163 - Mercury isotopic compositions in paired experimental forest watersheds with long-term forest management

We examined the stable mercury (Hg) isotopic compositions of abiotic and biotic environmental media from paired experimental forest watersheds in coastal South Carolina (USA). The treatment watershed (WS77; 155 ha) has been experimentally treated using prescribed burning and thinning over the last 1-2 decades while the control watershed (WS80; 160 ha) is an undisturbed forest. From 2018 to 2020, we collected air, precipitation, surface water, and aquatic food web members, which were analyzed for total Hg and methylmercury (MeHg) concentrations, and stable Hg isotopic compositions. Both Hg isotopic compositions of gaseous Hg (δ202Hg: +0.57 to +1.01 ‰; Δ199Hg: -0.22 to -0.14 ‰; n=3) and precipitation (δ202Hg: -0.24 to -0.14 ‰; Δ199Hg: +0.20 to +0.34 ‰; n=3) resembled those of other background sites in North America. The δ202Hg of surface water was more positive for the treatment watershed (WS77; δ202Hg: -1.45±0.14 ‰; n=4), than the control watershed (WS80; δ202Hg: -1.64±0.07 ‰; n=3), but this was not statistically significant (p=0.08), leading to a higher estimated contribution of wet deposition to the stream Hg export in the treatment watershed (~36% in WS77 vs. ~27% in WS80). The difference may be due to the different soil and tree compositions between the paired watersheds and/or altered hydrology in the treatment watershed compared to its native state. For the biota samples, all crayfish and fish samples in both watersheds had slightly negative Δ199Hg values, but we did not find any significant differences in Δ199Hg values between the paired watersheds (WS77; Δ199Hg: -0.26±0.12 ‰; n=9 vs. WS80; Δ199Hg: -0.23±0.10 ‰; n=10). These negative Δ199Hg values of MeHg strongly suggest a nearly absence of MeHg photodemethylation in these forested wetland ecosystems, which may explain the high MeHg levels in the resident aquatic food webs.

Authors: Martin Tsui, Joel Blum, Marcus Johnson, James S. Coleman, Peijia Ku, Sae Yun Kwon, Yener Ulus, Juang Chong, Troy Farmer, Alex Tat-Shing Chow, Amatya Devendra, Carl C. Trettin