An algal bloom that killed hundreds of thousands of fish in the river Oder between Germany and Poland in 2022 could reappear this summer with devastating consequences, scientists have warned.
About 360 tonnes of dead fish were hauled from the Oder, which runs for 840 kilometres along the German-Polish border, between July and August last year following a huge bloom of the toxic alga Prymnesium parvum.
The European Commission described it as “one of the largest ecological disasters in recent European river history”.
Prymnesium parvum is usually found in the brackish waters of estuaries. Scientists are unsure how it made its way to the Gliwice canal in Poland, a spur of the Oder that lies hundreds of kilometres from the coast and where the bloom is thought to have originated.
But now the alga is present in the waters, a fresh bloom could appear in the Oder or nearby rivers if conditions are right, scientists fear.
“That’s one of the main concerns that we have: that it reoccurs in this river, but also that it could spread to other polluted rivers,” says Gary Free at the European Commission’s Joint Research Centre (JRC), which has published a study into the 2022 fish deaths on the Oder.
The river’s poor water quality provided an “ideal soup” for Prymnesium parvum to bloom, Free and his colleagues concluded.
Due to discharges from agriculture and wastewater sites, the Oder was already suffering from excess levels of nitrogen and phosphorus, nutrients that make it possible for algal blooms to develop.
The problem was compounded by successive heatwaves and a long drought during July and August 2022, which depleted water levels throughout the river and concentrated pollution.
Discharges of salty wastewater from industrial sites near the Gliwice canal then caused a surge in the river’s salt levels, allowing the algae to bloom. It is unclear whether the discharges were illegal or within the permissions granted under permits, says Free. Polish records suggest at least 34 facilities in the Oder catchment have a licence to discharge salty waste.
The ecological damage was worsened by poor communication between the German and Polish authorities, the JRC report said, with “late and incomplete” information exchange stalling response efforts.
A spokesperson for Germany’s environment ministry said in a statement authorities were only notified about the incident on 11 August, after fish had already washed up dead downstream inside the country’s borders. It said the international alert system for pollution incidents was being revised, adding: “In the future, it shall be even clearer that also in events like for instance a fish die-off, a transboundary and timely warning shall be released.” Polish Waters, the national water management authority for Poland, was approached for comment.
Alongside the huge numbers of fish that were killed in the event, populations of invertebrates such as mussels and snails were also severely impacted, says Dietrich Borchardt at the Helmholtz Centre for Environmental Research in Germany. These filter-feeders usually help to control algal blooms, so the drop in their numbers leaves the river more susceptible to another bloom this summer and in future years, he says.
“Because of the loss of invertebrate life – which is not that visible compared to the fish, but which definitely is there – I think there is a significant likelihood that the river now is in a much more vulnerable condition compared to spring last year,” says Borchardt.
A further bloom could devastate the river’s ecology, as research suggests fish stocks in the river have already fallen by half since the first incident, according to research by the Leibniz Institute for Freshwater Ecology and Inland Fisheries in Germany.
Jan Köhler at the institute has just started research investigating the impact Prymnesium parvum has on the survival and efficiency of filter-feeders like mussels. He is also concerned about more blooms of the alga. “We are afraid that the reduction in filtration activity will favour future mass developments of Prymnesium,” he says.
The Oder needs to go into “intensive care” to prevent further blooms, says Free, with urgent work required to tackle industrial pollution along the river and reduce nutrient loads. For example, authorities should be able to pause industrial discharges of salty waste when the threat of an algal bloom is high, the JRC recommends.
Others believe a wholesale change in the rules governing industrial discharges will be necessary to protect river health in the future, especially as European summers become hotter and drier under climate change. Discharge licences need to be rewritten with the impacts of climate change in mind, says Borchardt. “We have to ask if the permissions, for example, for the wastewater release, are sufficient under conditions of climate change,” he says. “My guess is they are not.”
Reference: JRC Publications Repository, DOI: 10.2760/067386
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