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Nanipora Miyazaki & Reimer, 2015 is a recently discovered genus of aragonite-skeleton producing octocorals closely related to the blue coral genus Heliopora de Blainville, 1830. Since its discovery, Nanipora has been reported from coral reefs in Okinawa, Japan, and Thailand, and from seagrass beds in the northern South China Sea. However, it remains little known and studied. Here, we report on the unexpected discovery of an abundance of Nanipora colonies in shallow waters less than 2-m deep around a CO2 vent from the uninhabited volcanic island of Iwotorishima, Okinawa, in southern Japan. Nanipora colonies were found covering both coral rubble and hard substrates, alongside a few soft coral and zoantharian species. Polyps were pale white in color with none brown or darker in coloration as in some recent reports. As the original description of N. kamurai from Zamami Island in Okinawa describes the species as azooxanthellate, as the current Iwotorishima specimens also appear to be, and recently reported specimens from Thailand, Dongsha Atoll, and Yaeyama are zooxanthellate, it may be that there are more than one Nanipora species; the type species N. kamurai that is also likely at Iwotorishima, and a zooxanthellate species that constitutes the other records. Although Nanipora is not well studied, its presence at this volcanic CO2 seep suggests it has the ability to survive under unique and extreme environmental conditions, rendering it as a potentially important subject of study in the face of increasing ocean acidification.

International audience; Maintaining coral reef ecosystems is a social imperative, because so many people depend on coral reefs for food production, shoreline protection, and livelihoods. The survival of reefs this century, however, is threatened by the mounting effects of climate change. Climate mitigation is the foremost and essential action to prevent coral reef ecosystem collapse. Without it, reefs will become extremely diminished within the next 20-30 years. Even with strong climate mitigation, however, existing conservation measures such as marine protected areas and fisheries management are no longer sufficient to sustain the ecosystem and many additional and innovative actions to increase reef resilience must also be taken. In this paper we assess the suite of protections and actions in terms of their potential be effective according to a set of criteria that include effectiveness, readiness, co-benefits and disbenefits. Even with the best scientific innovation, saving coral reefs will require a well-funded, welldesigned, and rapidly executed strategy with political and social commitments at the level of other grand challenges.

Atoll islands are small, low-lying and highly vulnerable to sea level rise (SLR). Because these islands are fully composed of the skeletons from coral reef creatures, the healthy coral ecosystem plays a pivotal role in island resilience against SLR. The environmental deterioration of reefs caused by increases in the human population has been recently reported, but the timing and process are unknown. We investigated the annual black bands in a coral boring core from Fongafale Island, the capital of Tuvalu, which is a symbolic atoll country that is being submerged due to SLR. The iron redox state and microbial gene segments in the coral skeleton might be new environmental indicators that reveal the linkage between anthropogenic activity and coral reef ecosystems. Our findings provide the first demonstration that iron sulfide has formed concentrated black layers since 1991 under the seasonal anoxic conditions inside coral annual bands. Since the 1990s, increasing human activity and domestic waste-induced eutrophication has promoted sludge and/or turf algae proliferation with the subsequent seasonal destruction, resulting in sulfate reduction by anaerobic bacteria. With the recent climate variability, these anthropogenic effects have induced the mass mortality of branching corals, deteriorated the coral reef ecosystem and deprived the resilience of the island against SLR.

New coral microatoll data allow presenting an updated late Holocene sea-level curve for the Gilbert Islands of Kiribati. Examination of build-up elevation and spatial distribution of microatolls, along with radiocarbon age data from coral samples, suggest an approximately 1 m sea-level high stand, possibly lasting from ~3500 to 1900 cal yr BP. Our sea-level curve, which is similar to the one reported from the Marshall Islands, is a baseline to reconstruct the evolution of reef flats and reef islands. In addition, it provides important contextual data to infer human settlement on islands in the west-central Pacific.

Carbonate production by large benthic foraminifers is sometimes comparable to that of corals and coralline algae, and contributes to sedimentation on reef islands and beaches in the tropical Pacific. Population dynamic data, such as population density and size structure (size-frequency distribution), are vital for an accurate estimation of shell production of foraminifers. However, previous production estimates in tropical environments were based on a limited sampling period with no consideration of seasonality. In addition, no comparisons were made of various estimation methods to determine more accurate estimates. Here we present the annual gross shell production rate of Baculogypsina sphaerulata, estimated based on population dynamics studied over a 2-yr period on an ocean reef flat of Funafuti Atoll (Tuvalu, tropical South Pacific). The population density of B. sphaerulata increased from January to March, when northwest winds predominated and the study site was on the leeward side of reef islands, compared to other seasons when southeast trade winds predominated and the study site was on the windward side. This result suggested that wind-driven flows controlled the population density at the study site. The B. sphaerulata population had a relatively stationary size-frequency distribution throughout the study period, indicating no definite intensive reproductive period in the tropical population. Four methods were applied to estimate the annual gross shell production rates of B. sphaerulata. The production rates estimated by three of the four methods (using monthly biomass, life tables and growth increment rates) were in the order of hundreds of g CaCO3 m−2 yr−1 or cm−3 m−2 yr−1, and the simple method using turnover rates overestimated the values. This study suggests that seasonal surveys should be undertaken of population density and size structure as these can produce more accurate estimates of shell productivity of large benthic foraminifers.

Worldwide, coral reef ecosystems are experiencing increasing pressure from a variety of anthropogenic perturbations including ocean warming and acidification, increased sedimentation, eutrophication, and overfishing, which could shift reefs to a condition of net calcium carbonate (CaCO3) dissolution and erosion. Herein, we determine the net calcification potential and the relative balance of net organic carbon metabolism (net community production; NCP) and net inorganic carbon metabolism (net community calcification; NCC) within 23 coral reef locations across the globe. In light of these results, we consider the suitability of using these two metrics developed from total alkalinity (TA) and dissolved inorganic carbon (DIC) measurements collected on different spatiotemporal scales to monitor coral reef biogeochemistry under anthropogenic change. All reefs in this study were net calcifying for the majority of observations as inferred from alkalinity depletion relative to offshore, although occasional observations of net dissolution occurred at most locations. However, reefs with lower net calcification potential (i.e., lower TA depletion) could shift towards net dissolution sooner than reefs with a higher potential. The percent influence of organic carbon fluxes on total changes in dissolved inorganic carbon (DIC) (i.e., NCP compared to the sum of NCP and NCC) ranged from 32% to 88% and reflected inherent biogeochemical differences between reefs. Reefs with the largest relative percentage of NCP experienced the largest variability in seawater pH for a given change in DIC, which is directly related to the reefs ability to elevate or suppress local pH relative to the open ocean. This work highlights the value of measuring coral reef carbonate chemistry when evaluating their susceptibility to ongoing global environmental change and offers a baseline from which to guide future conservation efforts aimed at preserving these valuable ecosystems.

Low-lying, small reef islands on Pacific atolls are vulnerable to coastal erosion and flooding, mainly due to rises in sea level, as well as local stresses associated with overpopulation. Because reef island sediments on Pacific atolls are composed mostly of shells of large benthic foraminifers (LBFs), their future stability depends on understanding foraminiferal shell sources and their production rates around the islands. Here, we report on the distribution and population density of LBFs around Fongafale Island, the main populated island of Tuvalu in the South Pacific, where coastal erosion and inundation have occurred. We discuss their controlling factors, in particular, anthropogenic influences on foraminiferal distribution. The results based on multivariate analyses (canonical correlation analysis and multiple regression analysis) demonstrated that Baculogypsina sphaerulata and Amphistegina lobifera were more common on the high-energy, intertidal ocean reef flats (≤ 10 6 and ≤ 10 4 individuals m − 2 , respectively), and B. sphaerulata was less common around the populated area of the island. On the relatively low-energy subtidal lagoon reef flats, Amphistegina lessonii was the most common species, particularly in the dry (leeward) season, with an increasing density in deeper offshore zones (≤ 10 4 individuals m − 2 ). Sorites orbiculus was less common than the other species, but was found on both sides of the reef flats (≤ 10 3 individuals m − 2 ). Macroalgal δ 15 N values indicated that upwelling nutrient-rich deep water could be the source of nitrogen for oceanic waters, whereas nutrients in lagoon seawater could be partly derived from groundwater mixing with domestic wastewater, especially near the populated area. These results suggest that the distribution and population density of LBFs in the study area are influenced by spatial differences in water energy and water quality, and that B. sphaerulata is less tolerant of nutrient loading than other species. Comparison of distributions between living individuals and empty shells indicated that ocean reef flats are the primary source of foraminiferal shells for reef islands and lagoon beaches.

article i nfo Article history: The depositional processesthat formed LauraIsland,MajuroAtoll, Marshall Islands,were reconstructed basedon a facies analysis of island sediments and spine ratios, and radiocarbon ages of foraminifera. Sedimentary facies were analyzed from trenches and drill cores excavated on the island and its adjacent reef flat. Depositional ages were obtained using benthic foraminifera (Calcarina) whose spines had not been abraded. The facies were classified into two types: gravelly and sandy. The initial sediments of these sites consisted of gravelly facies in the lower horizon and sandy facies in the upper horizon. Their ages were approximately 2000 cal BP and coincident with the onset of a 1.1-m decline in regional relative sea level, which enabled deposition of the gravelly facies. Half of the sand fraction of the sediment was composed of larger benthic foraminifera. The spine ratioshowedthat theirsupply source on the reef flatwaslocated oceansideof theisland.The supplysource appears to have been caused by the relative sea-level fall. This indicates that the studied island was formed by a relative reduction in wave energy and enhanced foraminiferal supply, both of which were triggered by the late Holocene relative sea-level fall.

Research combining the analysis of coral distribution in volcanically acidified waters with laboratory culture experiments indicates that reef communities may shift from reef-building hard corals to non-reef-building soft corals under CO2 partial pressure (pCO2) levels of 550–970 μatm that are predicted to occur by the end of this century.

In 2007, high-temperature-induced mass coral mortality was observed in a well-developed fringing reef area on the southeastern coast of Ishigaki Island, Japan. To analyze the response of the corals to thermal stress, the coral cover was examined using Quickbird data, taken across the reef flat just before and after the bleaching event and performing a reef scale horizontal 2-dimensional thermal model simulation. The Quickbird data consisted of multispectral (MSS) imagery, which had a spatial resolution of 2.4 m, and panchromatic (PAN)-fused multispectral imagery, which had a 0.6-m spatial resolution. The observed changes in coral cover implied that the delineation of partially bleached coral was more precise with PAN + MSS. The classification accuracy achieved using PAN + MSS (93%) was superior to that obtained using MSS (88%). The in situ water temperature observations and 2-dimensional thermal model simulation results indicated that the water temperature fluctuated greatly in the inner reef area in late July 2007. Different thermal stress indices, including daily average temperature, daily maximum excess temperature, and daily accumulated temperature, were examined to define a suitable index that represented the severity of the thermal stress on coral cover. The results suggested that the daily accumulated temperature that occurred during the maximum sea surface temperature period of the bleaching season provided the best predictor of bleaching. The changes in water temperature, bathymetry, and coral patch size affected the severity of bleaching; therefore, the spatial dependence of these variables was examined using Moran’s I and Lagrange multiplier tests. An investigation of the effect of coral patch sizes on coral bleaching indicated that large coral patches were less affected than the small patches, which were more likely to suffer bleaching and coral mortality.

Cores from Holocene reefs provided a growth history and species-level identification of corals and demonstrated the most important reef builders during the formation stage. This knowledge is important to determine a principle for reef formation and to provide preservation plans in the near future. A biological and sedimentological study of sediment cores recovered from the Palau Islands and Yoron Island, northwest Pacific, revealed four major facies: corymbose Acropora, arborescent Acropora, massive Porites, and detritus. Species-level observations show that arborescent Acropora (A. muricata and A. intermedia) contributed to reef growth under low- to moderate-energy conditions, whereas corymbose and tabular Acropora (A. digitifera, A. hyacinthus, and A. robusta/A. abrotanoides) and I. palifera were key species for reef formation under high-energy conditions during Holocene sea-level rise and the ensuing period of sea-level stability. Once sea level had stabilized, massive Porites became restricted to areas subjected to low-energy, turbid conditions. These key species are successful corals because the ecological strategy is rapid growth, determinate growth, a high degree of colony integration, strongly resistant to wave action, and rapid local dispersion via fragmentation. Moreover, the western boundary current (Kuroshio) flows along the reefs in the northwest Pacific and it is easy for key species to distribute throughout the region during the period of Holocene sea-level rise and stabilization. These features are a principle for reef formation during sea-level changes. These key species played a significant role in Holocene reef formation in the northwest Pacific; however, coral mortality, caused by climate change, has recently been widely reported. Moreover, the decrease in key species abundance in present-day reefs has been more severe than that in any other species. These geological findings have important implications regarding the appropriate use of coral transplantation and decisions regarding the optimal location and size of marine protected areas.

Holocene sea-level change provides direct evidence of the melting of ice sheets; however, our understanding of the timing and course of such change is limited by a lack of information regarding environmental condition of corals. For example, Holocene sea-level curves are compiled based on water depths estimated from the occurrence of a small number of coral species, as species identification is difficult work. The error associated with this method of estimating sea level exceeds ± 2.5 m. In addition, there generally occurs a difference in the living-water depth of corals between high-energy reefs and low- to moderate-energy reefs. This difference in living-water depth also results in an error in reconstructed sea-level curves. In the present study, we analyzed drill core and recorded the vertical distribution of corals at the high-energy Ibaruma reef and low- to moderate-energy Fukido reef, Ishigaki Island, Ryukyu Islands, Japan. Analyses of corymbose Acropora (A. digitifera), massive Goniastrea (G. retiformis), massive Pocillopora (P. verrucosa), massive Platygyra (P. ryukyuensis), and other corals enables the accurate reconstruction of a sea-level curve because these species are distributed in a narrow range, with a weighted mean (Mw) error of between ± 0.5 and ± 2.5 m. These observation data from Ibaruma and Fukido reefs can be used to reconstruct Holocene sea-level curves based on drill core data from Ibaruma reef; however, the two reefs yield contrasting sea-level curves: the curve reconstructed based on data from Fukido reef is several meters deeper than that reconstructed based on data from Ibaruma reef. This discrepancy reflects the different energy levels of the two reefs (high-energy at Ibaruma reef and low- to moderate-energy at Fukido reef). Therefore, the accurate reconstruction of a Holocene sea-level curve requires the identification of species from drill core and observations of the vertical distribution of species at the present-day reef at a location close to the drilling site. The Holocene sea-level curve reconstructed based on drill core data from Ibaruma reef reveals a rapid sea-level rise of approximately 7.5 m kyr− 1 between 8000 and 6000 cal. years BP. The rate of sea-level rise decreased from 7.5 to 3.5 m kyr− 1 between 6000 and 5000 cal. years BP. A mid-Holocene highstand occurred at around 5000 cal. years BP, at a level of approximately 3 ± 2.5 m above the present mean sea level. The increase in ocean volume corresponding to this sea-level rise indicates that melting of the Antarctic ice sheet continued beyond 6000 cal. years BP, as most of the Northern Hemisphere ice sheets had melted by 6000 cal. years BP, resulting in sea levels similar to those of the present day.

Coral reefs are influenced by global and local factors, and living corals are currently faced with a potential loss of species diversity. Knowledge of the relationship between species diversity and reef growth during the Holocene is important in terms of accurately reconstructing natural conditions prior to recent disturbances (e.g., human impact, pollution, and over-harvesting) and in predicting future scenarios (e.g., abrupt sea-level rise, coastal change, and economic services). This study seeks to characterize the Holocene and present-day reef at Ishigaki Island in the Ryukyu Islands, focusing on spatial and temporal variations in the relationship between species diversity and reef growth. The analysis is based on a drilling core obtained for the Holocene reefs and quantitative species-diversity data (Shannon and Weaver's diversity index, H ′) obtained for the present-day reef. H ′ was calculated for four coral communities surveyed at the Ibaruma and Fukido reefs. The Holocene sequence was dominated by the corymbose coral community (e.g., Acropora digitifera , A. hyacinthus , Goniastrea retiformis , and Platygyra ryukyuensis ), yielding an H ′ value of 1.6. The encrusting coral community (e.g., Echinopora lamellose and Pachyseris rugosa ) showed the highest diversity at the reef ( H ′ = 2.2); however, this community was not one of the main reef builders during the Holocene. The massive coral community (e.g., Porites lutea and Favites chinensis ) showed the lowest diversity ( H ′ = 0.6). It also made a minor contribution to reef building; this community appeared in a shallow lagoon once sea level had stabilized. The arborescent coral community (e.g., A. formosa and A. nobilis ) was one of the main reef builders, although yielding an H ′ value of much less than 1.0. Species diversity is not a prerequisite in terms of Holocene reef growth. Thus, a few species (e.g., A. digitifera , A. hyacinthus , A. formosa , A. nobilis , G. retiformis , and P. ryukyuensis ) from two main reef-building communities are important for reef growth. These corals that act as reefs are characterized by high growth rates, having an upward reef growth rate of 2–12 m kyr − 1 in the Indo-Pacific during the Holocene.

Coral reef growth is largely controlled by the wave energy associated with ocean swells and local winds. This growth has responded to changes in wave conditions that evolved in concert with Holocene sea-level changes. We analyzed Holocene facies of the Ibaruma and Fukido reefs of Ishigaki Island, southern Ryukyu Islands, in the context of temporal and spatial changes in wave conditions by comparing present-day coral assemblages and internal (fossil) reef facies. Four major coral assemblages that developed under different wave-energy conditions were recognized. Six cores recorded a Holocene reef sequence consisting of four sedimentary facies that differed in the relative volumes of four coral taxa: Acropora digitifera and Acropora hyacinthus, and lesser amounts of Favia stelligera and Montastrea curta. Holocene windward reef growth began at 7820 cal. years BP after flooding of the Pleistocene reef limestone surface; the ensuing reef growth kept up with the rapid sea-level rise. A vertical growth rate of 7.5 m/1000 years during reef nucleation and early growth stages decreased to 3.5 m/1000 years during the main stage of reef framework development. The uppermost part of the windward reef consisted of a very robust framework, unlike the open framework of the leeward reef. Ages indicated that the uppermost leeward reef was ca. 3000 years younger than the uppermost windward reef. These variations in facies development and timing were caused by differences in local wind-driven wave conditions.

The distributions and population densities of large benthic foraminifers (LBFs) were investigated on reef flats of the Majuro Atoll, Marshall Islands. Annual sediment production by foraminifers was estimated based on population density data. Predominant LBFs were Calcarina and Amphistegina, and the population densities of these foraminifers varied with location and substratum type on reef flats. Both foraminifers primarily attached to macrophytes, particularly turf-forming algae, and were most abundant on an ocean reef flat (ORF) and in an inter-island channel near windward, sparsely populated islands. Calcarina density was higher on windward compared to leeward sides of ORFs, whereas Amphistegina density was similar on both sides of ORFs. These foraminifers were more common on the ocean side relative to the lagoon side of reef flats around a windward reef island, and both were rare or absent in nearshore zones around reef islands and on an ORF near windward, densely populated islands. Foraminiferal production rates varied with the degree to which habitats were subject to water motion and human influences. Highly productive sites (>103 g CaCO3 m−2 year−1) included an ORF and an inter-island channel near windward, sparsely populated islands, and a seaward area of a reef flat with no reef islands. Low-productivity sites (

Significant coral bleaching was observed in summer in Shiraho reef, a fringing reef in the east coast of Ishigaki Island, Okinawa, Japan. To find relationship between coral bleaching and thermal stress in the reef and its governing factors, we have examined data obtained by field surveys in July 2003 and 2007 at Shiraho reef and the corresponding numerical simulation results for spatio-temporal variations of water temperature. The results of analysis showed that the spatial distribution of the degree of coral bleaching in the reef is closely related with that of the mean water temperature, which is strongly governed not only by local topography but also by meteorological conditions, especially latent heat flux.

The reef islands formed on coral atolls are generally small, low, and flat, with elevations of only a few meters. These islands are thus highly vulnerable to elevated sea levels caused by extreme events and global warming. Such vulnerability was recently evidenced at Fongafale Islet, the capital of Tuvalu, when it flooded during accelerated spring high tides possibly related to sea level rise caused by global warming. Many factors, not only environmental but also economic and social, determine the vulnerability of an island to sea level rise. In this study, we used data spanning 108 yrs to reconstruct changes in topography, land use/cover, population, and the distribution of buildings at Fongafale Islet. The results indicate that the vulnerability of Fongafale Islet relates to its original landform characteristics: the central part of the island was formerly dominated by swampland that flooded at high tides. Fongafale Islet experienced greater population in-migration and centralization beginning in the 1970s following the independence of Tuvalu and Kiribati. Migrants were also responding to declines in overseas mining operations and limited options for paid employment. As the population increased, construction took place in vulnerable swampland areas. Our results clearly demonstrate that examinations of global environmental issues should focus on characteristics specific to the region of interest. These characteristics should be specified using historical reconstruction to understand and address the vulnerability of an area to global environmental changes.

We measured the air–water CO2 flux in four coastal regions (two coral reefs, one estuary, and one coastal brackish lake) using a chamber method, which has the highest spatial resolution of the methods available for measuring coastal air–water gas flux. Some of the measurements were considerably higher than expected from reported wind-dependent relationships. The average k600 values for Shiraho Reef, Fukido Reef, Fukido River, and Lake Nakaumi were 1.5 ± 0.6, 3.2 ± 0.3, 0.69 ± 0.26, and 2.2 ± 0.9 (mean ± S.D.) times larger than the wind-dependent relationships. Results were compared with current-dependent relationships and vertical turbulence intensity (VTI). VTI is an index of water-surface stirring and is calculated from near-surface vertical velocity. Although some measurements from the reefs and river closely matched those expected from wind-dependent relationships, others were considerably higher. All data were correlated with VTI and were qualitatively explained by bottom macro-roughness enhancement. In Lake Nakaumi, results tended to differ from the wind-dependent relationships, and the difference between the measured and expected gas-transfer velocity was correlated with biological DO changes and/or the intensity of density stratification. We found these factors to have important effects on coastal gas flux. In addition, the chamber method was an effective tool for evaluating coastal gas flux.

Concentration of dissolved and particulate carbon (C), nitrogen (N), and phosphorus (P) were measured daily and seasonally in the water column of a coral reef lagoon at Ishigaki Island. Concentrations of dissolved inorganic N and P (DIN, DIP), dissolved organic C and N (DOC, DON), particulate organic C, and particulate N and P (POC, PN, PP) were usually higher in the lagoon than at offshore stations, while dissolved organic P (DOP) was higher at the offshore stations than in the lagoon. Concentration ratios of DIN:DIP and PN:PP were near 16:1 in winter and >20:1 in summer. DON:DOP was always ≥30. Total P (DIP + DOP + PP) was fairly constant around 0.177 ± 0.025 μmol l–1 (mean ± SD), suggesting conservative behavior of water-column P around this reef. DIP and PP seemed to be produced at the expense of DOP, and PP rather than DIP increased when gross primary production (GPP) was high. In contrast, total N (DIN + DON + PN) showed large spatial and seasonal variation from 4.3 to 12.4 μmol l–1. DON and PN increased when GPP and DIN:DIP were high, which suggests that export production of organic N by the lagoon biota was enhanced by primary production and the availability of N relative to P. Estimation of C, N, and P exchange fluxes between the lagoon and the outer ocean using a simple hydrodynamic model showed that the lagoon ecosystem released N (especially DIN) but not P to the surrounding ocean. Oceanic DOP could be the major source for the P budget of the lagoon. While the lagoon was a net source of organic C to the outer ocean, the net export was only a minor fraction (

The ability of five satellite sensor bands (IKONOS band 4, Terra ASTER bands 3 and 4, and Landsat ETM+ bands 4 and 5) was examined to extract the waterline at coral reef coasts (Majuro Atoll, Marshall Islands) using different wavelength regions (near infrared [NIR] and shortwave infrared [SWIR]) and different spatial resolutions (4, 15, and 30 m). After performing georeferencing and normalization of the images, density slicing was used to extract the waterline. Comparisons of extracted waterline positions with ground-level data for eight transects and global positioning system (GPS) tracks of the island shorelines showed that NIR bands were superior to SWIR bands because of the characteristics of the coral reef coasts, including a lack of foam and suspended sediments (which can affect the NIR wavelength region, if present) and the presence of remnant water on reef flats during low tide (which can affect the SWIR wavelength region). A linear relationship was found between the estimation errors of waterline positions and the spatial resolutions of the NIR sensors. Analysis on estimation errors and image costs showed that Terra ASTER band 3 was the most cost-effective sensor for extracting waterlines with reasonable accuracy. The results serve as general guidelines for using satellite-derived data to estimate intertidal topography and detect and monitor shorelines in coral reef environments.

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.

Larval dispersal and recruitment are important factors that determine the distribution of adult corals. The relationships between larval dispersal, recruitment, and the adult distribution of the blue octocoral, Heliopora coerulea, were investigated on Shiraho Reef, Ishigaki Island, southwest Japan. Heliopora coerulea is a surface brooder that releases planulae in June or July on Shiraho Reef. We observed planulae between 1998 and 2000 and found that they did not swim actively; instead, they crawled into their settlement positions after becoming grounded on the substratum. Planulae occurred throughout the water column and were dispersed by tidal and wind-driven currents around the parent population on the reef flat. Recruitment was observed only within 350 m of the parent populations, including areas between the branches of the adult colony. The planulae of H. coerulea had a narrow dispersal range as a result of their mostly benthic, shorter larval duration, and the influence of weaker currents. Thus, the dispersal distance of larvae is determined by their position in the water column, the currents that deliver the larvae, and the competency period of the larvae. The narrow dispersal range of H. coerulea was consistent with recruitment of sexually derived larvae onto their natal reef.

Wave energy induced by ocean swell and local winds is an important factor controlling the coral reef development. Here we compare and contrast facies, ages, and accretion rates of reefs that developed independently along windward and leeward margins of the same island. As both reefs face the open ocean, swells provide a background of low- to medium-energy condition with additional energy along windward margins due to prevailing winds. This similarity of the wave-energy regimes (medium to high energy) produced a similar framework facies that was constructed by robust-branching Acropora. However, much greater lateral accretion rates were observed in the high-energy reef than in the medium-energy reef. Also, coral cobble facies were only found in the medium-energy reef, indicating that the reef framework was more susceptible to storm breakage than that in the high-energy reef. These differences are attributable to the differing biological and ecological responses of corals to wave energy and water motion.

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.

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.

Internal facies and development of an oceanic island's barrier reef were revealed by the stratigraphical study of six drill cores in Palau Islands, western Pacific. The Holocene reef development is primarily constrained at its foundation by the antecedent topography of Pleistocene substratum. Holocene barrier reef is an increment on the Pleistocene barrier reef, which had been subaerially exposed during glacial stages. About 8300 cal. year BP (calibrated calendar years B.P.), branching Acropora facies initially formed a bank on the seaward side of a Pleistocene limestone surface with a vertical accumulation rate as high as 30 m/ka (ka=1000 years). After 7200 cal. year BP, when the sea-level rise rate decreased, reef crest facies caught up with the sea surface with an accumulation rate of less than 2.2 m/ka. Corals found in the reef crest facies are similar to the present-day reef crest corals dominated by Acropora digitifera and A. humilis. After the reef crest was formed, bioclastic sand and gravel facies prograded lagoonward of the reef crest and consisted mostly of reef derived materials. The construction of patch reefs post-dated the barrier reef formation. The mature barrier reef provided calm conditions inside the lagoon, which then led to the construction of patch reefs and fringing reefs. Sea-level changes deduced from the accumulation curves show rapid rise before 7200 cal. year BP followed by a slight rise of 4 m at its maximum. This change in sea-level rise rate inspired the change in reef facies from branching Acropora to reef crest.

In order to investigate Pacific atoll lagoonal facies and temporal evolution, we collected and analyzed surface and core sediments from three Pacific atoll lagoons of various sizes and depths (Kayangel of Palau Islands, Enewetak and Majuro of the Marshall Islands). The lagoons were divided into three facies: Calcarina facies, Calcarina–Heterostegina mixed facies and Heterostegina facies. These were based on the presence or absence of the tests of two larger benthic foraminifera, Calcarina and Heterostegina , as indicators of reef flat and deep lagoon environments, respectively. The Calcarina facies is composed of allochthonous reef-derived materials, the Calcarina–Heterostegina mixed facies is a mixture of reef-derived and in situ lagoonal materials, and the Heterostegina facies is composed mainly of in situ lagoonal materials. As both foraminifera are widely distributed in the Western to Central Pacific region, the facies classification presented in this study can be applicable to other Pacific atolls. Based on the results of radiocarbon dating on samples obtained from cores at Majuro lagoon, the accumulation rates of the lagoonal sediments were estimated as 0.65 m kyr −1 . Calcium carbonate production was calculated on the basis of the accumulation rates and density of sediments to be 710 g CaCO 3 m −2 yr −1 .

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

The most extensive and severe bleaching of coral ever recorded occurred in 1997 and 1998 as a result of anomalously high sea-surface temperatures. In the Ryukyu Islands, extensive bleaching occurred from late July to early October in 1998. A time-series transect study on a reef flat revealed the different responses of coral populations among species to this event. Coral cover was monitored just before, during, and 6 times after the bleaching occurred, along 5 fixed transects, 710 to 800 m long, across the reef flat of Shiraho Reef, Ishigaki Island, in the Ryukyu Islands of Japan. The dominant corals were Heliopora coerulea, massive Porites, and branching Porites, Montipora, Acropora, and Pavona (these latter 4 genera could not always be distinguished to species level, and are thus grouped by genus and ecoform). H. coerulea was the least susceptible to bleaching and maintained almost constant coverage before and after the bleaching. Massive Porites were suscepti- ble to bleaching, but regained their algae after the bleaching and sustained their coverage. On the other hand, the branching Porites, Montipora, and Acropora were susceptible to bleaching and mortality was high. Coverage by branching Montipora was initially reduced by 66%, but by 2 yr after the bleaching it had recovered to pre-bleaching coverage. However, only large patches of more than 70 m along a transect, with coverage of 10 to 40%, recovered. Small patches of less than 30 m along a transect, with coverage of less than 10%, died and did not recover. The response of corals to bleach- ing differs according to the strategy the coral uses against bleaching; moderately frequent and severe bleaching might permit the coexistence of both types of corals on the same reef flat, as in the case of Shiraho Reef.

Organic carbon flux and community production rates were estimated on Shiraho coral reef (Ishigaki Island, Japan) from 19 to 26 September 1998. The daily net community production (P n ) and respiration rate (R) during the study period were 3 to 79 and 596 mmolC m -2 d -1 , respectively. This resulted in a daily gross community production (P g ) of 599 to 675 mmol C m -2 d -1 , The variation of P n associated with the uncertainty of the curve fitting parameters of light response curves for photosynthesis was estimated using an error propagation formula. The averaged P n ± SE was 36 ± 12 mmol C m -2 d -1 (n = 23), indicating that the P n was significantly positive (t-test, p < 0.05). The apparent fluxes of dissolved organic carbon (DOC) and particulate organic carbon (POC) on the reef were estimated as 30 to 36 and 5 to 7 mmol C m -2 d -1 , respectively; The sum of which was comparable with the P n during the study period. The sediment trap study conducted at 1 km off the reef and 40 m depth showed that the vertical flux of POC was 1.0 mmol C m -2 d -1 . The results indicated that 6 to 7 % of the P g was exported to offshore and about 14 to 20% of the POC exported from the reef flat and 0.2 % of the Pg reached 1 km off and 40 m depth.

Cores from Kabira Reef, Ishigaki Island, southwest Japan, reveal the internal structure and temporal changes in sedimentary process of a complete coral reef flat. The reef crest caught up with sea level at about 4000 yr BP. Since then, it expanded oceanward and the reef pavement has grown landward. The backreef structure is composed of bioclasts derived from the reef framework. Of all the bioclasts, corals and coralline algae are most abundant. Coral fragments coarser than -3.0 o have dominated the backreef sediments since about 2000 yr BP. Benthic foraminiferal tests first occurred at about 4000 yr BP, and their abundance increased significantly starting around 2000 yr BP. Shallowing of the reef crest is also indicated by the presence of the shallow-water benthic Foraminifera Baculogypsina sphaerulata tests after 2000 yr BP. The date 2000 yr BP is coincident with tectonic uplift at Kabira Reef (Kawana 1989). We consider this uplift to have caused a relative sea-level fall that aided the deposition of coral fragments transported from the reef pavement by storms. The relative sea-level fall also caused subaerial exposure of the reef crest during low tides and the transition of reef-building organisms from corals to shallow-water species of benthic Foraminifera, resulting in a change noticeable in the constituents of backreef sediments. The results of this study suggest that the coral reef flat, and especially the backreef, can be a faithful recorder of relative sea-level changes.

Changes in living coral coverage along five transects on Shiraho reef and one transect on Kabira reef, Ishigaki Island, Ryukyu Islands were investigated before and after the 1998 bleaching event. The coverage of living corals after the bleaching decreased by 50% of their original values. Branching types of Montipora, Acropora and Porites were most severely bleached and their mortality was also high. Bleaching was intermediate to high for massive Porites whose recovery was better than branching corals. Heliopora coerulea was less susceptible to bleaching and mortality was low. Higher than 30°C sea surface temperature prevailed over the south sea of Japan, which induced higher than 30°C reef water temperature on the coral reef flat and resulted in coral bleaching.

Kabira Reef is a well-developed fringing reef situated in a monsoon area where the dominant wind direction changes seasonally: south in summer and north in winter. Circulation in this reef shows a marked wind influence. The circulation pattern under calm wind conditions is characterized by an inflow of ocean waters into the moat over the reef crest and an outflow through a prominent channel. Current vectors change according to wind conditions, and this pattern is weakened and strengthened under southern and northern wind conditions, respectively. We establish a simple model to explain these circulation patterns with two factors: wind and a fundamental circulation pattern under calm conditions. We estimate the ratios of the component of wind to that of the fundamental circulation. The ratios reach 3 and 10 in absolute values under southern and northern wind conditions, respectively. These results can be applied to water circulation throughout the year, with the southern wind-driven circulation dominant in the summer, and the northern wind-driven circulation dominant in the winter. While trade wind conditions often result in a constant circulation pattern, monsoonal wind conditions make the circulation pattern vary according to the seasons.

Shiraho Reef, Ishigaki Island, Southwest Japan is characterized by (1) a lack of prominent channels, which causes ponding at low tide, (2) its location in a monsoon area, which causes the dominant wind direction to change seasonally, and (3) its orientation facing the Pacific Ocean, where swells originate in distant stormy areas in addition to those produced by local winds. The water circulation observed during three separate periods and the estimated residence time in Shiraho Reef are very sensitive to all three factors. Advanced satellite technology contributes significantly to the long-term monitoring of ocean swell states, and the water circulation observed during the three periods can be classified using swell state (high and low swell) and wind conditions (northerly and southerly winds). During high swell states, the water residence time ranged from 3 to 8 h. The shorter time occurred when ponding did not occur, whereas the longer time occurred when ponding was observed. The water circulation and residence time likely affect the coral distribution and resilience to bleaching.

Modern fringing reefs of Yoron Island, Ryukyu Arc, Japan, have been investigated for the purpose of understanding the geomorphic development of the surface features and shallow sedimentological structure of the coral reef. Field investigations have been conducted by means of geomorphological observations, scuba divings, bathymetrical soundings and multiple shallow drillings on a modern reef at the northeastern coast of the island. Modern fringing reefs of Yoron Island form a distinctive zonation of landforms, which are composed of sandy or rocky shores, a moat, a reef crest, a reef front with spurs and grooves, and outer slopes from the shore to the reef margin. The cored samples from the modern coral reef are divided into the following four facies: reef crest, reef pavement, bioclasts and moat. A rigid framework of the reef crest has been formed upon the unconsolidated bioclasts facies. The reef crest of Yoron Island started to grow upward from 3m below the sea level to near the sea surface for the period from 5, 260 to 3, 230yrs BP. The reef pavement behind the crest has extended toward the moat along with the reef crest formation. After 3, 230yrs BP, the crest is also extending its width by forming spurs and grooves toward the ocean. These results indicate that the geomorphic zonation of the modern fringing reef of Yoron Island has been formed in the recent 5, 000 years.

Holocene emergent reefs and notches are well distributed on Rota and Guam. Relative sea-level changes at these islands are reconstructed based on geomorphological observations and borings on present and emergent reefs, together with 54 radiocarbon dates. Sea level rose gradually to a maximum of 1.8 m between 6000 and 4200 yr B.P. and reached its highest level by 4200 yr B.P. on both islands. After 3200 yr B.P. abrupt uplift caused emergence of the reef. By subtracting the tectonic effect, we obtained the sea-level change in the Marianas: sea level reached its present level by 4200 yr B.P. and has remained almost stable since then. Reconstructed late Holocene sea-level change in the Mariana Islands provides constraints on geophysical models of sea-level variations.

Human impacts on sand-producing, large benthic foraminifers were investigated on ocean reef flats at the northeast Majuro Atoll, Marshall Islands, along a human population gradient. The densities of dominant foraminifers Calcarina and Amphistegina declined with distance from densely populated islands. Macrophyte composition on ocean reef flats differed between locations near sparsely or densely populated islands. Nutrient concentrations in reef-flat seawater and groundwater were high near or on densely populated islands. δ15N values in macroalgal tissues indicated that macroalgae in nearshore lagoons assimilate wastewater-derived nitrogen, whereas those on nearshore ocean reef flats assimilate nitrogen from other sources. These results suggest that increases in the human population result in high nutrient loading in groundwater and possibly into nearshore waters. High nutrient inputs into ambient seawater may have both direct and indirect negative effects on sand-producing foraminifers through habitat changes and/or the collapse of algal symbiosis.

[1] We measured the gas-transfer velocity (k) and analyzed factors regulating k at coral reefs and an estuary at Ishigaki Island, Japan, using the floating-chamber method and the measured energy-dissipation rate (ɛ) to represent turbulence in a small-eddy model. We confirmed the validity of the floating-chamber method quantitatively for the first time by the comparing ɛ values inside and outside the chamber device. We also compared k to ɛ and empirical parameters such as wind and current speeds. Measured k had a low correlation with the empirical parameters and a high correlation with ɛ, as indicated by the small-eddy model. The high ɛ values may have been regulated by topographic conditions, e.g., corals or seagrasses that generate wakes, and complex coastlines or large-scale (on the order of kilometers) topographic factors that generate horizontal current shear. Our measurements indicate that coastal k is regulated by ɛ and cannot be accurately determined using wind or current speeds. Topographic conditions in coastal regions are important factors that regulate ɛ; thus, a quantitative analysis of the effects of these conditions is necessary to accurately determine coastal air–water gas flux.

[1] Monitoring seawater CO2 for a full year with seasonal observations of community metabolism in Ishigaki Island, Japan, revealed seasonal variation and anomalous values owing to the bleaching event in 1998. The daily average pCO2 showed a seasonal pattern on an annual scale, 280 to 320 μatm in winter and 360 to 400 μatm in summer, which was determined primarily by the seasonal change in seawater temperature. By contrast, the range in the diel variation in pCO2, 400 to 500 μatm in summer 200 to 300 μatm in winter, was attributed to the seasonal variation in community metabolism: Gross primary production (Pg) and respiration (R) were high in summer and low in winter. During the 1998 bleaching event, although Pg and R increased, community excess organic production (E) decreased by three quarters compared with the same month in 1999, when the coral community showed high recovery. This change in metabolism led to large diel range and increased average value of pCO2 levels in the seawater on the reef flat. The decrease in the range and increase in the average value of pCO2 were observed by monitoring the Palau barrier reef flat, where overall mortality of corals occurred after the bleaching. All the metabolic parameters, Pg, R, E and calcification (G) were reduced by half after the bleaching, which increased the average pCO2 value by 10 μatm and decreased its diel range from 200–400 μatm to 100–200 μatm. Bleaching and resultant mortality of coral reefs led to degradation of their metabolic performance, and thus resulted in the loss of their active interaction with the carbon cycle.

Coral reefs are considered to be a source of atmospheric carbon dioxide because of their high calcium carbonate production and low net primary production. This was tested by direct measurement of diurnal changes in the partial pressure of carbon dioxide ( P co co2 ) in reef waters during two 3-day periods, one in March 1993 and one in March 1994, on Shiraho reef of the Ryukyu Islands, Japan. Although the P co co2 values in reef waters exhibited large diurnal changes ranging from 160 to 520 microatmospheres, they indicate that the reef flat area is a net sink for atmospheric carbon dioxide. This suggests that the net organic production rate of the reef community exceeded its calcium carbonate production rate during the observation periods.

Coral cays are low-lying islands formed by coral gravels deposited by the waves and currents on the coral reefs. Cases have also been reported that the coral cays were formed in the short time scales when high waves with storm surges. Once the formation of coral cays can be controlled, which can contribute to the protection, environment and use of the coast on coral reefs and the national land conservation of the Pacific Ocean island nations in danger of inundation by the sea level rise. In this paper, the formation mechanism of the coral cays is verified through hydraulic movable bed model tests and the numerical simulation aimed at Ballast Island in the north of Iriomote Island. As a result, we confirmed that the wave breaking on the reef edge, wave setup, currents, and gravel supplies on the coral reefs exert influence on the formation of coral cays.

Holocene relative sea levels in oceanic islands that are situated sufficiently far from glaciated regions provide basic information for the study of the melting histories of the continental ice sheets and of the rheological structure of the Earth. We have studied Mangaia Island, South Cooks, as one such oceanic island located in the middle Pacific Ocean. In addition to the usual geological and geomorphological observations of geomorphic features associated with former shorelines, we have used a portable drilling sampler for shallow borings of coral reefs. Geological and geomorphological studies of the coast of Mangaia Island have revealed that there was a higher sea level than the present in the mid-Holocene. The heights and ages of emerged microatolls on the emerged bench indicate that the sea reached a maximum level of +1.7 m around 4000-3400 yr B.P., and then emergence is considered to have occurred between 3400 and 2900 yr B.P. At the same time, the reef crest formed in the period from 5000 to 3400 yr B.P. emerged above the sea and suffered erosion. As a result of this change in sea level, upward reef growth from the fore reef slope began to form a new reef crest by 2000 yr B.P. to seaward of the old reef crest. The reef margin of Mangaia grew outward intermittently with seaward jump of the reef front, in accordance with a fall of sea level, in the late Holocene. The presence of fossil reef crests or eroded algal ridges on modern reef flats is a rather common feature in the South Pacific. The geomorphic development of such features on reef flats can be explained by the effect of the late Holocene fall in sea level.