Ocean warming and acidification may substantially affect the photophysiological performance of keystone species such as Fucus vesiculosus (Phaeophyceae) in shallow coastal waters. In four consecutive benthic mesocosm experiments (Kiel Outdoor Benthocosm, Kiel, Germany, 54°20'N; 10°09'E), we compared the photophysiological performance (i.e., oxygen production, in vivo chlorophyll a fluorescence, energy dissipation pathways and chlorophyll concentration) of Baltic Sea Fucus under the single and combined impact of elevated seawater temperature (Δ + 5°C) and pCO2 (1100 ppm). Fucus specimens were sampled, in each season (spring: April 2, 2013; summer: July 2, 2013; autumn: 8 October; winter: January 14, 2014) from a depth of 0.2–1 m in the Kiel Fjord (Bülk), western Baltic Sea, Germany (54°27'N; 10°11,5'E). Photosynthetic performance was measured with two different methods, one based on in vivo chlorophyll a fluorescence measurements of photosystem II (PSII), the other one based on oxygen production. For each experiment and treatment, three Fucus specimens 15–25 cm long with 91 ± 30 total apices and apparently equal vigor were chosen, each individually growing on a stone (10–15 cm in diameter) from a single holdfast. For details see material and methods in Graiff et al. 2021. Photosynthesis was highest in spring/early summer when water temperature and solar irradiance increases naturally, and was lowest in winter (December to January/February). Temperature had a stronger effect than pCO2 on photosynthetic performance of Fucus in all seasons. Photophysiological responses were generally positive during the cooler spring months, but strongly negatively affected during summer (due to a marine heat-wave). Especially, future summer temperatures exceeded the thermal tolerance threshold of western Baltic Sea Fucus and had a deleterious impact overall. Potential benefits of the combination of future ocean warming and increased pCO2 over most of the year for Fucus photophysiological performance are suggested by our study, but not during summer peak temperatures.
