Budget calculations of nitrogen, phosphorus and BOD5 passing through the Oder estuary

The Oder River estuary is a large and complex system composed of lagoons, lakes and river branches in which numerous biogeochemical processes lead to modification of loads of dissolved/suspended material brought in with the riverine waters. Budget calculations show that on an annual basis, 71–88% of total nitrogen, 73–89% of total phosphorus and 72–101% of BOD5 inflowing to the estuary are exported to the Baltic Sea. Among the inorganic nutrient species, nitrates exhibit the highest net transformation rate into organically bound forms (over 60%). The transformation could have been equally high or even higher in the case of ammonia and phosphates but these processes may have been compensated by intensive mineralization. The mechanisms responsible for the nutrient transformation patterns, as well as their net effect on the annual loads delivered into the Baltic Sea, are discussed in the paper. Phosphorus seemed to play a limiting role in phytoplankton production in the estuary in spring, while nitrogen did the same in summer.