Redefining what we consider a ‘blue carbon ecosystem’

Photoautotrophic marine ecosystems can lock up organic carbon in their biomass and the associated organic sediments they trap over millennia and are thus regarded as blue carbon ecosystems. Because of the ability of marine ecosystems to lock up organic carbon for millennia, blue carbon is receiving much attention within the United Nations’ 2030 Agenda for Sustainable Development as a natural climate solutions and possible nature-based solution, but classically still focuses on seagrass meadows, mangrove forests, and tidal marshes. However, other coastal ecosystems could also be important for blue carbon storage, but remain largely neglected in carbon cycling budgets. Using a meta-analysis of 253 research publications, we identify other coastal ecosystems – including mud flats, fjords, coralline algal (rhodolith) beds, and some coral reef systems – with a strong capacity to act as blue carbon sinks in certain situations. Features that promote blue carbon burial within these ‘non-classical’ blue carbon ecosystems included: (1) balancing of carbon release by calcification via carbon uptake at the individual and ecosystem levels; (2) high rates of allochthonous organic carbon supply because of high particle trapping capacity; (3) high rates of carbon preservation and low remineralisation rates; and (4) location in depositional environments. Some of these features are context-dependent, meaning that these ecosystems were blue carbon sinks in some locations, but not others. We provide a universal framework that can evaluate the likelihood of a given ecosystem to behave as a blue carbon sink for a given context. We seek to encourage consideration of non-classical blue carbon ecosystems within management strategies, allowing more complete blue carbon accounting.