Hydrochemical studies were performed from May to December 2004 at four stations in the polymictic, polyhumic Siemianówka Dam Reservoir (SDR) located on the upper Narew River in northeastern Poland. The total manganese (TMn) concentrations in the reservoir waters varied from 118 µg/dm₃ to 638 µg/dm₃, and their increased values are caused by forming complexes with organic matter present in waters flowing from wetlands and forests. Until the moment when the maximum manganese concentration occurred in the reservoir, its upper part was characterized by the higher manganese concentration than the lower part. A long retention time of water delays the occurrence of total manganese maximum in the lower part in SDR. Maximal TMn concentrations were noted in the lower part of the reservoir in the end of summer, while minimal for the upper part in winter. The organic dissolved manganese fractions (ODMn), were the largest part of TMn, slightly less was the particular manganese (PMn) fraction, and the smallest part constituted the reactive dissolved manganese fraction (RDMn). Low concentrations of reactive manganese in winter are due to a low pH value and low concentrations of dissolved organic matter (DOC). Manganese sorption on colloids increases in summer and is accompanied by a pH increase during an intensive photosynthesis of algae and cyanoprocaryota.