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The nutrient levels, notably phosphorus and nitrogen, found in the Danube River and its major tributaries hold significant international importance. This is because they directly contribute to the phenomenon of eutrophication in both the river itself and the Black Sea. Historical trend analysis of nutrient river loads over the past decades shows a significant reduction in the transported nutrient fluxes to the Black Sea. Overall nutrient emissions declined by 30% (nitrogen) and 50% (phosphorus) in the last 15 years. However, the current long-term fluxes are still considerably higher than those of the early 1960ies which represent river loads under low pressures, indicating a further load reduction potential that might be exploited for the benefit of the Black Sea. Nevertheless, in the last 5-10 years the measured loads are rather low and close to the Black Sea targets indicating significant water quality improvement.

The floodplains and wetlands of the Danube basin are uniquely valuable ecosystems in global terms, although few areas are still in their natural or near-natural state.

The Danube River is seeing an influx of new non-native species, stemming from the increasing interconnections with various European and global water bodies via canals and other waterways designed to facilitate navigation. It is currently believed that alien species are very likely to become even more significant in the future, as the importance of the Danube as an international waterway increases.

The habitats created by the Danube and its tributaries host a unique mix of species. But many habitats are degraded by man-made changes to the river profile and width, water depth and flow velocity following the construction of dams, weirs and canals. Many migratory fish including sturgeon species and the Danube Salmon are endangered or close to extinction by being disconnected from their spawning grounds and habitats or by being over-exploited.

The waters of Bosnia and Herzegovina are split between the Danube River Basin District and the Adriatic Sea Basin. Some 40.2% of the Sava river sub-basin, the second largest sub-basin of the Danube River Basin, lies in Bosnia and Herzegovina. The rest of the watershed is shared by Croatia, Serbia and Slovenia.

As both the largest remaining natural wetland and second largest river delta in Europe, the Danube Delta is one of the continent's most valuable habitats for wetland wildlife. Pollution and discharge manipulation from upstream has a huge effect on this area of high biodiversity, plus the delta continues to change ecologically itself – the Danube Delta spreads seaward at a rate of 10 to 25 metres every year.

The Sava is the Danube's largest tributary of the Danube in terms of discharge and the second largest in terms of catchment area. The Sava is shared by Slovenia, Croatia, Bosnia and Herzegovina, Serbia and Montenegro. The joint management arrangements acted as a pilot for the implementation of the European Union's Water Framework Directive for the Danube and Europe.

Covering an area of 157,186 km², the Tisza River Basin is the largest sub-basin of the Danube River Basin. The Tisza River is the longest tributary of the Danube (966 km), and second largest in terms of flow after the Sava. The countries of the Tisza Basin (Ukraine, Romania, Hungary, Slovakia, Serbia) agreed to close transboundary co-operation, aiming to achieve integrated water resources management of the Basin.

The Drava River is the fourth largest, as well as the fourth longest Danube tributary. It connects the Alps with the Danube and the Black Sea. The Drava has been considerably regulated with dams constructed to generate hydroelectricity and channels dredged to direct its flow. Nevertheless, natural habitats along the middle and lower reaches host unique varieties of flora and fauna, and several endemic species.

The International Commission for the Protection of the Danube River (ICPDR) works to ensure the sustainable and equitable use of waters in the Danube River Basin. The work of the ICPDR is based on the Danube River Protection Convention (DRPC), the major legal instrument for cooperation and transboundary water management in the Danube River Basin.