Abstract
The Oder River creates one of the largest estuaries in Poland. The estuary can be divided into three zones, each with a different hydrochemical regime: the Pomeranian Bay (salt water habitat), the Szczecin Lagoon (brackish water habitat), the Oder River up to the Widuchowa section (transitional environment - brackish and typical freshwater). The Pomeranian Bay was characterized by the lowest concentration of nutrients throughout the research series in comparison to the other regions. It had a low nitrogen to phosphorus proportion and the highest stability of variability in total nitrogen concentration. The Szczecin Lagoon was characterized by a higher concentration of nutrient loads in comparison to the Pomeranian Bay, by a lower nitrogen and phosphorus proportion than the Oder waters, and by the high stability of variability of concentrations of mineral nitrogen and total nitrogen. As the "nutrient trap" for the Oder estuarine system, the Oder River, with its highest nutrient concentration, is characterized by the highest stability of variability of total nitrogen concentration.