Climate's control of intra‐annual and interannual variability of total organic carbon concentration and flux in two contrasting boreal landscape elements

Large spatial and temporal variations in stream total organic carbon (TOC) concentration and export occurred during an 11‐year observation period (1993–2003) in a boreal headwater catchment. TOC flux and concentration patterns from mire‐ and forest‐dominated subcatchments differed (mean annual flux 8.2 g m −2 a −1 versus 5.8 g m −2 a −1 ). Temporal variations in stream TOC concentrations in both landscape types were primarily driven by variations in streamflow, with the mire stream generally diluting by half with increased runoff during spring flood and TOC from the forested landscape increasing during runoff peaks irrespective of season. Average TOC concentration in the mire stream in the snow‐free season increased with increased seasonal precipitation from around 20 to 40 mg L −1 but then dropped to around 35 mg L −1 during very wet years. Average snow‐free season TOC concentration at the forested site remained stable when summer precipitation was below average but then increased from 10 to around 25 mg L −1 during exceptionally wet years. For both the forested subcatchment and the whole catchment, TOC concentrations increased during the warm summer months during wet years, but no such increase occurred during dry years. Interannual variations in TOC flux were primarily driven by variations during the snow‐free period. Wet years decreased the relative TOC export from the mire and favored the relative export of TOC from areas dominated by forest, an observation that also held true on a larger scale when similar landscape types were considered. Predicted climate change in rainfall and temperature patterns will affect the amount and character of TOC exported downstream from boreal landscapes with a mix of forest and mire.