How Do Rates of Carbon Metabolism Vary over a Geological Gradient, and How does this Contribute to Riverine Greenhouse Gas Emissions?
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Rivers and streams are increasingly recognised as important components in the global carbon cycle, and act as net sources of CO2 and CH4 to the atmosphere. However, the origins and controls over the fate of these greenhouse gases are still poorly constrained. This thesis firstly explores the production of CO2 and CH4 in the sediment of several rivers over a geological gradient (chalk, sand and clay), to investigate the magnitude and controls on production. It was found that, whilst there are some general patterns due to geology, variables such as organic carbon content are much better predictors of production of CO2 and CH4 and these can vary widely within a single reach. The response of production to temperature was found to be very constant across rivers and in both summer and winter, suggesting a uniform increase in production of both CO2 and CH4 with projected future climate change. However, production of CH4 was much more responsive to temperature change than was CO2, meaning a greater proportion of carbon is mineralized as CH4 under warmer conditions and indicating a positive feedback with global warming. In addition, the amount of CO2 and CH4 out-gassed from the rivers to the atmosphere was measured. It was found that the amount out-gassed could not be explained by local sediment respiration alone: CO2 out-gassing was consistently higher (and CH4 consistently lower) than that produced by the riverbed. Instead, CO2 out-gassing was under hydrological control, and was correlated with rainfall. The source of this was ingress from the surrounding catchment, with higher out-gassing during periods of high rainfall. This strong hydrological controls on CO2 emissions were however modulated by biological processes, as lower emissions were measured during the day than night; and the difference could be accounted for by local in-stream GPP.
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