Microbial metabolic activity and nutrient turnover at groundwater-surface water interfaces

Rivers and streams have crucial ecohydrological functions and are much more than just transport systems for excess water. Active exchange across groundwater-surface water interfaces facilitates many biogeochemical processes. However, natural functions of microbial metabolic activity (MMA) are disturbed by excess nutrient inputs from anthropogenic disturbances such as agriculture or wastewater treatment plants. Key results of this thesis are: (1) streambed MMA and greenhouse gas production are driven by quantity and quality of the organic matter, and streambed CO2 production rates can account for up to 35% of estimated total stream evasion. (2) Streambed heterogeneity drives MMA and nutrient turnover. While dissolved organic carbon was not limiting, high organic matter structures can control nutrient concentrations. (3) Macrophyte cover was negatively related to hyporheic exchange. Mowing of macrophytes as a management practice does therefore not negatively influence MMA and can improve transient storage in urban streams. (4) MMA in dual-porosity Chalk groundwater aquifers is sensitive to fertilisation by carbon that can be introduced during groundwater flood events and can be responsible for problems drinking water production quality. These results improve understanding of MMA, nutrient turnover and greenhouse gas production at groundwater-surface water interface and anticipate on the impacts of anthropogenic disturbances.