“Marine heatwaves are relatively rare, but they are extreme water temperature anomalies, where water temperatures are significantly above long-term climate normals for at least five consecutive days. Water temperature is one of the main determinants of the distribution, functioning, and behavior of organisms and is also an important consideration for holidaymakers. Marine heatwaves can have extremely negative effects on both ecosystems (changes in biological production, harmful algal bloom (HAB), increased species mortality, etc.) and socio-economic activities (human health, fisheries, etc.). Increasing frequency and intensity of marine heat waves will also have an impact on the now much talked-about blue economy”, Dr. T. Dabulevičienė lists the negative effects of marine heatwaves.
According to the oceanographer, there is still a lack of knowledge and information about heatwaves and their consequences. This is why KU scientists analyze long-term measurements from coastal stations and use the latest research methods, such as satellite images of water surface temperature, which also allow them to observe spatial changes in water temperature.
Dr. Toma Dabulevičienė became interested in heatwaves a few years ago during an expedition in August. At that time, KU’s research vessel Mintis carried out studies on the temperature of the Baltic Sea, which showed unexpected results – the temperature reached 21°C at a depth of about 20 meters.
Baltic Sea heatwaves are getting longer and more intense
According to the scientist, such high bottom water temperatures are not typical for the Baltic Sea. Following such study results, Dr. T. Dabulevičienė continues her research. Students also contribute to the studies. Currently, Inesa Servaitė, a student at KU Marine Research Institute, is working on her master’s thesis on heatwaves.
The study analyses the water temperature of the Baltic Sea during the warm period 1993-2023 (May-August). The results show that the Lithuanian part of the Baltic Sea has experienced marine heatwaves almost every year. It is also observed that heatwaves in recent years last much longer than at the beginning of the study period.
For example, at the beginning of the research decade, heatwaves of the warm period of the year lasted up to a week, while 2021 was characterized by extreme heatwaves with a total of 60 days of heat.
It is in 2021, when the average water temperature in July was more than 4°C above the climatological average for the month, and the recorded maximum water temperature was as high as 25.5°C, which is more typical of tropical seas than of the Baltic Sea waters. The impact of marine heatwaves was felt not only in the surface waters, but also at a depth of 20 meters, where the temperature then reached around 21°C.
Such high, prolonged bottom temperatures can lead to a decrease in oxygen concentration in the water and contribute to the formation of increasing “dead zones” (with oxygen concentrations ≤ 2 mg/l). This has been a particularly sensitive issue in the Baltic Sea for some time.
“It is noticeable that the prolonged periods of marine heatwaves attract more and more coastal residents and tourists to swim in the sea. From one point of view, this seems like a positive aspect. However, it is worth pointing out that the increased water temperature may make it more attractive not only for humans, but also for certain species that are harmful to humans, such as Vibrio bacteria. They thrive in low-salinity, warm waters (>18°C),” says Dr. T. Dabulevičienė.
You can find out more about marine heatwaves in a recently published scientific paper (in English): Dabulevičienė, T. & Servaitė, I. Characteristics of Marine Heatwaves in the Southeastern Baltic Sea Based on Long-Term In Situ and Satellite Observations. J. mar. sci. eng. 12, 1109 (2024). DOI: 10.3390/jmse12071109.
