The spring and early summer of 2025 were cooler than average in Estonia and the slower warming was reflected in coastal waters. A more noticeable rise in temperatures came only in the second half of July with the arrival of hot weather. Since then, surface temperatures in Estonia's coastal sea have in many places remained above the long-term average.

For example, in Tallinn Bay and Pärnu Bay, water temperatures have been 2–3 degrees higher, while along Saaremaa's western coast they have stayed below average. In the northern part of the Gulf of Bothnia, the difference reached as much as four degrees, leading to the formation of a marine heatwave covering 40,000 square kilometers by mid-July.

A marine heatwave is defined as an abnormal situation lasting at least five days, when surface water temperatures are higher than in 90 percent of days in previous years. In the second half of August, another, smaller marine heatwave developed in the Gulf of Finland, and due to the continued warm weather, it is still ongoing.

Young fish perish and ecosystems damaged

Mariliis Kõuts, a researcher at Tallinn University of Technology's Department of Marine Systems, confirmed the situation is unusual and could affect marine life. "The fact that it is warmer than average means nature is facing different conditions. That does cause problems. For marine life, unusually warm waters trigger stress reactions," she explained.

For instance, fish are currently in an active juvenile development stage. Warmer temperatures may slow that development and increase mortality. "These are not things that can be easily measured in the sea, but it can be assumed that the conditions are not good for them," Kõuts added.

Marine heatwaves also affect underwater algae and plant communities, such as seagrass and bladderwrack meadows, which are cornerstones of the marine ecosystem. "If they are in poor condition, the impact shows up in every subsequent stage. For example, they serve as food for juvenile fish," Kõuts said.

In her view, warm sea temperatures in the fall may encourage further growth of phytoplankton and microalgae, especially if nutrients rise from the seabed. "That can lead to a larger autumn bloom, which in turn could result in oxygen shortages later in winter and spring," the researcher explained.

Weather, not climate

Although the summer of 2025 has not brought new heat records like in 2024, the frequency of marine heatwaves may still increase. "What we are seeing now is weather fluctuation, not climate change, but the overall trend is rising water temperatures, with peaks pushing them higher still. More frequent and widespread marine heatwaves are part of this extreme weather, which can now last longer," Kõuts explained.

The impact of such heatwaves can be assessed using a climate service developed at Tallinn University of Technology, which allows real-time monitoring of marine heatwave development and analysis of their extent, duration and frequency. The system is based on an autonomous data processing model that compares satellite-derived sea surface temperatures with long-term averages to detect anomalies across the Baltic Sea.

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