| Summary: | Lake ecosystems receive, transmit and process terrestrial carbon and thereby link terrestrial, aquatic and global carbon cycles. Most lakes evade CO2 to the atmosphere, but the annual magnitude of CO2 evasion, as well as sources and mechanisms underpinning CO2 evasion from lakes are still largely unresolved. CO2 evasion from lakes can be sourced from direct external input from the catchment, but CO2 can also be produced in-lake from organic carbon breakdown. Both sources have been shown to be of importance to individual systems, but a landscape perspective is still missing. Globally, most lakes are in northern high latitudes, but due to infrequent seasonal sampling the magnitude of CO2 evasion on an annual scale is largely unknown, as are constraining variables of in-lake metabolism (i.e. production and consumption of CO2). As a consequence of these knowledge gaps, there is little possibility to predict future lake carbon cycling, for instance due to changing dissolved organic carbon (DOC) input or lake temperature resulting from global warming. In this thesis I aim to resolve these knowledge gaps surrounding the magnitude, cycling and sources of CO2 evasion from high-latitude (mainly arctic) lakes. By combining the estimates of annual CO2 evasion and metabolism, I investigated the magnitude of CO2 evasion, as well as the contribution of the internal carbon processing to CO2 evasion. Inclusion of ice-melt evasion allows to assess the importance, and drivers, of ice-melt CO2 evasion on the annual scale. Furthermore, by pooling lakes from multiple different lake surveys I was able to analyse the lake and landscape variables associated with high-latitude lake metabolism. Finally, through use of an experimental pond facility I manipulated dissolved organic carbon input and temperature to explore the effects of future climate conditions on lake carbon cycling and CO2 evasion. I found that both external input and internal CO2 production can contribute to CO2 evasion from lakes, but it is often dominated (>75%) by a ...
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