Storage reservoirs play various roles in the circulation of biogenic substances in river and reservoir systems. Research into the infl uence of rubble (bottom deposits) was done in 2004 and covered the Pokrzywnica reservoir (max. area 154 ha, max. capacity 4.35 M m³) in the Pokrzywnica river catchment in the Wielkopolska Lowland. The scope of the research included determination of the physicochemical composition and the oxygen indexes of water above the bottom and on the surface; preparation of a bathymetric plan; determination of the granulometric composition of rubble, the chemical composition and ChZT-Cr of bottom deposits, the mean concentrations (oxygen indexes) of chemical components of infl owing and outfl owing waters, and the accumulation of total phosphorous and nitrogen in the reservoir. It was found that the considerable adsorption of clay minerals (particularly colloidal clay), delivered to the reservoir together with the suspended sediment load (varicoloured clays, so-called Poznań-type clays), and of humus substances (colloidal humus) delivered especially in the case of a fl ood wave, reduces the concentrations of biogenic substances in waters while increasing their contents in bottom deposits (sedimentation). For shallow reservoirs, biogenic substances accumulated in bottom deposits constitute a source of secondary pollution of the reservoir’s water (“internal” supply). The values of oxygen indexes and concentrations of biogenic compounds (and less frequently total nitrogen and Kjeldahl nitrogen) in the Pokrzywnica reservoir are higher above the bottom than in the surface water.

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