In most cases, eutrophication of a coastal zone is a multi-source phenomenon. The questions often raised by authorities about these different sources are: what are their marine area of influence and their respective role in the eutrophication process?

A first answer to these questions is proposed for the bay of Biscay-English Channel French coasts, using a hydrodynamical model alone with a passive tracer for each of the main 45 French watersheds of the domain. The statistical marine receiving area of each river is then defined over a whole decade (2000–2010) by using different percentiles (respectively the 10th, 50th, 90th ones) of the distribution of tracer values in each mesh of the grid: this enables to map the dilution areas of each river plume respectively for low water regime, mean flow rate regime and flood regime. If one can impose: 1/the marine winter concentration not to be exceeded to keep a Good Ecological Status, 2/a proxy for the cost of the unitary abatement of the nutrient concentration in each river, this linear dilution model can be coupled to a global optimisation method, in order to compute the set of concentrations in the 45 rivers which allows to obtain at the lowest price the best Ecological Status everywhere in the maritime domain under consideration. Two global optimisation techniques (linear Simplex method and quadratic Beale's method) have been compared on various marine target areas: the marine WFD water masses considered separately or together, the 3 MSFD French sub-regions of the Channel-Biscay area, or the coastal strip laying between the seashore and the 50 m isobath. Computations have been made using two successive nitrate and phosphate thresholds at sea, which are associated respectively to the WFD eutrophication High Status and Good Status. This exercise has shown that focusing on a single small marine target area can sometimes prescribe a strong abatement of concentrations in the sole small neighbouring rivers, but that dealing with a large marine target area points always to the biggest tributaries as being the main nutrient sources to be diminished, whereas small tributaries can be neglected. As it contains the whole big river plumes, the MSFD target requires stronger abatement of nitrate in some watersheds than the WFD target, which is limited to a thin, 1 nautical mile wide strip along the coasts. Whereas very few rivers (e.g. the Seine river) require some abatement of their phosphate loadings, almost all the medium and large French rivers would require very strong abatements (between 50 and 80%) of their nitrate concentrations.

To verify this quick linear approach restricted to winter nutrients, a full non-linear, coupled biogeochemical-hydrodynamical model has been used to compute the effect of these optimal nutrient reduction scenarios on the most characteristic descriptors of eutrophication: chlorophyll 90th percentile, dinoflagellate maximum and bottom oxygen concentration 10th percentile. The results show that nutrient loading reductions enabling a Good Ecological Status everywhere for winter marine nutrients would still leave some marine areas in a Bad Ecological Status in terms of chlorophyll content