Stream water hydrochemistry as an indicator of carbon flow paths in Finnish peatland catchments during a spring snowmelt event

Extreme hydrological events are known to contribute significantly to total annual carbon export, the largest of which in Arctic and boreal catchments is spring snowmelt. Whilst previous work has quantified the export of carbon during snowmelt, the source of the carbon remains unclear. Here we use cation hydrochemistry to trace the primary flowpaths which govern the export of carbon during the snowmelt period; specifically we aim to examine the importance of snowpack meltwater to catchment carbon export. The study was carried out in two forested peatland (drained and undrained) catchments in Eastern Finland. Both catchments were characterised by base-poor stream water chemistry, with cation concentrations generally decreasing in response to increasing discharge. Streamflow during the snowmelt period was best described as a mixture of three sources: pre-event water, snowpack meltwater and a third dilute component we attribute to the upper snow layer which was chemically similar to recent precipitation. Over the study period, pre-event water contributed 32% and 43% of the total stream runoff in Välipuro (undrained) and Suopuro (drained), respectively. The results also suggest a greater near-surface throughflow component in Suopuro, the drained catchment, prior to snowmelt. CO2 and DOC concentrations correlated positively with cation concentrations in both catchments indicating a common, peat/groundwater flowpath. CH4 concentrations were significantly higher in the drained catchment and appeared to be transported in near-surface throughflow. Meltwater from the snowpack represented an important source of stream water CO2 in both catchments, contributing up to 49% of total downstream CO2 export during the study period. We conclude that the snowpack represents a potentially important, and often overlooked, transient carbon store in boreal snow-covered catchments.

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► Soil water displacement by meltwater greater in drained catchment.
► Greater contribution of ‘new water’ from undrained catchment during snowmelt.
► CO2 and DOC associated with deep flowpaths; CO2 also sourced from snowpack itself.
► CH4 associated with near-surface throughflow.
► Snowpack meltwater contributed < 49% of total aquatic CO2 export during study period.