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 . 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 À1 . The sediment accumulation rates were estimated by radiocarbon dating of six selected cores (0.32-1.34 mm yr À1). 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 À1 yr À1 ) were considerably lower than the OC accumulation rates previously reported for Mediterranean Posidonia oceanica meadows (580 kg ha À1 yr À1 on average). Current estimates for the global carbon sink capacity of seagrass meadows, which rely largely on Mediterranean studies, may be considerable overestimations.

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 (r 2 5 0.816 and 0.755, respectively; p < 0.0001),with an average OC loading per sediment surface area of approximately 60 lmol m 22. In seagrass meadow sediments, the fraction of seagrass-derived OC was also greater in samples with a larger SSA, and the seagrassderived 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. Vegetated shallow coastal ecosystems, including intertidal salt marshes, mangroves, and seagrass meadows have been ranked among the most efficient biotic systems for accumulating organic carbon (OC) on an areal basis (McLeod et al. 2011;Fourqurean et al. 2012). It is estimated that these ecosystems may contribute almost half of OC burial in the global ocean even though they cover < 2% of the ocean surface (Duarte et al. 2005). Recent interest has focused on the potential to incorporate these ecosystems, called "blue forests," into policies for reducing carbon dioxide (CO 2 ) emissions. At the same time, there is increased concern about the possibility of CO 2 emissions caused by the decline of blue forest ecosystems, including the seagrass meadows (Pendleton et al. 2012;Grimsditch et al. 2013).High rates of OC accumulation in seagrass meadows are likely the result of specific ecosystem functions such as (1) extremely high primary productivity of seagrasses and associated microalgae, (2) efficient trapping of organic particles within the meadow sediment via its flow-regulation and bottom-stabilization effects, and (3) slowness of remineralization of OC within the meadow sediment due to the anoxic conditions that prevail (Duarte et al. 2013). Most of the OC stored in seagrass meadows exists as detrital OC derived from seagrasses...

SUMMARY “Monitoring Sites 1000” – Japan's long‐term monitoring survey was established in 2003, based on the Japanese Government policy for the conservation of biodiversity. Ecological surveys have been conducted on various types of ecosystems at approximately 1000 sites in Japan for 15 years now and are planned to be carried out for 100 years. Since 2008, seaweed communities had been monitored at six sites, featuring the kelp (e.g. Saccharina and Ecklonia; Laminariales) and Sargassum (Fucales) communities in the subarctic and temperate regions of Japan. Annual surveys were carried out during the season when these canopy‐forming seaweeds are most abundant. A non‐destructive quadrat sampling method, with permanent quadrats placed along transects perpendicular to the shoreline, was used to determine species composition, coverage, and vertical distribution of seaweeds at these sites; while destructive sampling was done every 5 years to determine biomass. The occurrence of canopy‐forming species Saccharina japonica (var. japonica) and Ecklonia cava have appeared to be stable at the Muroran (southwestern part of Hokkaido Island) and Shimoda (Pacific coast of middle Honshu Island) sites, respectively; whereas the coverage of Ecklonia radicosa (= Eckloniopsis radicosa) at the Satsuma‐Nagashima site in southern part of Kyushu Island was highly variable until its sudden disappearance from the habitat in 2016. Thalli of E. radicosa lost most of their blades through browsing by herbivorous fish, and thus, this may be one of the causes of the decline. A shift in the community structure related to environmental changes had also been observed at some other sites. Pre‐ and post‐disaster data revealed the impact of the 2011 earthquake and tsunami disasters, including a shift in the vertical distribution of Ecklonia bicyclis (= Eisenia bicyclis) to shallower depths at the Shizugawa site in the Pacific coast of northern Honshu Island, due to seafloor subsidence.

The sequestration of atmospheric CO 2 in seagrass meadows as organic carbon (OC) has been attracting more attention as a means for climate change mitigation and adaptation. A direct method to detect seagrass DNA in coastal sediments, which is essential to unravel long-term seagrass-derived OC accumulation, was developed based on environmental DNA (eDNA) detection techniques. Quantitative real-time polymerase chain reaction (qPCR) and droplet digital PCR (ddPCR) were applied to quantify Zostera marina DNA in coastal sediments, using species-specific primers and dual-labeled probes for one nuclear and one chloroplast gene. Suitable pretreatments and methods for extracting Z. marina DNA from coastal sediments were examined and their applicability to environmental samples was demonstrated. Surface sediments collected from Z. marina meadows contained about 2000 times more Z. marina DNA than the unvegetated tidal-flats in the Seto Inland Sea. Moreover, both qPCR and ddPCR successfully detected Z. marina DNA in ancient sediments (up to 5000 calibrated years before present), evidencing that Z. marina DNA can be preserved in temperate coastal sediments for several millennia. In addition, qPCR and ddPCR results obtained in the present study were highly correlated, although the latter was more accurate than qPCR, particularly at low eDNA concentrations in ancient sediments. This work opens avenues to explore and clarify the process of the sequestration of OC produced by Z. marina and demonstrate the presence of past seagrass meadows from several millennia.

Protists provide insights into the diversity and function of RNA viruses in marine systems. Among them, marine macroalgae are good targets for RNA virome analyses because they have a sufficient biomass in nature. However, RNA viruses in macroalgae have not yet been examined in detail, and only partial genome sequences have been reported for the majority of RNA viruses. Therefore, to obtain further insights into the distribution and diversity of RNA viruses associated with marine protists, we herein examined RNA viruses in macroalgae and a diatom. We report the putative complete genome sequences of six novel RNA viruses from two marine macroalgae and one diatom holobiont. Four viruses were not classified into established viral genera or families. Furthermore, a virus classified into Totiviridae showed a genome structure that has not yet been reported in this family. These results suggest that a number of distinct RNA viruses are widespread in a broad range of protists.