environment
Low water in rivers can kill animals
The Rhine currently has little water – like here in Bonn. photo
© Roberto Pfeil/dpa
The low water levels of the Rhine and other rivers affect inland navigation. Fish and mussels fare worse: high temperatures and little oxygen can have fatal consequences for them.
Midsummer, drought, low water: A huge area of gravel has dried up on the southern tip of the Rhine island of Niederwerth near Koblenz. Zoology professor Jochen Koop bends down at the waterline and picks up clams. Some are still alive, others are already dead.
In the low water, which was heated up more by the sun and with less dissolved oxygen, they had problems with their metabolism, as the biologist from the Koblenz Federal Institute for Hydrology (BfG) explains. “The temperature here in the shallow water is like in a bathtub.” The extremely low water levels in many rivers meanwhile annoy not only inland waterway boaters. Nature suffers too.
On the way to the fish kill
For example, in the hot summer of 2003, fish and shellfish died out in rivers. “Around 50,000 dead eels washed up all over the Rhine,” says Koop. Less water, sometimes more current, higher water temperature, less oxygen concentration: respiration and exercise require much more energy for animals, they cannot take in enough of it. They become weaker and more susceptible to disease. “In 2006 and 2007 there were periods of heat on the Rhine, but the water was not so low at the same time,” the biologist recalls. That was less dramatic for animals. “But in 2022 we will be on the way to the fish kill again,” Koop fears. If it doesn’t rain longer and river levels fall 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 of permanently more than 26 degrees Celsius. Nevertheless, the oxygen binding capacity in the water decreases with the warming. At the same time, as the water level drops, fish retreat into the deeper shipping channel, where they have to avoid the ships. Both stress her.
Karsten Rinke, Head of the Lake Research Department at the Helmholtz Center for Environmental Research (UFZ) in Magdeburg, also explains: “At low water levels, the proportion of treated wastewater in the discharge process increases percent of the discharge.” This increases the concentration of pollutants and nutrients in the water. This could “also lead to an accumulation of pollutants in the organisms that, in the case of fish, reach the human food supply”.
habitat is lost
According to Rinke, the increasing heating at low tide can lead to the formation of oxygen-free zones, for example on the river bed. “The affected areas are lost as a habitat for fish, mussels, insects and so on. While fish can escape to other areas of the body of water, this is not the case for mussels,” explains the doctor of biology. you can die
At the same time, there may be more animal immigrants from warmer areas. For example, the Black Sea goby, a fish named after its origin, has already spread to German rivers. “This will increase sharply in the future and the species communities will change significantly,” predicts Rinke.
There has just been a massive fish kill in the Oder. There was no clear explanation for the time being. The water authority in Wroclaw, Poland, announced at the beginning of August that a substance with strong oxidizing properties might have gotten into the water. In addition, the toxic substance mesitylene was detected in two places. investigations are ongoing. Many dead fish were also found in the Oder in Germany. The river also has low water. The resulting general relative increase in the proportion of pollutants could have promoted the death of fish.
Smaller bodies of water also affected
Summer drought also affects streams. “Small watercourses are often much more affected than large ones,” explains the biologist. The proportion of waste water could be higher here. In addition, some streams have dried up temporarily – also due to a lack of groundwater. “River beds that have dried up are a total loss for the flora and fauna of the water body,” warns Rinke. In Saxony-Anhalt and Brandenburg, for example, the groundwater has dropped by one meter since around 2010.
The Rhineland-Palatinate Environment Minister Katrin Eder (Greens) responds to a parliamentary question with a view to brook upper reaches in low mountain ranges, with the beginning of drying out, some fish could flee in time to deeper brook sections. “Others don’t manage to do this and they die in creek beds that dry up.” In the drought summer of 2020, this happened in the Hunsrück, for example.
According to Eder, the summer water temperatures of streams can be significantly reduced by shading trees and bushes in favor of the survival of animals. Comparative studies by the Rhineland-Palatinate State Office for the Environment in July 2022 on various bodies of water have shown this.
The Magdeburg biologist Rinke recommends holding back more water in the landscape – by seeping into the ground instead of sealing it. “This also helps in flood situations and heavy rain and supports agricultural production.” All water that gets into a river “is in the sea one to two weeks later and is therefore lost for the country”.