Description: Kelps (Phaeophyceae, Laminariales) function as ecosystem engineers along many Arctic rocky shores. With ongoing climate change, the frequency and intensity of marine heatwaves are increasing. Further, extensive meltwater plumes darken Arctic fjords. It was the aim of this study to analyse the future development of Arctic kelp forest ecosystems. We conducted a laboratory experiment, in which we determined physiological and biochemical responses of Agarum clathratum and Saccharina latissima to a marine heatwave scenario (4, 7, and 10°C) being acclimated to either low-light (3 µmol photons m²/s) or in-situ-light (120 µmol photons m²/s) conditions. Grown sporophytes were sampled in Nuup Kangerlua, Greenland from a sampling depth between 7-10 m. Meristematic discs were cut (diameter 2 cm) and distributed equally between treatments and replicates. The discs acclimated to the light conditions for two days, before the 12 days heatwave scenario started, followed by a five-day recovery period. Maximum quantum yield of photosystem II (Fv/Fm) measurements were measured every second day, using a pulse amplitude modulated fluorometer (Portable Chlorophyll Fluorometer PAM-2100, Heinz Walz GmbH, Effeltrich, Germany). The following kelp response parameters were measured before (day 6), at the peak (day 18) and after (day 23) the experimental heatwave. Growth was analysed by measuring the discs dry weight (g). Rapid light curves were measured with a pulse amplitude modulated fluorometer (Portable Chlorophyll Fluorometer PAM-2100, Heinz Walz GmbH, Effeltrich, Germany), using irradiance steps between 0-611 µmol photons m²/s to derive photosynthesis vs. irradiance curve parameters (Pmax, alpha, EK). Dark respiration and net photosynthetic rates were measured with a 4-channel optode set-up (FireStingO2 Fibre-Optic Oxygen Meter FSO2-C4, PyroScience Sensor technology, Aachen, Germany) by analysing the oxygen evolution in response to different light intensities within a 25 ml Schott bottle, each containing meristematic discs. Pigment content was analysed with a High-Performance Liquid Chromatograph (HPLC, LaChromElite® system, L-2200 autosampler (chilled), DA-detetctor L-2450; VWR-Hitachi International GmbH, Darmstadt, Germany). Total phlorotannin concentration was analysed with the Folin-Ciocalteau assay, using a phloroglucinol dilution series (C6H6O3, Sigma-Aldrich: 0–1000 µg/ml).