Gulf-4

Terrestrial organic matter (OM) concentrations have been increasing in coastal waters, such as the Gulf of Maine (GoM), over the past 30 years. The implications of this change on critical mechanisms controlling the levels of mercury (Hg), and its toxic form, methylmercury (MeHg), in marine waters and at the base of the food web, are not well understood. Given this lack of knowledge, this proposal will examine the effects of climate-driven increases in terrestrial OM concentrations in GoM waters on the complex and often competing processes influencing: 1) Hg cycling and distribution; 2) the formation of MeHg; and 3) MeHg uptake to microplankton. Our central hypothesis is that higher terrestrial OM inputs and concomitant shifts in coastal OM concentration and quality will cause increases in Hg inputs, MeHg production and bioaccumulation. Terrestrial OM is thought to be a conduit for transfer of Hg from watersheds to coastal and offshore waters and controls the formation of aqueous MeHg by providing a microenvironment that supports bacterial Hg methylation. Increasing dissolved OM concentrations have been associated with decreased bioaccumulation of MeHg into phytoplankton, the primary bioaccumulation step for uptake of MeHg into seafood. However, our preliminary results show this association is mediated by OM source more than concentration, as indicated by the sulfur content of the OM. Furthermore, mixotrophic microplankton can bioaccumulate MeHg complexed with OM while feeding heterotrophically. Mixotrophs can seasonally dominate phytoplantkon assemblages in coastal waters, and the impact of heterotrophic feeding modes on MeHg uptake at the base of the food web has not been explored. This proposal is focused on studying the effects of OM dynamics on Hg and MeHg cycling and bioaccumulation through 1) field surveys and shipboard experiments in the GOM, where significant increases in terrestrially-sourced OM have caused a “yellowing” effect, and 2) through laboratory microcosm experiments using autotrophic and mixotrophic microplankton taxa under contrasting carbon acquisition modes and OM characteristics and concentrations. This work builds on our prior and ongoing work studying Hg cycling and bioaccumulation in estuarine environments along the Northeastern U.S. and will apply novel Hg and MeHg-specific isotopic analyses and measures of OM quality to enhance understanding. This research will fill important gaps in predicting the effects of environmental perturbations on marine MeHg levels, providing critical information to mitigating Hg emissions and MeHg exposures.