Hundreds of tons of dead fish floating in the Oder in early August 2022. © Luc De Meester/IGB
A deadly wave hits the Oder
9 August 2022
A wave of dead fish reaches the border river. Before that – in late July – there had already been isolated incidents of fish mortality on Polish territory. In response, the municipality of Frankfurt/Oder warns its population not to come into contact with the Oder water or to eat fish from the river. Then everything happens very quickly: within just two weeks, hundreds of tonnes of dead fish are found floating on the river. More than 200 tonnes are recovered, and even greater quantities sink to the bottom or are washed up on the river banks. The creatures exhibit symptoms of suffocation.
Suspicion confirmed: researchers identify the brackish water alga Prymnesium parvum
15. August 2022
Data measured at the monitoring station in Frankfurt/Oder by the Brandenburg State Office for the Environment (LfU) reveal an unusual picture: “Oxygen levels were well above 100% saturation, and pH was elevated. At the same time, we noticed strong fluctuations over the course of the day,” explains Jan Koehler, algae specialist at the Institute of Freshwater Ecology and Inland Fisheries (IGB). “Such dynamics can only be explained by photosynthesis. To us, this meant that we were dealing with a massive algal bloom.” At first, the reason for the high electrical conductivity is unknown. The level increases from 800 to more than 2,000 micro-siemens per centimetre (µS/cm) – such high concentrations in a river can only occur as a result of industrial salt discharges.
Researchers of IGB immediately start evaluating samples, measuring the algal pigment content and fitness, taking photos and preserving samples for genetic analysis. What they find is more than unusual for European freshwaters: large numbers of Prymnesium parvum, a brackish water alga. This species of algae is known to produce a strong toxin that attacks mucous membranes and thin blood vessels, and, in particular, to suffocate fish and molluscs. This alga has the potential to proliferate when there is abundant light, and when the river exhibits low flow velocities, warm water temperatures, and comparatively high salt and nutrient loads. The salt soon proves to be sodium chloride – common “table salt”.
19 August 2022
IGB detects and microscopically identifies Prymnesium parvum in all samples collected from the middle reaches of the Oder. The samples are sent to a colleague at the University of Vienna’s Department of Food Chemistry and Toxicology for toxin detection. “We were able to unequivocally detect significant quantities of a subtype of the algal toxin, known as ‘prymnesins’, in samples taken from various parts of the Oder,” states Elisabeth Varga, a scientist from the University of Vienna. Tests on fish eggs with Oder water conducted by IGB confirm the lethal effect of the toxin.
In light of the findings obtained thus far, the researchers are convinced that it is not a natural phenomenon. “To my knowledge, such a mass development has never been observed before in our freshwaters,” remarks Jan Koehler. “This disaster would not have happened if there had not been man-made discharges and interventions in the river.”
A further indication: satellite data confirm a massive algae bloom
31 August 2022
IGB scientist Tobias Goldhammer has analysed data from the Copernicus satellite Sentinel-2 with colleagues from the University of Leipzig and Brockmann Consult, an environmental data analysis and software company from Hamburg. They looked for early signs of algal blooms and wanted to reconstruct the temporal and spatial development. Chlorophyll concentrations indicating the algal bloom can be calculated from the processed raw satellite data. The dataset is now complete. It shows that in the second half of July, the concentrations in the entire river course were at a moderate level, while in the upper course around the town of Opole (Poland), they were already elevated. At the beginning of August, the chlorophyll concentration spikes near Wrocław (Poland). The algal bloom then spreads very rapidly, covering almost the entire River Oder within a week. It is not until late August that chlorophyll concentrations return to the average level of early July.
According to the researchers' analysis, the environmental disaster was caused by several stress factors, all of which were caused by human activity. These include development measures that had already reduced the river's natural resistance to hydrological and climatic changes: "We see the Oder disaster as a multi-causal, man-made event. Increased salt levels due to industrial pollution in the upper reaches of the Oder have also occurred more frequently in the past, without such massive algal blooms having occurred. However, the general conditions now seem to have changed," explains Tobias Goldhammer.
An initial test fishing documents a massive drop in fish stocks
27 September to 19 October 2022
To get a better overview of the remaining fish populations, IGB performs several test fishing procedures in the River Oder. Their results are sobering: species such as burbot, loach, common bream and white bream have suffered massive losses. Larger fish with a body length of 10 centimetres or more are now virtually non-existent.
The researchers are particularly concerned about the Baltic golden loach, which in Germany occurs exclusively in the River Oder. The only known stable population near Reitwein was estimated to comprise about 500 individuals. Not a single specimen can be detected there now. Instead, the IGB team finds ten specimens at Ratzdorf and a single specimen in the lower River Neiße. “It is not known whether the fish fled and migrated from upstream stocks before the toxic wave struck, or whether it is a stable population that has already been existing for a while; nor how long the specimens have been present there,” remarks fish ecologist Christian Wolter.
Also in October, the LfU measuring station at Frankfurt/Oder again shows highly elevated conductivity, even though more water is flowing in the Oder than in the summer months. This means that even higher quantities of salt are being discharged into the river than at the time of the acute disaster situation.
No sign of recovery yet – salt loads remain too high
29 November 2022
Following the first scientific test fishing procedures along the banks in September and October, the first extensive fishing event in the middle of the Oder since the disaster now gets underway. This inventory is sobering: the scientists catch much fewer fish, and species such as blue bream and asp are lacking completely. In total, the researchers catch only half as many fish as in previous years on average. But not only are there fewer fish: mussels and snails have also almost disappeared. “Data from August already provided us with indications that the Oder disaster has reduced mussel biomass by half,” remarks Christian Wolter. They are the most important filter-feeders in the ecosystem. “It will take a very long time for stocks to replenish, because mussels are not mobile enough to leave their refuge and repopulate areas quickly. This is particularly the case for native large mussels, which also face competition from invasive mussel species,” says the ecologist.
Measurements of conductivity taken during fishing show once again that salinity levels are still far too high for the river ecosystem. At the Frankfurt/Oder gauge, conductivity has been more than 1,900 µS/cm since mid-November already; by the end of November, the figure even exceeds 2,000 µS/cm. The major component of the salt load continues to be sodium chloride. “We detected around 400 milligrammes of sodium chloride per litre of water in water samples from the Lower Oder, which equates to about half of the total amount of salt in these samples. Obviously, large quantities of salt are still being discharged into the river,” explains biogeochemist Tobias Goldhammer.