Nutrient concentrations and their annual/interannual variability in the Szczecin Lagoon are shaped by variable riverine water/nutrient discharges and bio-geochemical processes in the Lagoon which modify the dissolved/suspended matter introduced. On an interannual scale, nitrate and phosphate concentrations in the Lagoon in 1969-97 displayed quite considerable variability, with maxima of NO3 coinciding with minima of PO4 concentrations. Both NO3 concentrations in the Lagoon and total nitrogen loads showed a strong positive correlation with the water volumes discharged (r = 0.73 and 0.82, respectively). This dependence was much weaker and negative for PO4 (r = - 0.51), and weak and positive for total phosphorus loads (r = 0.35). It would appear, therefore, that in the catchment area of the Oder River, nitrogen sources are mainly diffuse, while phosphorus is derived from point sources. There was a considerable predominance of nitrates in the inorganic nitrogen pool, but the presence of ammonium in large concentrations in winter and summer was also evident. Among the inorganic nutrient species, nitrates exhibited the highest transformation rate into organically bound forms (over 60%) while passing through the Lagoon. The mechanisms responsible for the nutrient transformation patterns in the Lagoon - temperature, in particular - are discussed in the paper. Phytoplankton production in the Lagoon appeared to be phosphorus-limited in spring and nitrogen-limited in summer.