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The Asia‐Pacific blue carbon ecosystems named mangroves, seagrass meadows and salt marsh plays an important role in climate change mitigation and adaptation. However, still data gaps exist in this region to understand the blue carbon dynamic process and how future climate change will impact it. The special feature aims to fill the gaps and contributing to our knowledge from the Asia‐Pacific region and to stimulate and promote future research in blue carbon research. [ABSTRACT FROM AUTHOR]

Mangroves, seagrasses and macroalgae form inter‐connected coastal habitats. They are involved in the direct and indirect sequestration and accumulation of blue carbon and are recognized for their role in climate change mitigation. Macroalgae, though residing within or in close proximity to these ecosystems are, however, not perceived as contributing to blue carbon storage. Such habitat connectivity among the mangrove, seagrass, and macroalgal compartments complicates a straightforward assessment of blue carbon storage. In this study, we assessed the contributions of these macrophytes as endmembers to the bulk organic carbon (OC) stocks in surficial (top 30 cm) and deep (31–100 cm) sediment layers of seagrass beds within the Sungai Pulai estuary (Malaysia). Organic carbon stocks in the seagrass sediments ranged from 14.3 to 21.4 Mg OC ha−1 (surficial depths), and 137.9 ± 23.3 (mean ± SE) Mg OC ha−1 for the deep layer. Mangrove‐derived matter contributed a maximum of 65% to the bulk OC pool. The existence of downcore variability indicated that macroalgae contributed up to 16% and were higher than seagrass sources (12%). Our results suggest that macroalgal‐derived and mangrove‐derived organic matter are important contributors to OC sequestration in the estuary. In view of the heterogeneous OC pool that contributes to bulk OC stocks, placing importance on only one OC source may construe an incomplete profile of habitat‐specific capacity for OC storage. This has wider implications for understanding OC subsidies, the effects of which are particularly relevant when one considers the impacts of coastal land use changes on blue carbon storage. [ABSTRACT FROM AUTHOR]

Coastal vegetated ecosystems play an important role in carbon cycling and bacterial communities inhabiting coastal sediments are responsible for the remineralization and processing of organic carbon (OC). We collected 1 m-long sediment cores in Posidonia seagrass meadows from coastal and estuarine sites in Australia that differed in their sedimentary organic and inorganic carbon, nitrogen and mud contents. The metabolic diversity of sediment heterotrophic bacterial communities was characterized at different sediment depths, based on the utilization pattern of 31 individual carbon substrates using Biolog EcoPlates™. High metabolic diversity was recorded at both sites, but the carbon substrate utilization rates and the use of carbohydrates were higher at the coastal site compared to the estuarine site. The heterotrophic bacterial community in the coastal sediment appeared to metabolize a more diverse OC pool compared to the estuarine site, which might partly explain the differences in OC storage among the seagrass habitats studied. The Biolog EcoPlates™ provided a useful tool for characte rising the sediment heterotrophic bacterial communities in the meadows and sediment characteristics and biochemical composition of the organic matter played an important role in shaping heterotrophic bacterial communities and their carbon utilization rates, potentially affecting carbon accumulation and preservation within seagrass sediments. [ABSTRACT FROM AUTHOR]

The actual estimates of carbon stocks beneath seagrass meadows worldwide are derived from few data, resulting in a tendency to generalize global carbon stocks from a very limited number of seagrass habitats. We surveyed Posidonia oceanica and Posidonia sinuosa meadows along depth-induced gradients of light availability to assess the variability in their sedimentary organic carbon (Corg) stocks and accretion rates. This study showed a fourfold decrease in Corg stocks from 2-4 m to 6-8 m depth P. sinuosa meadows (averaging 7.0 and 1.8 kg m−2, respectively; top meter of sediment) and a fourteenfold to sixteenfold decrease from shallow (2 m) to deep (32 m) P. oceanica meadows (200 and 19 kg m−2 average, respectively; top 2.7 m of sediment). The average Corg accretion rates in shallow P. sinuosa meadows were higher (10.5 g m−2 yr−1) than in deeper meadows (2.1 g m−2 yr−1). The reduction of sedimentary Corg stocks and accretion rates along depth-related gradients of light reduction suggests that irradiance, controlling plant productivity, meadow density, and sediment accretion rates, is a key environmental factor affecting Corg storage potential of seagrasses. The results obtained highlighted the exceptional carbon storage capacity of P. oceanica meadows at Balearic Islands (Spain), containing the highest areal Corg stocks of all seagrasses (estimated in up to 691-770 kg m−2 in 8-13 m thick deposits). Seagrass communities are experiencing worldwide decline, and reduced irradiance (following e.g., eutrophication or sediment regime alterations) will lead to photoacclimation responses (i.e., reduced plant productivity and shoot density), which may impact the carbon sequestration capacity of seagrasses. [ABSTRACT FROM AUTHOR]