It is estimated that around 100 tons of dead fish have drifted down the Oder so far.

The first larger quantities of fish carcasses were found south of Breslau at the end of July, and the first finds in Germany were made near Frankfurt an der Oder on August 9th.

There, as early as August 7th, there were noticeable changes in some parameters, for example the pH value of the river water rose significantly, i.e. it became more alkaline.

It is also unclear whether the high water temperatures or the low water levels have anything to do with the phenomenon.

Ulf von Rauchhaupt

Editor in the “Science” section of the Frankfurter Allgemeine Sunday newspaper.

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Laboratories in Germany and Poland are currently examining cadavers and river water for around 300 different chemical substances.

However, the current data situation is still incomplete, said Jörg Oehlmann, head of the Aquatic Ecotoxicology department at the Institute for Ecology at the University of Frankfurt am Main, the Science Media Center Germany.

"Specifically, even basic data is missing, such as the levels of total phosphorus, orthophosphate and ammonium in the water.

In this respect, valuation is subject to precisely these uncertainties.”

There are two possible strategies for clarifying the cause of fish deaths: First, "target analysis" for hundreds of chemical compounds that may be the cause.

"However, there is a great risk that, given the around 350,000 substances in use worldwide, the chemical responsible is not covered," says Oehlmann.

An alternative is to search for conspicuous patterns using a combination of liquid chromatography and mass spectrometry.

"However, identifying the chemical behind a conspicuous peak is very time-consuming."

This typically takes up to several weeks.

The pathologists need fresh tissue

The second strategy, explains the ecotoxicologist, analyzes the symptoms of poisoning in the deceased animals in a similar way to a human corpse, where poisoning is the possible cause of death.

"This approach is very effective and can use the symptom pattern - changes in organs, including histological examinations - to identify the causes well.

This typically takes three to five working days.

With both strategies, however, it is essential that 'fresh' samples are available because suspected substances may degrade quickly and then no longer be detectable.”

However, Oehlmann considers it unlikely that the overlapping of several individual events is responsible for the fish kill.

"The theory that the low water level and the high temperatures amplify the effects of individual chemicals, which are not toxic under normal conditions, or of the complex cocktail of substances, through additional stress, so much that the fish die off is not plausible to me."

Rather, the coincidence of the fish die-off with the sudden change in several parameters at the measuring station in Frankfurt an der Oder is striking: the increase in oxygen content to up to 160 percent of saturation, the pH value, conductivity, turbidity and the total chlorophyll content as well as with the decrease in nitrate nitrogen.

Cascade Effect: Decay makes the broth even more toxic

The death of fish and river invertebrates could be a result of the release of toxins by cyanobacteria, which produce numerous substances that cause nerve, liver and cell damage.

Such substances were also responsible for fish deaths in the past, says Oehlmann.

Cascade effects are often observed: the release of the first cyanotoxins leads to the death of sensitive species and the decomposition of their carcasses to the formation of ammonium.

As a result of the algal bloom, the pH value also increases and the pH-dependent balance is shifted from weakly toxic ammonium to highly toxic ammonia in favor of ammonia, "so that other species die off and in the end the entire system 'tips'."