Skuteczność różnych metod rekultywacji jezior w celu poprawy czystości ich wód

Kajak, Zdzisław

doi:ISSN 0013-2969

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Title: Skuteczność różnych metod rekultywacji jezior w celu poprawy czystości ich wód

Abstract:

In general, allochthonus loading of phosphorus to lakes is positively correlated with concentrations of phosphorus and of phytoplankton in waters. However, the dispersion of data for particular lakes is quite large (Fig. 1). The improvement of water quality of moderately rich lakes can be achieved by decreasing the external loading of phosphorus to the value of “permissible loading” acc. to Vollenweider. This should decrease the concentration of total phosphorus, and in consequence also of phytoplankton below blooming level. However, if a lake is rich to an extent when the internal loading results in too high concentration of phosphorus in water (Fig. 4, Tables III, IV), decreasing the external loading of phosphorus would not improve the quality of water. Thus, the control of external loading is not always sufficient, but always necessary for improvement of water quality with any kind of recultivation method.The main pool of lake phosphorus, about 90%, is cumulated in bottom sediments (if only 10 cm surface layer is taken into account), its content in water with seston and in macrophytes as well as in fish does not exceed few percent of phosphorus in the whole ecosystem (Table II).The mam pool of lake phosphorus, about 90%, is cumulated in bottom sedi- waters; 90% or more of allochthonous phosphorus may be sedimented. The construction of settling reservoirs, where the majority of phosphorus and suspended matter is traped. above the main reservoir results in better quality of water in the latter. Sedimentation of phosphorus from surface waters by chemical means (compounds of iron, aluminium, calcium) or physical means (clay), although theoretically possible, is difficult and expensive in application. A large part of ses- ton and phosphorus contained in it sediments by itself (in form of faeces or as dead organisms and inorganic particles), but at present this process can not be controled.The removal of top layers of bottom sediments (usually with the highest phosphorus content) improves sufficiently quality of waters, but it is complicated, expensive and seldom applied method. The removal of hypolimnion waters, also rich in phosphorus, is much simpler and quite efficient. The removal of a 8 m layer of hypolimnion with concentration of phosphorus 1 mg-1-1 will result in removal of 22 g of P-m-2, thus about 1/3 of its content in 10 cm layer of sediments with 95% of water content and 0.4% of phosphorus in the dry mass. Such an amount is of the order of magnitude of the highest annual loading from the drainage basins. However, oligotrophication by the removal of hypolimnetic waters lasts longer than in the case of sediments removal, because with decreasing phosphorus concentration in sediments and in near bottom water, smaller and smaller amount of phosphorus are removed from a lake.The through-flow polymictic lakes, even very rich, will gradually turn toward lower trophy after stoppage of external loading, as phosphorus from sediments will intensively migrate to waters (Fig. 5) and will be exported from the lake.Aeration of hypolimnion and of bottom sediments efficiently precipitates and stores phosphorus in sediments if the applied method does not warm up the hypolimnion causing various processes of phosphorus liberation. A method of oxygenation of sediments by nitrates (Fig. 6) is very interesting, but was proved till now in only few situations.The amounts of phosphorus which can be removed by exploitation of macrophytes and fish are rather small (Tables II, V). […]