The Danube River has been turned into a series of lakes with the "Iron Gate" hydroelectric plant (Romania) being the largest impoundment on the river (Petrovic, 1996). At the same time, the Danube watershed is highly populated and industrialized and therefore the river receives considerable loads of nutrients with agriculture as the main source. These high loads of phosphorus and nitrogen lead to extensive diatom blooms in the reservoirs of the Iron Gate. With the sedimentation of these blooms, a significant quantity of dissolved silicate is removed from the reservoir system. Based on biogenic silicate concentrations in the sediment (Reschke, 1999) and the total amount of sediment trapped behind the dam (Panin et al., 1999), a biogenic silicate retention of about 500 kt Si per year can be estimated. This value agrees with what was estimated by Humborg et al., (1997) as "missing" silicate in the coastal Black Sea since the closure of the Iron Gate in 1975. This is an important part of dissolved silicate transported by the Danube. Between 1988 and 1992 about 300 kt Si per year reached the Black Sea.

While the Iron Gate reservoir seems to be an important sink for dissolved silicate, there are no data available on P or N retention within the impoundment. N/Si and Si/P ratios measured in the water column during a summer cruise in 1995 indicate that N and P are depleted, just as dissolved silicate. The nutrient ratios were 0,5 for N/Si and 200 for Si/P (Reschke 1999). At the river mouth N/Si values of 4-5 were reported (Humborg, 1995; Garnier et al., in press). P and N are supplied below the dam through agricultural activities and wastewater effluent whereas no significant source exists for Si (Brunner, 1997; Friedl et al., in press). As a result of the apparent depletion of dissolved silicate in the riverine delivery, average dissolved silicate concentrations measured in the Black Sea, offshore the Danube Delta, decreased from 55 uM before the closure of the Iron Gate to around 20 uM today. The frequency of diatom blooms over the last decades has decreased and dinoflagellates and gelatinous species have become more important (Humborg et al., 1997). As a result of the shift in phytoplankton communities, the food chain of the NW Black Sea has changed leading to a drastic decline in fishery since the 70s. Historically the Black Sea has been one of the most fish productive marine regions in the world. Since the 70s number and quality of fish catches declined. Whereas 30 years ago economical valuable mackerels and bonito were caught, today's catches consist only of anchovies (Tolmazin, 1985).

Cociasu, A., L. Dorogan, et al. (1996). "Long-term ecological changes in Romanian coastal waters of the Black Sea." Marine Pollution Bulletin 32: 32-38.

Garnier, J., G. Billen, et al. (2002). "Modelling transfer and retention of nutrients uin the drainage network of the Danube river." Estuarine, Coastal and Shelf Science 54(3).

Humborg, C., V. Ittekkot, et al. (1997). "Effect of Danube River dam on Black Sea biogeochemistry and ecosystem structure." Nature 386(27): 385-388.

Panin, N., D. C. Jipa, et al. (1999). Importance of sedimentary processes in environmental changes: Lower River Danube- Danube Delta - Western Black Sea System. Environmental Degradation of the Black Sea: Changes and Remedies. S. T. Besiktepe, U. Unluata and A. S. Bologa. Dordrecht, Kluwer academic Publishers. 56: 23-42.

Reschke, S. (1999). Biogeochemische Variabilitäten in der Schwebstofffracht der Donau und deren Einfluß auf das Sedimentationsgeschehen im nordwestlichen Schwarzen Meer, Hamburg.

Tolmazin, D. (1985). "Changing coastal oceanography of the Black Sea. I: Northwestern shelf." Progress in Oceanography 15: 217-276.