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Abstract Although the mesopelagic zone occupies a substantial volume of the world’s oceans, our results suggest that the livable portion may compress vertically by ~ 40 m or ~ 39% by the end of the century. Using an ensemble of three downscaled climate projections from a high emissions scenario, we evaluated the connection between anthropogenic greenhouse gas emissions and changes in light and oxygen at depth, which influence the upper and lower limits of mesopelagic habitat in the central California Current. Although the model projects a small deepening (~ 2 m) of the upper light boundary consistent with increased stratification and reduced upper ocean productivity, the main driver of vertical mesopelagic habitat compression is the significant shoaling (by ~ 44 m) of the hypoxic boundary over the course of the 21st century. Differences in dissolved oxygen across ensemble members highlight the potential influence of equatorial dynamics and the California Undercurrent in constraining the future availability of mesopelagic habitat along the U.S. west coast. Mesopelagic ecosystems connect the surface ocean to the deep sea, and a projected decrease in the vertical extent of mesopelagic habitat could have cascading effects on a broader range of marine ecosystem processes and carbon export. 

Abstract Measuring, reporting, and verification (MRV) of ocean-based carbon dioxide removal (CDR) presents challenges due to the dynamic nature of the ocean and the complex processes influencing marine carbonate chemistry. Given these challenges, finding the optimal sampling strategies and suite of parameters to be measured is a timely research question. While traditional carbonate parameters such as total alkalinity (TA), dissolved inorganic carbon (DIC), pH, and seawater pCO2 are commonly considered, exploring the potential of carbon isotopes for quantifying additional CO2 uptake remains a relatively unexplored research avenue. In this study, we use a coupled physical-biogeochemical model of the California Current System (CCS) to run a suite of Ocean Alkalinity Enhancement (OAE) simulations. The physical circulation for the CCS is generated using a nested implementation of the Regional Ocean Modeling System (ROMS) with an outer domain of 1/10 ̊ (~10 km) and an inner domain of 1/30 ̊ (~3 km) resolution. The biogeochemical model, NEMUCSC, is a customized version of the North Pacific Ecosystem Model for Understanding Regional Oceanography (NEMURO) that includes carbon cycling and carbon isotopes. The CCS is one of four global eastern boundary upwelling systems characterized by high biological activity and CO2 concentrations. Consequently, the CCS represents an essential test case for investigating the efficacy and potential side effects of OAE deployments. The study aims to address two key questions: (1) the relative merit of OAE to counter ocean acidification versus the additional sequestration of CO2 from the atmosphere, and (2) the footprint of potentially harmful seawater chemistry adjacent to OAE deployments. We plan to leverage these high-resolution model results to competitively evaluate different MRV strategies, with a specific focus on analyzing the spatiotemporal distribution of carbon isotopic signatures following OAE. In this talk, we will showcase our initial results and discuss challenges in integrating high-resolution regional modeling into models of the global carbon cycle. More broadly, this work aims to provide insights into the plausibility of OAE as a climate solution that maintains ocean health and to inform accurate quantification of carbon uptake for MRV purposes. https://agu.confex.com/agu/OSM24/prelim.cgi/Paper/1491096 

Abstract 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. 

Abstract In the California Current Ecosystem, krill represent a key link between primary production and higher trophic level species owing to their central position in the food web and tendency to form dense aggregations. However, the strongly advective circulation associated with coastal upwelling may decouple the timing, occurrence, and persistence of krill hotspots from phytoplankton biomass and nutrient sources. Results from a coupled physical‐biological model provide insights into fundamental mechanisms controlling the phenology of krill hotspots in the California Current Ecosystem, and their sensitivity to alongshore changes in coastal upwelling intensity. The simulation indicates that dynamics controlling krill hotspot formation, intensity, and persistence on seasonal and interannual timescales are strongly heterogeneous and related to alongshore variations in upwelling‐favorable winds, primary production, and ocean currents. Furthermore, regions promoting persistent krill hotspot formation coincide with increased observed abundance of top predators, indicating that the model resolves important ecosystem complexity and function.