Free flowing rivers are the "favourite habitat" of Sonja Jähnig and Christian Wolter I Foto: Angelina Tittmann
Almost unnoticed by the public, species extinction in freshwaters continues. On land, insect research (the Krefeld study) has generated attention about the losses in the nature reserves there. Are there similar exemplary studies in aquatic ecology? What is the status quo?
Sonja Jähnig: The dramatic loss of species in freshwaters is an important topic in research – but not in political discourse, you are absolutely right. Yet one third of the 28,000 freshwater species on the IUCN Red List are threatened with extinction. The WWF's Living Planet Index and its own studies show that the large animals in particular are highly threatened – their populations have declined by almost 90 percent in the last 40 years. The WWF report "The World's Forgotten Fishes" shows that 80 freshwater fish species are already extinct. However, this knowledge has not yet led to the issue receiving special attention.
What are the differences between standing and flowing waters in terms of biodiversity and its losses?
Sonja Jähnig: In lakes, for example, the temperature habitats are shifting due to climate change. As a consequence, many species have to move to other depths or change their seasonal appearance. Not all of them will be able to adapt. In rivers, it is above all the expansion of waterways or instream barriers such as weirs and dams that leads to habitat loss.
How do you explain to a layperson what the difference is between chemical and ecological water quality?
Sonja Jähnig: The ecological water quality is determined by biological, chemical, physical-chemical parameters, and information on the structure of the water body – so it is based on a complex assessment. In the case of the biological criteria, for example, we examine whether the biocoenoses are intact. In contrast, the chemical water quality is determined on the basis of pollutant concentrations and physico-chemical parameters such as oxygen or nutrient content. It is checked whether all values are within the defined limits.
Inputs from agriculture, industry, traffic and households make life difficult for water bodies. Which protective measures actually make a difference?
Christian Wolter: Nutrient inputs from industry and households are so-called point sources, while inputs from roads and agriculture enter water bodies diffusely. Pollution from point sources has decreased considerably in recent decades due to a better infrastructure of sewage treatment plants. The first sewage treatment plants are now being equipped with a fourth treatment stage which can remove more pollutants from the water. This is paying off: nationwide, water quality has improved. The situation is different with diffuse sources. Here, 5 to 10 meter wide riparian zones are the most effective measure to reduce inputs. However, this is in direct competition with agricultural land use.
How serious is the problem of invasive species in freshwaters?
Sonja Jähnig: Invasive species can change the food web, water quality and species composition of the colonised ecosystem, and also promote the spread of diseases – invasive crayfish transmit crayfish plague without becoming ill themselves. Invasive species will continue to increase or spread, also because climate change alters living conditions.
Environmental organisations are calling for the removal of structures in rivers. The EU Commission has proposed to restore about 25,000 river kilometres by 2030. Is that enough?
Christian Wolter: It is a first step in the right direction. However, about 700,000 river kilometres will not reach the environmental goals of the European Water Framework Directive (WFD). In view of this figure, 25,000 kilometres seems rather unambitious. Changes in watercourse structure and barriers are mainly responsible for the poor condition. In Germany, around 200,000 barriers fragment the flowing waters, in Europe there are a total of one million. Many of them no longer fulfil their function, but cause ecological damage. Incentives should be created to remove obsolete barriers.
What does a hydropower plant do to ecosystems?
Christian Wolter: The weir prevents the migration of living organisms and sediments. In the impounded area, the flow velocity and drag force of the water are reduced. This leads to temperature increases and the sedimentation of finer substrates and thus to the clogging of coarse substrates. In this way, habitats for species typical of flowing waters are lost. These effects occur at all transverse structures. Specific to hydropower are fish injuries and losses, e.g. during turbine passage. A certain form of hydropower, the hydropeaking operation, also significantly affects river sections downstream of the plant, because the water level is temporarily lowered considerably and habitats fall dry.
The EU wants to expand waterways, and the German government recently decided to speed up the implementation of such construction projects. At the same time, climate change and droughts regularly cause water levels to drop so that the so-called waterways are not navigable at all. How does that fit together?
Christian Wolter: Not at all! This is also shown by the current fitness check of the WFD: The main causes for the WFD's failure to achieve its environmental objectives are being promoted – through incentives for the expansion of waterways and hydropower, or, for example, a lack of cost allocation for irrigation in agriculture. The implementation of the WFD and climate protection must become a mandatory task across all sectors.
The European Water Framework Directive (WFD) should – originally by 2015 – bring water bodies into a "good ecological status". This goal has now been postponed to 2027. The audit has shown that the WFD as a law is good, but its implementation by the member states is poor. How do you assess the WFD from a water ecology perspective?
Christian Wolter: The WFD is a milestone in water management and marks a paradigm shift: for the first time, ecological objectives and water quality objectives are placed on an equal footing. Thanks also to an improvement in water quality – this is the only reason why we can seriously think about ecological improvements; which, by the way, are also irreplaceable for us humans! Ecologically functional water bodies and floodplains provide flood protection and water retention in the landscape and counteract the consequences of climate change. It is regrettable that the implementation of the WFD is so slow and unambitious.
In biology and nature conservation, work is often done with so-called indicator species, which are symbolic of a certain habitat and can indicate certain environmental influences well. Do you have a favourite species or habitat and - if so, why exactly this one?
Sonja Jähnig: Rivers fascinate me very much. Even as a child I was fascinated by the Danube – I grew up on the Brigach – and the idea that a drop of water crosses so many countries, "sees" so many people...
Christian Wolter: My favourite habitats are also free flowing rivers because they are subject to constant change due to their dynamics of water level, flow, form and structure, which also always holds surprises in store for the animal world, which temporarily migrate in or out, so as biotic communities they are always dynamic and remain exciting.
