Assessment of the long-term effectiveness of sediment dredging to reduce benthic phosphorus release in shallow Lake Muggelsee (Germany)

Two series of laboratory experiments mimicking dredging of the uppermost ph osphorus (P) rich sediment layers of the shallow eutrophic L. Muggelsee wer e carried out to study the extent of P release from deeper sediment layers, and changes in P mobility by means of fractional P composition of the in 1 0 cm steps 'dredged' sediment cores. In the first run over 38 days, the aer ated controls reached 55%, and the 'dredged' cores from 18.1% (-10 cm) down -core to 0.4% (-50 cm) of the non-aerated control (50.82 mg P m(-2) d(-1) = 100%). In the second run over one year, the fractional P composition in th e revealed sediment layers changed slightly. The water-soluble P (H2O-P) in creased for the respective 'dredged' horizon by between 1.5 and 5.6% TP. Th e redox-sensitive P (BD-P) increased in each horizon from the intact core t o the situation following 'dredging', as well as with depth in each horizon f rom 4.9% TP (-10 cm) to 11.4% TP (-40 cm). The organic bound P portion ( NaOH-NRP) decreased least (1.7% TP) in the uppermost layer and most (15.6% TP) in the deepest horizon exposed to water after 'dredging'. Based on the changes in P pools following dredging at the future sediment-water interfac e, it is predictable that dredging without reduction of the external loadin g may give only temporary improvement followed by a slow return to the pres ent situation.