Methylmercury Dynamics along the Land-to-Sea Aquatic Continuum

Mercury (Hg) cycling in aquatic environments is governed by a complex interplay of biogeochemical processes that influence its speciation, transport and bioavailability. This thesis integrates results from four papers with the aim of enhancing our understanding of how environmental factors control Hg dynamics across the aquatic continuum from terrestrially-influenced freshwaters to marine systems. A field study across a climate and vegetation gradient in the Swedish sub-Arctic revealed that catchment properties (associated with dominant terrestrial vegetation type) impact Hg levels and its chemical form in lakes and streams (Paper I). Among the studied properties in these waters, dissolved organic matter (DOM) characteristics were found to be especially useful in interpreting Hg dynamics. Through our photodegradation experiments of monomethylmercury (MMHg) we extended the knowledge regarding MMHg photochemical decomposition by focusing on the Land-to-Ocean aquatic continuum based on the changes in DOM quantity and quality as well as salinity (Paper II). Combining natural and artificial waters allowed us to test the role of environmental parameters in isolation while interpreting the results related to environmentally relevant systems. We observed that the stabilizing effect of salinity on MMHg was especially relevant when tested in isolation or at low DOM levels. The presence and composition of DOM had a stronger impact on changes in photodegradation rate than changes in its concentration (as long as light was not fully attenuated). Further, laboratory experiments confirmed that the photodegradation of Dimethylmercury (DMHg) can be a source of MMHg in sunlit surface waters (Paper III). We demonstrated that DMHg was readily photodegraded in a range of natural and artificial waters at rates comparable to, or exceeding, those of MMHg. Lastly, experiments exploring the partitioning of MMHg between environmental soils and waters were used to estimate the formation of refractory particulate MMHg pools following adsorption (Paper IV). The soils and waters represented a gradient from terrestrial to marine systems and the experiments confirmed that soil organic carbon content was not only an important factor in determining the partitioning of MMHg between solid and aqueous matrices but also increased the formation of refractory MMHg pools. Together, these studies underscore the multi-faceted controls on Hg cycling along the dynamic Land-to-Ocean aquatic continuum. The work highlights the importance of DOM and terrestrial particles in key processes in the biogeochemical cycle and environmental fate of Hg.