In the study, the researchers have taken into account important possible factors influencing the proportion of reeds in relation to the size of the riparian zone, which have been measured over 20 years: the status of the riparian zone, water quality and the type of riparian use. Some of the lakes were polluted with sulphate due to the water supply from the River Spree, others were not. This was also included in the investigation.
"The results of the scenario calculations for the sulphate-polluted lakes show a clear influence of the sulphate concentration, which has been increasing for ten years, on reed growth," explained author Lydia Roesel, a scientist at the HU Berlin and the Technical University of Braunschweig. This is problematic because Berlin's reed beds are also affected by a number of other negative factors, such as falling water levels, mechanical stress caused by waves from boat traffic, human alteration of the banks and water pollution. And they show that renaturation measures make sense: According to the calculations, brushwood fascines as breakwaters promote reed development.
Important ecosystem services provided by reed
Reeds are important in many ways: as a habitat, they provide breeding sites and shelter for water birds, fish and insects. An intact reed belt dampens the impact of waves caused by boat traffic or strong winds. It stabilises the shore, preventing severe erosion. At the same time, reeds support the self-purification of the freshwater system by removing excess nutrients.
Further reed decline puts focus on sulphate
Many European lakes have been affected by the decline of reeds since the 1950s. In Berlin, attempts have been made to respond to the reed decline. However, despite various efforts to stabilise and increase reed belts, a decline has been documented since 2010 on some of the shores of the lakes flowing through the Berlin urban area, after the reeds had previously recovered somewhat following a prolonged period of dieback. This prompted the scientists to investigate further - including the possible influence of sulphate on the plants.
"We were surprised that the link was so clear. If the sulphate concentrations had not increased, there would be around 20 percent more reeds on the lakes today according to our calculations," said author Dominik Zak, who does research at IGB and the University of Aarhus in Denmark. This calculation is based on the results of the statistical analysis and must be verified by further studies. High nutrient concentrations of phosphorus and nitrate, which had been identified in previous studies as influencing reed decline, did not play a significant role in the Berlin lakes studied.
Sulphate is normally non-toxic to reed plants even at very high concentrations and is stable in the presence of oxygen. However, under unoxic conditions and at high sulphate concentrations, the formation of plant-toxic sulphide can occur at the sediment. This oxygen depletion can occur when large amounts of organic material accumulate and decompose in the riparian zone. This also promotes the activity of sulphate-reducing bacteria.
The sulphide formed in the water sediment is usually rapidly bound by the iron present. "It would now be necessary to investigate whether this detoxification process in some of the sulphur-polluted lakes, such as Lake Müggelsee, has been disrupted", explained Dominik Zak. Especially in the area where drinking water is extracted through bank filtrate, the sulphide load could possibly increase. This is because there is more sulphate in the root zone and less iron, which chemically binds sulphide. The extent to which this known detoxification process is actually affected by bank filtration requires further investigation.
Other factors, such as riparian tree cover, are currently more relevant to reed decline, as the study has shown. However, sulphate inputs are expected to continue for an unknown period of time. The results of the study are therefore also relevant for the future. If the current sulphate concentrations already show a negative correlation with the reed population, an even stronger influence can be expected if the concentrations continue to increase.
Replanting and reed protection make sense, but are not always successful
The researchers' calculations also showed that protective measures are useful. For example, fascines used as wavebreakers promote the development of reed beds. "But such measures are not always successful, and the reasons for this are not always clear. That's why it's important to look more closely at other possible causes so that we can better target measures against reed dieback," said Dominik Zak. For example, the problem of grazing by water rats and nutrias is becoming increasingly serious. This factor also needs to be taken into account in the future. "But one key message is already clear: sulphate is partly to blame for the decline of the reeds, so emissions should be reduced urgently," Dominik Zak emphasised.
IGB researcher Tobias Goldhammer, who coordinates the Spree sulphate monitoring programme at IGB, added: "The study shows that the high sulphate loads in the River Spree are not only a problem for drinking water abstraction, but can also have effects on the natural biogeochemical cycles that have not been taken into account so far, and can have a negative impact on the ecology of the connected surface waters in Berlin."
The sulphate pollution of Berlin's waters originates from mining in the Lusatian region
The sulphate originates from the large former lignite mining areas in the upper catchment of the River Spree. It is formed from iron sulphides that are released from deeper layers of soil during mining. Under oxic conditions, sulphuric acid, sulphates and iron are formed, which then enter water bodies.
The iron compounds precipitate as brown-red ochre when exposed to air and neutral pH in the water body and are only partially transported. Sulphate, on the other hand, remains invisible and dissolved in water and can be transported over long distances due to its relatively low reactivity.
Because of its potential effects on human health, limits have been set for sulphate in drinking water supplies. However, there are also known ecological effects on water bodies, such as the leaching of phosphate from sediments and toxic effects on living organisms in water bodies due to the formation of hydrogen sulphide in anoxic conditions.