SEASONAL SUCCESSION OF DIATOMS AND CHLOROPHYCEAE IN THE DRAINAGE NETWORK OF THE SEINE RIVER - OBSERVATIONS AND MODELING

Seasonal succession of diatoms and Chlorophyceae have been analyzed in the Seine River system (France), which is characterized by a temperat e oceanic hydrological regime and high nutrient enrichment. Phytoplank ton development is invariably initiated by the decrease of discharge i n spring. When this occurs in early spring, the bloom is dominated by diatoms that severely deplete silica, and a regular increase of their biomass is observed along the river continuum from headwaters to the e stuary. The bloom occurs earlier downstream than upstream. Chlorophyce ae succeed the diatoms by the end of May and represent a significant c omponent of the summer phytoplankton population. Fluctuations of the p hytoplankton biomass are observed within the continuum in summer, with high biomass in 6th-order rivers, low biomass in 7th-order rivers, an d again high biomass in 8th-order rivers. These seasonal and spatial v ariations are interpreted with the aid of a mathematical model (the RI VERSTRAHLER model); the model calculates the development of diatoms an d Chlorophyceae within the whole drainage network which is represented as a regular pattern of confluences of tributaries with increasing st ream order. The model, taking into account both bottom-up and top-down regulating factors of phytoplankton, has proved to be a powerful tool in understanding the dynamics of a large drainage network.