According to scientists, a toxic species of algae could be a decisive factor in the death of fish in the Oder. A researcher at Berlin’s Leibniz Institute for Freshwater Ecology and Inland Fisheries identified the toxic species as a microalgae named Prymnesium parvum. This species has also been found in Poland. According to the water ecologist Christian Wolter, it is known for occasionally leading to fish deaths. This is confirmed by Jörg Oehlmann, head of the Aquatic Ecotoxicology department at the University of Frankfurt. However, it has not yet been proven that the poison of this alga is the reason for the fish deaths, only their mass development is proven.
According to the researchers, the algae species Prymnesium parvum actually only occurs in brackish water. It requires greatly increased salinity, which does not normally exist on the affected stretch of the Oder. At the official measuring station of the State Office for the Environment in Frankfurt an der Oder, however, massively increased, unnatural salt loads have been measured for around two weeks, which, according to the researchers, must have their origin upstream.
According to the scientists, the mass growth of the algae also caused significantly increased measured values for oxygen, pH and chlorophyll. There are many barrages in the upper part of the Oder. Due to the low water there is currently hardly any water exchange there.
An investigation by the Brandenburg state laboratory also revealed that water samples taken from the Oder between August 7th and 9th contained a certain pesticide in unusually high concentrations. According to the spokeswoman for the Brandenburg Ministry of the Environment, Frauke Zelt, on Friday evening, it is the active substance 2,4-dinitrophenylhydrazine.
The values exceeded the maximum limit of the environmental quality standard. This is one microgram per liter (µg/l). The measurements would have resulted in 9.14 micrograms per liter for August 8th. The previous day 6.41 µg/l and on August 9th 4.28 µg/l were found. According to the Ministry, the annual average environmental quality standard is 0.2 µg/l.
According to “chemie.de”, 2,4-dinitrophenylhydrazine is a “detection reagent for aldehydes and ketones”. A connection with pesticides is not described there. The Ministry of the Environment could no longer be reached on Friday evening for inquiries in this regard.
According to Zelt, the samples come from a routine measuring point in Frankfurt (Oder). It can therefore be assumed that the pesticide was already heavily diluted here. This assumption is based on the assumption that the entry must have taken place far upstream in Poland.
In other places in the upper reaches of the Oder, there should have been higher concentrations in the days before. The notification does not reveal whether there are still similar, so-called “reserve samples” from there that can now be supplied by the Polish authorities for independent, targeted analyses. However, it is also theoretically possible that the pesticide got into the river near Frankfurt from an unknown source and then had nothing to do with the fish deaths upstream.
Exceeding the values for several days at the measuring location “certainly had an effect on the biocenosis (animals, plants, microorganisms), especially on macrophytes (aquatic plants)”. However, the dose detected in Frankfurt could probably not trigger fish kills, as has been observed.
The experts working on behalf of the state of Bandenburg apparently continue to assume that the fish kill could have been a “multi-causal event”, according to Zelt, i.e. several causes could have added up and possibly increased.
According to the current status, a combination of pesticides, high salt load, the growth of algae that is toxic to fish, which may be promoted by this, high water temperatures that stress fish, increased mercury levels caused in places by the turbulence of the sediment and possible other factors are possible.
The other results from August 19th confirmed “by and large the previous knowledge”, according to the ministry. The values would not have deteriorated in the further tests either.
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The limit values were therefore complied with for all finds of organic substances. “Even the metal values do not reach the acutely toxic range.” At the Oder border crossing at Lomy, however, the electrical conductivity, a sign of high salt content, is still increased. The chlorophyll values in Frankfurt (Oder) are therefore still unusually high. These could be related to the high oxygen levels that have also been and are being measured.
Since the end of July, an increasing number of dead fish have been found on the Oder. In the meantime, probably several hundred tons, but also large quantities of mollusks and crustaceans have died. The Polish authorities had long neglected to report these findings.