<p>Coastal macroalgae are important for global warming mitigation due to their high carbon fixation efficiency. Stipitate laminarian kelps provide a large sink of carbon storage that consists of different thallus parts: blades, stipe and holdfast, where susceptibility to decomposition mainly depends on thallus rigidity. In this study, carbon retention was investigated in the stipe of the stipitate kelp <i>Eisenia bicyclis</i> by field decomposition experiments at three different times of the year. Stipes were examined with respect to their rigidity compared with blades and holdfasts. Cut pieces of stipe were suspended in the water column to prevent consumption by benthic grazers for the duration of the experiments. Stipe decomposition rate was found to be lower at lower water temperatures. Warm water temperatures accelerated the decomposition rate of the stipe and reduced the effectiveness of its function as carbon sink. Stipes of stipitate kelp (as the most rigid structure of marine algae) can function as a long-term carbon sink, but warming of the coastal environment may reduce the effectiveness of this potential function.</p>