DYNAMICS OF SESTON CONSTITUENTS IN THE ARIEGE AND GARONNE RIVERS (FRANCE)

Water contents of suspended matter, algal pigments, particulate organic carbon and particulate phosphorus were measured in the rivers Garonne (2 sites) and Ariège (1 site) throughout an annual cycle. The general trend of the parameters was similar at the three sites. Depending on the sites, the period of algal growth (chlorophyll a + phaeopigments > 25 μg l-1), lasted from two to six weeks in August-September. The algal peaks reached 50 to 90 μg 1-1 of total pigments. High contents of particulate organic carbon (> 2 mg 1-1) occurred at the end of summer (coinciding with algal growth), and during the November and May floods. In summer 50-75 % of the suspended matter was organic, in spring this was 10 times less. The high linear correlation between particulate organic carbon and pigment contents (r = 0.87; P = 0.0001) suggested an algal origin of at least part of the particulate carbon. Algal carbon was minor in the annual fluxes of particulate carbon (25 to 39% depending on the sites), but relatively high in comparison with other rivers. The mean particulate phosphorus content calculated over the year was 24 μg l-1 ; it varied from 15 μg l-1 during the high water period to 28 μg 1-1 during the low water period. Likewise the percentage of particulate phosphorus in the suspended matter varied from 0.17 to 0.40. A negative linear correlation existed between particulate phosphorus content and specific discharge (r = - 0.46; P = 0.0001). The very marked seasonal trend of the parameters and the interactions led us to differentiate two modes of the rivers' functioning: a 'hydrologic' phase and a 'biological' phase. The hydrologic phase (high water) was dominated by the processes of erosion and transfer over the whole catchment area and the flood plain, while the biological phase was characterized by a high primary production in the river bed. © 1990 Kluwer Academic Publishers.