Rising fish populations in the Oder River, yet algal peril persists
The Oder River, a vital waterway in Europe, experienced a significant environmental disaster in the summer of 2022, with an estimated 1,000 tons of fish, along with mussels and snails, perishing due to the toxin produced by the golden alga, Prymnesium parvum.
Three years on, the fish population in the Oder is showing signs of recovery, according to fish ecologist Christian Wolter from the Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB) in Berlin. The golden alga, found in the Oder for the first time in 2022, produces toxin, and while its production is not yet fully understood, researchers are working to unravel the mystery.
The federal government has been funding the research into the causes of the fish die-off and the golden alga since 2023. The project, which runs until 2026, boasts a budget of over 4.8 million euros. The Leibniz Institute, along with other researchers, are focusing on understanding when the golden alga produces toxin and how to prevent a mass development.
The golden alga is considered complex to research due to its toxin production. Prymnesium parvum typically produces toxins such as prymnesin-1 under certain stress or environmental conditions, linked to its biosynthesis pathway. While the exact seasonal timing is not fully detailed, P. parvum’s toxin production is known to be problematic during bloom events when environmental factors favour its proliferation, often in nutrient-rich and low-salinity waters.
To prevent mass development of P. parvum in the Oder River, chemical control using algaecides like peracetic acid (PAA) peroxide has been indicated as an effective approach to control toxin-producing blooms.
The high salt content of the Oder, a result of Poland's discharge of salty wastewater, has been identified as a significant factor contributing to the mass development of the golden alga. Former Federal Environment Minister Steffi Lemke (Greens) has repeatedly called for Poland to reduce the discharge of salty wastewater into the river.
Poland has announced plans to reduce salt discharges, especially from mining, into the Oder and its tributaries, starting in 2024. This move is expected to help decrease the high salt content of the Oder and create a more hospitable environment for the fish population.
Fishing has largely normalized since the environmental disaster, but professional fishermen are proceeding with caution to allow the fish population to reproduce well. Pike and zander, two common species in the Oder, are growing well, according to the expert.
While the recovery of the fish population is underway, it will take five to six years for the population to return to pre-die-off levels, according to the state fisheries association. More than a year after the environmental disaster, analyses showed that more than half of the fish were still missing in the Oder.
Environmentalists are concerned that a mass die-off of fish could recur in the Oder. However, if no further incidents occur, a full recovery of the fish population is expected by 2026 or 2027, according to Christian Wolter. The focus now is on monitoring water conditions, reducing nutrient inputs, and using algaecides such as PAA peroxide to prevent mass development of the golden alga and ensure the continued recovery of the Oder's fish population.
As the fish population in the Oder River gradually recovers, scientists from the Leibniz Institute are focusing on understanding the conditions that trigger the golden alga, Prymnesium parvum, to produce toxins. The goal is to develop strategies to prevent another mass development of the golden alga, which could potentially cause another health-and-wellness disaster, compromising the fitness-and-exercise opportunities for those who rely on the Oder for recreation. Meanwhile, ongoing climate-change scenarios could potentially exacerbate the proliferation of the golden alga, calling for increased environmental-science research on the long-term impact of the algae on the aquatic ecosystem and the food chain.
