They are, however, less known for their ability to retain and decompose nutrient (phosphorus and nitrogen) loads. The functional ability of wetlands to trap plant nutrients from households, industry and agriculture was most recently investigated in the framework of the UNDP-GEF Danube Regional Project/DRP (www.undp-drp.org).

Smart brains then had the idea to use the filtering capacity of wetlands as an inexpensive method for solving the problem of excessive nutrient loads (resulting in algal bloom) in the Danube Delta and on the western shores of the Black Sea. It is known that the catchment area of the Danube still lacks a large number of modern wastewater treatment plants – at a time when national economies in the region are gradually recovering from recession.

The aforementioned concept long failed to be developed due to a lack of quantitative data on this wetland function. It took several years to evaluate these data in the framework of the DRP research project “Wetlands in the Danube River Basin”, the results of which were recently presented. The project was powered by the University of Natural Resources and Applied Life Sciences (BOKU) Vienna, Wasserkluster Lunz and the Vienna University of Technology.

Wetlands in the Danube River Basin was carried out in five so-called demonstration zones in Austria (Regelsbrunn wetlands in the Donau-Auen National Park east of Vienna), Hungary, Bulgaria, Moldova, and the Ukraine. These are wetlands that have become reconnected to the Danube.

Targeted monitoring programmes are required for a quantitative assessment of nutrient retention. The water flow into and out of a demonstration zone, the temporal dynamics of transport and the relevant uptake processes are examined in detail. Water samples allow to draw conclusions on nutrient concentrations and nutrient loads. Nutrient retention can be evaluated based on the retention time of the water flowing through the wetlands.

Surface water flow and the impact of groundwater play as much a role as the spatial differentiation of connected water bodies in a pristine wetland section. Phosphorus is stored in the wetlands exclusively in particulate form. On the one hand, plants (algae and riparian plants) can take up and store dissolved phosphorus, on the other, phosphorus entering the wetlands through suspended solids can be eliminated from the water through sedimentation.

Phosphorus retention, for example, may therefore be most efficient during floods, because then larger amounts of suspended solids enter and are stored in the wetlands as a result of the reduced flow rate. Nitrogen enters the wetlands mostly in the form of nitrate.

Apart from plant uptake and sedimentation, another vital process of eliminating nitrogen from water is denitrification (= microbial decomposition of nitrate to atmospheric nitrogen). The experiment conducted in the Regelsbrunn wetlands revealed that in a year of extreme flood events, the Danube carries a total load of approx.

18,000 t of phosphorus, of which 500 t are stored in the adjoining wetlands. In a year with much dry weather flow, the phosphorus load transported in the Danube near Vienna is reduced to about 4,000 t. Some 15 t of this amount are retained in the Regelsbrunn wetlands.

After the upgrade of the Vienna Main Wastewater Treatment Plant, by comparison, the latter now has enough capacity to eliminate 1,200 t.p.a. of phosphorus from the Viennese wastewater; the remainder of 150 t.p.a. still flows into the Danube. Figures show that large amounts of phosphorus are only retained in the Regelsbrunn wetlands in case of extreme flood events. Under normal conditions, the wetlands reconnected with the Danube can only serve as a relevant nutrient trap collectively.

This also goes for nitrate. Wetlands, due to their diversity and high productivity, have a pronounced organic matter turnover and thus an effective retention capacity. Especially in terms of nitrate, microbial turnover along water-land boundaries is of crucial importance and leads to nitrogen removal from the water system.

10 to 20 % of the overall nitrate load entering the wetlands is eliminated from the water. Yet only a small fraction of the total water flow of the Danube ever reaches the wetlands. Therefore, only 0.1 to 0.2 % of the total nitrate transported in the Danube near Vienna is actually retained in the Regelsbrunn wetlands. The nutrient absorption capacity of wetlands is mainly determined by the existing wetland area and the natural exchange rate between the water body of a wetland and the Danube.

The research project Wetlands in the Danube River Basin was supported by ten partners. The International Commission for the Protection of the Danube River (ICPDR) is the central platform through which the results obtained are disseminated. The task of the latter also is to convince its member representatives of that the Danube wetlands, with regard to the nutrient situation in the Black Sea, must not be (ab)used as a lowcost option of wastewater treatment.

The research project has shown that wetlands, on condition that they preserve or regain their original dimension and quality, assume an important role in such cleanup collectively. But they, too, are unable to cope with excessive nutrient loads and rather suffer from the implications of the latter: eutrophication and terrestrialisation lead to a dramatic decrease in biodiversity.

Due to their function as valuable habitats for endangered animal and plant species, flood control systems, groundwater reservoirs, stabilising factors for the local climate and psychotopes for humans, wetlands are sensitive ecosystems and as such principally unsuited for wastewater purification. Decision-makers in the Danube Region are therefore well-advised to invest part of the increasing revenues of their national economies into improving integrated river management as well as building genuine wastewater treatment plants.
(Source: aqua press Int. 2/2007, Univ.-Prof. DI Dr. Matthias Zessner, Univ.-Doz. Dr. Thomas Hein)

Technische Universität Wien
Inst. f. Wassergüte, Ressourcenmanagement
und Abfallwirtschaft
ao. Univ.-Prof. DI Dr. Matthias Zessner
E-Mail: mzessner@iwag.tuwien.ac.at
Internet: www.iwag.tuwien.ac.at

Wasserkluster Lunz, AG BioFrames
Univ.-Doz. Dr. Thomas Hein
Internet: www.wasserkluster-lunz.ac.at

Inst. für Hydrobiologie & Gewässermanagement,
Universität f. Bodenkultur, Wien
Internet: www.boku.ac.at