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. Incident light was generated by a slide projector (Liesegang Dianfant, Leitz Prado, Germany) equipped with a halogen lamp (Osram Xenophot 400 W/36 V, Germany) and 11 Schott neutral gray filters. Eleven light steps were conducted between 0 and 560 µmol photons m-2 s-1. Dark respiration was measured first for 20 mins followed by increasing light steps in 10 min intervals. Oxygen concentration expressed in % air saturation was logged by the OxyView software (Presens, Regensburg, Germany) and corrected for air pressure, salinity and logged temperature according to Tengberg et al. (2006). During post-processing, the oxygen production rate for each photon flux density (PFD) level was calculated by plotting a linear regression model through all O2-values measured during the time interval, and was normalized to either fresh mass (FM, unit: µmol O2 g–1 h–1) or disc surface area (DA, unit: µmol O2 cm–2 h–1).

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