Greenhouse gases from human-impacted rivers and estuaries - identifying ‘hotspots’ and their drivers: a source-to-sea study of the Clyde river and estuary

There is growing global concern that greenhouse gas (GHG) emissions from water bodies are increasing because of interactions between nutrient levels and climate warming. This thesis investigates three types of aquatic systems, rivers, estuaries and mine water outflows to determine key sources and controls on GHGs in these environments. Riverine systems were studied by consideration of land-cover, seasonal and hydrological controls of GHGs, by comparison of the seminatural, agricultural and urban environments in a detailed source-to-sea study of the River Clyde, Scotland, home to one third of Scotland’s population. Estuarine systems, some of the most sensitive ecosystems to ecological degradation, were investigated by consideration of GHGs from estuaries across the UK and by a detailed investigation of the urban, mesotidal, stratified Clyde estuary. Surveys of the Clyde estuary were made longitudinally, through tidal-cycles and across the riverestuary transition. Mine waters, which can be the dominant pollutant source in mining catchments, were studied by measurement of mine water outflows from sixteen sites across the Midland Valley, Scotland, including radiogenic and stable carbon isotopes measurements to determine the sources of dissolved methane (CH4) and carbon dioxide (CO2). GHG concentrations were consistently oversaturated with respect to the atmosphere. Concentrations of CH4-C ranged from 0.1 to 44 µg l-1 in the riverine environment, 1.2 to 132 µg l-1 in estuarine waters and 20 to 215 µg l-1 in mine waters. Concentrations of CO2-C ranged from 0.1 to 2.6 mg l-1 in the riverine environment and 30 to 120 mg l-1 in mine waters. Concentrations of nitrous oxide (N2O-N) ranged between 0.3 - 3.4 µg l-1 in the riverine environment, 0.4 to 5.9 µg l-1 in estuarine waters and 0.4 to 5.3 µg l-1 in mine waters. In the riverine environment high concentrations of CH4 were primarily associated with point source inflows from urban wastewater (UWW) treatment, abandoned coal mines and lakes. Concentrations of CO2 and N2O were mainly driven by nitrogen concentrations, dominated by diffuse agricultural inputs in the upper catchment and supplemented by point source inputs from UWW in the lower urban catchment. Across the UK, estuarine GHG concentrations were highly correlated with UWW loading although the estuarine environment was highly variable. In the Clyde estuary persistent low river flows, increased freshwater flushing times, which impacted nutrient concentrations (primarily from UWW), salinity, and oxygen levels. Nitrogen processing in the upper freshwater layer occurred via nitrification, while in the lower saline layer denitrification dominated, quadrupling the N2O per unit available nitrogen with a significant inverse exponential correlation with dissolved oxygen. Methanogenesis in the estuarine surface waters was stimulated by a small (0.5 ppt) increase in salinity and positively correlated with water turbidity, however CH4 concentrations near the bed reduced exponentially with salinity persistence (continuously saline water > 5ppt) by about 50% after 10 days but were reinvigorated by freshwater flushing. Mine water CH4 composition included 51% modern biogenic, 41% thermogenic and 8% from methanogenesis of coal. Biogenic CH4 concentrations were inversely correlated with sulphate. Mine water CO2 included 64% from the dissolution of limestone, 21% from terrestrial organic carbon and 15% from coal, with sulphate increasing the dissolution of limestone. This study improves our understanding of aquatic GHG generation and dynamics, which contributes to our knowledge of their release to the atmosphere. GHGs were primarily associated with different sources of pollution including UWW, agriculture and legacy industry, and increased by interactions with physical conditions such as low river flow and low oxygen conditions with temperature a secondary cause. It identifies where actions could support reductions in aquatic GHG generation