Low water in rivers can kill animals

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Currently the Rhine has little water – as is the case here in Bonn. © Roberto Vail / dpa

The low water level of the Rhine and other rivers affects inland navigation. Fish and mussels are worse off: higher temperatures and less oxygen can have dire consequences.

Koblenz/Magdeburg – Midsummer, drought, low water: A large patch of gravel has dried up on the southern tip of the Rhine Niederworth Island near Koblenz. Zoology professor Yuchen Cope bends over at the waterline and picks up oysters. Some are still alive, others are already dead.

In lower waters, which were more heated by the sun and with less dissolved oxygen, they had metabolic problems, explains a biologist from the Koblenz Federal Institute of Hydrology (BfG). “The temperature here is in shallow water like a bathtub.” Nature suffers, too.

On the way to kill fish

For example, in the hot summer of 2003, fish and shellfish died in rivers. “All over the Rhine, about 50,000 snakes were washed away,” says Cobb. Less water, sometimes more current, higher water temperature, lower concentration of oxygen: Breathing and exercise require more energy for animals, and they cannot absorb enough of it. They become weaker and more susceptible to disease. The biologist recalls: “In 2006 and 2007 there were periods of heat on the Rhine, but the water was not very low at the same time.” This was less exciting for the animals. “But in 2022 we’ll be on our way to catching killer fish again,” Cope fears. Unless it rains longer and river levels drop further, “we could hit the tipping point again in late August, early September.”

The city of Düsseldorf reports that the water temperature in the Rhine here is still just below the critical threshold for fish permanently above 26°C. However, the oxygen-binding capacity of water decreases with increasing temperature. At the same time, as the water level drops, the fish retreat into the deep shipping channel, where they have to avoid ships. They both stress it out.

Karsten Rinke, Head of Lake Research at the Helmholtz Center for Environmental Research (UFZ) in Magdeburg explains: “When the water level is low, the proportion of treated wastewater in the discharge increases. In the Rhine, Elbe and Oder, it can account for more than 40 percent of the Discharge in drought conditions.This increases the concentration of pollutants and nutrients in the water.This may also lead to “the accumulation of pollutants in organisms that, in the case of fish, reach the human food supply.”

Habitat loss

According to Rinke, the rise in temperature at low tide can lead to the formation of areas devoid of oxygen, for example on the river bed. “Infected areas are lost as habitat for fish, mussels, insects, etc. While fish may be able to move to other areas of the water body, this is not the case for mussels,” explains the biologist. can die

At the same time, there may be more animals migrating from warmer regions. For example, the Black Sea guppy, a fish named after its origin, has already spread in German rivers. “This will increase exponentially in the future and species communities will change dramatically,” Reinke predicts.

There was just a huge fish killing in Oder. There was no clear explanation for the present. At the beginning of August, the Water Authority in Wroclaw, Poland announced that a substance with strong oxidizing properties may have entered the water. In addition, the toxin mesitylene was detected in two places. Investigations are underway. Many dead fish have also been found in the Oder River in Germany. The river water is also low. The resulting general relative increase in the proportion of pollutants could have resulted in the death of the fish.

Smaller bodies of water are also affected

Summer drought also affects streams. “Small streams are often affected more than large streams,” the biologist explains. The percentage of waste water could be higher here. In addition, some streams temporarily dried up – also due to a lack of groundwater. “The riverbeds that have dried up represent a complete loss of the flora and fauna of the water body,” warns Reinke. In Saxony-Anhalt and Brandenburg, for example, groundwater has decreased by one meter since around 2010.

The Minister of Environment of the Rhineland-Palatinate Catherine Eder (the Greens) responded to a parliamentary question with the aim of dredging the upper reaches of the lower mountain ranges, with the onset of drought, some fish can escape in time to deeper sections. “Others can’t do it and they die in the basins of streams that dry up.” In the dry summer of 2020, this happened in Hunsrück, for example.

According to Eder, summer water temperatures in streams can be lowered drastically by shading trees and shrubs in favor of animal survival. Comparative studies conducted by the Rhineland-Palatinate State Office of the Environment in July 2022 on different water bodies showed this.

Magdeburg biologist Rinke recommends trapping more water in the landscape – seeping into the ground rather than damming it. “This also helps with floods and heavy rains and supports agricultural production.” All the water that reaches the river “after one to two weeks is at sea and thus loses the country”. dpa

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