Organic carbon content drives methylmercury levels in the water column and in estuarine food webs across latitudes in the Northeast United States

Abstract

Estuaries are dynamic ecosystems which vary widely in loading of the contaminant methylmercury (MeHg), and in environmental factors which control MeHg exposure to the estuarine foodweb. Inputs of organic carbon and rates of primary production are important influences on MeHg loading and bioaccumulation, and are predicted to increase with changes in climate and land use pressures. To further understand these influences on MeHg levels in estuarine biota, we used a field study approach in sites across different temperature regions, and with varying organic carbon levels. In paired comparisons of sites with high vs. low organic carbon, fish had lower MeHg bioaccumulation factors (normalized to water concentrations) in high carbon sites, particularly subsites with large coastal wetlands and large variability in dissolved organic carbon levels in the water column. Across sites, MeHg level in the water column was strongly tied to dissolved organic carbon, and was the major driver of MeHg concentrations in fish and invertebrates. Higher primary productivity (chlorophyll-a) was associated with increased MeHg partitioning to suspended particulates, but not to the biota. These findings suggest that increased inputs of MeHg and loss of wetlands associated with climate change and anthropogenic land use pressure will increase MeHg concentrations in estuarine food webs.

Sediment organic carbon and temperature effects on methylmercury concentration: A mesocosm experiment

“…This is also likely the layer that is the food source for the amphipods, which may explain why even in these organisms, their MeHg burden reflects the water column concentrations rather than that of the bulk sediment. Overall, the results of the mesocosm study are consistent with our field studies across northeastern estuaries (Balcom et al, 2015;Buckman et al, 2017;Chen et al, 2014;Taylor et al, 2019), in that sediment MeHg concentrations are not the best predictors of biota body burdens, which is relevant to assessments of contaminated sites and Hg remediation strategies.…”

“…The results of the mesocosm experiment also demonstrate that MeHg bioaccumulation into primary consumers was driven by water column MeHg and not related to sediment MeHg concentration. While this has been shown by correlation in field studies (Chen et al, 2014;Taylor et al, 2019), sediments were likely the only source of MeHg in this experimental system, whereas watershed sources may also contribute under field conditions. Overall, the extent of the sediment impact on bioaccumulation is a function of flux which is primarily controlled by both methylation rates and the sediment chemistry, particularly its organic matter content and composition.…”

A National-Scale Assessment of Mercury Bioaccumulation in United States National Parks Using Dragonfly Larvae As Biosentinels through a Citizen-Science Framework

“… 36 − 38 Thus, high DOC in wetlands can result in lower MeHg bioaccumulation factors within wetlands themselves by binding MeHg and making it less bioavailable. 39 At the same time, DOC can facilitate THg and MeHg transport to surrounding rivers and streams where it may become more bioavailable in a lower DOC environment. Indeed, dragonfly THg concentrations were positively correlated with lake DOC concentrations across a series of lakes in the northeastern US, but the DOC inhibition effect was observed in bioaccumulation factors for dragonfly THg across the range of measured DOC.…”

Mercury Export From Freshwater to Estuary: Carbocentric Science Elucidates the Fate of a Toxic Compound in Aquatic Boreal Environments

“…But what happens to settled particles containing T-Hg and MeHg? According to several studies, MeHg concentrations in bulk estuarine/marine sediments are not related to MeHg concentrations in the water column (Taylor et al, 2019) and are not the primary determinants of MeHg entry into the food web (Amirbahman et al, 2013;Buckman et al, 2019). Instead, in some north Atlantic estuaries with low SPM concentrations and a high proportion of dissolved MeHg relative to particulate MeHg, MeHg levels in biota were found to be strongly related to dissolved MeHg concentrations (Taylor et al, 2019).…”

“…Although numerous studies deal with Hg accumulation in freshwater food webs, only a few focus on Hg export downstream of reservoirs to estuaries (Taylor et al, 2019). Some studies address the impact of reservoir impoundment in situ as well as on downstream ecosystems in Canada (Schetagne et al, 2000;Anderson, 2011), including some critical voices (Calder et al, 2016).…”