Understanding the Fluvial Loss of Carbon from UK Watersheds - Implications for Terrestrial Carbon; Greenhouse Gases and Water Quality.

This study develops a number of methods for estimating the fate of fluvial carbon through UK watersheds. The study considers dissolved organic carbon (DOC); particulate organic carbon (POC); and dissolved CO₂. The study considers the loss from the terrestrial source; in-stream losses and production; and export to the continental shelf. The study used multiple approaches, including: mass balance studies; modelling and experimentation.

i) Mass balance studies suggest that the UK losses 5 Mtonnes C/yr (21.8 tonnes C/km²/yr) from soils in the proportion of 7:22:4 (POC:DOC:diss. CO₂). The mass balance studies suggest 3.5 Mtonnes C/yr (15.2 tonnes C/km²/yr) is lost to the atmosphere in the proportion 8:75:17 (POC:DOC:diss. CO₂).

ii) UK rivers have short residence times (typically 1-2 days) and so the diurnal cycle becomes critical. Experiments show that turnover rates are close to zero overnight but that these can be periods of DOC production from turnover of POC.

iii) Development and modelling using physical rate showed that the loss of the DOC was between 24 and 37% - lower than that estimated from mass balance studies, but that the loss rate of TOC (DOC + POC) was between 57 and 80% - close to that estimated from mass balance studies.

iv) Studies of molecular change suggest that nutrient supply is not the limiting factor on turnover and that a position of “pseudo-equilibirum” is achieved that limits the net effect of the fluvial processes.