Concentrations and fluxes of dissolved organic carbon in drainage water from an upland peat system

Measurements were made of the concentrations of total and hydrophilic dissolved organic carbon (DOC) and optical absorbance at 340 nm (A 340) in two pools (X and Y) situated in an area of peat on Great Dun Fell, North Pennines, UK, between November 1992 and December 1996. Other chemical data were also collected. Pool X was acid (pH 3.9–5.3) while Pool Y was circumneutral (pH 5.8–6.9). Concentrations of DOC in the two pools displayed similar seasonal variations, ranging from as low as 5 mg L −1 in winter to 20 mg L −1 or more in late summer and early autumn. The ratio of hydrophilic to total DOC also showed a seasonal trend, with the highest values observed during the summer months. During the period to mid-1995, the ratio of A 340/DOC in both pools was nearly constant, but fell by 50% following a two-month period of drought, during which time the pools were dry, remaining at the lower value throughout the remainder of the sampling period. Through a series of salt addition experiments, estimates of water flow through the pools were made during two separate periods (2 weeks and 3 weeks). The results were consistent with flows calculated using a simple rainfall-runoff model, driven by rainfall inputs, and taking into account the size and slopes of the catchment. Water flow was strongly correlated with rainfall amount and frequency, and the model could therefore be used to estimate flows over the entire sampling period. The flows were combined with concentration data to estimate fluxes of DOC. The fluxes in 1993 and 1994 were similar (15 g m −2 a −1) and showed a strong seasonality, with most carbon passing through the pools during the late summer and autumn. In 1995, a year of low rainfall and high temperatures, the flux was only 7.0 g m −2 a −1, increasing again to 10 g m −2 a −1 in 1996. The results were interpreted in terms of production of dissolvable organic matter within the catchment soils during summer, and export associated with subsequent high rainfall events. The climatic conditions of 1995 appeared to have restricted the soil production of dissolvable organic matter.