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Oyster Harbour, on the south coast of Western Australia, supports 3.6–3.9 km 2 of seagrass meadows, following the loss of approximately 2.8–3.1 km 2 in the 1980s. This small area of prevailing meadows hold significant carbon stores accumulated over the past 3000 years. In this study, we sampled three sediment cores from a Posidonia australis meadow and analysed organic matter (OM), organic carbon (OC) and inorganic carbon (IC) contents in bulk sediments, and δ 13 C signatures of OM. The OM and OC contents (mean ± SE) in the cores were 9.07 ± 0.36% and 2.24 ± 0.05%, respectively. The mean IC content was 3.16 ± 0.17%. δ 13 C signatures of the sedimentary OM ranged from −10.01‰ to −13.28‰. Using a Bayesian isotopic mixing model, it is estimated that 57–67% of the OM in the seagrass sediments was derived from P . australis detrital matter. The total carbon (TC) stores in 150 cm-thick seagrass sediments averaged 27.92 kg TC m −2 (10.79 kg OC m −2 and 17.13 kg IC m −2 ). Based on radiocarbon dating, the mean sediment accumulation rate was 0.0494 cm yr −1 , which led to a long-term TC accumulation rate of 8.92 g TC m −2 yr −1 (3.45 g OC m −2 yr −1 and 5.47 g IC m −2 yr −1 ). Based on historical seagrass cover (3.6–6.7 km 2 during the 1960s to 1980s), the estimated TC stores in 150 cm-thick seagrass sediments at Oyster Harbour would have been 101–187 Gg TC. The eutrophication-driven loss of seagrasses during the 1980s resulted in the absence of OC accumulation capacity amounting to 280–310 Mg OC (over 29 years). The loss of seagrass area could also have resulted in the release of 37–41 Gg CO 2 , assuming that all of the OC in shallow sediment is remineralised after meadow disturbance. These results exemplify the importance of seagrasses meadows as important carbon sinks and the potential for losses of carbon stores due to ecosystem degradation. [ABSTRACT FROM AUTHOR]

The high organic carbon (OC) stores in seagrass meadows have led to their recognition as significant Blue Carbon sinks, though the diagenetic conditions that enable OC retention in seagrass soils remain poorly understood. In this study, seagrass soils were sampled from a Posidonia australis meadow in Oyster Harbour (Albany; south-western Australia) to investigate the preservation of sedimentary OC. We analysed soil characteristics (colour, grain size and redox potential), radiocarbon age, and characterised the soil organic matter (OM) using solid state CP/MAS 13C NMR spectroscopy to examine the preservation of OM down the soil profile. There was minimal change in organic composition over 1,700 years of accumulation, indicating long-term OM preservation. Primarily, this preservation appears to be driven by the recalcitrance of seagrass detritus buried in anoxic soils. The majority (70–83%) of total sedimentary OM comprised components directly attributable to seagrass origins (lignin, carbohydrate and black carbon-like matter), while the remainder consisted mostly of protein, some of which may have been present in seagrass biomass, along with likely contributions from algae and microbes. Although black carbon originates from organic matter combustion, here we provide evidence that the 13C NMR signal identified as black carbon-like matter in our soils is possibly associated with seagrass-derived organic matter consisting of degraded lignin products or other non-pyrogenic aromatics. The increase in the relative abundance of this black carbon-like matter with aging suggests its selective preservation. The relative abundances of carbohydrates significantly decreased with age down core (i.e. they appeared to be selectively decomposed), while lignin and protein did not show any quantitative changes in relative abundance (non-selective preservation). These findings demonstrate the exceptional preservation of P. australis derived OC, which contributes to our understanding of the higher OC storage capacity of Posidonia compared to other seagrass species. Image 1 • Biogeochemical basis to understand organic preservation in seagrass soils. • Exceptional preservation of P. australis matter in seagrass soils over 1,700 years. • 75% of soil organic matter made of lignin, carbohydrate and non-pyrogenic aromatics. • Selective preservation of aged black carbon-like matter identified as non-pyrogenic aromatics. • Non-selective preservation of carbohydrate, lignin and protein with ageing. [ABSTRACT FROM AUTHOR]

• An intertidal population of Halophila decipiens was discovered. • Intertidal exposure coincided with diel periods of low light irradiances. • This population formed a sub-clade with regional populations based on ITS data. • Macroalgae co-occurred with H. decipiens ; other seagrasses grew in shallower areas. • The occurrence on the mudflats shows site-specific and opportunistic colonisation. In this paper, we report the occurrence of Halophila decipiens Ostenfeld found growing in an intertidal environment. At the study site of Sungai Bahan estuary located along the Straits of Johor (Malaysia), the seagrass grew sparsely in the lower littoral zone of the mangrove coastline. Forming discontinuous patches, the seagrass was fully exposed on the mudflats during the lowest spring tides. Predictable times of exposure during low tides coincide with specific diel periods of lower light availability, which enabled avoidance of exposure to excessive irradiance. Hand-collected specimens of H. decipiens were initially defined through morphological and inflorescence characteristics. The identity of the species was confirmed based on analysis of the internal transcribed spacer sequence of its ribosomal DNA. Further analysis of this sequence demonstrated that among global populations, H. decipiens formed a sub-clade with populations from Malaysia and Australia. The colonisation by H. decipiens at the study site demonstrates its opportunistic acclimation to local environmental conditions. This discovery opens opportunities to further explore its ecology, as an instance of a less commonly studied seagrass within the region and to further investigate the unusual intertidal niche associated with the species. [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 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]

Algae form prolific coastal habitats and contribute the largest carbon dioxide (CO 2) flux globally. Most of the algal biomass is transported to the coastal ocean as particulate organic matter (OM), of which the contribution of carbon occurs mainly in depositional environments. Identifying the algal contribution alongside other major sources of organic carbon (OC) is crucial for determining allochthonous and autochthonous contributions in blue carbon habitats. In this study, we identified the algal contribution to OC storage in the seagrass meadow of Tanjung Adang Shoal (Johor, Malaysia) using dual-stable isotope measurements (δ13C and δ15N) and environmental DNA (eDNA) analysis. Stable isotope analysis showed that mangrove plants are the largest contributor to organic carbon pool in the sediments, followed by seston, macroalgae, epiphytes and seagrasses. The combined proportions of sestonic, macroalgal and epiphytic inputs as algal-derived organic matter had contributed a cumulative of 41.4–55.4% of organic matter in the bulk sediment. The eDNA method was used to examine bulk sediments and determine the types of algae present in the seagrass meadow at the genus/family level. Results showed that the main macroalgae present were from member of Coralinalles, Cladophoraceae and Ulvaceae, while diatoms were the predominant microalgae found in the sediment. These findings shed light on the importance of algae in carbon sequestration in the seagrass ecosystem. Understanding the role of algae in carbon storage and their interactions with sediment is crucial in the face of global and local environmental changes. • Quantifying algal contributions provide burial insights into blue carbon storage. • The proportion of algal organic carbon in seagrass sediments is presented. • Algae contributes more organic matter into the sediments compared to seagrass. • Diatoms dominate the algal DNA pool by having the highest relative abundance. [ABSTRACT FROM AUTHOR]

The capacity of macroalgae to absorb, hold, and release the relative amounts of carbon (C) and nitrogen (N) in its environment varies. C and N stable isotopes can be used to gain a better understanding of the ecosystem services of nutrient cycling by quantifying its usage in macroalgae. In this study, we conducted an in situ δ13C and δ15N tracer study of the macroalgae Ulva reticulata and Jania adhaerens in the seagrass meadows of Sungai Pulai estuary, Johor. Bulk tissues were labelled with enriched sodium bicarbonate (NaH13CO 3) and ammonium chloride (15NH 4 Cl). Within 1 to 18 days post-enrichment, the macroalgae were measured for C N contents and stable isotope signatures. U. reticulata showed more bulk losses for excess 13C and 15N compared to J. adhaerens. J. adhaerens tended to retain greater %C and %N incorporated in its tissues. There was a clear distinction between the amount of C and N retained and lost in the tissues of each species. The amount of nutrients, especially C, that remained at the end of the experiment demonstrated that both macroalgae species are capable of short-term carbon sequestration, with implications for meadow-scale nutrient subsidies. Higher N uptake and retention in the biomass of the macroalgae, especially in U. reticulata , suggest their relatively high growth capacity. Understanding the C and N dynamics and loss rate of the whole plant provides an estimation of the nutrient uptake and retention capacity of the macroalgae. Such insights are important as a means to formulating steps in preserving the ecological balance of marine ecosystems. [Display omitted] • C and N isotope signatures of Jania adhaerens and Ulva reticulata are reported. • After pulsing with 13C and 15N, these macroalgae were placed in a seagrass meadow. • Variations of δ13C and δ15N in their tissues were measured over 18 days. • J. adhaerens retained more of the initial 13C and 15N tracers than U. reticulata. • C and N changes allow insights into ecological roles of nutrient cycling. [ABSTRACT FROM AUTHOR]

The study of a Posidonia australis sedimentary archive has provided a record of changes in element concentrations (Al, Fe, Mn, Pb, Zn, Cr, Cd, Co, As, Cu, Ni and S) over the last 3000 years in the Australian marine environment. Human-derived contamination in Oyster Harbor (SW Australia) started ~ 100 years ago (AD ~ 1900) and exponentially increased until present. This appears to be related to European colonization of Australia and the subsequent impact of human activities, namely mining, coal and metal production, and extensive agriculture. Two contamination periods of different magnitude have been identified: Expansion period (EXP, AD ~ 1900–1970) and Establishment period (EST, AD ~ 1970 to present). Enrichments of chemical elements with respect to baseline concentrations (in samples older than ~ 115 cal years BP) were found for all elements studied in both periods, except for Ni, As and S. The highest enrichment factors were obtained for the EST period (ranging from 1.3-fold increase in Cu to 7.2-fold in Zn concentrations) compared to the EXP period (1.1-fold increase for Cu and Cr to 2.4-fold increase for Pb). Zinc, Pb, Mn and Co concentrations during both periods were 2- to 7-fold higher than baseline levels. This study demonstrates the value of Posidonia mats as long-term archives of element concentrations and trends in coastal ecosystems. We also provide preliminary evidence on the potential for Posidonia meadows to act as significant long-term biogeochemical sinks of chemical elements. [ABSTRACT FROM AUTHOR]

The recent focus on carbon trading has intensified interest in ‘Blue Carbon’–carbon sequestered by coastal vegetated ecosystems, particularly seagrasses. Most information on seagrass carbon storage is derived from studies of a single species, Posidonia oceanica, from the Mediterranean Sea. We surveyed 17 Australian seagrass habitats to assess the variability in their sedimentary organic carbon (Corg) stocks. The habitats encompassed 10 species, in mono-specific or mixed meadows, depositional to exposed habitats and temperate to tropical habitats. There was an 18-fold difference in the Corg stock (1.09–20.14 mg Corg cm−3 for a temperate Posidonia sinuosa and a temperate, estuarine P. australis meadow, respectively). Integrated over the top 25 cm of sediment, this equated to an areal stock of 262–4833 g Corg m−2. For some species, there was an effect of water depth on the Corg stocks, with greater stocks in deeper sites; no differences were found among sub-tidal and inter-tidal habitats. The estimated carbon storage in Australian seagrass ecosystems, taking into account inter-habitat variability, was 155 Mt. At a 2014–15 fixed carbon price of A$25.40 t−1 and an estimated market price of $35 t−1 in 2020, the Corg stock in the top 25 cm of seagrass habitats has a potential value of $AUD 3.9–5.4 bill. The estimates of annual Corg accumulation by Australian seagrasses ranged from 0.093 to 6.15 Mt, with a most probable estimate of 0.93 Mt y−1 (10.1 t. km−2 y−1). These estimates, while large, were one-third of those that would be calculated if inter-habitat variability in carbon stocks were not taken into account. We conclude that there is an urgent need for more information on the variability in seagrass carbon stock and accumulation rates, and the factors driving this variability, in order to improve global estimates of seagrass Blue Carbon storage. [ABSTRACT FROM AUTHOR]

Coralline macroalgae are globally distributed rhodopyhtes that remove carbon from their immediate environment and transform it into carbonate sediments through the senescence of their calcified tissues. In this study, the calcium carbonate (CaCO 3) stocks in the tissue of Jania adhaerens and sediments in Tanjung Adang Shoal, Johor were quantified for a 13-month study period. The detailed maps of the geographical distribution based on the spatial and temporal variations of biomass and CaCO 3 were also assessed. The highest amount of biomass, CaCO 3 and organic carbon (OC) stocks in the tissues showed the highest in May 2018 and May 2019. The biomass values ranged from 65 to 143 g DW m−2, which contained 53–147 g CaCO 3 m−2 and 3–11 g OC m−2. These findings provided insights into the biogeochemical cycling of these inputs, which can be used to estimate the overall carbon budget of the macrophyte meadow. • Monsoon events affect the variability of Jania adhaerens distribution. • Drift accumulation of J. adhaerens was highest during the inter-monsoon season. • Senescence of J. adhaerens contributes to carbonate deposition. • The calcified tissues of J. adhaerens are susceptible to ocean acidification. • Measuring CaCO 3 stocks of J. adhaerens helps in understanding local carbon budgets. [ABSTRACT FROM AUTHOR]