Toshihiro Miyajima

Importance of foraminifera for the formation and maintenance of a coral sand cay: Green Island, Australia

Abstract CaCO3 production by reef-building organisms on Green Island Reef in the Great Barrier Reef of Australia is estimated and compared with the contribution of benthic foraminifera to the sediment mass of the vegetated sand cay. Major constituents of the cay are benthic foraminifera (mainly Amphistegina lessonii, Baculogypsina sphaerulata, and Calcarina hispida), calcareous algae (Halimeda and coralline algae), hermatypic corals, and molluscs. Among these reef-building organisms, benthic foraminifera are the single most important contributor to the sediment mass of the island (ca. 30% of total sediments), although their production of CaCO3 is smaller than other reef-building organisms. Water current measurements and sediment traps indicate that the velocity of the current around Green Island is suitable for transportation and deposition of foraminiferal tests. Abundant foraminifera presently live in association with algal turf on the shallow exposed reef flat, whose tests were accumulated by waves resulting in the formation and maintenance of the coral sand cay.

Significance of groundwater nitrogen discharge into coral reefs at Ishigaki island, southwest of Japan

Abstract. Groundwater discharge from adjacent terrestrial areas can be a potentially important nutrient source to coastal coral reefs, since adjacent lands are often overlaid with permeable bedrock such as limestone. The quantity of groundwater nitrogen discharged into the Shiraho and Kabira coral reefs from their namesake watersheds on Ishigaki Island southwest of the Ryukyu Islands, Japan (24°19′–37′N, 124°4′–21′E) was monitored. These watersheds were subject to different types of nitrogen loading. The groundwater nitrogen discharge was compared by two independent methods, one based on measuring dissolved inorganic nitrogen (DIN) concentrations in the groundwater near the coastline, the other by estimating nitrogen loading from various land uses within the watershed. For a common watershed, the two methods agreed within a factor of two. The Shiraho reef received 4- or 5.5-fold more nitrogen than the Kabira reef. Groundwater discharge contributes significantly to the reef nitrogen budget, and is potentially a key factor controlling the biomass and succession of aquatic vegetation of the reefs.

Anaerobic mineralization of indigenous organic matters and methanogenesis in tropical wetland soils

Tropical wetlands are one of the largest natural sources in the global methane budget due to high biological activities and the anaerobiosis in soil. We studied mineralization and gas production during the early stage of anaerobic decomposition of indigenous organic matters in soils of Narathiwat, southern Thailand, to clarify the significance of the substrate quality in controlling decomposition and methanogenesis in some different tropical wetland soils. The optimal temperature of decomposition was around 35°C, while methanogenesis did not proceed at 45°C. During the first 50 days of anaerobic incubation, 5 ∼ 63% (carbon basis) of indigenous plant leaves were mineralized. The mineralization rate was strongly and negatively correlated with the lignin and/or fiber contents, but not theC/N ratio, of the substrate plant materials. Difference in δ13C between the substrate and the produced CH4 was generally greater (more negative in CH4) for more recalcitrant substrates, indicating that H2 as opposed to acetate becomes a more important metabolic intermediate in the anaerobic food web when the decomposition rate is limited by substrate recalcitrance. Thus, the CH4 isotope signature may be used to evaluate the importance of new vs. old organic matter as CH4 source in natural soils. The mineralization rate was higher, and the isotopic difference between the substrate and CH4 was smaller when plant materials were incubated with sulfate-contaminated soils than with native peat soils. The isotopic difference between the substrate and CH4 was significantly different between native peat soils. Results of a tracer experiment using 13C-labeled substrates indicated that these differences could be ascribed to difference in the mode of acetate metabolism between soils.

Geographic variability in organic carbon stock and accumulation rate in sediments of East and Southeast Asian seagrass meadows

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.

Dynamics of dissolved O2, CO2, CH4, and N2O in a tropical coastal swamp in southern Thailand

We studied the distribution of dissolved O2, CO2, CH4, and N2O in a coastal swamp system in Thailand with the goal to characterize the dynamics of these gases within the system. The gas concentrations varied spatially and seasonally in both surface and ground waters. The entire system was a strong sourcefor CO2 and CH4, and a possible sink for atmospheric N2O. Seasonal variation in precipitation primarily regulated the redox conditions in the system. However, distributions of CO2, CH4, and N2O in the river that received swamp waters were not always in agreement with redox conditions indicated by dissolvedO2 concentrations. Sulfate production through pyriteoxidation occurred in the swamp with thin peat layerunder aerobic conditions and was reflected by elevatedSO42−/Cl− in the river water. When SO42−/Cl− was high, CO2 and CH4 concentrations decreased, whereas the N2O concentration increased. The excess SO42− in the river water was thus identified as a potential indicator for gas dynamics in this coastal swamp system.

Distribution of greenhouse gases, nitrite, and δ13C of dissolved inorganic carbon in Lake Biwa: Implications for hypolimnetic metabolism

AbstractThe vertical distribution of dissolved greenhouse gases (CH4,ΣCO2 and N2O), NO

Alkali-extractable polysaccharides in marine sediments: Abundance, molecular size distribution, and monosaccharide composition

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VARIATION IN INTERNAL δ15N AND δ13C DISTRIBUTIONS AND THEIR BULK VALUES IN THE BROWN MACROALGA PADINA AUSTRALIS GROWING IN SUBTROPICAL OLIGOTROPHIC WATERS1

,and δ13C of ΣCO2 in Lake Biwa during a stagnantperiod was precisely determined. ΣCO2 as well as NO