Long-term changes in water physicochemistry in the Adour-Garonne hydrographic network during the last three decades.

This study details a trend analysis covering a 30-year period (1975-2004), for 19 physicochemical parameters at 45 surface water sites in the Adour-Garonne basin, south-west France. To perform statistical analysis, we used the annual average of each variable. The analysis revealed sites affected by strong patterns of temporal variation and sites with weak or imperceptible changes of water quality. More than half the studied sites were affected by chemical changes. Trends were generally clearest in the River Garonne continuum, but similar tendencies could also be identified in tributaries. The overall trends indicated the onset of an increase of water temperature starting about 20 years ago and partial recovery from eutrophication during the last decade. As expected, the strongest trend affected the temperature regime of the hydrosystems, there being a more significant warming during the second decade of the study (1984-1994). Additionally, at many sites nutrient loads were lower between 1995 and 2004. This confirms a downward trend in eutrophication status resulting from more stringent control of sewage treatment despite the constant increase of anthropic pressure. Sites that did not present any trends are extreme sites located at each end of the river gradient: headwater and downstream sites under tidal influence. Other sites not affected by changes are those strongly perturbed by human activities showing a high level of degradation.