Laminaria hyperborea off the island of Helgoland (North Sea, Germany) was sampled along a depth gradient (0.5, 2, 4, 6 m) throughout summer 2014. Discs were cut from three different blade regions (5, 25 and 50 cm above the stipe-blade transition zone) and set into photosynthesis versus irradiance (PI) curves. PI curves were fitted by minimizing the sum of differences between the measured oxygen flux and the model proposed by Jassby and Platt (1976). Parameters were normalized to fresh mass and area.

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