A year after the Odra flood: Baltic Sea shows strong resilience

Experts confirm that last year’s Odra River flood had no lasting impact on the Baltic Sea. Despite the influx of freshwater and pollutants, the sea absorbed the disturbance and recovered within months.

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14 september 2025   |   13:06   |   Source: Gazeta Morska / PAP   |   Prepared by: Marek Matacz   |   Print

fot. Regionalny Zarząd Gospodarki Wodnej w Gdańsku

- If we look at the entire Baltic ecosystem – fish, crustaceans, plankton, benthic organisms – there is virtually no trace of the flood’s impact today, said Professor Jan Marcin Węsławski of the Institute of Oceanology at the Polish Academy of Sciences. - The Baltic, though relatively small and shallow, holds a huge water volume compared to what flowed in. After just a few months, no significant biological effects could be observed.

Nutrient load is a chronic, not flood-driven problem

The temporary influx of nutrients (biogenic substances) did not significantly worsen eutrophication risk.

- The Vistula River supplies far more nutrients to the Baltic on a daily basis than the Odra did even during the flood. The event merely caused a short-lived increase, explained Professor Jan Marcin Węsławski.

He noted that nutrient oversaturation will remain a challenge for decades – possibly the entire century – even with drastic reductions in inputs.

Szczecin Lagoon acted as a natural filter

Professor Karol Kuliński stressed that the flood’s greatest impact was felt in the Szczecin Lagoon rather than the open Baltic.

- The lagoon acts as a biogeochemical filter. Most of the excess pollutants, nitrogen, phosphorus, and organic matter were trapped and deposited there, which was clearly visible in sediment analyses, said Professor Karol Kuliński.

During the flood peak, the Odra’s flow increased from an average of 550 m³/s to more than 2,500 m³/s. The majority of material carried during this time was processed and stored within the lagoon ecosystem.

Hypoxia remains a structural feature of the Baltic

According to Professor Karol Kuliński, oxygen deficits near the seabed – so-called “dead zones” – are one of the Baltic’s major environmental challenges but are largely natural.

- The Baltic’s semi-enclosed nature and strong stratification create a ‘second bottom’ around 70–80 meters deep, preventing oxygen from reaching the deepest layers. Floods have little effect on this phenomenon, he explained.

Protecting rivers and wetlands is key

Both researchers emphasize the importance of keeping rivers in as natural a state as possible.

- The more we straighten rivers and turn them into shipping channels, the more polluted the water becomes during floods. Natural floodplains and wetlands, like the Szczecin Lagoon, are crucial for protecting the Baltic, said Professor Jan Marcin Węsławski.

Climate change means more extreme events

With rising global temperatures, the frequency of extreme events is expected to grow.

- Floods are simply the atmosphere releasing excess energy. As temperatures rise, we should prepare for more such events, warned Professor Karol Kuliński.

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