Differences in catchment properties may be major drivers in mercury (Hg) cycling. However, the complex interplay of these environmental drivers with Hg speciation, transport and bioavailability is still not fully understood. To relate Hg speciation to different catchment types (tundra, birch, boreal) and their inherent differences in stream and lake chemistry, we studied Hg speciation and concentrations along a climate and vegetation gradient in the sub-arctic northern Sweden (including 18 streams and 8 lakes). We find differences in Hg concentrations aligning with differences in water chemistry between the studied catchment types. All observed differences between catchments align with the gradient in biological productivity (tundra<birch<boreal). Moreover, we find higher methylmercury (MeHg) concentrations in lakes compared to streams. Overall, our data suggests, that DOM components play a crucial role for (I) the concentrations of total Hg and MeHg in the studied waters (especially allochthonous DOM) and (II) Hg methylation (especially autochthonous DOM).

In surface waters, photodegradation is a major abiotic removal pathway of the neurotoxin monomethylmercury (MMHg), acting as a key control on the amounts of MMHg available for biological uptake. Different environmental factors can alter the rate of MMHg photodegradation. However, our understanding of how MMHg photodegradation pathways in complex matrixes along the land-to-ocean aquatic continuum respond to changes in salinity, dissolved organic carbon (DOC) concentration and dissolved organic matter (DOM) composition is incomplete. In a set of laboratory experiments combining several artificial and natural waters, we demonstrate that the interplay of DOC concentration, DOM composition, and salinity affects the photodegradation rate of MMHg. The presence of DOM was found to facilitate MMHg photodegradation, but degradation rates were not altered by varying DOC concentrations over two orders of magnitude. We found DOM composition to have a stronger effect on MMHg photodegradation rates than DOC concentration. However, at high DOC levels, where most UV radiation was lost within the first cm of the reaction vessels, lower MMHg photodegradation rates were observed. When moving from terrestrially influenced waters, characterized by a high degree of humification, towards marine conditions with a protein-rich DOM pool, MMHg photodegradation rates increased. In contrast, salinity had a stabilizing effect on MMHg. Hence, especially in systems with low salt and DOC concentrations, changes in either salinity or DOC concentration can impact the photodegradation rates of MMHg.

Photochemical demethylation of dimethylmercury (DMHg) could potentially be an important source of monomethylmercury (MMHg) in sunlit water. Whether or not DMHg is photochemically degraded when dissolved in water is, however, debated. While an early study suggested DMHg dissolved in natural waters to readily degrade, later work claimed DMHg to be stable in seawater under natural sunlight and that early observations may be due to experimental artifacts. Here, we present experimental data showing that DMHg is readily degraded by photochemical processes in different natural waters (including water from a DOC-rich stream, the Baltic Sea, and the Arctic Ocean) as well as in artificial seawater and purified water. For most of the waters, the degradation rate constant (k_d) for DMHg measured in indoor experiments exceeded, or was close to, the k_d observed for MMHg. Outdoor incubations of DMHg in purified water and Arctic Ocean surface water further confirmed that DMHg is photochemically degraded under natural sunlight. Our study shows that DMHg is photochemically degraded in a range of natural waters and that this process may be a source of MMHg in sunlit waters where the supply or formation of DMHg is sufficient. 

Adsorption of mercury (Hg) onto particles is well known to limit the availability of Hg for important biogeochemical processes in natural systems, including the methylation of divalent Hg (HgII) and biological uptake of monomethylmercury (MMHg). To what extent particle adsorption leads to the formation of refractory Hg pools, i.e. pools not readily available for desorption, is, however, largely unexplored. We here present novel data to study the formation of refractory particle-bound MMHg in slurries prepared from environmental soils and waters which represent a gradient from terrestrially- influenced to marine systems. The refractory pools were quantified based on the difference in partitioning at equilibrium between adsorption (up to 48 hours) and desorption (up to 8 weeks) using an isotopically labelled MMHg tracer. In all slurries (combinations of the different soils and waters), refractory particle-bound MMHg pools were formed. While the organic carbon content in soil and water were found to increase and decrease partitioning coefficients, respectively, the effect on the formation of refractory particulate MMHg was less pronounced. Evolving this approach further can help improving our understanding of the link between the export of terrestrial particulate MMHg in aqueous environments and the risks associated with MMHg biological uptake in downstream ecosystems.