Open ocean biogeochemical impacts of extreme terrestrial precipitation

Mara A Freilich, Shailja Gangrade, Élise Beaudin, Kelly Luis, Ines Leyba, Melissa Omand, Sarah Lang, Larry O'Neill, Dante Capone

Extreme events reshape ocean ecosystems with significant implications for nutrient and carbon cycling. Here we demonstrate that flooding on land can be a significant driver of biogeochemical variability in the open ocean, even in dry climates. Using satellite observations, in situ measurements, and Lagrangian particle tracking, we present evidence that freshwater discharge from extreme precipitation during the 2023 water year was transported into the oligotrophic region of the California Current System. This transport was facilitated by submesoscale eddies and resulted in significant anomalies in surface salinity, stratification, nutrient concentrations and phytoplankton pigments. These observations, combined with an Earth system model, highlight the importance of episodic river discharge events in shaping offshore biogeochemical fluxes and ecosystem structure in eastern boundary current systems under a changing climate.