Sedimentation-induced eutrophication in large river floodplains - An obstacle to restoration?

We found a remarkably strong transversal gradient in the functional floodplain. Extremely high concentrations of CAL-soluble phosphorous (more than 150 mg kg−1) were measured in close proximity to the main channel. These sites were characterized by low vascular plant species richness and a significantly higher proportion of species with high nutrient demands. At a distance of 300 m from the main channel P concentrations in the soil dropped to a quarter of this figure, whereas the average species richness per plot strongly increased. Contrary to our expectations, the influence of the flooding duration was of minor importance for the nutrient status. We can specify that strong eutrophication effects induced by sediment deposits are confined to the close proximity to the main channel within a distance of 200-300 m. At more distant sites, target vegetation of flood meadows was obviously not affected by recent nutrient input in the course of flooding. Concerns by conservationists that the restoration of active floodplains by the removal of dykes will generally lead to unwanted eutrophication by river water and sediments require revision and thorough specification.