Modelling phytoplankton development in whole drainage networks: the RIVERSTRAHLER Model applied to the Seine river system

Phytoplankton development in river systems is under the control of various meteorological, hydrological, chemical and biological factors. Because of the continuity of the aquatic systems which progress from headwaters to the largest rivers, the interplay of these control factors can only be understood at the scale of the entire drainage network. The RIVERSTRAHLER Model, based on the concept of stream-order, has been established for that purpose. It has been applied here on two rivers from the Seine basin: rivers Marne and Oise. It is shown that hydrological factors determine the time of onset, and the position within the drainage network, of the spring algal bloom. Phosphorus availability, when limiting, controls the intensity of the bloom. During summer, top-down control, linked to grazing and other causes of mortality, has a marked impact on algal dynamics.