Understanding the long-term concentration, flux, composition and processing of dissolved organic carbon in UK rivers

Dissolved organic carbon (DOC) represents an important component of the terrestrial and fluvial carbon cycle as it represents a flux from terrestrial carbon stores and while it transfers through the fluvial network it can be processed to release greenhouse gases to the atmosphere. Furthermore, DOC is a major water resource limitation as the dissolved organic matter (DOM) has to be removed prior to treatment. Therefore, we need to understand the concentration and fluxes of DOC and they change across a landscape between the terrestrial source and the tidal limit.

Our ability to understand the processing of terrestrial and fluvial carbon has been limited by the range of catchments that have been considered and the time scale over which they have been considered. Studies focused on similar catchment types and very little means of comparing between catchments. However, if we can access and understand large datasets we can find general principles which control DOC and the relative importance of these controls. In this study we use two datasets. The first, is a dataset sampled across the UK for major rivers (270 catchments) from 1974 and this dataset is ideal for understanding flux to the continental shelf and this dataset has over 50000 datapoints. Secondly, many of these sites are monitored for a range of other parameters that are related to the composition of the DOM. The important covariates for DOM composition are BOD, which is a measure of DOM decomposition, and COD which is measure of the oxidation state of the DOM. All the study catchments could be characterised by a range of covariate information, eg. soil cover, land use, hydro-climatology. To make maximum use of this data the dataset was considered within a Bayesian hierarchical framework.

The study can show that DOC flux from the UK has been declining since 1986. There are two exceptions to the observed decline in flux, in 1994 and 1995, which were years of severe drought in the UK. The change in flux between catchments shows that the decline is driven by changes in urban sources, particular by improvements in sewage treatment. The analysis showed that there was a significant change overtime in the composition of the DOC with two independent components. However, variations in these components are driven by increases in DOC concentration and cannot explain the trend in the DOM composition.