A climate change profiteer? Temperature and light effects on primary production in non-native Vaucheria sp. turfs in the European Wadden Sea

Climate change profoundly impacts coastal ecosystems by altering temperature, light, and nutrient conditions that regulate primary production. In the European Wadden Sea, seawater temperatures have risen by nearly 2 °C over the past 60 years, promoting shifts in macroalgal communities, including the recent spread of the non-native yellow-green alga Vaucheria sp. (Xanthophyceae). We investigated how temperature and light affect Vaucheria sp. primary production under controlled laboratory conditions. Therefore, we measured the O2 concentration in the water as a proxy for primary production using fiber-optic oxygen meter. Primary production increased with temperature and reached a maximum at 32 °C, though individual variability was high. At high light intensities (500 and 900 µmol photons m-² s-¹), primary production increased with temperature, whereas at low light (100 µmol photons m-² s-¹), the highest values occurred at 16 °C. Photosynthetic parameters were stable between 20 °C and 32 °C but declined sharply at 36 °C, when thalli were nearly degraded. These results indicate a broad thermal tolerance and strong photo acclimation capacity of Vaucheria sp. Its ability to maintain high productivity under elevated temperature and irradiance suggests that it may benefit from regional warming. However, as extreme heat events could even exceed the high thermo-tolerance of Vaucheria sp., the need to understand its long-term acclimation potential and ecological impacts in a rapidly changing Wadden Sea is indispensable.