Effects of long-term forest management on total mercury and methylmercury in stream water and biota: A case study in a paired watershed (unmanaged vs. managed) in the lower coastal plain of South Carolina

Land use changes and forest management (prescribed burn, clear-cutting, etc.) are known to affect ecosystem productivity and water quality, but less is known about its short- and long-term impacts on the biogeochemical cycling of atmospheric contaminants such as mercury (Hg). In Santee Experimental Forest on the lower coastal plain of South Carolina, a paired experimental watershed (unmanaged WS80 vs. managed WS77) have been monitored for the export of dissolved organic carbon (DOC) since 2005, in which unmanaged WS80 generally had higher DOC levels than managed WS77. We examined total Hg (THg) and methyl Hg (MeHg) in stream water from both watersheds along with DOC since September 2017, and also MeHg in stream biota in both watersheds. Analyses of filtered water samples show that THg:DOC ratios were significantly lower (p<0.05) in managed WS77 (0.11 ng Hg/mg OC) than in unmanaged WS80 (0.22 ng Hg/mg OC), implying that long-term forest management may decrease stream export of THg. For MeHg, however, we did not observe a significant relationship between MeHg and DOC in unmanaged WS80 but that may be hampered by limited sampling and analyses at this point (n=14). At the time of presentation, one-year of monitoring data will be provided. Interestingly, we found some contrasts in MeHg levels in certain stream biota between watersheds - (i) crayfish in managed WS77 (111±19 ng/g, n=4) had almost five times of MeHg in tissues as found in unmanaged WS80 (23±5 ng/g, n=2), but we observed less but still considerable differences in dragonfly larvae (185 ng/g, n=1, in WS77 vs. 91±19 ng/g, n=3, in WS80), and almost no differences in water scorpions (166 ng/g, n=1, in WS77 vs. 153 ng/g, n=1, in WS80). However, we found similar levels of MeHg in biofilm (6.7-7.0 ng/g) in both sites, representing the entry point of MeHg into the food webs. Thus, there are subtle differences in THg, and more importantly, MeHg bioavailability in this paired watershed indicating that there is an effect of the different forest stands that have resulted from management. In the next 1-2 years, there will be a whole watershed tree harvest in WS77 at Santee Experimental Forest. We believe that these initial Hg data will serve as a reference benchmark to better understand short- and long-term effects of forest management on the Hg cycles and its bioavailability at the watershed scale.