| | | 6.1 Abstract |
A modelling approach has been developed with special emphasis on the understanding of the nutrient dynamics and fate of the phytoplankton, often leading to eutrophication problems. The stream-order concept used by geomorphologists has proved very useful for modelling -- the ecological and biogeochemical functioning of hydrosystems at the basin scale. A basic hypothesis to the approach is that related ecological processes are the same throughout the whole hydrographic network.
The RIVERSTRAHLER model, validated on several European rivers, can be used to determine the temporal variations of the equilibrium between phytoplankton production and heterotrophic respiration which controls the oxygen levels at the scale of the hydrographic network. P/R diagrams are good indicators of the ecological functioning of an aquatic systems. In addition, the model leads to the calculation of the Redfield ratios, that describe the respective role of nitrogen, phosphorus and silica in nutrient limitation of algal growth.
The RIVERSTRAHLER model can be also used to test management scenarios aimed at reducing eutrophication problems at the scale of large river basins. In addition, it has been recently coupled to a model of the marine systems. Application of these coupled models to the case of the Seine river and Seine Bight system shows that domestic reduction of phosphates inputs is only efficient if it is carried out at the basin scale: such a reduction should banishphosphates detergents and should be accompanied by a program of reduction of diffuse phosphorus sources. The scenarios also showed that only a drastic program of nitrogen reduction by tertiary treatment in wastewater treatment plants (at least 70% reduction) would be efficient in preventing marine eutrophication.
| | | 6.1 Abstract |