Influences of Elevated Nutrients and Water Temperature from Wastewater Effluent on River Ecosystem Metabolism

Abstract

AbstractRiver 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.