Human activities such as agriculture, transport, energy production or urban development exert pressures on the water environment. When addressing pressures at the basin-wide scale, it has to be considered that cumulative effects may occur. Effects can occur both downstream (e.g. pollutant concentrations) and/ upstream (e.g. river continuity) of a particular pressure. Addressing these issues effectively requires a basin-wide perspective and cooperation between countries.
Organic pollution refers to emissions of non-toxic organic substances that can be biologically decomposed by bacteria to a high extent. The key emitters of organic pollution are point sources like untreated or not sufficiently treated municipal wastewater from households, industries and major agricultural farms. The primary impact of organic pollution on the aquatic environment is dissolved oxygen depletion due to biochemical decomposition of organic matter. In the most severe cases this can lead to anaerobic conditions, to which only some specific organism can accommodate. The pollution with organic substances can therefore cause changes in the natural composition of the aquatic flora and fauna. It can also be associated with health hazards due to possible microbiological contamination of waters.
Nutrient pollution is caused by releases of nitrogen (N) and phosphorus (P) into the aquatic environment. Nutrient emissions can originate from both point and diffuse sources. Point sources of nutrient pollution are similar to those of the organic pollution. Diffuse pathways such as overland flow, urban runoff, soil erosion, tile drainage flow and groundwater flow can remarkably contribute to the emissions into surface waters transporting nutrients from agriculture, urban areas, atmosphere and even from naturally covered areas. Impacts on water status caused by nutrient pollution can be recognized through substantial changes in water ecosystems. In case of nutrient enrichment, water bodies can turn to eutrophic state where the growth of algae and/or macrophytes is substantially accelerated. Eutrophication severely impairs water quality and ecosystem functioning (e.g. oxygen depletion, toxicity, overpopulation of species) and might limit or even hinder human water uses as well (e.g. recreation, fisheries, drinking water supply).
Hazardous substances pollution involves contamination with priority substances and other specific pollutants with toxic effects on aquatic organisms and humans. Hazardous substances can be emitted from both point and diffuse sources. The most important sources of hazardous substances pollution are industrial facilities, agriculture (pesticide and contaminated sludge application), contaminated and mining sites, households and public buildings via municipal wastewater and urban run-off (deposited air pollutants, litter, combined sewer overflows). Moreover, significant volumes of deadly toxins can be unexpectedly flushed directly into watercourses by occasional industrial accidents or floods. Hazardous substances can pose serious threat to the aquatic environment. Depending on their concentration and the actual environmental conditions, they can cause acute (immediate) or chronic (latent) toxicity. Some of the hazardous substances are persistent, slowly degradable and can accumulate in the ecosystem.
Surface waters suffer significantly from hydromorphological alterations. Interruptions to river and habitat continuity, disconnection of adjacent wetland/floodplains, hydrological alterations and future infrastructure can hugely impact water status and therefore will need to be addressed in future.
It is assumed that the effects of the floods that impacted the countries in the Danube River Basin in recent years were worsened due to deforestation, the destruction of natural floodplains and climate change. Preserving the natural habitats of the many species living in the basin is a constant struggle. The habitats of pelicans in the Danube Delta and sturgeon species are particularly under threat.
Major problems affecting aquatic ecosystems in the Danube River Basin
- Excessive nutrient loads (particularly nitrogen and phosphorous)
- High amounts of organic substances originating from untreated or poorly treated wastewater
- Changes in river flow patterns (hydromorphological alterations) and its effect on sediment transportation
- Contamination with hazardous substances (including heavy metals and organic micropollutants)
- Accidental pollution from industrial and mining facilities
- Degradation and loss of wetlands
Human pressures and impacts are investigated and addressed in the frame of the Danube River Basin Management Plan (according to the WFD).
Water quality in the Danube River Basin is largely influenced by the inputs of pollutants - particularly excessive nutrients, organic material, and hazardous substances. According to the ICPDR’s Emission Inventory 2002, which considers point and diffuse sources of pollution in the whole Danube River Basin, municipalities and industry released fewer pollutants than in previous years.
Since the 16th century, people have been changing the natural course of the rivers in the Danube River Basin, mainly for flood defence, hydropower generation and navigation. All these changes affect the ecological quality of the rivers. Changes in the depth or width of a river typically reduce flow rates, interrupting natural sediment transportation as well as the migration routes of animals.
Article in Danube Watch 02/2006