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Ocean acidification decreases the pH of seawater and the seawater saturation state with respect to CaCO3 minerals. In the event of ocean acidification, Mg-calcite is considered to be the first mineral to dissolve. Dissolution of Mg-calcite is more prevalent at depth in the sediment than at the sediment interface because of production of CO2 resulting from microbial decomposition of organic matter. Rates of CaCO3 dissolution can be estimated from total alkalinity (AT) fluxes calculated from concentration gradients and diffusion coefficients. We estimated AT flux in a sandy area of the Shiraho coral reef under natural hydrodynamic conditions using eddy covariance and sedimentary AT profiles. The calculated nighttime AT flux at the sediment–water interface was 0.4–2.6 mmol m−2 h−1. Analysis of the sedimentary profile at a depth of 0–20 mm indicated that respiration by organisms consumed oxygen and produced CO2 during night and that photosynthesis enhanced O2 concentrations during the day. However, dissolved oxygen was depleted at all times in sediments deeper than 20 mm. The pore-water aragonite saturation state (=Ωa) was constant at ∼ 3.0, which is equivalent to a value of 1.0 for the saturation state with respect to foraminifera (=Ωfora), as determined in a previous study. Both organic reactions (e.g., respiration) and inorganic Mg-calcite dissolution occur in the sediment, leading to a constant Ωfora value in the sediment. These data confirm the metastable equilibrium of pore water with respect to Mg-calcite from foraminifera, which is the most soluble phase in the sediment.

We studied the seawater carbon dioxide (CO2) system in the Palau coral reef by measuring total alkalinity (TA) and total dissolved inorganic carbon (DIC). Variation in the CO2 system on the reef flat and in the lagoon was analyzed by TA–DIC diagrams, taking into accounts the differing residence times of seawater. CO2 in the offshore water was relatively stable in space and time, but on the reef flat it was subject to rapid (about 3 h) and substantial changes due to photosynthesis and calcification during the day and due to respiration and calcification at night. Water flowed into the lagoon where decomposition of organic matter and continuing calcification occurred over relatively long residence times (,30 d). Despite the spatial and temporal variations, the center of the lagoon had relatively constant TA and DIC values similar to the mean values for the entire lagoon. A long-term 30–40% decrease in reef productivity and calcification has occurred over the last decade, primarily a result of degradation of the reef environment following a major coral reef bleaching event in 1998. This is reflected in decreases in the differences in TA and DIC between offshore lagoon waters and those in center of the lagoon.

We measured the C/N ratio and d 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 2 3 reefs seemed to indicate differences in nitrogen loadings from the land. The d 15 N values of these algae, on the other hand, linearly or curvilinearly decreased from 18‰ to 12‰ 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 d 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.

The phosphorus (P) cycle that maintains the production of microphytobenthic communities in carbonate sediments was investigated in the fringing Shiraho coral reef system located off the east coast of Ishigaki Island, Japan. Production rates of the diatom-dominated microphytobenthic communities and the P flux at the sediment‐water interface were determined by in situ benthic enclosure experiments. The daily net production (6.9‐24 mmol C m 22

A rapid technique is presented for measuring organic C and N in carbonate-bearing sediments. Organic C and N were determined with a Yanaco MT-5 CHN analyzer after removing carbonate by acidification of samples in ceramic sample boats with HCI vapor. This procedure eliminates loss of acid-soluble organic matter during carbonate dissolution. The accuracy of this new technique and two earlier methods of determining organic C and N concentrations was verified with standard chemicals and carbonate-bearing sediments (>80 wt% CaCO 3 ). With the new technique there was no contamination or loss of organic C and N. The method is suitable for acidifying sediments with a wide range of carbonate content.