The potential contribution of kelp to blue carbon sinks is currently of great interest. In the Northeast Atlantic, kelp forest composition is changing due to climate-driven poleward range shifts of cold temperate Laminaria digitata and L. hyperborea and warm temperate L. ochroleuca. To understand how this might affect carbon sequestration potential, we quantified interspecific differences in carbon export and decomposition alongside changes in detrital photophysiology and biochemistry. We found that warm temperate kelp decomposes up to 155% faster than its boreal congeners, likely due to lower carbon and polyphenolic content. Faster decomposition further causes its detrital photosynthetic apparatus to be overwhelmed after 20 d and lose integrity after 36 d, while cold temperate species maintain carbon assimilation. Besides climate-driven phase shifts, heatwaves, forest miniaturisation, decompositional acceleration and coastal darkening, compositional change such as the predicted dominance of L. ochroleuca will likely reduce the carbon sequestration potential of these temperate forests.