Do concepts about catchment cycling of methylmercury and mercury in boreal catchments stand the test of time? Six years of atmospheric inputs and runoff export at Svartberget, northern Sweden

Previous studies at the Svartberget catchment in northern Sweden have identified potential terrestrial sources of methylmercury MeHg. and total mercury THg. in runoff as well as processes controlling MeHgrHg transfers from soil to runoff water. This paper considers whether the concepts based on a few seasons of observations are consistent with catchment budgets of MeHgrTHg over half a decade. Inter-annual and seasonal variations in the input and output fluxes of THgrMeHg, in open field wet deposition OF., litterfall LF. and runoff water are evaluated together with more recently measured concentrations that include the throughfall water TF. data. The input and output flux data of THg and MeHg from the Svartberget catchment are also compared to those from the G°ardsj¨on Catchment. The average annual MeHg input fluxes in OF, TF and LF are 0.08, 0.17 and 0.3 g kmy2 yeary1, respectively. The comparable inputs for THg are 7, 15 and 17 g kmy2 yeary1. Thus, LF is as important as TF for THg inputs, while LF is twice as important as TF for MeHg inputs. The annual output flux of MeHg varied between 0.05 and 0.14 g kmy2 yeary1. The annual output flux of THg varied between 1 and 3.4 g kmy2 yeary1. The large inter-annual variations in catchment output did not follow the smaller variations in atmospheric input. This suggests that changes in climate can effect terrestrial outputs of THgrMeHg to surface water more than atmospheric deposition. These data do not contradict the earlier findings that it is the hydrological and biogeochemical processes in the riparian zone that have a central role in determining the amount of MeHg reaching surface waters from forested catchments. The juxtaposition of major flow paths and organic-rich soils in the riparian zone may create the locations of most importance for net MeHg production, runoff export and a larger pool of MeHg.