Sustainable management of natural water resources should include environmentally sound dam construction and operation with respect to both upstream and downstream management. Because of slowly evolving alterations in riverine ecosystems following the construction of a dam -due to the sometimes large distances between dams and affected areas, and the interference with other anthropogenic activities -some of the effects of damming may be overlooked. Constructing reservoirs modifies the biogeochemical cycles, such as interrupting the flow of organic carbon, changing the nutrient balance, Aquat. Sci. 64 (2002) 55 -65

Benthic fluxes of nutrients and metals were measured in the coastal zone of the north-western Black Sea, which is influenced by the Danube and Dniestr rivers. The results from the benthic flux chambers deployed during two EROS 21 cruises in summer 1995 and in spring 1997 yield information on benthic nutrient cycling and diagenetic pathways at the sediment-water interface. This information is discussed in the light of benthic activity as well as pore-water data. The benthic recycling of nutrients varied seasonally due to the availability of oxygen and organic material, and spatially due to river influence. Areas of high benthic fluxes near-shore and of low benthic fluxes offshore on the shelf were distinguished. Nutrients and suspended particulate matter discharged by the Danube is kept nearshore by the coastal current. The oxygen concentrations in the bottom water varied strongly between the two seasons. In summer, the bottom water at the near-shore stations turned anoxic. In spring, the higher oxygen concentrations are related to more intense mixing of the water column due to stormy periods and high river discharge. Highest oxygen concentrations were found on the offshore continental shelf. However, the benthic oxygen consumption rates in spring (13-23 mmol m 2 day 1 ) were as high as in summer. Areas with highest nutrient concentrations in the overlaying bottom water were found at the Danube delta front and Danube prodelta. On the Danube delta front and the Dniestr mouth, ammonia fluxes were lower in spring (1·1-1·7 mmol m 2 day 1 ) than in the summer (2·6-4·4 mmol m 2 day 1 ) due to higher nitrification rates and the lower influx of organic matter. In spring, the concentrations of dissolved iron and manganese in the bottom water were one order of magnitude lower than during the summer. This decrease in flux rates is related to the higher oxygen concentrations in the bottom water in spring. A rough comparison of the river's nutrient load and the benthic recycling over a certain area showed that the near-shore benthic phosphate and silica recycling account for 50% and 35% of the Danube input in summer, respectively. The fluxes of ammonia from benthic recycling and from the Danube discharge were at the same order of magnitude in both seasons. Nitrogen is introduced by the rivers mostly as nitrate. Benthic recycling is the dominant source of ammonia. The Danube input is phosphate deficient. In brief, benthic nutrient recycling is an important factor in sustaining high productivity of the system.

Damming rivers changes sediment and nutrient cycles downstream of a dam in many direct and indirect ways. The Iron Gates I reservoir on the Yugoslavian-Romanian border is the largest impoundment by volume on the Danube River holding 3.2 billion m 3 of water. Silica retention within the reservoir in the form of diatom frustules was postulated to be as high as 600 kt year À1 in previous studies using indirect methods. This amount of dissolved silicate was not delivered to the coastal Black Sea, and presumably caused a shift in the phytoplankton community there, and subsequent drastic decline in fishery. We directly quantified the amount of dissolved silicate (DSi) entering and leaving the reservoir for 11 continuous months. The budget based on these data reveals two important facts: (1) only about 4% of incoming DSi was retained in the reservoir; (2) the DSi concentrations were relatively low in the rivers upstream of the reservoir compared to regional and global averages. Thus damming the Danube at the Iron Gates could not have caused the decline in DSi concentrations documented downstream of the impoundment. Rather, this change in DSi must have occurred in the headwaters of the Danube River. Potential reasons include the construction of many dams upstream of the Iron Gates, hydrologic changes resulting in lower groundwater levels, and clogging of the riverbed limiting groundwater-river exchange.

Manganese(II) oxidation rates in a eutrophiclake were calculated from a 4-year record of sediment-trap data, and the structure of the pre vailing manganese oxides were determined by extended X-ray absorp tion fine structure (EXAFS) spectroscopy. The oxidation rate near the sediment surface showed a distinct seasonal pattern, with maxima of up to 2.8 mmol/m 2 per day during summer. The average half-life of Mn(II) during stagnation in summer was 1.4 days. A review of pub lished oxidation rates showed that this half-life, which cannot be ex plained with available data of abiotic surface catalysis, is within the typical range of microbiological oxidation. EXAFS revealed that the oxidation product consists mainly of vernadite (δ-MnO2), an X-ray -amorphous Mn(IV) oxide. INTEREST IN THE AQUATIC REDOX CHEMISTRY of manganese is at least as old as Werner Stumms scientific career. The first Ph.D. student in his laboratory at Harvard University worked on the chemistry of aqueous Mn(II) and Mn(IV) (J). Since then aquatic chemists have refined their analytical tools (2), their conceptual (3) and numerical (4) models of manganese cycling, and their understanding of heterogeneous redox reactions in general (5,6). This chapter examines the biogeochemical and mineralogical aspects of the manganese re-OO65-2393/95/0244-Olll$O8.72/O

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