Search

Recent studies report trends of strongly increasing iron (Fe) concentrations in freshwaters. Since Fe is a key element with a decisive role in the biogeochemical cycling of major elements, it is important to understand the mechanisms behind these trends. We hypothesized that variations in Fe concentration are driven mainly by redox dynamics in hydraulically connected soils. Notably, Fe(III), which is the favored oxidation state except in environments where microbial activity provide strong reducing intensity, has several orders of magnitude lower water solubility than Fe(II). To test our hypothesis, seasonal variation in water chemistry, discharge, and air temperature was studied in three Swedish rivers. Methylmercury and sulfate were used as indicators of seasonal redox changes. Seasonal variability in water chemistry, discharge, and air temperature in the Eman and Lyckeby Rivers implied that the variation in Fe was primarily driven by the prevalence of reducing conditions in the catchment. In general, high Fe concentrations were observed when methylmercury was high and sulfate was low, indicative of reducing conditions. The Fe concentrations showed no or weak relationships with variations in dissolved organic matter concentration and aromaticity. The seasonal variation in Fe concentration of the Ume river was primarily dependent on timing of the snowmelt in high- versus low-altitude areas of the catchment. There were long-term trends of increasing temperature in all catchments and also trends of increasing discharge in the southern rivers, which should increase the probability for anaerobic conditions in space and time and thereby increase Fe transport to the aquatic systems.

Rates of Hg methylation and demethylation were measured in anoxic hypolimnetic waters of two pristine Wisconsin lakes using stable isotopes of Hg as tracers. One of the lakes is a clear-water seepage lake situated in sandy terrain with minimal wetland influence. The other is a dark-water lake receiving channelized inputs from a relatively large terrestrial wetland. Methyl mercury (MeHg) accumulated in the anoxic hypolimnia of both lakes during summer stratification, reaching concentrations of 0.8 ng center dot L-1 in the clear-water lake and 5 ng center dot L-1 in the dark-water lake. The stable isotopic assays indicated that rate constants of Hg-(II) methylation (K-m) ranged from 0.01 to 0.04 center dot day(-1) in the clear-water lake and from 0.01 to 0.09 center dot day(-1) in the dark-water lake, depending on the depth stratum. On average, K-m was threefold greater in the dark-water lake. Hypolimnetic demethylation rate constants (K-dm) averaged 0.03 center dot day(-1) in the clear-water lake and 0.05 center dot day(-1) in the dark-water lake. These methylation rates were sufficient to account for the observed accumulation of MeHg in hypolimnetic water during summer in both lakes. Despite substantial export of MeHg from the wetland to the dark-water lake, our study indicates that in-lake production and decomposition of MeHg dominated the MeHg cycle in both lakes.

During most of an annual cycle, we studied the temporal variation of total mercury (Hg-T) and methyl mercury (MeHg) in unfiltered and filtered (0.45 mu) peat water from a minerotrophic peat bog in southeastern Sweden. MeHg in bulk water ([MeHgT]) and total Hg in filtered water in discharge water from the peat bog ([Hg-D]) were an order of magnitude higher than in upland runoff water entering the peat bog. At the discharge end, peat-water [Hg-D] and [MeHgD] ranged from 8 to 54 pmol.L-1 and from 1 to 32 pmol.L-1, respectively. Whereas the variation of [MeHgT] was explained by changes in [MeHgD], the variation of inorganic Hg-T [IHgT] = [Hg-T] - [MeHgT] was explained by changes in particle-bound IHg [IHgP] = [IHgT] - [IHgD]. Filterable organic matter and sulfide in the water both correlated poorly with [Hg-D]. Neither did the amount of Hg-T in precipitation and upland runoff water correlate well with the estimated discharge of Hg-D from the peat bog. However, there was a strong correlation between [Hg-D] and [MeHgT] in the peat water (r = 0.96). Furthermore, a significant fraction of Hg-D was MeHg (mean 28%; range 8-60%). These results suggest that methylation increased the mobility of Hg.