highly productive and diverse marine ecosystem. Here we used a 25-year model simulation (1996-2020) to explore how its structure and variability (from phytoplankton to krill) are affected by the highly variable environmental conditions experienced over the continental shelf each season and across years. A statistical analysis identifies light intensity as a main driver for small phytoplankton variability during spring, and nutrient availability as a main driver for large phytoplankton (diatoms) during spring and for both phytoplankton types during summer. Changes in zooplankton are influenced by abundances from the previous year and prey availability from the current year. In general, larger zooplankton species (copepods and krill) respond to large phytoplankton availability and smaller zooplankton species respond to small phytoplankton availability. Finally, the biomass of large zooplankton increases more significantly during years when large phytoplankton is relatively more abundant, as opposed to years when more nutrients are available. By advancing knowledge about the seasonal and year-to-year response of several key planktonic functional groups over the recent historical period (1996-2020), this work lays the foundation for an improved understanding of the long-term impacts of climate change in the northern Gulf of Alaska region. Drivers of recent seasonal and interannual variability of the Gulf of Alaska planktonic food web are explored with a 25-year hindcast Light and nutrient availability are the main drivers of phytoplankton production, but their effect varies by season and functional group Trophic energy transfer to larger zooplankton is more sensitive to changes in phytoplankton composition than to changes in abundance alone