We welcome the recent synthesis by Howard et al. (2017), which drew attention to the role of marine systems and natural carbon sequestration in the oceans as a fundamental aspect of climate‐change mitigation. The importance of long‐term carbon storage in marine habitats (ie “blue carbon”) is rapidly gaining recognition (Figure 1a) and is increasingly a focus of national and international attempts to mitigate rising atmospheric emissions of carbon dioxide. However, effectively managing blue carbon requires an appreciation of the inherent connectivity between marine populations and habitats. More so than their terrestrial counterparts, marine ecosystems are “open”, with high rates of transfer of energy, matter, genetic material, and species across regional seascapes (Kinlan and Gaines 2003). We suggest that policy frameworks, and the science underpinning them, should focus not only on carbon sink habitats but also on carbon source habitats, which play critical roles in marine carbon cycling and natural carbon sequestration in the oceans. Howard et al. (2017) concluded that certain habitats and taxa (eg kelp forests, large vertebrates) are “unimportant” in natural carbon sequestration, which we argue is an oversimplification that fails to account for not only the magnitude of carbon transfer between living components of the cycle but also the interconnectedness of the highly dynamic and open marine environment. Crucially, developing carbon budgets for habitats in isolation – without considering their connectivity and functioning as carbon “fixers”, “donors”, and “recipients” – is neither representative of marine ecosystems, nor a useful approach for prioritizing management. Here, we highlight the importance of carbon transfer between habitats, which is not currently recognized within policy frameworks, through two pertinent and widespread processes.