Release of nitrogen and phosphorus from macrophyte stands of summer dried out sediments of a eutrophic reservoir

Spremberg Reservoir, Lusatia, Germany, is a linear reservoir which impounds the lowland River Spree to a length of 8km. Due to low summer runoff and continuous extraction from it for public water supply ca. 21 % of the total area (6.8 km 2 ) dried out from May/June to October/November in 2000. The seasonal P and N accumulation and release by the macrophytes growing on these areas were investigated. Two major groups of macrophyte stands, dominated either by water smartweed Polygonum amphibium L. (50-100 %) or by marsh cudweed Gnaphalium uliginosum L. (50-75 %), were distinguished. In September 2000, their above-ground biomass was harvested from from 15 representative 5 x 5 m plots. It varied between 34-420 g d.w. m -2 for P. amphibium and 76-120 g d.w. m -2 for G. uliginosum. Plant elemental composition, incubation of undisturbed sediment cores and litter bag exposure showed that the mean P accumulation rate of both stands (7.2 mg Pm -2 d -1 ) was less than the release rate (9.7 mg P m -2 d -1 ), whereas the N accumulation rate (57.4 mg N m -2 d -1 ) was higher than the release rate (43.4 mg N m -2 d -1 ). The temporary storage of 0.52 t P and 6.51 t N by the macrophyte stands was compensated by release rates orders of magnitude higher than the corresponding benthic release rates (0.03-0.06 mg Pm -2 d -1 , 1.03-2.08 mg N m -2 d -1 ) influencing the overall water quality of the reservoir periodically. Riparian zone fluxes apparently added little to the reservoir-wide internal loading since the high Fe supply guarantees an efficient P retention in the sediments.