Anthropogenic, climate, and meso and submesoscale influences on diatom productivity in the Southern California Bight, with implications for domoic acid producing harmful algal blooms

Blooms of Pseudo-nitzschia (PN), a toxigenic marine diatom genus, produce the neurotoxin domoic acid (DA) that causes nearly annual shellfishery closures and wildlife illnesses and deaths within the Southern California Bight (SCB), an urbanized marine embayment supporting a coastal population of more than 23-million people. Understanding the mechanisms that control these DA-producing harmful algal bloom (HAB) events is essential for shifting from a reactive to an adaptive management approach, yet knowledge remains limited by gaps in observational data. Because DA-producing PN strains are part of the broader diatom community, this study used a validated coupled physical-biogeochemical model to disentangle the environmental drivers of diatom productivity and their influence on the risk of DA-producing HABs. Model simulations, with and without anthropogenically enhanced terrestrial nutrient sources, were used to (1) investigate spatial and temporal patterns governing diatom productivity, (2) evaluate how upwelling, cyclonic eddies, climate regimes, and local anthropogenic nutrients contribute to those patterns, and (3) quantify the relative contribution of natural versus anthropogenic forcing on the risk of DA-producing HABs and attribute effects to specific nutrient sources. Results show that diatom production is primarily controlled by upwelling and eddies that modulate the vertical delivery of dissolved inorganic nitrogen (DIN) to the surface; Climate regimes further modulate DIN fluxes by altering oceanic energy, upwelling strength, stratification, and nitracline depth. Vertically integrated DIN concentration, combined with a regional climate index, accounts for 85% of the interannual variability in annual maximum DA. Together, variability in these processes creates spatial and temporal gradients in diatom productivity that influence the likelihood of DA events. In the SCB nearshore, anthropogenic nutrient inputs are elevating diatom biomass by up to 45 percent over five years on average, and up to 67 percent in a single year. Applying a chlorophyll-a threshold associated with a 50 percent increased risk of DA detection, model results indicate that anthropogenic nutrient inputs have widened the natural ocean’s window of opportunity for DA events by expanding their spatial footprint, seasonal duration, and intensity. This work highlights the coupled natural-human dynamics driving HAB risk and the value of numerical models for informing adaptive coastal management.