A programme of field sampling was undertaken to quantify annual organic carbon fluxes at 23 sites from the headwaters to the lower reaches of the river Dee and river Don in northeastern Scotland. The annual fluxes of both dissolved organic carbon (DOC) and particulate organic carbon (POC) were found to increase downstream in both rivers. In headwaters and tributaries of the river Dee, annual DOC fluxes showed as much variation between catchments as seen in data from river systems worldwide and were positively related to the amount of wetland in the catchment area. This indicated that the size of the soil organic carbon pool was the single most important factor in determining annual riverine organic carbon fluxes. This hypothesis was tested by using archive information to estimate organic carbon exports for 85 large British rivers. The resulting flux estimates, along with data on soil organic carbon content were used to develop a predictive model of British riverine DOC fluxes. A model based on soil organic carbon storage and annual precipitation data from the catchments of 17 river systems was successful in explaining 94% of the variation in annual DOC fluxes between the sample catchments.

During the study, the potential importance of dissolved free carbon dioxide as an additional form of carbon transport in both rivers and lakes was highlighted. A direct method for measuring the partial pressure of carbon dioxide in freshwaters was also developed. Application of this methodology to samples from the river Dee and river Don indicated that free carbon dioxide in rivers may constitute an additional export equivalent to 10% of the annual organic carbon flux in British rivers. The relationship between dissolved carbon dioxide, DOC and watershed characteristics was further investigated in a study of 27 lakes in northern Wisconsin, the United States. The findings showed a significant positive correlation between the partial pressure of dissolved carbon dioxide, DOC, and the extent of wetland in catchments.

Overall the findings from this work have shown that lakes and rivers can act as important conduits for carbon transport (particularly from wetlands) both to the atmosphere and oceans. The link that has been established between soil carbon pools and riverine fluxes, should help to integrate rivers into future models of global carbon cycling. Furthermore, this work suggests that rivers (and lakes) may regulate increases in soil organic carbon pools induced by climate change.