In the search for the cause of the fish deaths in the Oder, scientists from the Berlin Leibniz Institute for Freshwater Ecology and Inland Fisheries (IGB) suspect an alga. They have detected high levels of the microalgae Prymnesium parvum in the Oder water, which is known to produce a toxin that is deadly to fish.
The question is why the alga, which actually does not occur in rivers, suddenly appeared in the Oder. Prymnesium parvum normally grows in brackish water with high salinity. “The type of algae requires increased salinity, which does not naturally exist at all on the affected stretch of the Oder,” IGB researcher Jan Köhler is quoted as saying in a press release.
However, favorable conditions for the alga could possibly have arisen at the barrages in the upper part of the Oder – for example, if saline sewage had been discharged into the dammed water. Mineral fertilizers that could have reached the Oder from agriculture also have the potential to promote massive algae growth. In principle, the nutrients contained in it fertilize the water in the same way as the fields.
Elevated water temperatures may also have contributed to the algal bloom
The low water levels may have amplified the effects of such chemicals because the less water in the river, the less the substances are diluted. Heat also promotes the growth of algae, partly because metabolic processes accelerate at high temperatures, so that they multiply faster. The heat and the associated increased water temperatures could also have contributed to the algal bloom.
However, all this has not yet been proven. So far, the researchers have only detected the algae, not the toxins. Why and under what conditions Prymnesium parvum produces toxins is not yet fully understood. One guess is that the toxins paralyze prey so the algae can consume them in peace.
Of course, the alga, which is only about ten micrometers in size, does not eat fish, but other single-celled creatures. However, in the event of an algal bloom, the concentration of toxins in the water can rise to such an extent that they are also fatal to fish. It seems certain that there was an algae bloom in the Oder. After all, the Berlin scientists found more than 100,000 algae per milliliter of water. And that at a point where the Warthe has already flowed into the Oder. Algae concentrations may be even higher further upstream.
The venom first attacks the gills and then works its way further inside the fish
As a rule, the algal toxin attacks the gills and fins of the fish first, simply because these areas are most exposed. If the top layers of cells are destroyed, the poison works its way further inwards. If it hits a blood vessel, the fish start bleeding. The poison also enters the bloodstream via the destroyed gills and damages the internal organs of the animals. The condition of the dead fish fits the assumption that Prymnesium parvum played a role in the fish die-off on the Oder, says Köhler.
Theoretically, the mass proliferation of this alga could also explain the unusually high oxygen concentrations measured in the Oder. Prymnesium parvum can photosynthesize like plants. This means that it can use sunlight to produce carbohydrates from carbon dioxide and water, and oxygen is also produced as a by-product. The increased chlorophyll content, which was registered at the measuring station for water quality in Frankfurt an der Oder from August 7th, also fits into the picture. Like many other algae and plants, Prymnesium parvum contains this pigment, which plays an important role in photosynthesis.
So there is much to suggest that the algae are at least involved in the environmental catastrophe in the Oder. Researchers are currently working on identifying the toxins typical of Prymnesium parvum. If their theory is confirmed, something else will also be clear: the catastrophe on the Oder is the fault of man, who made it possible for this alga to grow in an atypical place.