Plenary Speaker III

Ting Wang UMass Lowel

Title : Mercury Dynamics in a Salt Marsh Estuary in Massachusetts, USA 

Abstract : Salt marsh estuaries serve as sources and sinks for nutrients and elements to and from estuarine water, which may enhance or alleviate watershed fluxes to the coastal ocean. Here, we quantified sources and sinks of mercury (Hg) in the intertidal Plum Island Sound estuary in Massachusetts. We first quantified pattern of dissolved (FHg) and total (THg) mercury concentrations in water across large tidal transects and found that concentrations were highest and strongly enhanced in upper marshes (1.31 ± 0.20 ng L−1 [mean±SD] and 6.56 ± 3.70 ng L−1, respectively), compared to riverine Hg concentrations (0.86 ± 0.17 ng L−1 and 0.88 ± 0.34 ng L−1, respectively). Hg concentrations declined from upper to lower marshes and were lowest in ocean water (0.38 ± 0.10 ng L−1 and 0.56 ± 0.25 ng L−1, respectively). Conservative mixing models using river and ocean water as endmembers indicate internal estuarine Hg sources which enhanced estuarine water Hg concentrations from riverine to the ocean (e.g., for FHg by +44%, for THg by +86%). We suggest that high Hg concentrations in salt marsh soils (average 200 ± 225 μg kg−1) lead to lateral Hg export to estuarine water re-mobilizing previous legacy sources accumulated in soils. 

We also hypothesize that high Hg levels observed in salt marsh soils are derived in parts from atmospheric gaseous elemental Hg (GEM) uptake by salt marsh vegetation, similar to the dominant vegetation Hg source reported from upland ecosystems. We conducted monthly quantitative harvests of aboveground biomass and Hg concentration analyses throughout a full growing season, along with belowground biomass. Results showed that seasonally, Hg concentration and mass in current-season aboveground biomass near-linearly increased from June (3.9±0.2 µg kg-1 and 0.7±0.4 µg m-2, respectively) to November (16.2±2.0 µg kg-1 and 5.7±2.1 µg m-2, respectively) and resulted in substantial end-of-season pools sizes of Hg. In the next season, these Hg pools are senesced, and they decreased subsequently throughout the season due to slow transfer to soils and wrack export, suggesting an important role of plants as Hg sources to marshes. Ongoing stable mercury isotope analyses and continuous surface-atmosphere exchange fluxes are intended to further quantify various sources origins of Hg in these salt marshes.