Seasonality influences the effect of warming on kelp photosynthesis

The consequences of a year-round warming scenario on the photosynthetic performance of kelp species are unknown. In times of global warming, fundamental photosynthesis data are important due to the high temperature dependence. Seasonal photosynthetic performance was quantified in the kelp Laminaria hyperborea sampled along a depth gradient (2, 4 and 6 m below mean low water spring tide) at the island of Helgoland (North Sea, Germany). Blade discs were cultivated at in situ irradiances and temperatures (spring: 7°C; summer: 16°C; autumn: 14°C; winter: 6°C) and in parallel under warming scenarios (+4°C) for 3 d. The results highlighted that seasonality and collection depth had a stronger effect on oxygen formation (photosynthesis versus irradiance curve) and pigmentation than short-term warming, as there is a stronger correlation with changing irradiance levels than with temperature. Photosynthesis was highest in spring and summer, with small impacts of warming, indicating a high resilience to short-term temperature increases. In summer, algae exhibited reduced maximum quantum yields ( F v /F m ) at sub-lethal temperatures (20°C), pointing to a moderate stress response. Photosynthetic acclimation along the depth gradient was intensified by short-term warming, reflected in a stronger decrease in P max and chlorophyll content. Overall, the influence of short-term warming on the depth acclimation varied seasonally, underlining the importance of considering the season. In conclusion, L. hyperborea revealed distinct seasonal photosynthetic response patterns and a high photoacclimation potential at different depths. Therefore, seasonal and depth effects need to be considered when predicting the effects of warming on kelp primary production.