319 - Exploring the Role of the Upwelling on Particulate Mercury Dynamics in the California Current Ecosystem

Mercury (Hg) cycling is essential to explore in marine ecosystems because of the adverse effects of Hg on health of humans and marine organisms via bioaccumulation. Coastal California Current Ecosystem (CCE), which experiences seasonal coastal upwelling, is commercially and ecologically important and exhibits Hg bioaccumulation. Coastal upwelling provides a pathway for Hg to be supplied into the mixed layer of the ocean, but it is poorly studied in regards to its effect on the Hg cycle including particle scavenging that drives the sinking flux. So far, Hg sinking flux is poorly constrained due to lack of consistent sampling efforts. We collected particle samples following two upwelling water parcels and one non-upwelling water parcel during the 2021 CCE Long Term Ecological Research Process Cruise. Here, we present the profiles of total and mono-methylmercury (MMHg) in both small size-fraction (SSF) and large size-fraction (LSF) suspended particles. There was relatively high total mercury in the upwelling region (SSF: 90-400 fM; LSF: 3-25 fM) compared to open ocean (SSF: 90-300 fM; LSF: 2-8 fM). The fraction of MMHg to THg varied greatly (3-50%) depending on depth and region. In addition, high sinking flux in the upwelling region suggested the important role of particle scavenging in Hg removal from the ocean. Finally, we applied a mass balance model that accounts for external inputs (i.e., riverine input, precipitation), gas exchange rates, sinking fluxes, and upwelling velocity to explore the dynamics of Hg species in the mixed layer. The difference between inputs and outputs can shed light on how much Hg is produced and transported in and out of the ecosystem. This study addresses gaps in particle scavenging and upwelling and expands our understanding of Hg cycling in productive coastal upwelling regions.

Authors: Xinyun Cui, Hannah Adams, Carl Lamborg, Amina Schartup