In-situ photosynetically active radiation (PAR) was measured in different depths (1.2, 2.9, 4.4, 6.6 m) every 10-15 min during summer 2014. Odyssey PAR loggers were calibrated against a cosine-corrected planar PAR sensor (LI-190SA quantum sensor, LI-COR Inc., USA) over a 24 h period at 4 m depth in the Helgolandic South harbor. During the Laminaria hyperborea sampling period (seven weeks), incoming PAR was recorded continuously every 15 min at 1.2 and 2.9 m, and every 30 min at 4.4 and 6.6 m near the sampling area of sporophytes. To avoid biofouling of the sensor heads, PAR loggers were cleaned every week (1.2 m) or every second week (all other depths) by SCUBA divers.

Additional funding was provided by the Postdoctoral fellowship grant of the Fonds de recherche du Québec - Nature et technologies and the Weston Foundation.