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Abstract Title: Mercury isotopic compositions in paired experimental forest watersheds with long-term forest management
Presenter Name: Martin Tsui
Company/Institution: Chinese University of Hong Kong
Session: Mercury in Freshwater Ecosystems
Day and Session: Monday 25th July - Session Two
Start Time: 0830 UTC
Co-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

Abstract Information :

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 (?202Hg: +0.57 to +1.01 ?; ?199Hg: -0.22 to -0.14 ?; n=3) and precipitation (?202Hg: -0.24 to -0.14 ?; ?199Hg: +0.20 to +0.34 ?; n=3) resembled those of other background sites in North America. The ?202Hg of surface water was more positive for the treatment watershed (WS77; ?202Hg: -1.45ñ0.14 ?; n=4), than the control watershed (WS80; ?202Hg: -1.64ñ0.07 ?; 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 ?199Hg values, but we did not find any significant differences in ?199Hg values between the paired watersheds (WS77; ?199Hg: -0.26ñ0.12 ?; n=9 vs. WS80; ?199Hg: -0.23ñ0.10 ?; n=10). These negative ?199Hg 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.

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