Environment: Low water in rivers can kill animals

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.

Midsummer, drought, low water: a huge expanse of gravel has dried up on the southern tip of the Rhine Island in Niederwirth 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.” Meanwhile, extremely low water levels in many rivers not only disturb inland waterway boats. Nature suffers, too.

Back 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.” That was less exciting for the animals. “But in 2022 we’ll be on our way to killing fish again,” Cope fears. If it doesn’t rain any longer and river levels drop more, “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.

“At lower water levels, the proportion of treated wastewater in the discharge process increases per cent of the discharge,” explains Carsten Reinke, Head of Lakes Research at the Helmholtz Center for Environmental Research (UFZ) in Magdeburg. This increases the concentration of pollutants and nutrients in the water. This may also lead to an “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. The biologist explains: “Infected areas are lost as habitat for fish, mussels, insects, etc. While fish can escape to other areas of the water body, this is not the case for mussels.” 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 sharply in the future and species communities will change dramatically,” Renke predicts.

There has just been a huge killing of fish in the 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 would 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. “River courses 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 don’t manage to do it and they die in the creek beds 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. This was shown by comparative studies conducted by the Rhineland-Palatinate State Office of the Environment in July 2022 on different water bodies.

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

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