Auen können Wasser speichern und reinigen. Ein Team unter Leitung des Leibniz-Instituts für Gewässerökologie und Binnenfischerei (IGB) hat in enger Zusammenarbeit mit dem Nationalpark Unteres Odertal untersucht, welche Rolle die ausgedehnten Flussauen der Oder für den Wasserhaushalt und den Umsatz von Nähr- und Schadstoffen spielen.
Flooding weir of a polder in the Lower Oder Valley National Park. | Photo: Dörthe Tetzlaff
Floodplains can store and purify water. A team led by the Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB), in close cooperation with the Lower Oder Valley National Park, has investigated the role of the Oder's extensive floodplains for the water balance and the turnover of nutrients and pollutants. The results indicate that a permanent opening of the flooding weirs could be beneficial in order to keep at least some of the floodplains in contact with the river for longer periods of the year and maintain their important function. This provides scientific support for a key project of the national park administration: one of the polders investigated will be opened all year round for five years to allow more water to flow in over a longer period of time.
Like other large floodplains, the Lower Oder Valley National Park provides important wetland habitats for animals and plants. Thanks to the floodplains and the free passage of the river in the lower and middle reaches, fish and other organisms have found important refuges here during and after the environmental disaster on the Oder in 2022, allowing populations to gradually recover. The floodplains also provide natural flood protection, as they can mitigate a flood wave. However, they also fulfil other important functions: "The results of our study confirm that the floodplains along the Oder are very important for water and nutrient retention", explained IGB researcher Prof. Dörthe Tetzlaff, who led the study. The dynamics of high and low water play an important role in the turnover of substances and therefore in water quality.
Water flow into the floodplain is regulated over the course of the year by flood barriers
In the Lower Oder Valley National Park, the floodplain areas are diked - a compromise between the natural floodplain and agricultural use, as Dr Jana Chmieleski, Deputy Director of the National Park, explained: "These polders are closed from mid-May to mid-November and serve as floodplains during the remaining months." The researchers spent two years in two alluvial polders quantifying where the water comes from, how old it is, how much of it evaporates and what the water quality and exchange with the groundwater is like. The role of floodplains in the retention and turnover of substances was also investigated. The most important findings on the prevailing patterns and processes were summarised in a concept that can be used to better assess these ecosystem services under changing environmental conditions.
Flooded floodplain: nutrients are carried in from the river, good conditions for fish fry and other organisms
As the study shows, high water levels in the Oder in winter allow the floodplain polders to be flooded. Along with the water, many dissolved substances from both natural organic material and fertilisers are also transported into the floodplains. During this time, however, the water is well supplied with oxygen, which is particularly important for fish and other animals that grow up or overwinter in the floodplains.
Floods recede: nutrient decomposition in floodplains improves water quality
As the floodwaters recede in late spring, the decomposition of organic matter and dissolved substances and the photosynthesis of plants in the floodplains intensify. This process consumes more oxygen than is produced by photosynthesis. Under these anaerobic conditions, biogeochemical processes that are crucial for improving water quality in floodplains take place. "For example, nitrogen bound in nitrate is converted to molecular nitrogen by denitrification, i.e. it is converted to a form that is relatively inert and cannot be used by most living organisms. In water and soil, it is no longer available as a fertiliser and is no longer of environmental concern", explained Professor Chris Soulsby from the University of Aberdeen, one of the authors of the study.
Upper Oder river basin brings many pollutants
"The ecosystem services of the floodplains are particularly important for the Oder, as the industrialised and urbanised region in the upper Oder catchment discharges water with a broad spectrum of pollutants, even if the situation has improved since the 1990s", said Dr Jana Chmieleski.
Nevertheless, the Szczecin Lagoon downstream, as a transitional zone between the river and the sea, has long suffered from considerable nutrient pollution. The negative highlight was the environmental disaster in the Oder in 2022, caused by a toxic algae bloom due to the high salt content of mine wastewater and excessive nutrient loads.
The water quality of the Oder is still poor: even in the summers of 2023 and 2024, for example, the conductivity as an indicator of salt pollution reached values similar to those in the initial phase of the ecological disaster several times at around 1,500 microsiemens per centimetre.
There is only a small exchange of water with the groundwater in the Oder floodplains
The isotope analysis on the origin and age of the water also showed that the floodplain polders of the Oder in the study area are hardly fed by groundwater, but that the water mainly originates from flooding; in summer, the proportion of precipitation still plays a role. The exchange with groundwater is limited due to the low runoff and the relatively shallow nature of the wetlands with clay-rich sediments in the upper floodplain. This emphasises the importance of surface water for the landscape water balance.
The Oder in a changing climate: opening the polders for longer can have advantages
As the floodplain polders are barely fed by groundwater, they are dependent on regular flooding in order to survive as ecosystems with diverse flora and fauna. However, in the course of climate change, the biogeochemical processes and the entire water balance will change significantly, the authors explain. Reduced water flow in dry periods and increased evaporation losses will reduce the connectivity between the floodplain and the river, which will impair all the processes described above.
A longer opening of the polders and the associated increase in connectivity in the lower Oder floodplain could therefore have clear advantages over the current summer closure. The results of the study underpin one of the national park administration's key plans: to keep one of the polders analysed open all year round for five years. IGB plans to scientifically monitor this opening period.
Hanwu Zheng; Doerthe Tetzlaff; Christian Birkel; Jana Chmieleski; Jean-Christophe Comte; Jonas Freymueller; Tobias Goldhammer; Axel Schmidt; Ellen Wohl; Chris Soulsby
Journal of Hydrology. - 653(2025), Art. 132708
doi.org/10.1016/j.jhydrol.2025.132708
