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Łukasz Bryl*,**, Tadeusz Sobczyński*
*Adam Mickiewicz University in Poznan, Poland; **PROTE Technologies for our Environment, Ltd., Poland
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Abstract
The role of phosphorus contained in bottom sediments of water reservoirs is crucial in the whole eutrophication process. It is an element responsible for the proper functioning of aquatic ecosystems, and its excess leads to deterioration of water quality. The release of phosphorus from bottom sediments directly into the water depends on many factors. This process is particularly intensified at the moment of anaerobic conditions occurring in the bottom layer, or the phenomenon of resuspension of sediments due to external factors, such as intense waving. The persistence of phosphorus deposition in sediments also depends on the nature of chemical connections and changing oxidation-reduction conditions, temperature and pH.

Bottom sediments from eight selected lakes of Wielkopolskie Lakeland were investigated for their total phosphorus content and the possibility of its re-release into water. The subject of the study were the following lakes: Wolsztyńskie, Winiary, Jelonek, Siekiera, Pniewskie, Umultowskie, Góreckie, Strykowskie. The investigated water reservoirs are glacial lakes, usually shallow and characterized by large variations in water level.

The samples of bottom sediments were collected from profundal zone with a tube probe from the surface layer of 10 cm thickness, i.e. the layer that exchanges matter with the surrounding water. The collected sediments were placed in sealed, plastic containers to their full capacity. Before proceeding to carry out detailed laboratory analyzes, samples were prepared accordingly. First, the bottom sediments were thoroughly homogenized. Then, they were dried and subjected to further testing. The obtained results were statistically analyzed in the program Statistica 6 PL.

Phosphorus was fractionated by means of a sequential extraction described by Psenner. The sediments were also analyzed for the content of phosphorus binding components such as organic matter, iron, aluminium and calcium. Phosphorus was found to be bound mostly to organic matter, aluminium and calcium – the fractions from which it was difficult to release into water. The share of exchangeable and most mobile and bioavailable iron-bound fraction of phosphorus was the lowest but it was still high enough to cause phytoplankton blooms. No clear relationships between the content of individual phosphorus-binding components and the share of phosphorus fractions related to them in total phosphorus pool were identified. Total content of phosphorus in the sediments of the investigated lakes was analogue and similar to that determined in bottom sediments of other eutrophic lakes of Wielkopolskie Lakeland. The content of aluminium, calcium, iron and organic matter were much more variable.

Keywords 
lake, bottom sediments, phosphorus, fractionation of phosphorus, organic matter, iron, aluminium, calcium

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