At around 380 kilometres, the Spree is not one of Germany’s longest rivers, but it is one of its most famous. As the capital’s river, it is representative of water bodies in Germany and around the world that are under increasing pressure. While private and commercial interests in its water resources continue to grow, it is also clear that there will be significantly less water available in the future. What needs to be done to counteract this is the subject of controversial debate. The river itself is often neglected. Yet its environmental flow requirements, which secure the functions and services of the river ecosystem for humans and nature, are essential. IGB is located directly on Berlin’s Lake Müggelsee, through which the Spree flows. This is not the only reason why scientists here have been conducting research in and on the Spree for decades. They are helping to better understand the interrelationships and to develop sustainable solutions.
The Berlin-Brandenburg region is rich in water bodies, but poor in rainfall. Ms Tetzlaff, you are a hydrologist: How much water actually reaches the Spree and what water levels can we expect in the future?
Dörthe Tetzlaff: Berlin-Brandenburg is one of the driest regions of Germany. What little rainfall there is is quickly lost due to very high evaporation rates. Compared to other well-known rivers, the Spree carries very little water on average, less than 10 per cent of the annual precipitation.
A large part of the discharge – up to 40 per cent in the summer months – currently comes from the so-called sump water of the lignite mining area. However, the mining area is increasingly turning from a water source into a water sink, with the creation of large post-mining lakes that evaporate additional water over their large surface area. As a result, we expect the water resources of the Spree River, which is also an important part of Berlin and Brandenburg’s water supply, to become even more vulnerable in the future. This will require careful planning and management of resources as the effects of climate change and continued population growth are already being felt in Berlin.
Are there ways to adapt land management and land use in the Spree region to improve water retention?
Different types of land use, such as forest, grassland, agroforestry and arable farming, can indeed influence the hydrology of landscapes – for example, how precipitation is distributed and partitioned, and how much of it evaporates.
Tracer-based ecohydrological modelling allows us to map the differences between these various land uses, as well as the temporal dynamics of soil moisture, water levels and groundwater recharge. Compared to other forms of vegetation, large-scale pine monocultures lead to high evaporation losses through transpiration and interception, which is reflected in reduced infiltration rates and groundwater recharge. On arable and pasture land, especially in spring when the vegetation cover is not yet very dense, a lot of water can be extracted from the soil by evaporation, which is then also not available for groundwater recharge.
We therefore recommend a so-called land use mosaic, i.e. away from monotonous large areas and towards a varied land use such as agroforestry or mixed forests with different tree species and ages and the best possible distribution in terms of soil cover, increased infiltration and groundwater recharge, while at the same time minimising evaporation losses.
If the Spree River is supplying less and less water to the capital, can Berlin’s stormwater be part of the solution, Ms Spahr?
Stephanie Spahr: Stormwater is a valuable but underutilised resource for urban water supplies. However, stormwater also washes away pollutants from roads and buildings, for example, which can end up in freshwaters or groundwater. During heavy rainfall events, there is also a large influx into wastewater treatment plants, which can then overflow. Discharging untreated wastewater into freshwaters can have serious impacts on ecosystems, including fish kills, as we see time and again in Berlin after rainfall.
To avoid such events and to use stormwater as a resource, Berlin is aiming for decentralised stormwater management. Multifunctional blue-green infrastructure such as ponds and vegetated swales play a central role. They provide valuable habitats, improve the quality of urban life, and keep rainwater in the city. One challenge remains: persistent, mobile and potentially toxic organic substances are difficult to remove from stormwater. To safely use stormwater for urban water supplies or discharge it into the Spree, these pollutants need to be systematically monitored, their emissions prevented, and purification processes improved.
It is not just rain that leads to the discharge of contaminants; the Spree is also polluted with sulphate, iron, heavy metals, nutrients and agricultural and industrial run-off. Mr Goldhammer, what is the current state of the river’s chemical water quality?
Tobias Goldhammer: Unfortunately, water quality has not improved over the last ten years in terms of the main pollutants. The Spree is mainly characterised by the effects of open-cast lignite mining in the Lusatian mining region, which discharges large amounts of iron and sulphate. While iron no longer plays a major role downstream of the Spreewald, sulphate is distributed throughout the river system up to the urban Spree in Berlin. Concentrations are increasingly reaching levels where some waterworks have to add groundwater with a lower sulphate content when extracting drinking water from the Spree bank filtrate.
Mining also emits heavy metals. However, the main sources are of urban-industrial origin and therefore mainly affect the urban Spree. It remains to be seen how the pollution situation will develop in the coming years as a result of the establishment of large industrial enterprises in the catchment area. Concentrations of nutrients, especially nitrogen from agriculture and phosphorus from domestic sewage, have decreased compared to previous decades, but the guidance values for good ecological status are not yet met everywhere. The overall picture is still problematic and will worsen with the challenges of climate change unless action is taken.
A river that is running out of water can hardly dilute nutrient and pollutant inputs. Is that right, Mr Lewandowski?
Jörg Lewandowski: Exactly, water quality depends very much on water quantity. Take, for example, the purified wastewater from sewage treatment plants that Tobias Goldhammer mentioned, the so-called clear water that is discharged into the Spree and its tributaries. Purified does not mean that the water is really completely clean. Many problematic water constituents are not removed by sewage treatment plants, such as organic micropollutants like pharmaceuticals or industrial chemicals. The lower the flow of water in the Spree, the less this clear water is diluted.
However, the quality of the clear water could be significantly improved by retrofitting a fourth purification stage in wastewater treatment plants. In addition, the natural purification capacity of the hyporheic zone, i.e. the river bed, can be increased by restoration measures in the tributaries of the Spree. In times of climate change and increasing water consumption, well-purified clear water is becoming an increasingly important resource for aquatic ecosystems.
Even today, the proportion of treated wastewater in the Spree increases massively in some places during dry summers, and can amount to over 50 per cent in Berlin-Köpenick, for example. At the same time, the water flow in the Spree is sometimes no longer sufficient to compensate for the water losses from Lake Müggelsee through evaporation and bank filtration, so that the flow direction downstream of Lake Müggelsee can even be reversed in some places.
The clear water from the Münchehofe sewage treatment plant then flows in the opposite direction via the Erpe and the Spree into the Müggelsee, from which drinking water is drawn. Mr Pusch, what would you recommend to politicians and authorities to deal with this situation?
Martin Pusch: Because water is extracted from the river in many places, it was already flowing backwards in some stretches in 2003. Today, the Spree is only kept flowing in summer by sump water and would otherwise regularly dry up. In order to preserve the Spree’s waters with their rich flora and fauna and their diverse ecosystem services, efforts are needed on several levels: technical water retention will not be sufficient and must be supplemented by other measures, in particular water saving, further wastewater treatment, water recycling, but also nature-based solutions such as water retention in the catchment area, supplemented by active groundwater recharge in times of water surpluses.
There is also an urgent need to adapt the river bed to the reduced water flow so that the Spree can continue to flow and thus remain a river ecosystem. Of course, such a range of measures can only be implemented if all relevant stakeholders are involved and if water management alternatives and their respective advantages and disadvantages are transparently weighed up.
Iron inputs lead to the brown colouring of the water in the Spree. Is this just a very local problem and what is known about the effects, Mr Hupfer?
Michael Hupfer: Sediment analyses along the Spree River showed typical patterns and signatures indicating that mining activities in Lusatia are still clearly detectable at a distance of more than 90 km. After that, urban influences increasingly dominate the geochemical composition of sediments up to the mouth of the Havel. Up to the Spremberg reservoir, iron deposition and redistribution create hostile conditions for many organisms. Along the Spree, iron and sulphur compounds from pyrite weathering are subject to different transport mechanisms, which we were able to demonstrate using remote sensing techniques.
While the high sulphate concentrations in Berlin are still strongly influenced by mining, the additional iron input after the Spremberg reservoir no longer plays a significant role in the matter balance of the Spree. The expectation that the increased iron discharge from mining would bind so much phosphorus that it would have a positive effect on the eutrophication of the lakes through which the river flows could not be confirmed on the basis of laboratory experiments and model calculations.
Ms Hilt, you have observed the return of some rare aquatic plants in the Spree, which should be a sign of improved water quality. At the same time, however, reeds are dying and massive algal blooms are occurring. How do you think these observations fit together?
Sabine Hilt: The nutrient inputs of nitrogen and phosphorus into the upper part of Lake Müggelsee have decreased overall over the last four decades – so the amount of algae turbidifying the water in this area has also decreased. As a result, sunlight is more available and this has led to a positive feedback loop: More plants in the summer act like a sieve, holding back more particles and increasing the clarity of the water. They also naturally slow down the flow and retain the water in the river system, and increase the structural diversity of the river bed.
Aquatic plants also provide additional habitat for many animals and micro-organisms. Mechanical removal of these plants, e.g. for flood protection, should therefore be done as infrequently and as sparingly as possible. However, large quantities of nutrients, especially phosphorus, still accumulate in the sediments of the lakes through which the Spree flows, such as Lake Müggelsee. In summer, this can lead to mass blooms of cyanobacteria – commonly known as blue-green algae – as water temperatures rise.
The decline of the reeds is partly due to predation by herbivores such as muskrats and nutria, but other factors also play a role.
In the face of water scarcity, the concept of environmental flow requirements is also being discussed. Ms Jähnig, what exactly does this term mean?
Sonja Jähnig: This is the amount of freshwater that aquatic habitats and water-bound terrestrial ecosystems need to maintain their ecological functions. For the Spree, the scientifically recommended minimum water flow is 5-8 cubic metres per second – in spring and summer, the Spree often carries less water.
However, it is important to consider not only the surface waters themselves but also their catchment areas, i.e. not only the watercourses but also their floodplains and other groundwater-dependent terrestrial ecosystems. The quantity, quality and timing of water supply, sediment transport and connectivity of water bodies all play a role. The underlying idea is that only if aquatic ecosystems have sufficient water for their basic functions can humans sustainably use them as a basis for life
