Long-term trends in dissolved organic matter in French rivers. In : American Geophysical Union (AGU) Fall Meeting, San Francisco, 11-15 Déc. 2006

This study presents long-term records of dissolved organic matter (DOM) as indicated by oxydability measurements that were constructed for four rivers in western France as back as 1979. Data show contrasted evolutions with three rivers exemplifying large increases in DOM concentrations over the period of study (a doubling of the mean annual concentration over a period of 25 years for one river), while the fourth shows decreasing DOM concentrations with time. All rivers show common inter-annual control on DOM concentration in response to a succession of dry and wet periods with a cyclicity of 5-7 years. The same cyclicity in DOM concentrations is apparent in the long-term records of DOM for rivers located in northern England. The evidence support the view that this cyclicity is climatic in origin being likely a consequence of the North Altantic Oscillation that controls the yearly amount of precipitation that falls over Europe. As regards the significance of long-term trends, we note that the onset of DOM increase in those rivers showing positive long- term DOM trends is in phase with an increase in average annual temperature. This might suggest that the change in temperature is the key mechanism that causes these trends, suggesting that the climate is also the driving force of long-term DOM trends in rivers. However, such an hypothesis is faced with the problem of explaining why one of the studied rivers shows a divergent evolution, i.e. a long-term DOM decrease. A change in hydrologic regimes must be also discarded. Alternative explanations other than climate change must thus be found. One such alternative explanation could include changes in land use and agricultural practices on river catchments. A survey of agricultural practices in the catchments drained by the four studied rivers reveals that the catchments of the river showing a long-term decrease of DOM receives massive pig manure spreading, a practice that does not occur in the other three catchments. Spreading of pig manure may acidify catchment soils, thereby promoting adsorption of organic matter on soil minerals which could ultimately limit the export of DOM by rivers. The evidence supports a view that DOM export by rivers is under the control of global, climatic factors mediated by local land use factors which can cause divergent long-term evolutions.