Organic carbon (OC) stored in the sediments of seagrass meadows has been considered a globally significant OC reservoir. However, the sparsity and regional bias of studies on long‐term OC accumulation in coastal sediments have limited reliable estimation of the capacity of seagrass meadows as a global OC sink. We evaluated the amount and accumulation rate of OC in sediment of seagrass meadows and adjacent areas in East and Southeast Asia. In temperate sites, the average OC concentration in the top 30 cm of sediment was higher in seagrass meadows (780–1080 μmol g⁻¹) than in sediments without seagrass cover (52–430 μmol g⁻¹). The average OC in the top 30 cm of subtropical and tropical seagrass meadow sediments ranged from 140 to 440 μmol g⁻¹. Carbon isotope mass balancing suggested that the contribution of seagrass‐derived carbon to OC stored in sediments was often relatively minor (temperate: 10–40%; subtropical: 35–82%; tropical: 4–34%) and correlated to the habitat type, being particularly low in estuarine habitats. Stock of OC in the top meter of sediment of all the studied meadows ranged from 38 to 120 Mg ha⁻¹. The sediment accumulation rates were estimated by radiocarbon dating of six selected cores (0.32–1.34 mm yr⁻¹). The long‐term OC accumulation rates calculated from the sediment accumulation rate and the top 30 cm average OC concentration for the seagrass meadows (24–101 kg ha⁻¹ yr⁻¹) were considerably lower than the OC accumulation rates previously reported for Mediterranean Posidonia oceanica meadows (580 kg ha⁻¹ yr⁻¹ on average). Current estimates for the global carbon sink capacity of seagrass meadows, which rely largely on Mediterranean studies, may be considerable overestimations.

Our knowledge of the factors that can influence the stock of organic carbon (OC) that is stored in the soil of seagrass meadows is evolving, and several causal effects have been used to explain the variation of stocks observed at local to national scales. To gain a global‐scale appreciation of the drivers that cause variation in soil OC stocks, we compiled data on published species‐specific traits and OC stocks from monospecific and mixed meadows at multiple geomorphological settings. Species identity was recognized as an influential driver of soil OC stocks, despite their large intraspecific variation. The most important seagrass species traits associated with OC stocks were the number of leaves per seagrass shoot, belowground biomass, leaf lifespan, aboveground biomass, leaf lignin, leaf breaking force and leaf OC plus the coastal geomorphology of the area, particularly for lagoon environments. A revised estimate of the global average soil OC stock to 20 cm depth of 15.4 Mg C ha⁻¹ is lower than previously reported. The largest stocks were still recorded in Mediterranean seagrass meadows. Our results specifically identify Posidonia oceanica from the Mediterranean and, more generally, large and persistent species as key in providing climate regulation services, and as priority species for conservation for this specific ecosystem service.

To elucidate the factors determining the organic carbon (OC) sequestration capacity of seagrass meadows, the distribution of OC and the fraction of seagrass‐derived OC in sediments of the temperate cosmopolitan seagrass Zostera marina meadows and surrounding habitats were investigated in relation to physical properties of sedimentary materials. On average, seagrass meadow sediments showed OC levels twofold higher than other shallow nearshore habitats. However, offshore sediments often showed greater OC concentrations than average seagrass meadow sediments. According to estimations of OC sources based on carbon isotope ratios, 8–55% and 14–24% of OC in nonestuarine seagrass meadow sediments and < 30 m deep offshore sediments, respectively, were assigned to seagrass origin. The OC concentration in seagrass meadow and offshore sediments closely correlated to the specific surface area (SSA) of sediment (r² = 0.816 and 0.755, respectively; p < 0.0001), with an average OC loading per sediment surface area of approximately 60 μmol m⁻². In seagrass meadow sediments, the fraction of seagrass‐derived OC was also greater in samples with a larger SSA, and the seagrass‐derived OC occurred preferentially in sediment grains that had a specific gravity exceeding 2.0, namely, in a form closely associated with sediment minerals. The OC concentration, the fraction of seagrass‐derived OC, and the SSA were positively correlated to the logarithm of areal extent of individual seagrass meadows (p < 0.01). These findings suggest that the OC sequestration capacity of nearshore vegetated habitats is under the primary control of geophysical constraints such as sediment supply rate and depositional conditions.

The elemental (C/N) and stable isotopic (δ¹³C, δ¹⁵N) compositions and compound-specific δ¹⁵N values of amino acids (δ¹⁵N_AA) were evaluated for coral holobionts as diagnostic tools to detect spatiotemporal environmental heterogeneity and its effects on coral health. Hermatypic coral samples of eight species were collected at 12 reef sites with differing levels of pollution stress. The C/N ratios, δ¹³C values, and δ¹⁵N values of coral tissues and endosymbiotic algae were determined for 193 coral holobionts, and the amino acid composition and δ¹⁵N_AA values of selected samples were analyzed. δ¹⁵N values were influenced most by pollution stress, while C/N ratios and δ¹³C values depended most strongly on species. The results imply that δ¹³C and δ¹⁵N values are useful indicators for distinguishing the ecological niches of sympatric coral species based on microhabitat preference and resource selectivity. Using δ¹⁵N_AA values, the trophic level (TL) of the examined coral samples was estimated to be 0.71 to 1.53, i.e., purely autotrophic to partially heterotrophic. Significant portions of the variation in bulk δ¹⁵N and δ¹³C values could be explained by the influence of heterotrophy. The TL of symbionts covaried with that of their hosts, implying that amino acids acquired through host heterotrophy are translocated to symbionts. Dependence on heterotrophy was stronger at polluted sites, indicating that the ecological role of corals changes in response to eutrophication.