Résumé
Climate change is projected to increase the frequency and intensity of marine heatwaves, potentially having drastic consequences on coastal environments. To assess how heatwaves of various intensities affect plankton communities in the Baltic Sea, an indoor mesocosm experiment was conducted with natural plankton communities sampled in the water column of the Gulf of Finland (N 59.91687, E 25.03022, max 82 m). Mesocosms were subjected to four different temperature levels: in situ Gulf of Finland water temperature at the moment of the experiment (16°C, control), and three levels of heatwaves: a mild (+2°C, 18°C), a moderate (+4°C, 20°C) and an extreme (+6°C, 22°C) heatwave. High-frequency monitoring from automated sensors immersed in the mesocosms was used to measure temperature, salinity, dissolved oxygen and chlorophyll-a concentration. The sensors were immersed at a depth of 1m inside each mesocosm, and made one measurement every minute during 10 days (from August 22nd to September 2nd, 2022).