Seasonal variations of total- and methyl-mercury in restored urban wetlands of different maturity in Piedmont Triad of North Carolina

Wetland restoration can help provide and maintain unique ecosystem services to urbanized areas such as increasing biodiversity and improving water quality. However, it is widely known that wetlands can be hotspots of the production of methylmercury (MeHg), a highly toxic neurotoxin, through the anaerobic methylation of mercury (Hg) under nutrient-rich, and organic matter-rich conditions. The urban campus of the University of North Carolina at Greensboro (UNCG), restored two on-campus sites in March 2017 (also known as "The Wetlands Project", both the shady and open site are roughly 232 m² each in surface area). While the project resulted in observable increases in biodiversity after 1 year of establishment, we still know very little about the alterations in Hg cycling and how total-Hg (THg) and MeHg levels may vary over time within each wetland site and nearby connected water ways. Since late 2017 water samples from each site, as well as nearby water ways have been collected monthly and analyzed for filtered THg, MeHg, and other water chemistry parameters. We also collected samples from more established urban wetlands in Greensboro, NC for comparison. Results so far have shown that the shady wetland site in the forested area has slightly higher (by an average of 12%) monthly filtered concentrations of THg compared to the open wetland site in the recreation field. While downstream water ways closely reflect concentrations of THg found in the respective wetland site. However, MeHg levels remained low in these newly restored wetland sites (<0.1 ng/L) but the downstream water ways contain considerably less MeHg (below detection limit, <0.02 ng/L) potentially due to dilution with stream water of very low MeHg, suggesting that each wetland site has internal net production of MeHg. We are currently collecting data for the summer 2018 in which we already observed anoxic conditions in the water (dissolved oxygen: <1 mg/L), and we hypothesize that active microbial methylation of Hg may occur and can be reflected by higher MeHg concentrations in water, sediment and macroinvertebrates, which we will report in the presentation. Therefore, restored urban wetlands have the potential to produce MeHg, but we need longer term data to better understand the pros and cons of restoring wetlands to the overall health of urban ecosystems.