Evaluation de l'impact des cours d'eau sur l'eutrophisation dans la bande c tière : nécessité de raisonner sur les concentrations et sur les flux

Assessing the impact of streams on eutrophication in the coastal strip: the need to consider concentrations and loads. The approach of using measurements of water quality concentrations has been favored to date because of the current standards for drinking water, which are defined in terms of concentration. For example, the standard for the concentration of nitrate is 50 mg·l-1, although this standard, in fact, derives from a limited quantity of nitrate absorbed by drinking water with a concentration of 20 mg nitrogen per day. The dual assessment of flow and concentration is essential when studying environmental impacts because the intensity of these impacts is mainly linked to the quantity of nutrients reaching a compartment of the environment. This link between nutrient load and environmental impact is particularly established in coastal marine environments exposed to the phenomena of green tides, diatoms or Phaeocystis blooms. At the watershed scale, flow control presents some metrological problems but assessment based on flow has the advantage of providing a better comparison of the overall impact (at the coastal band scale) of several watersheds. At the catchment scale, in order to reduce the outflow, it is important to intervene at the level of the inflow by reducing any excesses in the N and P balance. This approach is illustrated by the example of the eutrophication of coastal marine water in the Eastern English Channel and at the mouth of the North Sea.