Application of Mercury Stable Isotopes: Temporal Field Assessment of Two Systems of Contrasting Mercury Inputs

In aquatic ecosystems, planktonic uptake of methylmercury (MeHg) is a key step linking mercury (Hg) sources to bioaccumulation and subsequent human and wildlife health concerns. Despite the importance of plankton in aquatic Hg cycling, little information is available regarding temporal shifts in Hg source accumulation as it relates to seasonal patterns or changes in algal community composition. This study examines how Hg and MeHg concentrations and isotopic signatures vary temporally with the algal succession (cryptophytes → diatoms → greens → blue-greens) in two inland lakes of contrasting Hg source profiles. The systems examined include two Wisconsin inland freshwater lakes, Lake Mendota and Lake Monona, located in urban and suburban watersheds prone to eutrophication and summer anoxia. Mercury sources to these lakes include legacy contamination, watershed inputs, stormwater runoff, and atmospheric deposition. Initial results demonstrate these lakes have different historic Hg profiles, with Lake Monona (176.3±62.9 ng g-1) having higher Hg concentrations in sediment compared to Mendota (34.2±0.9 ng g-1). This difference is not strongly observed in water or zooplankton, suggesting that factors controlling contemporary source inputs and water chemistry drive Hg bioaccumulation in the lower food web. To understand Hg bioaccumulation within these lakes, we are employing a multi-analysis approach examining Hg concentrations in tandem with other trace metal (iron, manganese, lead, and cobalt) concentrations, water quality parameters, and Hg and MeHg stable isotopes. Plankton were collected from the lakes during spring-summer months to compare how Hg species cycle and are transformed within ecosystems with contrasting Hg inputs. This presentation will discuss the resulting temporal patterns and statistical relations between concentration, isotopic, and algal speciation data from these two systems. Ultimately, this field assessment allows us to gain insight into the reactivity and bioavailability of varying Hg sources, a piece currently missing in our current understanding of Hg biogeochemical cycling.