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Uptake and regeneration fluxes and concentrations of nutrients, i.e., nitrate (NO3−), ammonium (NH4+), phosphate (PO43−) and dissolved organic carbon (DOC), were evaluated upstream and downstream of a wastewater treatment plant (WWTP) in the River Wandle, UK, from July to October 2019. Using chamber techniques, water-specific nutrient concentrations were measured at two exposures (3 and 10 min) to calculate fluxes. The WWTP effluent contributed to elevated concentrations and modified flux rates, resulting in significant differences at the study sites. Compared with summer, the concentrations of NO3− and DOC increased while NH4+ and PO43− decreased in autumn. Nutrient fluxes varied both temporally and spatially in uptake (i.e., storage in sediments) or regeneration (i.e., release into river water). Under the actions of physical and biological processes, the fluxes of NO3− and NH4+ showed opposite flux directions. Dissolved oxygen (DO) and bioabsorption mainly affected PO43− and DOC fluxes, respectively. Specifically, across all sites, NO3− was −0.01 to +0.02 mg/(m2 s), NH4+ was −29 to +2 μg/(m2 s), PO43− was −2.0 to +0.5 μg/(m2 s), and DOC was −0.01 to +0.05 mg/(m2 s). Further, we did find that these variations were related to nutrient concentrations in the overlying water. Our results provide further evidence to show that reductions in river nutrients are paramount for improving river ecological conditions. Additionally, we suggest that more research is needed to evaluate chamber-based experimental approaches to make them more comparable to in-situ flux methods. Highlights • Sewage effluent resulted in elevated nutrient concentrations and modified fluxes. • Flux was affected by initial nutrient concentrations, DO and microbial activity. • Inexpensive approaches to study nutrient dynamics are needed for river restoration.

River ecosystem metabolism (REM) is a measure of ecological function which integrates gross primary production (GPP) and ecosystem respiration (ER). Urban rivers often receive effluents from wastewater treatment plants (WWTP) which frequently alter nutrient concentrations and modify temperature regimes of receiving water bodies. To investigate how variations in nutrients and water temperature affect REM, we applied the night-time slope modelling to estimate diurnal REM at sites above and below a wastewater outfall on the River Wandle, UK. Overall, estimated GPP (0–21.2 mgO2·L− 1·d− 1) and ER (5.5–10.1 mgO2·L− 1·d− 1) from our study sites were similar to those of urban impacted rivers in other countries. GPP values were similar between sites, but downstream ER values were significantly higher affected by the WWTP effluent. GPP/ER ratios were < 1 indicating heterotrophic conditions and the river as a carbon source during the study. We found that sites had similar activation energy associated with ER suggesting our work provides a useful reference for estimating temperature corrected metabolic processes for other urban rivers in the region. Furthermore, structural equation modelling revealed that nutrient supply, water temperature and light availability were the main factors driving REM. This research highlights the major environmental factors affecting REM, which helps to understand the response of river metabolism and river regulation of regional carbon cycle to future climate change and provide evidence to inform river restoration and future in-stream management.

No abstract

River ecosystem metabolism (REM) is a measure of ecological function which integrates gross primary production (GPP) and ecosystem respiration (ER). Urban rivers often receive effluents from wastewater treatment plants (WWTP) which frequently alter nutrient concentrations and modify temperature regimes of receiving water bodies. In this study, we applied the nighttime slope method (NSM) to estimate diurnal REM at sites above and below a wastewater outfall on the River Wandle, a tributary to the River Thames, and structural equation modelling (SEM) revealed that nutrient supply, water temperature and light availability were the main factors driving REM. Overall estimated GPP and ER from our study sites were like those of urban impacted rivers in other countries. Upstream to downstream, GPP values (0 ~ 21.2 mgO2·L− 1·d− 1) were similar, but ER values (5.5 ~ 10.1 mgO2·L− 1·d− 1) were significantly higher at the downstream site receiving WWTP effluents. GPP/ER ratios were > 1 indicating heterotrophic conditions during the study. We found that sites had similar activation energy associated with ER suggesting our work provides a useful reference for estimating temperature corrected metabolic processes for other urban rivers in the region. Structural equation modelling (SEM) revealed that nutrient supply, water temperature and light availability were the main factors driving REM. This research highlights the major environmental factors affecting REM providing needed evidence to inform river restoration and future in-stream management.