Spatial variation in discharge and concentrations of organic carbon in a catchment network of boreal streams in northern Sweden

SummaryTotal organic carbon (TOC) is an important feature of water quality in Fenno-Scandia’s till catchments. In this study we estimated the contribution of headwater streams to downstream TOC, tested the hypothesis that TOC-concentration decreases downstream and explored mechanisms for the observed patterns. The drainage network of a boreal catchment (66 sites) in northern Sweden, with subcatchment sizes 0.11–78 km2, was sampled in August 2002. In the headwaters there was a large variation in TOC-concentration (4–66 mg l−1) as well as other chemical parameters and specific discharge (0.13–8.2 l s−1 km−2). Further downstream there was less variation in both chemistry and specific discharge. Both flow and chemistry stabilized at catchment areas larger than 5 km2. No clear indication of in-stream processing effects on downstream TOC was observed, though there was TOC loss at lakes and some stream junctions. To test whether the observed downstream decrease is different than that expected from conservative mixing along the stream network, we used a Monte Carlo approach to simulate downstream conservative mixing. The observed spatial variability was higher than the simulated, indicating that landscape-scale patterns are more than conservative mixing of random inputs. While the importance of in-stream processes that alter TOC-concentrations cannot be ruled out, and loci of TOC loss do exist, we propose that headwater/downstream patterns in TOC, and related parameters depend largely on the mosaic of landscape elements (mires, lakes and forest) together with specific discharge.