Sediment phosphorus stock shows a shift at the river – floodplain interface, accompanied by high vegetation biodiversity

Floodplains can contribute enormously to nutrient reduction in streams. The phosphorus cycle at the aquatic–terrestrial interface is driven by hydrology and vegetation. Our aim was to assess conditions relevant to the phosphorus cycle prior to a floodplain restoration. The phosphorus cycle was studied by means of measuring several phosphorus fractions and adsorption/desorption experiments using floodplain soil and river sediment. The study area was located at the Mulde River near Dessau, Germany, and covered different inundation patterns, vegetation and reaches with or without embankment. A shoreline ecotone was identified by means of high vegetation biodiversity with a distinct plant community. In the ecotone, the P cycle was influenced by accumulation of relatively high bioavailable P in the soil (equilibrium phosphate concentration, EPC0) and reduction of the soluble reactive phosphorus (SRP) concentration in the pore water. Both suggested a high productivity of the vegetation. The ecotone also acted as a delineation between the stream sediments with low organic matter and inorganic P and the floodplain soil with high organic matter and inorganic P. Additionally, the study demonstrated a lower SRP concentration and EPC0 in the parts of the floodplain without bank fortification.

Since floodplains were considered ecotones before, we identified another ecotone at another scale, i.e. between floodplain and river. The ecotone does not only show the area with a favourable ratio of disturbance and resource availability but also acts as a location for biogeochemical exchange processes between rivers and floodplains. The identified shoreline ecotone offered a habitat for a specifically diverse vegetation, which itself influenced the P cycle by high biological turnover. As a highly biogeochemically active part of the floodplain, the shoreline ecotone could help in mitigating high nutrient loads in anthropogenically impacted water sheds and provide a habitat for diverse vegetation.