1238  2003 Estuarine Research Federation Estuaries Vol. 26, No. 5, p. 1238–1254 October 2003 Nutrient Dynamics in the Pomeranian Bay (Southern Baltic): Impact of the Oder River Outflow MARIANNA PASTUSZAK 1 *, KLAUS NAGEL 2 ,ALFRED GRELOWSKI 1 ,VOLKER MOHRHOLZ 2 , and MARIUSZ ZALEWSKI 1 1 Sea Fisheries Institute, Department of Fisheries Oceanography and Marine Ecology, ul. Kołła ˛taja 1, 81-332 Gdynia, Poland 2 Baltic Sea Research Institute, Sektion Meereschemie, Seestr. 15, D-18119 Warnemu ¨nde, Postfach 301161, D—18112 Rostock, Germany ABSTRACT: The Pomeranian Bay is a coastal region fed by the Oder River, one of the seven largest Baltic rivers, whose waters flow through a large and complex estuarine system before entering the bay. Nutrients (NO 3 - , NO 2 - , NH 4  , N tot. , PO 4 3- ,P tot. , DSi), chlorophyll a concentrations, oxygen content, salinity, and temperature were measured in the Pomeranian Bay in nine seasonally distributed cruises during 1993–1997. Strong spatial and temporal patterns were observed and they were governed by: the seasonally variable riverine water-nutrient discharges, the seasonally variable uptake of nutrients and their cycling in the river estuary and the Bay, the character of water exchange between the Pomeranian Bay and the Szczecin Lagoon, and the water flow patterns in the Bay that are dominated by wind-driven circulation. Easterly winds resulted in water and nutrient transport along the German coastline, while westerly winds confined the nutrient rich riverine waters to the Polish coast and transported them eastward beyond the study area. Two water masses, coastal and open, characterized by different chemical and physical parameters and chl a content were found in the Bay independently of the season. The role of the Oder estuary in nutrient transformation, as well as the role of temperature in transformation processes is stressed in the paper. The DIN:DIP:DSi ratio indicated that phos- phorus most probably played a limiting role in phytoplankton production in the Bay in spring, while nitrogen did the same in summer. During the spring bloom, predominated by diatoms, the DSi:DIN ratio dropped to 0.1 in the coastal waters and to 0.6 in the open bay waters, pointing to silicon limitation of diatom growth, similar to what is being observed in other Baltic regions. Introduction The Baltic Sea is one of the major brackish water basins of the world. For the almost 85 million peo- ple living in the drainage area, which is shared by 15 countries with highly-developed industry and agriculture, the Baltic Sea is also exploited as a huge natural waste-water treatment plant that is ex- pected to cope with discharges of different origin and composition. Since the turn of the century, the Baltic Sea has changed from an oligotrophic clear-water sea into an eutrophic one (Larsson et al. 1985). Many of the sub-regions in the Baltic Sea have become overloaded with nutrients (HELCOM 1998b). In 1995 the inputs of N tot. and P tot. amount- ed to 760,750 t and 37,650 t, respectively, with 90% of N tot. and 81% of P tot. entering the Baltic Sea with rivers (HELCOM 1996a,b). Although nitrogen and phosphorus as such do not pose any direct hazard to marine organisms or people, their excessive in- puts may disturb the balance of the ecosystem. Ex- cessive primary production (Renk 2000), caused by high concentrations of phosphorus and nitrogen * Corresponding author: tele: +48 58 620 17 28 ext. 126; fax +48 58 620 28 31; e-mail: marianna@mir.gdynia.pl (HELCOM 1998b; Łysiak-Pastuszak 2000), has caused an increase in phytoplankton biomass and abundance, especially blue greens (Kahru et al. 1994; Niemkiewicz and Wrzołek 1998; Wasmund et al. 1998; Plin ´ski and Jo ´z ´wiak 1999; Hajdu et al. 2000), the development of anoxic and hypoxic conditions in deeper waters (HELCOM 1996b), and consequent declines in commercially and re- creationally valuable fish species (ICES 2001) in the Baltic Sea. Long-term changes in nutrient discharges in the Baltic region concern not only loads of N and P but also modification of dissolved inorganic nitro- gen (DIN): dissolved inorganic phosphorus (DIP): dissolved silicate (DSi) ratio. A decrease in DSi: DIN ratio has been reported for the Baltic Sea by Rahm et al. (1996) and the first hints of DIN:DIP modification of the ratio were noticed in the Pom- eranian Bay in 1989–1993 (Trzosin ´ ska and Łysiak- Pastuszak 1996). The Pomeranian Bay—together with the Szcze- cin Lagoon, the Oder River branches, and a couple of lakes—form a huge complex estuarine system supplied by the Oder River, one of the seven larg- est rivers flowing into the Baltic. The annual nu-