The main aim of this case study was to estimate the nutrient concentration variability in the Szczecin Lagoon. This amount was calculated with use of the basic knowledge of some variable riverine water and nutrient outflow and biogeochemical processes in the Lagoon. On the interannual scale nitrate and phosphate concentrations measured in the Lagoon showed quite considerable variability, with maxima of NO3 coinciding with minima of PO4 concentrations. Concentrations of NO3 in the Lagoon showed a strong positive correlation with the volumes of water discharged, this dependence was much weaker and negative for PO4. That speaks for mainly diffuse sources of nitrogen and point sources of phosphorus in the catchment basin of the Oder River.
Riverine water discharges, strongly positively correlated with total nitrogen loads and rather weakly correlated with total phosphorus loads, showed substantial variability over the years 1969-1997; annual volumes of water feeding the Lagoon ranged from 9.6 to 26.5 km³*year-1. Annual loads of Ntot. and Ptot. introduced to the Oder Estuary over the years 1987-1998 varied from 40*10³ to 100*10³ tonnes N, and from 4.5*10³ to 8.5*10³ tonnes P. There was considerable predominance of nitrates in the pool of inorganic nitrogen, but the presence of ammonium in great concentrations in winter and summer is also evident.
Among the inorganic nutrient species nitrates exhibit the highest transformation rate into organically bound forms (over 60 %) while passing the Lagoon. The transformation in the case of ammonium and phosphates seemed to be balanced by intensive processes of mineralization. The mechanisms responsible for nutrient transformation patterns in the Lagoon (in particular role of temperature) are discussed in the paper. Phosphorus seemed to play a limiting role in phytoplankton production in the Lagoon in spring, while nitrogen did the same in summer.
Future studies on the role of the Lagoon in transformation/transport of the dissolved/suspended matter introduced should include biogeochemical processes taking place in the Lagoon and on mathematical modelling allowing a quantitative evaluation of water/material exchange between the Lagoon and the Pomeranian Bay.
The transformation of Polish agriculture into intensive production (larger farms, greater use of fertilizers) may result in increased discharges of nutrients by Polish rivers, and future studies in the rivers' catchment areas should include these aspects.