The potential contribution of kelp forests to blue carbon sinks is currently of great interest but interspecific variance has received no attention. 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 the carbon sequestration potential of these ecosystems, we quantified interspecific differences in carbon export and decomposition alongside changes in detrital photophysiology and biochemistry. We found that while warm temperate kelp exports up to 71% more carbon per plant, it decomposes up to 155% faster than its boreal congeners. Elemental stoichiometry and polyphenolic content cannot fully explain faster carbon turnover, which may be attributable to contrasting tissue toughness or unknown biochemical and structural defences. Faster decomposition causes the detrital photosynthetic apparatus of L. ochroleuca to be overwhelmed after 20 d and lose integrity after 36 d, while detritus of cold temperate species maintains carbon assimilation. Depending on the photoenvironment, detrital photosynthesis could further exacerbate interspecific differences in decomposition via a potential positive feedback loop. Via compositional change such as the predicted prevalence of L. ochroleuca, ocean warming will therefore likely reduce the carbon sequestration potential of these temperate marine forests.