Sink or source? - The effect of hydrology on phosphorus release in the cultivated riverine wetland Spreewald (Germany)

The cultivated riverine wetland region Spreewald faces detrimental changes in the hydrological conditions due to a significant discharge reduction. With its dense network of impounded waterways and a forced tendency of sedimentation of soluble reactive phosphorus adsorbed to large amounts of FeOH/FeOOH available from mining water and groundwater discharges the 320 km² region is favoured to accumulate large amounts of total phosphorus (TR) and thus act as an effective phosphorus sink. The change of conditions strongly challenges this function hereafter. This is especially important because eutrophication of lakes downstream the Spreewald region is controlled by phosphorus. Phosphorus balances at a testfield situated in a polder area typical for the central Spreewald region point out that hydrological and consequently hydraulic conditions are the key factors for the phosphorus sink or source behaviour. This is true for the main processes determine P retention and release at the sediment-surface water transition zone as well as for the dominant phosphorus release and retention pathways: groundwater emissions and sedimentation. In the context of hydrological changes in the Spree river catchment results from point scale and river reach scale point out the need for an adapted water management in the Spreewald region to prevent risk of extended eutrophication tendencies downstream due to forced SRP emissions.