Sediment flux and photodegradation of methylmercury in two boreal drainage lakes

Abstract

Current trends in the anthropogenic mobilization of mercury into the environment suggest that the widespread problem of mercury pollution will persist for decades. Methylmercury is of particular concern because this the form of mercury that accumulates in fish and is toxic to fish consumers. Therefore, methylmercury is a potential health threat to humans who eat fish. Methylmercury accumulates in the fish of many lakes, but study of its production and loss has been hampered by the lack of quantitative measurement techniques. In this study, trace-level analytical techniques were used to estimate fluxes of methylmercury in lakes by two different methods: Direct measurement of sediment and water fluxes by in situ incubations of sediment cores and water column samples, and measurement of whole-lake rates of methylmercury production by whole-lake chemical mass balance. This study was conducted at the Experimental Lakes Area, northwestern Ontario. Results of the sediment core incubations showed that the sediment-water methylmercury flux was naturally variable from site-to-site within a lake. This variability was likely due to small scale chemical and/or biological variability at the sediment-water interface, and means that extrapolation of core results to larger areas and time periods is uncertain. During incubations of water column samples in situ, I discovered that methylmercury is abiotically photodegraded by sunlight. Methylmercury was primarily photodegraded by solar UV-A, and was directly proportional to methylmercury concentration and to solar irradiance. The photodegradation rate was not affected by major differences in water chemistry, which means that in situ rates were easily predicted from methylmercury concentration and solar irradiance alone. Knowledge of this process has significantly changed our understanding of factors controlling methylmercury concentrations in different types of lakes, especially brown vs. clear lakes. Measurements of methylmercury photodegradation, and inputs and outputs of methylmercury in an annual mass balance study showed that the methylmercury in an oligotrophic lake (Lake 240) was controlled by both in-lake processes (photodegradation and in-lake production) and external inputs. The relative importance of external inputs, photodegradation, and internal production changed from year to year, primarily due to differences in precipitation that affected external inputs. Previous mass balance studies have underestimated internal production because photodegradation was not included as a loss term.