Hydrology and Seasonality Shape the Coupling of Dissolved Hg and Methyl‐Hg With DOC in Boreal Rivers in Northern Québec

Co‐loading of mercury (Hg) with dissolved organic carbon (DOC) is a key driver of the observed spatial and temporal Hg patterns among aquatic ecosystems. Their strong biogeochemical coupling has spurred the use of DOC as a predictor of Hg concentrations and exports in boreal regions where sampling logistics for Hg are costly and complex. Yet relationships between Hg and methylmercury (MeHg) with DOC have recently been shown to be highly variable in terms of slope and strength, suggesting that mechanisms other than co‐transport along the land‐water continuum may drive the relationship between Hg and DOC across landscapes. In this study, we explore the relationship between Hg and MeHg with DOC across 18 boreal rivers collectively draining over 350,000 km2 of the eastern James Bay territory (Québec), comprising watersheds with a wide range of vegetation, water residence time and riverine DOC concentrations and optical properties. Our results show that although a large portion of the variation in Hg and MeHg is explained by concentrations of DOC, Hg‐DOC and MeHg‐DOC relationships and ratios vary greatly both spatially and temporally. We show that ratios and strength of the coupling can be predicted from system hydrology, with declines in Hg:DOC and increase in MeHg:DOC ratios and stronger coupling during the seasonal progression to warmer temperatures, higher water evaporation, and longer residence time. Our study highlights the role of seasonal hydrology and biogeochemical processing in governing Hg, MeHg and DOC patterns in boreal rivers. Plain Language Summary Different forms of mercury and organic carbon may have similar behaviors in rivers, which facilitates understanding of the processes in the landscapes and in the water that controls the inputs and the losses of these elements. Seasonal and geographic differences in the factors behind this behavior, however, may break this similarity and highlight key processes of environmental importance. Here we show how organic carbon and mercury loaded from land tend to be strongly associated across 18 rivers found across a >350,000 km2 boreal landscape, especially during periods of high flow and low temperature that favor passive transport of these elements. Higher concentrations and proportions of biomagnifiable neurotoxic methylated mercury relative to total mercury and organic carbon concentrations, were found during warm and low flow seasons that favored high biological transformation of the mercury and carbon