Water Resources Research Act Program

Details for Project ID 2016NH205G

Effects of dissolved organic carbon on methylmercury bioavailability in stream ecosystems

Institute: New Hampshire
USGS Grant Number: G16AP00195
Year Established: 2016 Start Date: 2016-09-01 End Date: 2018-08-31
Total Federal Funds: $126,875 Total Non-Federal Funds: $126,877

Principal Investigators: Kathryn Cottingham, Celia Chen, James Shanley

Abstract: Watersheds across the northeastern United States are impacted by the mobilization of mercury (Hg) and dissolved organic carbon (DOC) resulting from land use/land cover modification, climate change-induced increases in precipitation events and runoff, and the lasting effects of acid rain. Hg is currently the primary cause of fish consumption advisories in the U.S., with statewide freshwater fish advisories in all 9 northeastern states (U.S. EPA 2011). The northeastern landscape is prone to Hg hotspots (Driscoll et al. 2007, Evers et al. 2007) where Hg is efficiently converted to its toxic form, methylmercury (MeHg), and where Hg levels in the biota frequently exceed critical effect levels for wildlife and human consumption. In these landscapes, DOC erosion and dissolution drive Hg inputs into streams – yet inputs of MeHg to the water column and bioaccumulation in the biota can have contrasting relationships with DOC. Bioaccumulation tends to initially increase with MeHg and DOC in the water column, but at very high DOC concentrations, MeHg uptake can inhibited (Driscoll et al. 1995, French et al. 2014, Tsui and Finlay 2011). This apparent threshold effect, together with the observed spatial variability in bioaccumulation, suggest that different sources and fractions of DOC control Hg and MeHg transport, versus MeHg availability to organisms. Our research goals are to identify the key biogeochemical factors that link land cover and water quality parameters to Hg concentrations in biota. We hypothesize that DOC quality is the missing link in our understanding of MeHg transport and bioavailability to aquatic food webs. DOC quality influences Hg binding ability, and varies with the source of DOC. Although terrestrially derived, large, aromatic DOC fractions are positively correlated with dissolved Hg in streams and runoff, the MeHg bound to small labile organic molecules is thought to be more bioavailable to algae and bacteria, potentially explaining the contrasting effects of DOC on abiotic Hg concentrations and uptake into the biota. In this study, we will use advanced analytical techniques, controlled laboratory studies, and field work to further our understanding of the controls of MeHg bioavailability, a crucial contribution to monitoring and managing Hg hotspots. This project builds on our recent work in the tributaries of Lake Sunapee, N.H., a forested watershed interspersed with wetlands, which has shown an apparent non-linear relationship between water column DOC concentrations and MeHg in both stream-dwelling and riparian biota (Broadley et al. in prep, Chaves-Ulloa et al. accepted). We will also extend our collaboration with USGS NH-VT Water Resource scientists, who lend expertise on Hg cycling and DOC quality analysis, and provide an opportunity to relate our results to their studies in other watersheds. Our specific objectives are to: 1) identify and characterize fractions of DOC in streams and relate DOC quality to the distribution of MeHg and Hg in the water column; and 2) determine the effects of DOC quality and quantity on MeHg uptake by primary producers and consumers in stream ecosystems.