Dynamics and Drivers: Carbon fluxes from temperate freshwater reservoirs

Freshwaters are significant sources of methane (CH4) and carbon dioxide (CO2) in the global carbon cycle, influencing the earth’s atmospheric energy budget. When considering freshwater reservoirs, these greenhouse gas emissions are anthropogenic, yet have received little research focus or integration into carbon budgets, despite having potential implications for policy makers. This thesis investigates the CH4 and CO2 flux dynamics from three reservoir systems in the south east of England, and the physicochemical conditions that influence these flux dynamics, during the summer of 2018. Using the closed dynamic floating chamber method in tandem with physicochemical analysis, an in-situ investigation found rates of CH4 efflux among the highest recorded in the literature from temperate reservoir systems (means up to 48 mg m-2 day-1), while CO2 rates of influx were among the highest (means up to 687 mg m-2 day-1). However, these flux rates were heterogeneous between reservoirs and within reservoirs, dependent upon the littoral or limnetic habitats investigated. Most significantly, efflux was correlated with increased phosphorous concentrations within the system (CH4 and CO2), pH (CO2) and the occurrence of physical ebullition events (CH4), all of which were accentuated by large water drawdowns during the sampling period. This study suggests that system productivity is likely to be the dominant driver of CH4 and CO2 production in reservoirs of the south east of England, and highlights the significant role of extreme climate events in potentially driving a positive feedback loop between climate change and Greenhouse Gas (GHG) release. Through water management strategies, the source of GHGs from reservoir systems could be minimised by controlling reservoir water levels and eutrophication, however further research on annual and diel temporal scales are required to incorporate reservoirs into regional carbon budgets accurately. In addition, water column CH4 and CO2 dynamics were investigated on a lab scale through a microcosm experiment. Using headspace analysis, the effect of control, cellobiose, acetate, phosphate and light experimental treatments upon water column CH4 and CO2 efflux were investigated. Dark conditions were observed to significantly increase the rate of CO2 efflux relative to light conditions, emphasising the need for CO2 flux dynamics in field investigations to be on a diel scale, to prevent the underestimating of lakes as sources of CO2. Minimal CH4 release was observed intermittently throughout the study, however experimental treatments had no statistically significant effect upon the headspace CH4 concentrations, suggesting that any release resulted from water disturbance rather than biogenic production. As such, this study supports the paradigm that CH4 production from the sediment, not the water column, drives the CH4 efflux observed from reservoirs of the south east of England.