A Multi-Year Study of Methylmercury and Nutrient Export in Surface Water from Big Meadow Bog, Brier Island, Nova Scotia

Mercury (Hg) is a naturally-occurring, toxic contaminant that accumulates in ecosystems, potentially having deleterious effects on the health of wildlife and habitat quality. Significant advances have been made in understanding Hg cycling in freshwater and marine ecosystems, but the processes controlling Hg accumulation in bogs and the confounding effects of both nutrient subsidies and increased flooding are not well known. Previous research by our group has shown that feces from avian biovectors are likely increasing certain trace metals and nutrients in shallow groundwater within the northern portion of recently restored Big Meadow Bog (BMB). There was significant variation in Hg, trace metals, and orthophosphate (PO43−) concentrations, with higher concentrations of these elements located in the highest gull nesting density areas. Since then, our analyses have shown that gull guano is very high in water soluble PO43− and is most likely the nutrient source to shallow groundwater. Total mercury (THg) and methylmercury (MeHg) concentrations were found to be low in guano but indicated a shift in food sources over a season. In this chapter we examine data from the bog’s northern outflow which shows high concentrations of MeHg in the water from the fall of 2018 to summer 2019 (immediately post-restoration), and declining concentrations thereafter. Seasonal cycles in nutrients (PO43−, total organic carbon (TOC)) were observed from 2019 to 2022 with decreasing summer maximums through time. The MeHg concentration in 2019 approached the Canadian Water Quality Guideline for the protection of low trophic level freshwater aquatic life (4 ng/L MeHg). Our data are consistent with various studies examining terrestrial freshwater catchment flooding that indicate a short-term increase in the export of MeHg followed by a gradual decline over a decade in otherwise undisturbed catchments.