Protection of seabed sediments in Canada's marine conservation network for potential climate change mitigation co-benefit

Marine conserved areas (MCAs) can provide a range of ecological and socio-economic benefits, including climate change mitigation from the protection and enhancement of natural carbon storage. Canada's MCA network is expanding to encompass 30% of its Exclusive Economic Zone by 2030. At present, the network aims to integrate climate change mitigation by protecting coastal vegetated blue carbon ecosystems (saltmarsh, seagrass, kelp). Here, we argue that incorporating unvegetated seabed sediments could bring similar benefits. Seabed sediments can store and/or accumulate high densities of organic carbon, and due to their large spatial extent, contain carbon stores orders of magnitude larger than coastal vegetated habitats. We estimate that currently designated MCAs encompass only 10.8% of Canada's seabed sediment organic carbon stocks on the continental margin, and only 13.4% of areas with high carbon densities. Proposed MCAs would cover an additional 8.8% and 6.1% of total stocks and high carbon areas, respectively. We identify an additional set of high-priority seabed areas for future research and potential protection, ranking their importance based on carbon stocks, proxies for lability, and ecological/biological significance. The incorporation of seabed sediments into MCA networks could support climate change mitigation by preventing future releases of stored carbon.