Increased concentrations of dissolved organic carbon (DOC) have been reported for many catchments across the northern hemisphere. Hypotheses to explain the increase have varied but one test of alternative hypotheses is the trend over the recent decade, with the competing hypotheses predicting continuing increase; rate of increase declining with time; and even decrease in concentration. In this study, records of DOC concentration in non-tidal rivers across the UK were examined for the period 2003–2012. The study found that: Of the 62 decade-long concentration trends that could be examined, 3 showed a significant increase, 17 experienced no significant change and 42 showed a significant decrease; in 28 of the 42 a significant step change was apparent with step changes being a decrease in concentration in every case. Of the 197 sites where annual flux and concentration records were available from 1974, 97 showed a significant step change down in flux and 107 showed a step down in concentration. The modal year of the step changes was 1997 with no step changes observed before 1982. At the UK national scale, DOC flux has declined from a peak of 2248 ktonnes C/yr in 1990 to 432 ktonnes C/yr in 2006. The study suggests that there is a disconnection between DOC records from large catchments at their tidal limits and complementary records from headwater catchments, which means that mechanisms believed to be driving increases in DOC concentrations in headwaters will not necessarily be those controlling trends in DOC concentration further downstream. We propose that the changes identified here have been driven by changes in in-stream processing and changes brought about by the Urban Waste Water Treatment Directive. Therefore, signals identified in headwater catchments may bear little relation to those observed in large rivers much further downstream and vice versa.
Bibliographical noteWithdrawn from publication
- Urban wastewater