Literature Review: Blue Carbon Research in the Tropical Western Indian Ocean (WIO)

The tropical Western Indian Ocean (WIO) is rich in blue carbon ecosystems. It hosts dense mangrove forests covering more than 733,000 ha (5.3% of the total mangrove cover worldwide) and diverse seagrass meadows reaching more than 40 m deep. Given their carbon sink potential, there is growing interest in including these blue carbon ecosystems into national climate adaptation and mitigations strategies. Accounting for the ocean’s carbon offsetting capacity can help many countries reduce their net greenhouse gas emissions and achieve their Nationally Determined Contributions (NDCs). Further, many of the costs of achieving emission reductions in the energy and transport sector could be met through Blue Carbon Markets or international climate financing such as the UN’s Reducing Emissions from Deforestation and Forest Degradation (REDD+). However, to participate from these programs and capitalise from the carbon sink capacity of coastal ecosystems, countries require accurate accounting of the current blue carbon stocks and robust reporting of their changes through time.Seychelles is a world leader in Blue Economy and the pioneer of sovereign blue bonds. Yet, little research has focussed on quantifying the carbon stocks held within Seychelles mangrove and seagrass ecosystems, thereby hindering the possibility of accounting for the nation’s natural carbon offsetting potential and the inclusion of blue carbon ecosystems in Seychelles’ revised NDCs. Considering the ecological similarities of blue carbon ecosystems within the tropical WIO (e.g., species occurrence, climate conditions), this report reviews mangrove and seagrass literature in the region to (i) identify trends and knowledge gaps in blue carbon research and (ii) compile data on the regional carbon pools (i.e., aboveground biomass, belowground biomass, soil organic matter).Key findings:(i)Trends in the literature• Through a comprehensive search in the ISI Webs of Science and Google Scholar, we identified 633 studies of potential relevance. From this total, 131 contained relevant blue carbon information from the tropical WIO (e.g., allometric equations, plant biometrics), but only 102 included unique datasets of the biomass or carbon stored within the mangroves and seagrass beds.• Interest in blue carbon is growing rapidly in the region, with almost 50% of the studies published within the past 10 years (2011-2021).• Most of the research has focused on mangrove ecosystems (62.6% of the studies) and aboveground carbon stocks (74.8% of the studies). While 53 studies have assessed the soil carbon pool, research on soil accretion rates is extremely rare.• Studies from Tanzania and Kenya dominate the blue carbon literature, with very little research originating from Island States. Many of the most complete and robust blue carbon datasets were fuelled by peer-reviewed publications arising from academic theses.• Several mangrove studies have developed species-specific allometric equations and estimated local wood density and carbon fractions.(ii) Regional carbon pools• Mangrove aboveground (AGC) and belowground carbon stocks (BGC) within the tropical WIO ranged between 0.05 ‒ 303.9 tonnes C ha-1 and 0.01 ‒ 598 tonnes C ha-1, respectively, which sit within the global ranges reported. Mangrove soil carbon stocks (SOC) ranged from 87.5 to 848.2 tonnes C ha-1 (within 60 – 200 cm cores) depending on the species, the site, and the methods used to collect soil samples.• The highest mean mangrove AGC and BGC stocks were recorded in Tanzania (303.9 tonnes C ha-1) and Madagascar (157.5 tonnes C ha-1), respectively. In relation to the carbon stored in the soil, the highest values were recorded in the mangrove forests of Kenya (405 tonnes C ha-1).• The mangrove species Rhizophora mucronata recorded the highest AGC (67.38 tonnes C ha-1) and SOC stocks (562.8 tonnes C ha-1), while Ceriops tagal had the highest mean BGC stock (295.83 tonnes C ha-1).• Seagrass AGC and BGC within the tropical WIO had a mean of 0.70 ± 0.03 tonnes C ha-1 and 2.21 ± 0.11 tonnes C ha-1, respectively. Seagrass SOC had a mean stock of 116 ± 24.1 tonnes C ha-1.• The highest mean seagrass AGC, BGC and SOC stocks were recorded in Kenya (0.89 tonnes C ha-1, 4.95 tonnes C ha-1, and 294.03 4.95 tonnes C ha-1 respectively).• The seagrass species Thalassodendron ciliatum and Cymodocea rotundata recorded the highest AGC (1.06 tonnes C ha-1) and BGC stocks (5.99 tonnes C ha-1) respectively. Stands of Thalassia hemprichii reached mean SOC stock of 362.34 tonnes C ha-1