Intra-annual variability of natural organic matter in boreal streams

Natural organic matter (NOM) is a key component in aquatic ecosystems. It influences for example acidity, mobility and toxicity of metals and organic pollutants, energy input to aquatic food webs, weathering, and water light conditions. There are also considerable costs associated with removing NOM in drinking water production. Furthermore, NOM is an integral part of the carbon cycle with possible indirect effects on climate. During recent decades, there have been observations of increasing concentrations of NOM in surface waters in parts of North America and Europe. The causes of these trends are not fully understood, but are thought to be related to climate change and recovery from anthropogenic acidification. This thesis presents results from studies on intra-annual NOM cycling in more than 130 boreal streams and rivers. It also presents developments of the Riparian flow-concentration Integration Model (RIM). Detailed studies on five forested headwater catchments revealed that stream discharge and soil temperature were the main drivers of NOM variability. In addition, a small headwater catchment at the Swedish West Coast was substantially influenced by sea-salt deposition, which suppressed NOM mobilization. A modified version of RIM with discharge and soil temperature as variables could successfully simulate NOM dynamics in all five catchments. Riparian soil organic matter content and distribution was hypothesized to be the underlying control on NOM response to discharge and soil temperature. Catchments where NOM was sensitive to discharge displayed stronger gradients in soil NOM concentrations than did catchments with weak discharge sensitivity. A large scale study of 136 streams and rivers indicated common relationships among NOM, discharge and temperature. Conversely, there was no geographical pattern in NOM trends. Relative trends were weakly related to NOM response to flow and temperature. There were also clear relationships among intra-annual NOM dynamics, temperature, flow, and catchment landscape characteristics, indicating that catchments can be classified based on NOM dynamics. Taken together, this implies that NOM dynamics could change in ways not reflected in inter-annual trends due to climate change