Nitrogen imports cause pollution in many rivers. The extent of these inputs, the degree to which they are decreased, and the contribution made by floodplain regions to this in the Danube catchment area have now been examined for the first time by an international study initiative that the IGB is a part of. The outcomes demonstrate the value of extensive renaturation of river floodplains for raising water quality.
Among all habitats on earth, riverine floodplains are among the richest in biodiversity. They are at the center of biodiversity and nutrient cycling because they are where land and water meet. Many floodplains along rivers have been separated from waterways or put to other purposes. At the same time, too many nutrients, particularly nitrogen, enter the water. Both decrease water quality and endanger biodiversity in both the rivers’ ecosystems and the seas they run into.
Rivers have a limited capacity to degrade nutrients in the river’s water and floodplains. Researchers working on the international IDES collaborative effort have determined how much the floodplains help to decrease nitrogen for the Danube River basin.
The special feature of our study is that we looked at such a large area for the first time because the Danube has the second largest catchment area in Europe.
Dr Andreas Gericke, Study Co-Author and Scientist, Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB)
Over 800,000 km2 of land is included in the Danube catchment, which spans 19 countries. In recent decades, between 70 and 80% of its floodplains have been shut off from the river or turned into agricultural land, denying it of the ecosystem’s services and activities.
Scientists have attempted to answer how much of the nutrient retention is given by the still-active floodplains.
The researchers did this by using the MONERIS model, which was created at the IGB, and estimated the fate and movement of nutrients in the river system by calculating their inputs from different sources, such as the atmosphere, fertilizer use in agriculture, and sewage treatment facilities.
The report suggested that 500,000 metric tons of nitrogen, mostly in the form of nitrate, enter the waterways of the Danube River Basin each year. Urban sources and agriculture account for 44% of the inputs (30%). One-third, or 160 000 metric tons, of these inputs, are degraded in aquatic bodies, and two-thirds make it to the Black Sea.
The team included additional models for the Danube and its tributaries the Sava, Tisza, and Jantra to the MONERIS computations to determine the size of the share of floodplains in nitrate retention.
Roughly, half of the active floodplains in the Danube basin are located there, covering 3842 km2.
“Most nitrate is degraded in the water network, for example by nitrogen being taken up by plankton or converted by bacteria (denitrification). But floodplains can also contribute to a not inconsiderable extent to nutrient retention,” Andreas Gericke conveyed.
According to the findings, active floodplains decompose 33,200 tons of nitrate or 6.5% of the input every year. According to model estimates, the approximately 1300 km2 of potentially repairable floodplains and oxbow lakes might boost nitrate removal by 14.5% if they were linked to the major streams.
“Our results impressively show that it makes sense to preserve floodplains and restore their functions - not only because of their ability to break down nutrients but also to preserve biodiversity among many other ecosystem services,” Martin Tschikof from BOKU’s Institute of Hydrobiology and Water Management stressed the point. He is the study’s principal author.
Only a few generalizations are possible given the simplified assumptions and data. Moreover, they provide a solid foundation for considering floodplains and reconnecting those to significant European river basins to maintain high water quality.
Tschikof, M., et al. (2022) The potential of large floodplains to remove nitrate in river basins – The Danube case. Science of the Total Environment. doi.org/10.1016/j.scitotenv.2022.156879.