| | | 6.2 Introduction |
The functioning of aquatic ecosystems is strongly conditioned by the levels and proportions of nitrogen, phosphorous and silica in surface waters. They are themselves determined by the lithology and the land use of the watershed. The problem of eutrophication resulting from nutrient excess is wide-spread throughout the north-western Europe and the northern Gulf of Mexico where the population density is especially high and the agriculture intensive [][]. In north-western Europe, most of the large rivers suffer from eutrophication []: the Elbe, the Rhine, the Scheldt, the Seine, the Loire, the Garonne, etc. Since the 1960s, the role of nutrients as factors controlling the trophic state of aquatic systems has given rise to numerous discussions and research efforts; however, the research has often been focused mainly on lake environments. The interest in river nutrient supply to eutrophication is more recent because rivers have been seen as systems capable of transporting, purifying and eventually, evacuating downstream all the pollutants discharged into them. Similarly, the link between the eutrophication of coastal zones and the fluxes from the watershed basins is a relatively new line of research [][][347][430]. Eutrophication of large river is linked to the development of microscopic planktonic algae, as opposed to eutrophication by macrophytes in small ones. Because they have a high growth rate, phytoplankton organisms respond immediately to environmental constraints and are therefore an excellent reference indicator of water quality, both by their biomass and their composition. In critical eutrophication conditions, algal bloom is often a great nuisance to river and coastal zones, preventing normal water use particularly for drinking purposes, recreation, shell fish production, etc. [].
All along the hydrographic network and in its stagnant annexes such as ponds or reservoirs, mechanisms of retention or elimination of nutrients are at work, greatly modifying the fluxes that are transferred downstream. The riparian zones that form the interface between the soil of the watershed basin and the surface water act as very efficient filters for nitrates of agricultural origin [][]. Since nitrogen, phosphorus and silica have different origin and circulate in different ways, the ratios between these elements change considerably from the upstream to the downstream sectors of the hydrographical network. These ratios should be compared to the average composition of the algal biomass which determines the proportions of nutrients taken up by phytoplankton. The mean N:P ratio of aquatic plants is on the order of 7 (in N and P weight) []; the mean Si:N ratio of diatom populations in freshwater is 5.5 (in Si and N weight) whereas it is about 2 for marine diatoms []. Whereas phytoplankton bloom intensity and duration is mainly driven by the nutrient loading, the composition of the phytoplankton blooms depends more on their ratios. In strongly human impacted systems, silica issued of diffuse natural origin, from weathering of the rocks, may be limiting compared to the phosphorus and nitrogen leading to a shift from diatom blooms to undesirable non-diatom blooms (often mucilaginous Phaeocystis and/or toxic Dinophysis at the coastal zone [][][].
Although a great number of lake systems have been successfully restored by reducing point discharges of nutrients (domestic discharges), rehabilitating rivers is more complicated. On the basis of the river continuum concept or RCC [], the river, from the headwaters to the estuary, can be represented as a succession of interdependent systems, which means that to quantify the nutrient load and understand eutrophication phenomena at any point in the river, one must take into account the processes occurring upstream.
| | | 6.2 Introduction |