biology Article Climate Mitigation through Biological Conservation: Extensive and Valuable Blue Carbon Natural Capital in Tristan da Cunha’s Giant Marine Protected Zone David K. A. Barnes 1, * , James B. Bell 2 , Amelia E. Bridges 3 , Louise Ireland 1 , Kerry L. Howell 3 , Stephanie M. Martin 4 , Chester J. Sands 1 , Alejandra Mora Soto 5 , Terri Souster 6 , Gareth Flint 1 and Simon A. Morley 1   Citation: Barnes, D.K.A.; Bell, J.B.; Bridges, A.E.; Ireland, L.; Howell, K.L.; Martin, S.M.; Sands, C.J.; Mora Soto, A.; Souster, T.; Flint, G.; et al. Climate Mitigation through Biological Conservation: Extensive and Valuable Blue Carbon Natural Capital in Tristan da Cunha’s Giant Marine Protected Zone. Biology 2021, 10, 1339. https://doi.org/10.3390/ biology10121339 Received: 1 November 2021 Accepted: 9 December 2021 Published: 16 December 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 British Antarctic Survey, NERC, Madingley Road, Cambridge CB3 0ET, UK; louela@bas.ac.uk (L.I.); cjsan@bas.ac.uk (C.J.S.); gflint@bas.ac.uk (G.F.); smor@bas.ac.uk (S.A.M.) 2 School of Geography, University of Leeds, Leeds LS2 9JT, UK; james.bell@cefas.co.uk 3 School of Biological and Marine Sciences, University of Plymouth, Plymouth PL4 8AA, UK; amelia.bridges@plymouth.ac.uk (A.E.B.); kerry.howell@plymouth.ac.uk (K.L.H.) 4 Tristan da Cunha Conservation Department, Edinburgh TDCU 1ZZ, UK; environment.policy@tdc.uk.com 5 School of Geography and the Environment, University of Oxford, Oxford OX1 3QY, UK; alejandra.morasoto@spc.ox.ac.uk 6 Faculty of Biosciences, Fisheries and Economics, Norges Arktisk Universitet, veg18, 9019 Tromso, Norway; terri.souster@uit.no * Correspondence: dkab@bas.ac.uk Simple Summary: Solving biodiversity loss and climate change are part of the same problem; intact natural habitats can provide powerful and efficient climate mitigation if protected. Beyond the land (forests), there is little appreciation of just how important ocean nature is to climate mitigation. Carbon captured, stored and the rate at which it is buried (sequestration) by marine organisms is called blue carbon. We measured how much blue carbon occurs around the remote islands and seamounts of the Tristan da Cunha archipelago Marine Protected Zone (MPZ). We estimated that there are 300 tonnes of carbon (tC) captured in seaweed biomass each year, a proportion of which will sink and become a part of the long-term sediment carbon store. In deeper water we found a standing stock of ~2.3 million tC in the shallowest 1000 m depths, of which equivalent to 0.8 million t of carbon dioxide has the potential to be sequestered. At current carbon prices, and were it to attract blue carbon credits, £24 million worth of blue carbon can potentially be sequestered from the standing stock of this small United Kingdom Overseas Territory. This standing stock is protected and growth could, therefore, generate an additional £3.5 million worth of sequestered carbon a year, making it an unrecognized major component of the local economy. The economic return on this example MPZ probably ranks highly amongst climate mitigation schemes. The message is that placing meaningful protection to carbon-rich natural habitats can massively help society fight climate change and biodiversity loss. Nations who provide this protection should be fairly compensated, particularly where it comes at the detriment of other economic uses of marine habitats. Abstract: Carbon-rich habitats can provide powerful climate mitigation if meaningful protection is put in place. We attempted to quantify this around the Tristan da Cunha archipelago Marine Protected Area. Its shallows (<1000 m depth) are varied and productive. The 5.4 km 2 of kelp stores ~60 tonnes of carbon (tC) and may export ~240 tC into surrounding depths. In deep-waters we analysed seabed data collected from three research cruises, including seabed mapping, camera imagery, seabed oceanography and benthic samples from mini-Agassiz trawl. Rich biological assemblages on seamounts significantly differed to the islands and carbon storage had complex drivers. We estimate ~2.3 million tC are stored in benthic biodiversity of waters <1000 m, which includes >0.22 million tC that can be sequestered (the proportion of the carbon captured that is expected to become buried in sediment or locked away in skeletal tissue for at least 100 years). Much of this carbon is captured by cold-water coral reefs as a mixture of inorganic (largely calcium carbonate) and organic compounds. As part of its 2020 Marine Protection Strategy, these deep-water reef systems are now protected by Biology 2021, 10, 1339. https://doi.org/10.3390/biology10121339 https://www.mdpi.com/journal/biology