L'eutrophisation des eaux marines et saumâtres en Europe, en particulier en France

In order to be able to count the cases of coastal eutrophication and to propose methods to monitor as well as to reduce these phenomena, first of all it is necessary to precisely define the word eutrophication itself. Instead of the strict etymological definition, that is to say a progression of the enrichment of a medium, we will rather retain the concept of a state enriched at a point such as it causes harmful effects on the ecosystem, and nuisances to man's activities. Based on the average chemical equations of the organic matter synthesis/degradation in the sea, this operational definition thus puts forwards the harmful consequences of enrichment, i.e. the production of an excessive algal biomass, sometimes unbalanced from the point of view of biodiversity, and the more or less severe hypoxia which results from the degradation of this organic matter in excess. The manifestations of coastal marine eutrophication can classically follow two main paths, according to whether the proliferating algae are planktonic or macrophytic ; both these forms are to be found in France. With regard to the phytoplanktonic mass blooms ("coloured water "), some coastal sites, especially the outlet of estuaries, are regularly hit; the only site having reached once at least the lethal anoxia is the Vilaine bay, where a massive kill of fish and benthic invertebrates took place at the end of July 1982. The dilution plumes of the Loire and the Seine rivers exhibit frequent phytoplanctonic blooms during spring and summer, but this induces only a slight hypoxia of the bottom water layer. The coastal waters along the French side of the Straits of Dover show every year, in April-May, large quantities of scum resulting from the proliferation of Phaeocystis. This case represents a particular example of the more general phenomenon of modification of the flora, favorable to the increase in the abundance of not-siliceous species, especially dinoflagellates in summer. Some dinoflagellates producing hazardous toxins for marine organisms (Gymnodinium mikimotoï) or for human consumers of infested shells (Dinophysis sp., Alexandrium sp., Pseudonitzschia sp.) are the target of a regular monitoring program in many sites of the French coast since the end of the Eighties (REPHY network). The geographic distribution of Dinophysis sp. clearly corresponds to the dilution plumes of the 4 main French rivers ; Alexandrium is restricted to specific very coastal, enriched sites, particularly in Brittany and in Mediterranean lagoons; Gymnodinium mikimotoï is concentrated on the western zone going from Brittany to Charente, while the Pseudonitzschia genus is present all over the coasts, but toxic species rarely occur. The massive proliferations of green macroalgae can be found in a recurring way in some Mediterranean lagoons, in the eastern part of the Arcachon embayment and on more than 50 Breton beaches. The associated species belong primarily to the Ulva genus, except in Arcachon, where it is the Monostroma genus which has dominated some years. The Breton sites do not undergo massive deoxygenation due to the decomposition of dead algal mats, thanks to the powerful tidal mixing. On the contrary, some Mediterranean lagoons can locally be damaged by summer anoxias (« malaïgues ») initiated by large amounts of decomposing organic matter, where the green algae can be dominant ; these « malaïgues » are responsible for massive fauna and flora kills. janvier-2001 Copyright The mechanisms which lead to eutrophication, for the macroalgal as well as for the phytoplanktonic forms, are: 1/ a containment of the water mass ; in lagoons, the terrestrial border provides a static containment, whereas in open embayments, hydrodynamic phenomena can create a dynamic containment. Thus, the horizontal trapping of the water masses in the areas with very weak residual current explains the occurrence of green tides in many Breton bays, as well as the phytoplanktonic blooms in the Vilaine bay. Horizontal or vertical density gradients generate also accumulation zones, which appear to be favorable to some dinoflagellates (Gymnodinium mikimotoï, Dinophysis sp.) 2 / a good illumination of the suspended algae : this explains the restriction of Ulva green tides to either lagoons or very shallow clear waters, which can be well mixed by the swell and the wind. In the dilution plumes of rivers (the Vilaine, the Loire, the Seine), the containment of brackish, enriched water in a surface layer limited by a pycnocline contributes to ensure the surface phytoplankton an average illumination which is higher than in well-mixed water columns. 3 / terrigenous nutrient loadings in excess compared to the flushing or the dilution capacities of the site. The nitrogen loadings are known to be responsible for the proliferation of the nitrophilic macrophytes (Ulva, Monostroma, Enteromorpha) and of Phaeocystis sp.: this not-siliceous phytoplanktonic species takes advantage of nitrate in excess remaining after exhaustion of silicate by the spring diatom bloom. In the same way, Alexandrium minutum, endowed with a great capacity for nitrate uptake, seems to grow well in nitrogen highly enriched sites. The control of the phytoplanktonic proliferation in enriched coastal areas is however variable in space and time: in the plume of the Seine or in the Vilaine bay, the limiting nutrient during spring and close to the mouth is currently phosphorus, while nitrogen becomes limiting offshore, especially in summer. The catchment area characteristics (geological background and capability of the ground water to sustain summer flow rate, agricultural practices, industrial and urban wastes) modulate the seasonal variation of the nitrogen loading, whereas the phosphorus loading remains more stable. With regard to the Ulva green tides in Brittany, one can define the concept of « coastal zone sensitive to eutrophication » as being the conjunction of a marine area having a weak hydraulic renewal and a catchment area producing stable flow rates, ensuring a constant nitrate flux from spring to summer. The monitoring of the eutrophication status classically relies on the chemical parameters betraying the degree of abnormal operation of the ecosystem: dissolved oxygen and phytoplankton biomass estimated by chlorophyll a concentration. As one estimates at 5 mg/l the dissolved oxygen content below which the ecosystem begins suffering, and at 2mg/l the level that marks the entrance of the system into serious hypoxia, it is obvious that the eutrophicated plumes of the Seine and the Loire are not yet at the stage of beginning suffering; on the other hand, the Vilaine bay frequently undergoes serious but short hypoxias. As 20µg/l chlorophyll a is considered as the low threshold value for eutrophication in coastal zone, one can say that the plume of the Seine, and sometimes those of the Vilaine and the Loire show clear episodes of eutrophication; the French coastal strip in the Straits of Dover requires for its follow-up the joined chlorophyll measurement and Phaeocystis counts, because this species does not contain much chlorophyll. For macroalgae, it seems that the follow-up of the proliferating species may allow status determination on an increasing eutrophication scale. Mapping of stranded biomasses by airborne photography, or by underwater video for the subtidal zone, remains the only means of evaluating the produced biomasses each summer. Generally, the nutrient concentrations in the coastal sea represent a bad indicator of the real eutrophication status of a site. In lagoons however, specificities resulting from the strong hydrodynamic containment made it possible to propose thresholds for various nutrients in order to define 5 quality stages with respect to eutrophication. Forecasting the evolution of the eutrophication status of some coastal zones is based on the numerical models of biogeochemical cycles. The application of these models to the French Channel-Atlantic coasts shows that the reduction of phosphorus anthropic loadings, for instance in the Bay of Seine, may have caused phosphorus concentrations to be the controling factor of the spring eutrophication events in the river dilution plume. These models also reveal that significant reductions in nitrogen as well as phosphorus loadings would have only little effect on dominant diatom blooms, whereas they would provide a significant decrease in dinoflagellate blooms, which are sensitive to nitrogen loadings.