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The littoral sediment of Lake Constance, Germany, is influenced by varying light conditions and mechanical perturbation. This study documents the effects of these changes on phosphate exchange between sediment and lake water. The concentration of total phosphate measured in sediment-near water was 50 ± 50 μg total P L-l in early spring, and 16 ± 9 μg total P L-l in late summer. In the interstitial water of the sediment, the dissolved orthophosphate content was 17 ± 2 mg PO4 - P L-l in both profundal and littoral sediment, and the total phosphate was 163 ± 4 mg total P L-1 and 79 ± 2 mg total P L-1 in profundal and littoral sediment, respectively. The release and uptake of phosphate compounds was measured with sediment cores incubated under gently flowing water under the influence of light, erosion, and sedimentation. In all experiments, phosphate was released to the overlying water by sediment sampled in late summer, but was taken up from the overlying water by sediment sampled in early spring. Illumination of the sediment surface enhanced the uptake of orthophosphate and total phosphate in early spring, and decreased the release of total phosphate in late summer. Erosion of the sediment surface did not enhance phosphate release or uptake. After exposure to light or dark and after erosion of surface sediment, both uptake and release were nearly constant over time. By resuspension of the eroded sediment, more orthophosphate was released from sediment sampled during early spring than from sediment sampled during late summer, but the amount of total P released by the sediment was about the same at both times of the year. During the first hours after re-sedimentation of the eroded sediment, the phosphate release from sediment sampled in late summer was enhanced and the phosphate uptake from sediment sampled in early spring was reduced. Our results document that the uptake and release of phosphate in littoral sediments is influenced by the season and differs between early spring and late summer. On the short time scale, the specific rates of uptake and release are influenced by light-dark changes and by mechanical perturbation.
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