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.

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We measured the C/N ratio and ␦ 15 N values of two brown macroalgae-Padina spp. and Dictyota sp., which are distributed over all the subtropical fringing reefs of the Ryukyu Islands, Japan-to evaluate the feasibility of these algae as indicators of the terrestrial nitrogen load to the reef. The correlations between the distance from the shoreline and algal C/N ratio and surrounding NO concentrations were not clear, although their average values among the Ϫ 3 reefs seemed to indicate differences in nitrogen loadings from the land. The ␦ 15 N values of these algae, on the other hand, linearly or curvilinearly decreased from ϩ8‰ to ϩ2‰ with increasing distance from the shoreline, indicating the difference in nitrogen sources available to macroalgae. The slope of the decline among eight study areas had different characters, which seemed to depend on the residence time of reef seawater and the fluxes of terrestrial nitrogen. Using ␦ 15 N values of brown algae as an indicator, we confirmed that primary producers, such as macroalgae on the reefs, assimilated land-derived nitrogen and successfully evaluated time-integrated effects of terrestrial nitrogen on coral reef algae, which had been missed by conventional monitoring of the water column nutrients.Direct anthropogenic influences on coral reefs are major concerns because the global population is increasing, especially in tropical and subtropical countries with coral reefs. Furthermore, coral reefs are threatened by global climate

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A simple, precise method for determining the carbon stable isotope ratio of total dissolved inorganic C (ΣCO2) in freshwater samples is described. Water samples are packed in airtight glass bottles of known inner volume (∼70 ml) with no air bubbles. Subsequently, a headspace of 5.0 ml is created inside each bottle with pure helium gas, and each sample is acidified by adding 0.5 ml of a CO2‐free, 6.0 N HCl solution. After the original dissolved CO2 has equilibrated with the headspace gas, a portion of this headspace gas is subsampled and injected into the GC/C/IRMS (gas chromatograph/combustion furnace/isotope‐ratio mass spectrometer) system to determine the carbon isotope ratio of the CO2. The isotope ratio of CO2 remaining in the liquid phase is calculated by temperature‐dependent isotope discrimination between gas and aqueous phases. The isotope ratio of ΣCO2 of the original sample is then derived assuming isotope mass balance. The analytical precision of this method is ±0.1‰. The method enables a single operator to determine the isotopic ratio in at least 60 lake‐water samples within 3 d of sampling.

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Effects of moderate nutrient enrichment (NO { 3 : ,5 mmol L 21 , PO 3{ 4 : ,0.3 mmol L 21 ) on two carbon (C) fixation rates (photosynthesis and calcification) of the zooxanthellate coral Acropora pulchra were investigated under laboratory conditions. The coral branches were incubated in the nutrient condition for three different periods (0, 5, 10 d) to observe changes in tissue biomass and zooxanthellate chlorophyll a (Chl a) concentration. Next, the incubated corals were simultaneously transferred to nutrient-depleted seawater containing 13 C-labeled dissolved inorganic carbon to assay net photosynthesis and calcification rates. Chl a concentration per unit surface area increased 2.6-fold for the 10-d enrichment, and net photosynthetic rates were also stimulated up to a similar level (2.8-fold). Tissue biomass of the host coral and zooxanthellae was approximately doubled during the period. On the other hand, calcification rates only increased 1.3-fold, suggesting that even moderate nutrient loading resulted in one-sided enhancement of the algal photosynthetic activity. The measured C fixation ratios of organic C : skeletal C were higher than the structural ratios, and the inconsistency became greater as Chl a concentration increased. The increased photosynthetic products could be excessively stored in the organic tissue and/or released into the ambient seawater.