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Could a fish kill like in the Oder be repeated?


At least 350 tons of fish died in the Oder last summer – the cause was probably a poisonous algae that was able to flourish in the salty low water. Could such a disaster happen again?

The Oder has always been a heavily polluted river. Large industrial and mining areas on the Polish side have been discharging saline sewage for years. That didn’t change last summer. But there was one thing: It was even hotter and even drier. The river carried low water. And the more water evaporated, the higher the salinity rose. The freshwater researcher Martin Pusch from the Leibniz Institute for Freshwater Ecology and Inland Fisheries says the high salinity could have been prevented.

According to Pusch, every authority would actually take into account the effects that discharges would have on the river – “that has apparently been neglected on the Oder in recent decades, although this is required by the EU Water Framework Directive, i.e. the applicable law”.

gaps in water protection

The little water and the resulting high salt concentration created good living conditions for a toxic alga. Ultimately she was the reason for the massive fish kills. On behalf of the Federal Environment Agency, researchers analyzed how global warming is affecting all of our water bodies and whether the existing control mechanisms are still sufficient.

The scientists Sabine Wollrab and Marco Neubert come to the conclusion that there are fundamental gaps in the protection of all our bodies of water. Even the strict requirements of the European Water Framework Directive would no longer be sufficient – or not sufficiently complied with.

“The ecological balance of the waters is already in danger today due to use – and this will become even worse as a result of climate change,” Neubert is certain.

Researchers complain about data shortages

According to Neubert, there is far too little data. Some rivers and lakes are only sampled every three years, others only every six years. This must be increased significantly. Above all, one should include climate indicators in the assessment – i.e. animals and plants that show whether bodies of water are under heat stress. According to researcher Wollrab, rivers are just as affected as lakes. The latter need the alternation of cold winters and warm summers to mix.

“In winter, when I have very cold temperatures and even ice cover, I have a so-called inverse, i.e. a reverse stratification, where the coldest water is at the top, around zero degrees, and just below the four degrees cold water” , explains Wollrab.

All the water in the lake is circulated in this way. If that doesn’t happen because the winter was too warm, for example, there is a lack of oxygen in the depths. Anoxic states could even arise, i.e. areas completely without oxygen. Plants and animals would die there.

More than 350 tons of fish died in the Oder in the summer of 2022, and helpers were on duty for weeks.

Image: dpa

Pusch: Elbe not endangered at the moment

The water bodies must be relieved accordingly, write the researchers in their analysis and demand that more water must be kept in the area. This includes getting moors wet again. Farmers would have to stop draining their fields. They do this to drive heavy machinery there. According to Wollrab, shady bushes and trees are also necessary by the water.

“Or that at low water there are still pools left in certain areas or deeper sections where species can retreat,” says Wollrab.

That’s what happened in the Oder. Many fish found refuge in marginal waters. Researcher Pusch says that saved their lives. A catastrophe like the one on the Oder cannot currently occur in the Elbe. A mass development of the blue-green algae is possible, but that is purely speculative.

Authorities should check discharges

Above all, the Elbe has problems as a result of its deepening into a shipping lane, as which it is hardly used anymore. Because the Elbe has also had low water levels for years.

“That means we can no longer discharge such large quantities of pollutants into the water. We also have to give the inhabitants of the rivers better living conditions,” demands Pusch. The waters would have to be renatured so that there would be refuges and retreat areas in which at least small parts of the population could survive such stressful situations.

The topic was addressed at the environment ministers’ conference of the federal states last November. The authorities should now check whether discharges from industry should be throttled depending on the water level. However, they do not have to report until autumn 2024.