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 temperate oceanic hydrological regime and high nutrient enrichment. Phytoplankton development is invariably initiated by the decrease of discharge in 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 estuary. The bloom occurs earlier downstream than upstream. Chlorophyceae succeed the diatoms by the end of May and represent a significant component of the summer phytoplankton population. Fluctuations of the phytoplankton biomass are observed within the continuum in summer, with high biomass in 6th-order rivers, low biomass in 7th-order rivers, and again high biomass in 8th-order rivers. These seasonal and spatial variations are interpreted with the aid of a mathematical model (the RIVERSTRAHLER model); the model calculates the development of diatoms and Chlorophyceae within the whole drainage network which is represented as a regular pattern of confluences of tributaries with increasing stream 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.