Dissolved organic carbon dynamics in the peat–streamwater interface

Abstract

A series of experiments were conducted to address the fate of dissolved organic carbon (DOC) in the peat - stream interface zone linking a minerotrophic poor fen and an ombrotrophic mire with surrounding stream water in the drainage area of Lake Ortrasket in northern Sweden. Transport and mineralisation of DOC were quantified in peat - stream interface cores in response to variations in pore water velocity, DOC concentration and the molecular size and source of DOC. Mineralisation and CH4 production were positively correlated with pore water velocity at rates between 0.08 and 0.20 cm h(-1) and negatively correlated at rates between 0.20 and 0.40 cm h(-1). The DOC concentration of the effluent from the peat cores was independent of the pore water velocity but proportional to the DOC concentration of the source water. Higher concentrations of DOC were exported from than imported to the peat cores, and the cores exported DOC molecules of smaller average molecular size than received. Carbon mineralisation in the peat, assessed in a static system, was independent of the concentration of DOC. DOC with a nominal cutoff at 100 Da was mineralised faster by streamwater bacteria than DOC dialysed with a cutoff at 3500 Da, and their mineralisation rate was positively correlated with the DOC concentration. Streamwater bacteria mineralised streamwater DOC at a lower rate than the peat - stream interface zone pore water DOC. The pattern of velocity dependence of mineralisation was the same for both sources of peat DOC but the mineralisation rates, average molecular size, and bioavailability of DOC were different, emphasising the importance of the compositional heterogeneity of the peat - stream interface zone for the DOC budget of streamwater.