Watershed and discharge influences on the phase distribution and tributary loading of total mercury and methylmercury into Lake Superior

Knowledge of the partitioning and sources of mercury are important to understanding the human impact on mercury levels in Lake Superior wildlife. Fluvial fluxes of total mercury (HgT) and methylmercury (MeHg) were compared to discharge and partitioning trends in 20 sub-basins having contrasting land uses and geological substrates. The annual tributary yield was correlated with watershed characteristics and scaled up to estimate the basin-wide loading. Tributaries with clay sediments and agricultural land use had the largest daily yields with maxima observed near the peak in water discharge. Roughly 42% of HgT and 57% of MeHg was delivered in the colloidal phase. Tributary inputs, which are confined to near-shore zones of the lake, may be more important to the food-web than atmospheric sources. The annual basin-wide loading from tributaries was estimated to be 277 kg yr−1 HgT and 3.4 kg yr−1 MeHg (5.5 and 0.07 mg km−2 d−1, respectively).

► The highest mercury yields occurred during spring melt except in forested watersheds.
► Roughly half of the mercury yield occurred in the colloidal phase.
► About 277 kg of HgT and 3.4 kg of MeHg were delivered annually via tributaries.
► Whole-water MeHg loading was roughly equivalent to the estimated atmospheric loading.
► Watersheds with peat, loam or sandy soils deliver more MeHg than those with clays.