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Nutrient loads to the Belgian coastal zone

par Brion, N.;Jans, S.;Chou, Lei ;Rousseau, Véronique
Editeur scientifique Rousseau, Véronique ;Lancelot, Christiane ;Cox, D.
Référence Current status of eutrophication in the Belgian coastal zone, Presses Universitaires de Bruxelles, Bruxelles, page (17-43)
Publication Publié, 2006

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Résumé :

Anthropogenic eutrophication in coastal environment results from increased delivery of land-based nutrients considerably enriched in nitrogen (N) and phosphorus (P) compared to silicon (Si). These nutrient inputs strongly modify the nutrient balance N:P:Si of coastal waters with respect to phytoplankton stoechiometry, i.e. N:P=16 for marine phytoplankton (Redfield et al., 1963) and N:Si=1 for coastal diatoms (Bzrezinski, 1985). This in turn modifies the composition of the phytoplankton community characterized by a dominance of opportunistic non-siliceous species (e.g. Officer & Ryther, 1980; Billen et al., 1991). Coastal waters are enriched by nutrients delivered by rivers and canals, coastal tributaries, atmospheric deposition, and advection from adjacent areas (Fig. 2.1). River nutrient loads are largely influenced by human activity and depend on the population density in the watershed but also on environmental drivers such as land use and agriculture practices, industrialization, and waste water treatment management (Fig. 2.1). Nutrients are released to surface waters from point sources as domestic and industrial effluents (mainly NH4 + and PO4 3-), and diffuse sources through soil leaching and ground water contamination by fertilizers and manure spreading (mainly NO3 - and Si(OH)4 ). Once released in the river system, nutrients are involved in physico-chemical and biological processes leading to their transformation, retention or elimination during their transfer along the aquatic continuum (Fig. 2.1; Billen et al., 1991). Atmospheric deposition occurs mostly as N oxide (NO, N2O, NO2) originating from industrial and traffic combustion processes and as ammoniac (NH3) resulting of animal breeding and manure spreading. Once in the atmosphere these gasses are transformed and transported with the air masses before their wet or dry deposition onto coastal areas (Spokes & Jickells, 2005). Nutrient enrichment of the Belgian coastal zone (BCZ; Fig. 2.2) results from local riverine inputs of the Scheldt, the IJzer and the coastal tributaries, from atmospheric deposition and from transboundary fluxes brought by the Southwesterly Atlantic waters enriched by the rivers Seine, Somme, Authie and Canche, and Rhine (Lacroix et al., 2004). The relative importance of these different nutrient sources varies depending on change in human activity in the watersheds and on the North Atlantic Oscillation (NAO) which determines the weather conditions over North-western Europe and the hydrological budget of BCZ (Breton et al., 2006). This chapter synthesizes the information on N, P and Si delivery to the BCZ and resulting enrichment of coastal waters. It compares the present situation with available historical data in order to evaluate their long term changes. Quantitative changes in riverine loads are analysed in relationship with human pressure and biogeochemical transformations within the aquatic continuum. Qualitative changes in nutrients are also considered, in particular changes in the N:P:Si molar ratio determining the limiting element for phytoplankton growth and triggering harmful algal blooms (Billen & Garnier, 1997; 2007).