Distribution and Cycling of Mercury Species in Wetlands and Reservoirs in Northern Mississippi

Author(s): Cizdziel, J.; Brown, G.

Methylmercury (MeHg) is a neurotoxin that accumulates in tissues and biomagnifies up the aquatic food chain. Fish consumption advisories have been issued Enid Lake and the Yocona River, a large reservoir and its tributary in north-central Mississippi. This study examined the origin, distribution, and cycling of mercury species in the Yocona River, Enid Lake and associated wetlands. Environmental conditions can have a dramatic impact on the production, transport and fate of Hg species in a given area. Wetlands play a critical role in the cycling of Hg in watersheds and have been shown to be net sources of MeHg to ecosystems. Total-Hg and MeHg were determined seasonally over the course of two years in the inflow and outflow of Enid and Sardis Lakes, in associated wetlands. The Hg species were also measured during storm events (i.e., in runoff from urban, agricultural, and wetland/forest areas). A range of water quality parameters were measured to determine the primary factors controlling the distribution and transport of Hg species in the watershed. The project served as an important step toward building a mass balance for mercury in Enid Lake. Key findings include: • Wetland areas were determined to be hotspots for MeHg in the watershed with relatively high concentrations in water and fish • Levels of Hg in river water were highest at peak flows during storm events. • Forest soil and wetland sediment had higher levels of Hg and organic matter than agriculture soils. • Hg levels were highest in the urban runoff, followed by forest/wetland, and agriculture. • Runoff from highly erodible agricultural areas likely provides the largest input of Hg to Enid Lake by transport of particle-bound-Hg. • MeHg in wetland water was about double that found in lake water, and both spiked during the summer months, with wetlands reaching as high as 1.3 ng/L. • MeHg in the wetland water was negatively correlated with oxidizing reducing potential. • The net flux of T-Hg in Enid Lake was the most negative in the winter due to lowering of lake water levels to accommodate spring rains, and most positive during storm events, suggesting that rain storm events contribute a significant portion of Hg to the lakes. • The net flux of MeHg in Enid Lake was more negative in the summer than fall and spring due to higher methylation and evaporation rates.

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