Collaborative Research: Vegetation assimilation as a source of mercury in a salt marsh ecosystem and implications for soil and tidal water exposures

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The goal of this award is to determine the origin and fate of mercury in a salt marsh ecosystem in the Plum Island Sound of Massachusetts and potential export to near shore coastal oceans. Mercury is a potent neurotoxin and leading cause for fish consumption advisories in the U.S and across the world. Along the U.S. East Coast, salt marsh sparrows have shown harmful exposures to mercury with elevated blood levels posing risks for reproductive outputs. The origin of mercury in these salt marshes is largely unknown. Possible reasons for high mercury exposure are contributions from runoff from adjacent watersheds or atmospheric inputs from legacy industrial impacts. This award will study the role of direct plant uptake of atmospheric mercury by salt marsh plants as the mercury source for these salt marsh ecosystems. In this tidal system, an additional focus lies on the transfer of mercury to soils by the plants and with death of the plants its export to the coastal ocean. The award will train and involve graduate and undergraduate students in the research.This award aims to quantify mercury sinks and sources and their relationships to plant primary productivity, tissue turnover rates, and soil dynamics, in a salt marsh ecosystem where some of the highest biological mercury exposures have been reported. The first hypothesis is that that plant mercury assimilation is the dominant source of mercury in this estuary where salt marshes dominate in areal extent (50 to 90% of surface area). The second hypothesis is that vegetation assimilation of mercury is incorporated into soils upon plant senesce leading to high mercury accumulation in soils, which ultimately is mobilized to tidal water leading to net export of mercury from salt marshes to tidal water. This is analogue to the “outwelling” hypothesis for carbon that proposes that salt marshes produce an excess of autochthonous carbon over what is degraded and stored resulting in net export to the coastal ocean where it stimulates ocean net primary production. Proposed measurements include quantification of atmospheric mercury deposition inputs via deployment of a micrometeorological flux-gradient system that allows to measure ecosystem-level gaseous atmospheric mercury fluxes including plant mercury assimilation, along with detailed vegetation mercury dynamics and other deposition processes. Spatial transect sampling of tidal water from the ocean through the tidal water up to the freshwater headwater will be conducted and analyzed for mercury species (dissolved, particulate, methyl-mercury and mercury bound contained in wrack detritus) in order to assess lateral exchanges between salt marsh and tidal water. Finally, distribution patterns of total mercury and methylated mercury in salt marsh soils will be characterized to constrain processes — both natural and anthropogenic — that lead to mercury accumulation in these 2,500-to-3,500 year old soils.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.