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Aquatic Microbial Ecology

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AME 13:3-17 (1997)  -  doi:10.3354/ame013003

The Phison River plume: coastal eutrophication in response to changes in land use and water management in the watershed

Billen G, Garnier J

Human impact on the coastal zone is mainly exerted through riverine nutrient delivery, dependent on land use and management of the watershed. The link between human activity and coastal eutrophication is not direct, however, because of the complexity of the processes involved in the retention or elimination of nutrients during their transfer along the aquatic continuum from land to sea. In order to explore this link, we make use of 2 idealized models respectively representing the ecological functioning of whole drainage networks including their stagnants annexes (the Riverstrahler model) and of river impacted coastal systems (the Zoco model). Coupling of these 2 models, which use exactly the same kinetic formulation for all microbiological processes with only slightly different parameter values, allows simulation of the major trends of the seasonal variations of coastal algal blooms in response to changes in land use in the watershed. The models were applied to a hypothetical river-coastal zone system (the 'Phison River' and its plume in 'Eden Bight'), for a chronological succession of land use and management scenarios, ranging from the pristine forested watershed to the dense urbanization conditions of Western European industrialized countries. The results show that, as early as the beginning of the 19th century, development of intense blooms of non-diatoms, accompanied by a shift from phosphorus limitation to silica/nitrogen limitation, might have occurred in the coastal zone. In the second half of the 20th century, increased use of nitrogen fertilizers, coming after the suppression of wetlands and buffer strips along rivers, might have again pushed the coastal system to phosphorus limitation, while the introduction of phosphorus-containing washing powders (from the mid 1960s) again reversed this trend. Under the chosen conditions of land use and population density, it was found that elimination of nitrogen from wastewater through generalized tertiary treatment would not produce very significant changes in the eutrophication status of the coastal zone. On the other hand, phosphorus removal from point sources, preferably combined with wetlands restoration, would lead to a drastic reduction of algal blooms with a new shift to phosphorus limitation.

Eutrophication · Riverine delivery · Silica · Nitrogen · Phosphorus · Land use

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