Hypolimnetic withdrawal increases the P export since nutrient rich hypolimnetic water instead P poor epilim netic water is removed from the lake. Continuous application leads to a new steady state at a lower P value, and the adaptation time following external P load reduc tion is shortened. Hypolimnetic withdrawal improves the redox conditions in the lake, as the hypolimnetic water shows a high oxygen deficiency and an accumulation of reduced substances (e.g., Mn2+, NH|, Fe2+, H2S).
Scientific background. The main preconditions for the application of this method are a stable stratification during summer and an enrichment of P in the hypolimnion caused by release from the sediment or by sedimentation of par ticles from the epilimnion. The efficiency depends on the water retention time and the differences in P concentra tion between the epilimnion and hypolimnion. Hypolimnetic withdrawal can force the P release from sediments so that temporarily stored P is exhausted sooner.
Techniques. A withdrawal pipe is installed near the dee pest point of the lake. The outlet is usually situated below the lake level, so that the device acts as a siphon. Pumping stations are necessary when the outlet is higher than the lake water level or when the pumped water is used for irrigation or is processed in a treatment plant. The introduction of the nutrient rich, oxygen depleted, and strong smelling water into downstream waters has to be considered carefully. In reservoirs the hypolimnetic withdrawal can be achieved with a variable deep water outlet in the dam.
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