Biogeochemical Effects of Golden Gate Strait Exchange and Other Land‐Based Inputs to the San Francisco and Monterey Bay Coasts

Abstract The central California coast between San Francisco Bay (SFB) and Monterey Bay (MB) is an upwelling‐dominated marine ecosystem with a coastal population of 8.5 million. Terrestrial nutrients enter the ocean via three primary pathways, representing natural and anthropogenic sources: (a) SFB net export across the Golden Gate Strait, (b) coastal rivers, and (c) municipal wastewater discharged to ocean outfalls. The consequences of these inputs on primary production, acidification, and hypoxia remain poorly understood. Here, we investigate these effects with a submesoscale‐resolving biogeochemical ocean model. Simulations suggest that while terrestrial nutrient inputs collectively affect a broad region, stronger impacts occur in nearshore waters, increasing dissolved inorganic nitrogen by 11.4%, primary production by 6.5%, and chlorophyll concentration by 4.5% along a 15‐km coastal band. While exchanges from the SFB dominate these effects, all sources, including coastal rivers and ocean outfalls, produce distinct, localized footprints. Subsurface oxygen and pH decline due to terrestrial nutrient loading, but these changes are small relative to vigorous upwelling and circulation. The resulting nutrient enrichment could promote conditions favorable for diatom growth, including toxigenic species such as Pseudo‐nitzschia spp ., creating an environment predicted to elevate the risk of domoic acid (DA) events. Model results indicate that chlorophyll concentrations exceed the threshold associated with elevated DA risk on 10%–45% more days under nutrient‐enriched conditions. These findings highlight the need for expanded observational and modeling efforts to better understand the ecological consequences of terrestrial nutrient pathways and their anthropogenic contributions along the central California coast.