Riverine water transformation during retention in small lowland reservoirs

Six small dam reservoirs in northeastern Poland, with different hydrologic and catchment characteristics were selected for research from March to October (2001-2003). There was a significant correlation between DOC and TP in the reservoirs and the water retention time (WRT). The highest values of electrical conductivity (EC) were recorded in reservoirs with an agricultural basin. In almost all reservoirs a statistically significant decrease of EC, compared to inflowing waters, was noted. Nitrates were substantially reduced in afforested catchment reservoirs. The highest TP concentration was recorded in a newly flooded reservoir. In reservoirs with long retention time, phosphorus sedimentation takes place more intensively. This caused phosphorus reduction in outflowing waters.