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, l akes and river branches in which numerous biogeochemical processes lead to modification of loads of dissolved/suspended material brought in with the r iverine waters. Budget calculations show that on an annual basis, 71-88% of total nitrogen, 73-89% of total phosphorus and 72-101% of BOD, inflowing t o the estuary are exported to the Baltic Sea. Among the inorganic nutrient species, nitrates exhibit the highest net transformation rate into organica lly bound forms (over 60%). The transformation could have been equally high or even higher in the case of ammonia and phosphates but these processes m ay have been compensated by intensive mineralization. The mechanisms respon sible far the nutrient transformation patterns, as well as their net effect on the annual loads delivered into the Baltic Sea, are discussed in the pa per. Phosphorus seemed to play a limiting role in phytoplankton production in the estuary in spring, while nitrogen did the same in summer. (C) 2000 E lsevier Science B.V. All rights reserved.