In the summer of 2022, a catastrophic environmental disaster unfolded along the Oder River, which forms a significant portion of the border between Germany and Poland. An estimated 1,000 tons of dead fish, along with countless mussels and snails, washed ashore, signaling a severe ecological crisis. The event, which began in the Polish section of the river and extended downstream into Germany, prompted urgent investigations from both nations to determine the cause of the widespread die-off that stunned scientists and the public alike.
Subsequent analysis and reports from scientific bodies in both Germany and Poland ultimately identified the primary culprit as a toxic bloom of a specific type of algae. This was not a typical freshwater algal bloom, but rather one involving a species that thrives in brackish, or semi-salty, water. The disaster was determined to be a man-made event, resulting from a complex interplay of industrial pollution, climate-related factors, and the unique biological properties of the offending microorganism. The scale of the fish kill highlighted the vulnerability of major river ecosystems to industrial discharges and the cascading effects of environmental stressors.
Unprecedented Salinity in a Freshwater River
The key factor that enabled the deadly algal bloom was an unusually high concentration of salt in the Oder’s waters. German and Polish investigations concluded that a sudden increase in salinity created the ideal conditions for the toxic algae to proliferate. While the exact source of the salt discharge remains a point of contention and investigation, industrial activities are considered the primary cause. Salts can enter river systems through various channels, including discharges from sewage treatment plants and mining operations. In the case of the Oder, rock salt, or sodium chloride, from mining activities is believed to be a significant contributor to the river’s salinity. Some reports specifically pointed to high salt levels at a copper mine in the Polish city of Glogow as a potential source.
The elevated salinity effectively transformed sections of the freshwater river into a brackish environment. This change in water chemistry was a critical precondition for the specific type of algae that caused the disaster. The salt content made the water toxic to many freshwater species and simultaneously created a favorable habitat for the salt-tolerant algae, allowing it to outcompete other microorganisms and grow to catastrophic levels.
The Culprit: Prymnesium parvum
The organism responsible for the devastation was identified as Prymnesium parvum, a species of microscopic algae commonly known as “golden algae.” This single-celled organism is typically found in brackish and marine waters around the world, with documented occurrences in Europe, North America, Asia, and Australia. While it can exist in freshwater, it usually does so in much lower, harmless concentrations. The high salinity in the Oder, however, allowed it to flourish. There are at least 40 genetically distinct strains of Prymnesium parvum, and the genotype that bloomed in the Oder River was identified as the B-type.
A Toxin Lethal to Gilled Organisms
What makes Prymnesium parvum so dangerous is its ability to produce potent toxins called prymnesins. Under certain conditions, the algae release these toxins into the water, where they have a devastating effect on gill-breathing organisms. The prymnesins destroy the gill tissues of fish, mussels, and snails, leading to a lack of oxygen and circulatory failure. The toxins then enter the bloodstream and decompose internal organs. This explains why the die-off was so extensive and affected such a wide range of aquatic life. The toxins also harm competing algae species and zooplankton, small organisms floating in the water, further disrupting the river’s food web.
A Confluence of Environmental Stressors
While high salinity and the presence of Prymnesium parvum were the direct causes of the disaster, other environmental factors created a perfect storm for the toxic bloom. The summer of 2022 was marked by a severe drought and high temperatures across Europe, which led to unusually low water levels in the Oder River. This reduced flow meant that pollutants, including the salt discharges, were less diluted and remained in the river for longer periods. The high water temperatures, combined with ample sunlight, further accelerated the growth of the algae.
In addition to salt, the river also contained high levels of nutrients like nitrogen and phosphorus, likely from agricultural runoff and wastewater treatment plants. These nutrients acted as a fertilizer for the algae, fueling its rapid multiplication. A study coordinated by the Helmholtz Centre for Environmental Research also found high concentrations of organic micropollutants, such as flame retardants and polymer additives, which may have exacerbated the lethal effects of the prymnesin toxins.
The Ecological and Political Aftermath
The ecological damage to the Oder River was immense. In some affected areas, the disaster wiped out up to 60% of the fish biomass and as much as 85% of the mussel and snail biomass. The sheer volume of dead animals—with over 100 tonnes of fish removed from the Polish side and another 35 tonnes from the German side—underscores the severity of the event. The long-term consequences for the river’s ecosystem and its recovery are still being studied.
The disaster also created political tensions between Poland and Germany. German officials initially criticized their Polish counterparts for a lack of communication about the unfolding crisis. In turn, Poland accused Germany of spreading “fake news” about the presence of herbicides and pesticides in the water. The two countries released separate reports on the disaster, and a German news magazine, Der Spiegel, accused Polish authorities of being uncooperative and secretive during the investigation. The Polish government was also criticized internally for its slow reaction, which led to the dismissal of officials responsible for water management and environmental protection.
Pathways to Prevention
In the wake of the disaster, environmental agencies and scientific institutions have issued recommendations to prevent a recurrence. A primary focus is on the significant and permanent reduction of salt discharges into the Oder River. This involves stricter regulation of industrial wastewater and mining byproducts. Establishing clear limits for salinity levels is seen as a crucial step to improve water quality and reduce the risk of future toxic algal blooms.
In the short term, recommendations include the ability to halt or severely restrict industrial discharges during periods of high risk, such as droughts or heatwaves. In the long term, the focus is on strengthening the overall resilience of the river’s ecosystem. This includes reducing nutrient pollution from agriculture and improving wastewater treatment to create a healthier, more robust aquatic environment that is less susceptible to the kind of cascading failure seen in the summer of 2022.