Dissolved organic matter (DOM) represents the largest fraction of organic carbon in most aquatic ecosystems and consists of a wide and complex array of compounds that varies according to its source material. Large loads of DOM are transported annually from terrestrial to aquatic environments. Human induced changes within catchments affect these exports and may also have large implications in regional carbon cycling. Carbon export calculations have so far focused on the quantity (DOC) and not the composition. Studying the DOM composition exports provides insight in global carbon cycling and serves as a tool that may further reveal the effect of human disturbance on carbon transport.
Using field data and Generalized Additive Models, we present a comprehensive analysis of temporal variability in nutrient concentrations, DOM composition, and exports while capturing seasonal hydrological variability in two contrasting catchments. The two streams varied in land use intensity, riparian vegetation and shading, stream bank erosion, and nutrient concentrations, but had similar DOC concentrations. We hypothesized that (1) the rate of change of DOM composition throughout the year would be higher in the agriculture stream, and (2) water temperature and nutrient concentrations would influence DOM composition export fluxes.
Results/Conclusions
We found that discharge and stream water temperature influenced DOM composition. The contribution of terrestrial allochthonous-like DOM was strongly and positively related to stream discharge. While average DOC loads were the same in both streams, the composition of the carbon being exported was significantly different. We found that nutrient enrichment simulates the export of labile DOM. The nutrient-rich stream exported greater amounts of microbial-like DOM while the forested stream exported large amounts of humic-like DOM. Patterns of DOM were consistent across seasons in both streams, implying that the different catchments and nutrient concentrations regulate DOM composition and export, while also being modulated by variables influenced by seasonality like discharge and water temperature the importance of different variables influenced by seasonality, such as discharge and water temperature, on the amount and composition of carbon fluxes from terrestrial to aquatic ecosystems. The variable DOM composition export may have important environmental and ecological consequences by affecting stream metabolism in nutrient enriched headwaters and the biogeochemistry in downstream receiving ecosystems.
