Origin and transport of phytoplankton in a large river:: the Garonne, France

Patterns in chlorophyll-a concentration, discharge and composition of suspended algae were used to examine the origin and transport of phytoplankton in a 291 km stretch of the River Garonne, south-west France during the summer-autumn of 1996. The importance of tributary inputs was also investigated. At higher altitudes, algal biomass (mean of 4.4 mug l(-1) chlorophyll-a) consisted essentially of the drift of benthic diatoms derived from reservoirs. The hydraulic conditions (high turbulence, short residence time) were not favourable for the development of a true phytoplankton in the river, except in segments of greater retentiveness (e.g. Malause reservoir, mean of 23 mug l(-1) chlorophyll-a). Some tributaries were occasionally eutrophic (e.g. Tolzac, 70 mug l(-1) chlorophyll-a), but dilution of these waters was too great for these tributaries to act as significant sources of algal biomass. However, they did contribute to diversity of planktonic algal assemblages. Spatial patterns in algal biomass (chlorophyll-a) were consistent with the drift of predominantly benthic algae followed by an input of the phytoplankton of the large tributaries (the Ariege and the Tarn) and with the euplanktonic species of Mancies and Malause reservoirs. Light intensity, sediments, nutrients and grazing do not seem to be limiting for algal development. The river hydrodynamics would not favour a high development of suspended algae, except maybe in lateral retention zones. Therefore, manifestations of eutrophication in the river Garonne are low.