Freshwater discharges drive high levels of methylmercury in Arctic marine biota

Abstract

Elevated levels of neurotoxic methylmercury in Arctic food-webs pose health risks for indigenous populations that consume large quantities of marine mammals and fish. Estuaries provide critical hunting and fishing territory for these populations, and, until recently, benthic sediment was thought to be the main methylmercury source for coastal fish. New hydroelectric developments are being proposed in many northern ecosystems, and the ecological impacts of this industry relative to accelerating climate changes are poorly characterized. Here we evaluate the competing impacts of climate-driven changes in northern ecosystems and reservoir flooding on methylmercury production and bioaccumulation through a case study of a stratified sub-Arctic estuarine fjord in Labrador, Canada. Methylmercury bioaccumulation in zooplankton is higher than in midlatitude ecosystems. Direct measurements and modeling show that currently the largest methylmercury source is production in oxic surface seawater. Water-column methylation is highest in stratified surface waters near the river mouth because of the stimulating effects of terrestrial organic matter on methylating microbes. We attribute enhanced biomagnification in plankton to a thin layer of marine snow widely observed in stratified systems that concentrates microbial methylation and multiple trophic levels of zooplankton in a vertically restricted zone. Large freshwater inputs and the extensive Arctic Ocean continental shelf mean these processes are likely widespread and will be enhanced by future increases in water-column stratification, exacerbating high biological methylmercury concentrations. Soil flooding experiments indicate that near-term changes expected from reservoir creation will increase methylmercury inputs to the estuary by 25-200%, overwhelming climate-driven changes over the next decade. mercury | plankton | estuary | biomagnification | hydroelectric reservoir

Methylmercury Bioaccumulation in an Urban Estuary: Delaware River, USA

“…Other studies of watershed impacts on Hg and MeHg levels in tributaries to Lake Michigan and the Chesapeake Bay (Hurley et al 1995; Lawson et al 2001) similarly found a correlation between MeHg concentration, %MeHg, and the extent of marsh area and/or agricultural activity within the watershed. Recently, Schartup et al (2015) showed the decoupling of MeHg in sediment and water compartments, and the relatively greater importance of water column Hg methylation to MeHg inputs at sites of large freshwater inflow into a stratified shallow Arctic estuary.…”

“…The data suggest that the Delaware River estuary shifts from dominantly sediment sources of increased dissolved MeHg in the ETM (observed in forage fish) to a relatively greater influence of wetland sources of water column MeHg in the lower portion (reflected in shrimp tissue concentrations). These water column dynamics, including interactions and the influence of complexation with changing DOC character and salinity on MeHg bioavailability (Schartup et al 2015), will impact uptake and transfer through the food chain.…”

Methylmercury uptake and degradation by methanotrophs

“…It can accumulate and biomagnify at high levels in fish as well as in rice grains, and human consumption can cause neurological damage ( 4 – 7 ). Our understanding of the mechanisms responsible for Hg methylation has greatly improved recently through the identification of the genetic basis ( 1 – 3 ) and factors affecting Hg methylation ( 8 – 12 ).…”

Mercury flux from salt marsh sediments: Insights from a comparison between 224Ra/228Th disequilibrium and core incubation methods

“…For example, a rainstorm prior to sampling, as was observed in August, could enhance the contribution of the watershed. In June, unidentified sources were again a substantial source of MeHg, but less so for THg, suggesting that while external terrestrial inputs could have contributed MeHg, other processes such as methylation in the water column could be another additional source of MeHg (Ortiz et al, 2015; Schartup et al, 2015), or else there was substantial methylation of Hg and subsequent transport from the upper marsh to the sampling site.…”