Long term records of riverine dissolved organic matter

This presents the longest, consistent records of dissolved organic carbon in rivers ever published. This study presents long-term records of organic matter as indicated by water colour that were constructed for three catchments in Northern England for as far back as 1962. Observations show that there have been large increases in DOC concentrations over the period of study with in one case a doubling of the concentration over a period of 29 years. However, in one of the catchments no significant change was observed over a 31-year period. All catchments show common inter-annual control on carbon release in response to droughts, but no step increases in DOC concentrations were observed in response to such perturbations with pre-drought levels being restored within a period 3-4 years. Observed increasing trends do not correlate with changes in river discharge, pH, alkalinity or rainfall, but do coincide with increasing average summer temperatures in the region. The times series of DOC concentration over the period of the record appears stationary, but the distribution of daily values suggests a change in sources of colour over the increasing trend. The evidence supports a view that increases in carbon release are in equilibrium with temperature increases accentuated by land-use factors.