Expert: Marine heatwaves can have devastating effects on life in Baltic Sea

The resilience limit of the biota in the Baltic Sea is relatively high, but marine biologist Jonne Kotta from the University of Tartu says recent marine heatwaves could cause extensive damage if they persist for a long time.
Last week, the Tallinn University of Technology's Institute of Marine Systems announced that satellite imagery shows that marine heat waves developed in the Baltic Sea. Heat waves can occur all year round, requiring water temperatures to significantly exceed the long-term average for at least five consecutive days. The duration and magnitude of marine heat waves in the Baltic Sea have increased in recent decades.
According to Kotta, marine heatwaves are not rare in the context of global warming. As a result of uneven regional warming, extreme weather events, such as storms, droughts, etc., are becoming more frequent. "The seasons have already become strange for us. I don't think there was any spring this year; winter was followed by summer," she said.
The Baltic Sea biota is relatively resilient to heat waves, Kotta said, but within limits. Seasonal variations already mean that all organisms in the sea have to tolerate some changing conditions. This gives them a certain degree of resilience to temperature fluctuations.
"The Baltic Sea is actually very young – only about 10,000 years old." During that time, it has gone through very fresh water periods when it was an iceberg lake, as well as saltier periods. It's that in-between time now as well," the marine biologist explained.
"Everyone who lives here has come from somewhere else, but the conditions here are very different. In addition to the variation in salinity, look at the seasons: in winter we sometimes have -30 degrees and in summer +30 degrees. That is a huge range that organisms have to endure, especially in coastal waters. Anyone who comes to live in the Baltic Sea has to be very resilient," says Kotta.
Baltic Sea temperatures have reached tolerable limits
According to Kotta, the heatwave that began last week brought such a dramatic change in temperature that it exceeded the tolerance limit even of the Baltic Sea's biota. This could be quite devastating. Keep in mind that the sea typically exhibits greater inertivity than the air, resulting in lower temperature amplitudes.
"Ten days ago, I went diving in the open Baltic Sea, and the water temperature was four degrees. Three days ago, I went diving near Veere in Saaremaa, and the water temperature was already 15 degrees. No normal form of marine life likes such a rapid temperature change. They are certainly under stress," the biologist said.
On the southern coast of Saaremaa, however, the sea temperature is already 20-25 degrees, according to Kotta. "Spring is a time of big changes in coastal nature; when plants start to bloom, invertebrates develop, and fish spawn. All these life forms have evolved over a long evolutionary time to adapt to an environment where sea temperatures do not exceed 10 degrees Celsius at any given time of life."
The marine biologist suggested imagining that it was 20 or 25 degrees instead of the usual 10. The researcher explained that organisms accustomed to lower temperatures use an enormous amount of energy for respiration and other life activities, which they cannot use for growth and development.
Such temperature changes kill many animals in the wild, she said. For example, mussels, an important species in our marine aquaculture, begin to die at around 25 degrees Celsius.
"Feeding mussels attach themselves to the seafloor with threads, but when the temperature exceeds 25 degrees Celsius, they simply let go and are carried by the waves to deeper depths where, unfortunately, they later die for lack of food," she said.
Likewise, many species that provide habitat for other animals, such as blackthorn and seagrass, are dying because of the heat. Their biomass will decline, as will the ability of such habitats to provide a variety of ecosystem services.
"As you know, nature does not tolerate empty spaces. At their expense, there is more room for opportunistic species to dominate the system," she went on.
According to the scientist, the disappearance of shellfish and some other species from the seabed releases huge amounts of nutrients that these species had previously locked up. "In this way, there will be more frequent blooms. If the summer heatwaves persist and there are not many waves, we will again have blue-green algae and a huge amount of stinking sludge from the algae mowing the shores."
"It's still spring and the water is still cool in some places, but if we get another heatwave of similar intensity in the summer, of course, it's going to be a mess. We are already at the high end of the temperature range of the summer, and if you add five degrees to that, you get mass mortality," she said.
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Editor: Kristina Kersa