Trends in DOC concentration in Great Britain

SummaryIncreasing DOC concentrations in rivers of boreal and sub-boreal regions have been a widely observed phenomena. However, individual studies have been limited in time and space and this has limited the ability to test hypotheses of the explanation of the observed increases. This study examines 315 records from across Britain all of which are at least 10 years in length up to 2002, but with some records extending back to 1975 and combines these with results from 198 sites from a previous study. The study covers catchment areas up to 9948 km2. The results show that:(i)Of the 315 records analysed, 216 showed a significant increase but 55 showed a significant decrease. This is the first time that significant long-term decreases in DOC concentration have been documented for British rivers. Trends in DOC concentration ranged from −5.7% to 16% annual percentage change.(ii)Although increasing trends are predominantly in catchments with peat covered headwaters, some peat catchment also show significant decreases.(iii)Some sites which show significant decreases over 10-year time periods exhibit significant increases over longer term records (23 years).(iv)Regional trends in DOC concentration cannot be explained by spatial or temporal trends in atmospheric deposition of acidity, oxides of N, and non-marine S. Further, trends in DOC concentrations do not appear to correlate with the occurrence of droughts.The study concludes that there is long-term increases in DOC are not ubiquitous for all organic soils across the UK and that even where long-term increases are apparent these trends are overprinted by changes in river flow.