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.

Dimethylmercury as a Source of Monomethylmercury in a Highly Productive Upwelling System

“…In contrast to THg and DMHg concentrations, dissolved MMHg concentrations are the same in the California Undercurrent and the California Current ( p > 0.05, Table , Figure ). As the water parcel moves from the California Undercurrent, we would expect MMHg losses from scavenging, photodemethylation, and dark demethylation. ,,− The lack of apparent change in MMHg concentrations between the water masses suggests a resupply of MMHg through atmospheric deposition, upwelling, Hg II methylation, or DMHg demethylation. To quantify the relative contribution of each source of MMHg to surface waters, we developed an empirically constrained mass budget model that encompasses both the productive California Undercurrent and the oligotrophic California Current.…”

“…Greater biological activity in upwelled nutrient-rich waters leads to increased particle production and sinking within the California Undercurrent. ,, The biological particle flux removes dissolved THg through scavenging and could explain the difference in THg concentrations between the California Undercurrent and the California Current. Using ship-board wind speeds, surface Hg 0 and DMHg concentrations, suspended and sinking particle THg concentrations, and published air Hg 0 and DMHg concentrations, , we calculate that 123 pmol m –2 d –1 of Hg evades and 1621 pmol m –2 d –1 scavenges as the upwelled waters advect offshore and merge with the California Current (eqs S1–S9). Accounting for these removal processes, we calculate that it takes 47 ± 4 days for the dissolved THg concentrations in the upwelled water parcel to match those in the California Current (Supplemental Figure S8, eqs S12 and S13, Table S11).…”

“…Atmospheric inputs were determined from literature values, while upwelling inputs were determined from depths between 300 and 600 m from one station in the California Undercurrent. Hg II and MMHg removal from scavenging was determined from our solids budget (Figure S4) and suspended and sinking particle data published in Cui et al ( Preprint ) . Hg 0 and DMHg, as dissolved gases, were not assumed to be present in the particulate phase.…”