Nitrogen and carbon fluxes from degraded aquatic ecosystems - the interplay between catchment and in-channel factors and remediation

Our efforts to reduce anthropogenic pressures on aquatic ecosystems and their catchments are still unsatisfactory as aquatic nutrient and pollutant exports remain constant or continue to increase. Geomorphologic modifications and diffuse pollution from agricultural land use are the two dominant pressures responsible for aquatic degradation and water pollution. In recent years the main focus has been on mitigating these pressures through various measures implemented at the catchment level, including rewetting/constructing wetlands and channel restoration and remediation. In this study we looked at the environmental effects of channel remediation for over 30 Swedish agricultural headwater streams and  ditches. Our aim was two-fold: 1) to evaluate the effects of channel remediation on chemical and ecological conditions in streams and ditches; and 2) to evaluate the linkages between stream and ditch exports of N and C and catchment and in-stream properties. The aquatic sites analysed in our study all shared high levels of anthropogenic disturbance, with high levels of nutrients and suspended sediments. Despite these common pressures, we found large variations in N and C exports through the stream network (as nitrate nitrogen NO₃-N and dissolved organic carbon DOC) and gaseous losses (as CO₂, CH₄ and N₂O). Using our extensive dataset, we were able to link these differences to catchment and in-stream properties describing N and C transport (e.g., flow discharge, contributing area) and processing (e.g., channel area, channel substrate, macrophyte stands) and carbon quality measured as fluorescent dissolved organic matter. Together, our results indicate that both intrinsic and extrinsic factors control catchment N and C losses to water and air, leading to a large variation in observed fluxes and the effects of remediation.