Divergent fish mercury trends in two South-Central Ontario lakes and the relationship to changes in atmospheric sulfate deposition
Kelly, Mark David
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Environmental conditions can lead to mercury accumulations in fish that are highly variability among spatially close lakes in Ontario, Canada. Mercury methylation caused by sulfate reduction can influence total mercury concentrations in fish (THgfish). Kahshe and Mountain lakes were chosen to assess temporal trends of THgfish and using paleolimnological tools described the lake environment in terms of total mercury (THgsed), total sulfur (TSsed), chromium-reducible sulfur (CRSsed), and chrysophyte-inferred lake-water pH (CI-pH). THgfish were standardized by length (LS-THgfish) to identify trends and make comparisons with archived fish from the Royal Ontario Museum. In Kahshe Lake, LS-THgfish for 30-cm smallmouth bass and 40-cm walleye displayed peak concentrations in the early 1980s (0.74 +/- 0.17 µg/g d.w; n=41 and 1.40 +/- 0.42 µg/g d.w; n=16, respectively) with clear declines starting in 1999 (0.33 µg/g d.w +/- 0.13, n=12). At Mountain Lake, LS-THgfish in smallmouth bass (0.31 – 0.45 µg/g) and walleye (0.30 – 0.49 µg/g) were low and stable through the period of observation. Peak concentrations at Kahshe Lake were over 300% higher than the 1926 archival sample (0.22 µg/g d.w, 22.1 cm total length), while the 1948 Mountain Lake sample (0.28 µg/g, 24.4 cm total length) is consistent with modern concentrations. As expected from LS-THgfish , THgsed was higher in Kahshe Lake, although enrichment was seen at both sites. Temporal trends for TSsed and CRSsed diverged with enrichment in Kahshe Lake and depletion in Mountain Lake. CI-pH showed minimal temporal changes. Although trends of CRSsed indicate sulfate is processed differently among the lakes, geochronology highlighted a consistent spatio-temporal relationship between THgfish and CRSsed. This strongly supports a relationship between sulfate reduction rates and Hg methylation, independent of THgsed and pH. Therefore, if THgfish is used as an indicator of aquatic health CRSsed may be a useful tool for rapid assessment.