The influence of summer seasonal extremes on dissolved organic carbon export from a boreal peatland catchment: evidence from one dry and one wet growing season.

A small peatland catchment in eastern Finland was monitored for runoff and export of dissolved organic carbon. To exemplify the response of a peatland under years of different meteorological conditions, data from one very dry and one wet growing season are presented. Runoff was 194 and 387 mm, and DOC export was 4.2 and 11.3 gC m(-2) in the dry and wet year respectively. Analysis of the hydrographs showed that in both years most runoff and DOC export was generated during peak flow events, with low baseflow in the intermediate periods. Runoff response to rain events was strongest under a high water table in the spring and autumn, with a summer runoff minimum. The effect of drought on runoff occurred only after mid June when the effects of snowmelt had dissipated. Snowmelt therefore dominated DOC export in the dry year (61%) but contributed much less (29%) in the wet year. The relationship between runoff and the water table was highly similar in both years. Any variation that was observed herein was potentially related to a superficial subsidence of the peat surface under drier summer conditions and to raised spring water levels, causing restricted drainage due to flooding. The observed variation of DOC concentrations in both years generally had a limited impact on DOC export. During peak runoff events, concentrations dropped proportionally to the magnitude of the events. This decrease in available DOC is argued to be caused by a dominance of leaching over supply and production and by the specific hydraulic characteristics of the peatland surface. DOC concentrations were some 25% higher under drought conditions. These elevated concentrations were dispelled by spring snowmelt of the next year, demonstrating that a winter snowpack in boreal regions may be instrumental in limiting the longer term hydrological and biochemical effects of a drought.