Urban settings contribute to multiple nutrient sources of urban rivers. Excessive nutrient concentrations can cause water environment problems and have negative impacts on the ecological functions of rivers. To deeply understand the responses and mechanisms of river ecological processes to high concentrations of nutrients and environmental changes, this thesis investigated the spatiotemporal variations of nutrient concentrations (i.e., nitrate (NO3-), ammonium (NH4+), phosphate (PO43-), dissolved organic carbon (DOC)), their fluxes (i.e., uptake and regeneration) at water-sediment interface and river ecosystem metabolism (REM) in which these nutrients participate. The River Wandle, a tributary of the River Thames in London, and the three tributaries of the River Beiyun in Beijing - the River Liangshui, the River Tonghui and the River Wenyu were selected in this study. They all receive wastewater treatment plant (WWTP) effluents and tributary inflows, providing natural conditions with high nutrient loads. Using chamber techniques, water-specific nutrient concentrations were measured at two exposures (3 and 10 min) to calculate flux. The WWTP effluents or the tributary inflow contributed to elevated concentrations and modified flux rates, resulting in significant differences among the study sites. Under the actions of physical and biological processes, the fluxes of NO3- and NH4+ are primarily controlled by equilibrium concentration and nitrogen form transformation within the N-cycle. The mean NO3- concentration was the lowest in summer and the highest in winter, and the mean NH4+ concentration was on the contrary. Dissolved oxygen (DO) and bioabsorption mainly affected PO43- and DOC fluxes, respectively, resulting in the highest mean concentrations of both in winter. Specifically, in London, 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). Among all sites in Beijing, NO3- was -0.31 to +0.30 mg/(m2·s), NH4+ was -0.01 to +0.05 mg/(m2·s), PO43- was -0.01 to +0.01 mg/(m2·s), and DOC was -0.04 to +0.13 mg/(m2·s). Regarding REM, a holistic measure of ecological function which integrates gross primary production (GPP) and ecosystem respiration (ER), I applied the nighttime slope modelling (NSM) to estimate diurnal and seasonal REM patterns at each site, and structural equation modelling (SEM) revealed that nutrient supply, water temperature and light availability were the main factors driving REM, which was also confirmed by the metabolism review of China's major rivers and coastal ecosystems during 2000-2020. Overall GPP and ER were similar to those of urban impacted rivers in other countries. GPP was similar at each study site, but ER was significantly higher at the sites affected by the WWTP effluents and the tributary inflow. In summer, the mean net ecosystem production (NEP) was greater than 0 at each site, indicating that the study reaches were autotrophic as overall sinks of atmospheric CO2, while the mean NEP less than 0 in other seasons indicated that these reaches were transformed into heterotrophic as sources of CO2. Our results provide further evidence to show that reductions in river nutrients are paramount for improving river ecological conditions and highlight the response of aquatic ecosystem metabolism to environment and climate change, which allows for a holistic understanding of the mechanisms and influencing factors of important river ecological processes, so as to suggest options for in-stream management.
|Date of Award
|1 Sept 2022
|Robert Francis (Supervisor) & Michael Chadwick (Supervisor)