Upwelling Enhances Hg Particle Scavenging in the California Current Ecosystem

Abstract Mercury (Hg) cycling is essential to explore in marine ecosystems because of the adverse effects of Hg on the health of humans and marine organisms via bioaccumulation. Coastal upwelling could supply Hg into the mixed layer of the ocean, however, its impact on Hg cycling, encompassing particle scavenging, water mass transport, and sea-air exchange, remains understudied. Additionally, particles supplying monomethylmercury (MMHg) in low oxygen water pose a significant environmental concern. During the 2021 California Current Ecosystem (CCE) Long-Term Ecological Research (LTER) Cruise, we investigated two upwelled water parcels and one non-upwelled water parcel, collecting suspended and sinking particle samples. We observed higher total particulate Hg and sinking flux in the upwelling region compared to the open ocean, indicating upwelling enhances particle scavenging. To better understand the intricate interplay between Hg particle scavenging and upwelling, we further modeled Hg inventories and fluxes in the upper ocean under two scenarios: upwelling and non-upwelling. The model simulations supported the hypothesis that upwelling enhances sinking fluxes by 40% through elevated primary production. The combined effect of these competing forcings results in an increased delivery of Hg to low oxygen regions where net methylation occurs.