A blueprint for blue carbon: toward an improved understanding of the role of vegetated coastal habitats in sequestering CO2

Large conservation opportunities exist in >90% of tropic-subtropic coastal habitats adjacent to cities

Krabi River Estuary, a ramsar site located in southern Thailand, is dominated by two major natural vegetation assemblages, i.e. mangrove forests and seagrass beds. According to the cluster development strategy, the cabinet has approved in principle to develop Krabi as the global marine tourism center and economic gateway of Asia. This study aimed to assess the economic value of the site, using market price and benefit transfer. The impact on economic activities with the 5% annual decline of mangrove forests was evaluated. The estimated annual use value of the site was $9.7 million for recreation and tourism. The economic value of mangrove forests was $758/ha. The net present value of mangrove forests was $73.1 million based on 7% discount rate and 15-year time line. An approximate impact of mangrove change follows the study of Ruitenbeek (1992). It was assumed that the impact on local direct resource extraction and loss in biodiversity would occur at the year after the depletion of the forest. The impact on the productivity of local fishery would occur in the fifth year and on tourism in the tenth year. Thus, value loss of mangrove at the annual rate of 5% in the net present term became $21.0 million or $2.3 million per year. The results imply that a development project that causes the same rate of mangrove destruction must generate a least an income of $2.3 million per year to be considered as an economically feasible project.

The scale at which New Zealand is currently storing and sequestering blue carbon, and could create additional blue carbon via restoration, has been unclear. Here, we calculate a preliminary estimate for the current extent of three key blue carbon ecosystems (saltmarshes, mangrove forests and seagrass meadows), their carbon stocks and their carbon sequestration rates using the best available data to provide a preliminary estimate of blue carbon in New Zealand. We also use local examples to explore opportunities to create additional blue carbon. Based on the available literature, we estimate the current extent of New Zealand’s blue carbon ecosystems to be 76,152 ha, which is 1.0% of the area of terrestrial native forests. Our preliminary estimate of New Zealand’s blue carbon stock is 2.66–3.76 Mt of carbon, with a current carbon sequestration rate of 0.12 (0.05–0.26) Mt/CO2/yr, which is equivalent to 0.16% of New Zealand’s 2021 gross emissions. Restoration of saltmarshes could enhance their carbon sink capacity, mangrove forests are naturally expanding and seagrass meadow restoration techniques at scale are still in development. Developing a national framework for blue carbon protection, monitoring and restoration is important as part of New Zealand’s climate change mitigation and adaptation efforts.

Mangroves, salt marshes and seagrasses are conspicuous and important features of the Sri Lankan coastline, playing an important role in delivering multiple ecosystem services including carbon sequestration (the so-called blue carbon ecosystems). Together, they support coastal biodiversity, provide habitat and nutrition for numerous plant and animal species, protect vulnerable coastlines and support millions of human beings. Sri Lanka is now placing itself at the forefront of marine conservation by becoming the first nation to legislatively protect all its remaining mangrove forests. The current area of mangrove forest in Sri Lanka (19,500 ha) is less than that of salt marsh (33,573 ha) and seagrass meadows (23,819 ha). There is, however, considerable uncertainty in these estimates, particularly in some regions, which hampers policy development and conservation goals. While knowledge of the value of ecosystem services provided by mangrove forests is improving (e.g. value to fisheries, national estimates of carbon stocks and sequestration rates), comparable studies on saltmarsh and seagrass ecosystems are rare and this is reflected in their governance and legal protection. Further, while there are considerable efforts to restore lost mangrove forests (i.e. more than 1200 ha restored, with ambitious plans for 10,000 ha), restoration of seagrasses and salt marshes remains largely unattempted. The current disparity in the state of knowledge of blue carbon ecosystems must be addressed through application of robust science that extends descriptive data and theoretical assumptions to generate deeper knowledge on the structure, function and socio-economic value of each of these important ecosystems. Overall, this review seeks to summarise the state of these important higher-plant-dominated coastal ecosystems in Sri Lanka. We seek to help set a research agenda to ensure the nation becomes a regional leader with respect the protection, conservation and restoration of its coastal zone, thereby ensuring continued delivery of important ecosystem services, especially blue carbon sequestration.

Blue carbon ecosystems require conservation and restoration to maximize organic carbon (CORG) sequestration to ameliorate greenhouse gas emissions. Salt marshes, mangrove forests and seagrass meadows are all autotrophic and are considered blue carbon ecosystems. Macroalgae and tidal flats are currently not considered blue carbon habitats. Blue carbon ecosystems contribute globally to climate change mitigation and at local and national scales, especially in the provision of other ecosystem goods and services. Financial investment is constrained by large uncertainties in CORG dynamics and best practices in restoration, rehabilitation and conservation. Several key emerging perspectives include (1) the fact that groundwater discharge of dissolved carbon is a major pathway of blue carbon loss; (2) allochthonous CORG inputs are required to achieve ecosystem carbon mass balance; (3) blue carbon dynamics are enhanced by habitat connectivity and biotic activities; (4) CH4 and N2O emissions reduce blue carbon potential; (5) habitat destruction causes blue carbon stock losses, but variable gas emissions; (6) sediment blue carbon stocks are increasing at the poles; and (7) land-use and land-cover changes (LULCC) drive changes in blue carbon stocks and emissions. Further research is needed to clarify the applicability of these emerging perspectives.

Blue carbon has made significant contributions to climate change adaptation and mitigation while assisting in achieving co-benefits such as aquaculture development and coastal restoration, winning international recognition. Climate change mitigation and co-benefits from blue carbon ecosystems are highlighted in the recent Intergovernmental Panel on Climate Change Special Report on Ocean and Cryosphere in a Changing Climate. Its diverse nature has resulted in unprecedented collaboration across disciplines, with conservationists, academics, and politicians working together to achieve common goals such as climate change mitigation and adaptation, which need proper policy regulations, funding, and multi-prong and multi-dimensional strategies to deal with. An overview of blue carbon habitats such as seagrass beds, mangrove forests, and salt marshes, the critical role of blue carbon ecosystems in mitigating plastic/micro-plastic pollution, as well as the utilization of the above-mentioned blue carbon resources for biofuel production, are critically presented in this research. It also highlights the concerns about blue carbon habitats. Identifying and addressing these issues might help preserve and enhance the ocean’s ability to store carbon and combat climate change and mitigate plastic/micro-plastic pollution. Checking out their role in carbon sequestration and how they act as the major carbon sinks of the world are integral parts of this study. In light of the global frameworks for blue carbon and the inclusion of microalgae in blue carbon, blue carbon ecosystems must be protected and restored as part of carbon stock conservation efforts and the mitigation of plastic/micro-plastic pollution. When compared to the ecosystem services offered by terrestrial ecosystems, the ecosystem services provided by coastal ecosystems, such as the sequestration of carbon, the production of biofuels, and the remediation of pollution, among other things, are enormous. The primary purpose of this research is to bring awareness to the extensive range of beneficial effects that can be traced back to ecosystems found in coastal environments.

Globally, blue carbon (i.e., carbon in coastal and marine ecosystems) emissions have been seriously augmented due to the devastating effects of anthropogenic pressures on coastal ecosystems including mangrove swamps, salt marshes, and seagrass meadows. The greening of aquaculture, however, including an ecosystem approach to Integrated Aquaculture-Agriculture (IAA) and Integrated Multi-Trophic Aquaculture (IMTA) could play a significant role in reversing this trend, enhancing coastal ecosystems, and sequestering blue carbon. Ponds within IAA farming systems sequester more carbon per unit area than conventional fish ponds, natural lakes, and inland seas. The translocation of shrimp culture from mangrove swamps to offshore IMTA could reduce mangrove loss, reverse blue carbon emissions, and in turn increase storage of blue carbon through restoration of mangroves. Moreover, offshore IMTA may create a barrier to trawl fishing which in turn could help restore seagrasses and further enhance blue carbon sequestration. Seaweed and shellfish culture within IMTA could also help to sequester more blue carbon. The greening of aquaculture could face several challenges that need to be addressed in order to realize substantial benefits from enhanced blue carbon sequestration and eventually contribute to global climate change mitigation.

Blue carbon is the carbon stored in coastal and marine ecosystems. Coastal ecosystems such as mangroves, tidal marshes, seagrass meadows and algae sequester and store more carbon per unit area than terrestrial forests and are now being recognised for their role in mitigating climate change. The objective of this study was to assess the biodiversity and carbon storage capacities in these ecosystems. The study was conducted in Vidathalathivu in Mannar, Kiranchi in Kilinochchi and Pooneryn. In mangroves, belt transects were taken from the shore towards the land. Nested circular plots having 7 m radius were established, large trees were sampled for diameter. Small trees (<3 cm dbh) were sampled in 2 m radius. Litter, pneumatophores and seedlings were sampled in plots of 30×30 cm area. Soil samples were taken at depths; 0-30 cm, 30-60 cm, 60-100 cm for the measurement of bulk density and carbon content. The biomass of the mangrove plants was measured using species specific allometric equations. In the salt marshes, 1×1 m plots were sampled. In the seagrass beds and algae, several 30×30 cm grids were sampled. In each plot, the species composition and the number of plants in each species were assessed. Samples of seagrasses and salt marsh vegetation were uprooted and taken to the laboratory for oven dry analysis. Three soil cores were taken using the soil auger which was inserted to a depth of 1 m in each ecosystem. The soil samples were taken from the core to represent the following depths; 0-30 and 30-60, 60-90. The data was analysed using MINITAB statistical software. The results showed natural mangroves in the study sites comprised of Avicennia marina, Ceriops tagal, Excoecaria agallocha, Luminetzera racemosa, Rhizophora murconata and Pemphis acidula. In sea grasses, Thalassima hemprichii, Cymodocea serrulate, Ruppia maritima and Syringodium isoiifolium were observed. Kappaphycus alvarezii and Eucheuma spinosum were the cultivated algae and Padina antillarum, Caulerpa racemosa, Ulva lactuca, Sargassum illifolium and Gelidium sp. were natural ones observed. Suaeda sp. and Salicornia sp. were the salt marsh species observed. The carbon content of these ecosystems were assessed. Keywords: Blue carbon, Mangroves, Seagrasses, Salt marshes, Algae, Carbon sequestration

Seagrass meadows are among the most efficient and long-term carbon sinks on earth, but disturbances could threaten this capacity, so understanding the impacts of disturbance on carbon stored within seagrass meadows—‘blue carbon’—is of prime importance. To date, there have been no published studies on the impacts of seagrass loss on ‘blue carbon’ stocks. We experimentally created several kinds of small-scale disturbances, representative of common grazer and boating impacts, within seagrass (Zostera nigracaulis) meadows in Port Phillip Bay (Australia) and measured the impacts on sediment organic carbon stocks (‘C org’, and other geochemical variables—%N, δ13C, δ15N). Disturbance had no detectable effect on C org levels within seagrass sediments, even for high-intensity disturbance treatments, which remained bare (i.e. no seagrass recovery) for 2 years after the disturbance. These findings challenge the widely held assumption that disturbance and concomitant loss of seagrass habitat cause release of carbon, at least for small-scale disturbances. We suggest that larger (e.g. meadow scale) disturbances may be required to trigger losses of ‘blue carbon’ from seagrass meadows.

Coastal wetlands, such as salt marsh and seagrass beds, are important sedimentary carbon sinks attributed to their inherent sequestration proficiencies. Their spatial heterogeneity however, introduces uncertainties in sediment blue carbon assessments, requiring explanatory understanding of causal factors, particularly where environmental gradients are evident. This study measured the spatial variability in sediment carbon stocks associated with seagrass, salt marsh, and degraded (disturbed, unvegetated sites) habitats, identifying causal geochemical factors. Significant intra-habitat sediment carbon partitioning was observed (mean SE), where degraded habitats (83.63 9.67 Mg C ha -1 ) and supratidal Salicornia pillansii marsh (65.36 8.62 Mg C ha -1 ) had significantly lower carbon stocks than intertidal salt marsh, Spartina maritima (203.25 35.03 Mg C ha -1 ) and Salicornia tegetaria (243.58 67.84 Mg C ha -1 ), as well as seagrass Zostera capensis (211.4 20.12 Mg C ha -1 ). At the estuary scale statistical correlations were found between sedimentary stocks and several explanatory biogeochemical factors including sediment bulk density (SDB, p < 0.01), particle size ( p < 0.05), total organic matter ( p < 0.001), and salinity ( p < 0.05). Furthermore, a significant inverse association was found between sediment carbon stocks and landward oceanic distance – stressing the effect of geomorphic setting on net estuarine sedimentary carbon accretion. By interpolating the spatial variability in sediment carbon stocks, this study identified opportunities for enhancing salt marsh carbon stock through restoration (385.4ha). The potential carbon emissions, attributed to possible degradation and loss of all habitats from the Swartkops Estuary, was predicted to be 2962.51 Mg CO 2 e ha -1 yr -1 . This further highlights the value of extant carbon stocks, opportunities for restoration of disturbed habitats, and the implications for potential carbon emissions from blue carbon habitats. Furthermore, it reinforces the importance of accounting for local carbon stock variability for blue carbon restoration and/or conservation actions. • Percentage organic carbon partitioned by habitats identified blue carbon hotspots. • Organic carbon decreased landwards and was greater in intertidal marshes. • Carbon stocks were positively correlated with finer grain sediments. • Percentage organic carbon showed strong inter-habitat and spatial heterogeneity. • Salt marsh disproportionately contributes to carbon storage in semi-arid estuaries.

Abstract. In this study we report fluxes of chloromethane (CH3Cl), bromomethane (CH3Br), iodomethane (CH3I), and bromoform (CHBr3) from two sampling campaigns (summer and spring) in the seagrass dominated subtropical lagoon Ria Formosa, Portugal. Dynamic flux chamber measurements were performed when seagrass patches were either air-exposed or submerged. Overall, we observed highly variable fluxes from the seagrass meadows and attributed them to diurnal cycles, tidal effects, and the variety of possible sources and sinks in the seagrass meadows. The highest emissions with up to 130 nmol m−2 h−1 for CH3Br were observed during tidal changes, from air exposure to submergence and conversely. Furthermore, during the spring campaign, the emissions of halocarbons were significantly elevated during tidal inundation as compared to air exposure. Accompanying water sampling performed during both campaigns revealed elevated concentrations of CH3Cl and CH3Br, indicating productive sources within the lagoon. Stable carbon isotopes of halocarbons from the air and water phase along with source signatures were used to allocate the distinctive sources and sinks in the lagoon. Results suggest that CH3Cl was rather originating from seagrass meadows and water column than from salt marshes. Aqueous and atmospheric CH3Br was substantially enriched in 13C in comparison to source signatures for seagrass meadows and salt marshes. This suggests a significant contribution from the water phase on the atmospheric CH3Br in the lagoon. A rough global upscaling yields annual productions from seagrass meadows of 2.3–4.5 Gg yr−1, 0.5–1.0 Gg yr−1, 0.6–1.2 Gg yr−1, and 1.9–3.7 Gg yr−1 for CH3Cl, CH3Br, CH3I, and CHBr3 respectively. This suggests a minor contribution from seagrass meadows to the global production of CH3Cl and CH3Br with about 0.1 and 0.7%, respectively. In comparison to the known marine sources for CH3I and CHBr3, seagrass meadows are rather small sources.

Intertidal habitats provide numerous ecosystem services, including the sequestration and storage of carbon, a topic of great recent interest owing to land‐cover transitions and climate change. Mangrove forests and seagrass meadows form a continuum of intertidal habitats, alongside unvegetated mudflats and sandbars, however, studies that consider carbon stocks across these spatially‐linked, threatened ecosystems are limited world‐wide. This paper presents the results of a field‐based carbon stock assessment of aboveground, belowground and sediment organic carbon stock to a depth of 1 m at Chek Jawa, Singapore. It is the first study of ecosystem carbon stocks of both vegetated and unvegetated intertidal habitats in the tropics. Ecosystem carbon stocks were 497 Mg C ha‐1 in the mangrove forest and 138 Mg C ha‐1 in the seagrass meadow. Sediment organic carbon stock dominated the total storage in both habitats, constituting 62% and &gt;99% in the mangrove forest and seagrass meadow, respectively. In the adjacent mudflat and sandbars, which had no vegetative components, sediment organic carbon stock ranged from 124–143 Mg C ha‐1, suggesting that unvegetated habitats have a carbon storage role on the same order of importance as seagrass meadows. This study reinforces the importance of sediment in carbon storage within the intertidal ecosystem, and demonstrates the need to consider unvegetated habitats in intertidal ‘blue carbon’ stock assessments. Copyright © 2015 John Wiley &amp; Sons, Ltd.

Shifts in ecosystem structure have been observed over recent decades as woody plants encroach upon grasslands and wetlands globally. The migration of mangrove forests into salt marsh ecosystems is one such shift which could have important implications for global 'blue carbon' stocks. To date, attempts to quantify changes in ecosystem function are essentially constrained to climate-mediated pulses (30years or less) of encroachment occurring at the thermal limits of mangroves. In this study, we track the continuous, lateral encroachment of mangroves into two south-eastern Australian salt marshes over a period of 70years and quantify corresponding changes in biomass and belowground C stores. Substantial increases in biomass and belowground C stores have resulted as mangroves replaced salt marsh at both marine and estuarine sites. After 30years, aboveground biomass was significantly higher than salt marsh, with biomass continuing to increase with mangrove age. Biomass increased at the mesohaline river site by 130±18Mg biomass km(-2) yr(-1) (mean±SE), a 2.5 times higher rate than the marine embayment site (52±10Mg biomass km(-2) yr(-1) ), suggesting local constraints on biomass production. At both sites, and across all vegetation categories, belowground C considerably outweighed aboveground biomass stocks, with belowground C stocks increasing at up to 230±62MgCkm(-2) yr(-1) (±SE) as mangrove forests developed. Over the past 70years, we estimate mangrove encroachment may have already enhanced intertidal biomass by up to 283097Mg and belowground C stocks by over 500000Mg in the state of New South Wales alone. Under changing climatic conditions and rising sea levels, global blue carbon storage may be enhanced as mangrove encroachment becomes more widespread, thereby countering global warming.

Coastal ecosystems including coral reefs, mangrove forests, seagrass meadows, and salt marshes are being lost at alarming rates, and increased scientific understanding of causes has failed to stem these losses. Coastal habitats receive contrasting research effort, with 60% of all of the published research carried out on coral reefs, compared to 11–14% of the records for each of salt marshes, mangrove forests, and seagrass meadows. In addition, these highly connected and interdependent coastal ecosystems receive widely contrasting media attention that is disproportional to their scientific attention. Seagrass ecosystems receive the least attention in the media (1.3% of the media reports) with greater attention on salt marshes (6.5%), considerably more attention on mangroves (20%), and a dominant focus on coral reefs, which are the subject of three in every four media reports on coastal ecosystems (72.5%). There are approximately tenfold lower reports on seagrass meadows in the media for every scientific paper published (ten), than the 130–150 media reports per scientific paper for mangroves and coral reefs. The lack of public awareness of losses of less charismatic ecosystems results in the continuation of detrimental practices and therefore contributes to continued declines of coastal ecosystems. More effective communication of scientific knowledge about these uncharismatic but ecologically important coastal habitats is required. Effective use of formal (e.g., school curricula, media) and informal (e.g., web) education avenues and an effective partnership between scientists and media communicators are essential to raise public awareness of issues, concerns, and solutions within coastal ecosystems. Only increased public understanding can ultimately inform and motivate effective management of these ecologically important coastal ecosystems.

Blue carbon ecosystems in Sri Lanka: A review