Bioavailability of stream dissolved organic carbon (DOC) during spring flood and base flow in high-latitude streams

An important component of the carbon cycle is the lateral flow of dissolved organic carbon (DOC) from terrestrial ecosystems to streams and rivers. The fate of this carbon depends very much on the bioavailability of DOC (BDOC), which may determine whether DOC is returned to the atmosphere as CO2 or deposited in sediments. This study focuses on the linkages between stream DOC composition, optical characteristics and bioavailability along vegetation gradients in subarctic Sweden. We sampled streams from tundra, birch forest, and boreal forest ecoregions, which encompass large differences in C:N ratios (6.4-30.1) and spectroscopic characteristics, all related to variation in landscape properties. The DOC bioavailability was determined through laboratory bioassays carried out twice during the year (spring-flood and base flow). During spring flood, DOC concentration varied between 0.5 - 6.7 mg L-1 and the BDOC ranged between 3- 24 %, with the highest BDOC from birch forest/tundra and tundra streams. Results suggest that broad-scale transitions in vegetation structure across sub-arctic landscapes have important implications for the quantity and quality of DOC delivered to aquatic ecosystems.