Phosphorus balance of a polytrophic shallow lake with the consideration of phosphorus release

In the course of an one-year investigation (April 1994 to March1995) the phosphorus balance of a polytrophic shallow lake withconsideration of phosphorus release (P-release) was studied. Theinvestigated Lake Blankensee covers an area of 2.9 km2 with anaverage depth of 1.2 m (volume 3.43 106 m3).The averagesummer chlorophyll-a concentrations range from 104 to418 µg l−1 (1992–1995). Secchi-depths from 0.2 to0.3 mare typical in summer. Massive algae blooms occur frequently andsubmerse macrophytes can no longer exist. The trophic state ispolytrophic.In summer phosphorus concentrations in the lake were controlled bythe internal processes, in winter by the inflowing River Nieplitz.The increasing total phosphorus concentrations and the solublereactive fraction of phosphorus in the lake water from winter tosummer are partly a function of net internal loading.According to the detailed chemical studies which are representativeonly for this investigated period the Lake Blankensee acts as aphosphorus source, which also affects the down-stream lakes and theriver system. The P-balance is determined by the river input of16.84 103 kg P and by an output of 20.71 103kg P.Therefore the export of about 3.87 103 kg P characterises thenegative P-retention rate due to the strong internal loading duringsummer.More than 60% of the sediment phosphorus is bound to theiron-redox system. Therefore it indicates a high potential mobilityof phosphorus under anaerobic conditions in the sediment surfacezone in summer during special weather conditions. This process islimited due to the high Fe:P ratio of about 21 (by weight) in thesurface sediments of the Lake Blankensee (0–5 cm).