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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.

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.

International audience; This study focuses on Okinawa and Yoron islands, in order to better understand tectonics in the Ryukyu Arc related to the subduction zone. We used coral microatolls-known for their centimetric accuracy in the record of relative sea-level (RSL) changes-to reconstruct RSL changes over the last century from living microatolls. A fossil microatoll in Yoron was used to discuss possible RSL changes beyond the last century. The signal consists of emergence whose rate varies through time, interrupted by decadal to multidecadal periods of sudden and/or gradual submergence and by interannual sea-level falls. Comparison with other existing RSL records in the arc highlights RSL variability along the arc. This pattern contrasts with the homogeneous and linear absolute regional sea-level rise, implying that this latter signal cannot fully explain our observations and that an additional process is required. We suggest the subduction zone as a possible source for generating centimetric scale RSL changes observed in coral microatolls. We tested how the Ryukyu megathrust could explain our observations with elastic dislocation modeling. The emergence trend could be explained by interseismic loading on the plate interface with a minimum coupling rate of 25%, which is higher than previous estimates based on short instrumental records. As for the submergence events that occurred regularly in the coral record, we show that they could be explained by slow slip events on the shallower part of the megathrust or in the transition zone. Such process could accommodate a significant part of the total convergence.

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.

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 atmospheric CO2 crossed over 400 ppm and we must prepare for a “+2 °C world” during this century. Coral reefs are directly related to each scenario of global warming: increase in CO2 results in ocean acidification and suppresses calcification, rise in sea surface temperature leads to severe bleaching, and sea level rise causes submergence of coral reefs and atoll islands. They are the most sensitive ecosystem and act as an early warning system to examine and predict response of ecosystem to the global warming. Records of bleaching events and SST for the last 17 years in the northwest Pacific show that 2 °C SST rise would induce severe bleaching of coral reefs. Reduction of ocean surface water pH by 0.3 would cause shift from hard coral to non-calcifying macroalgae or soft coral populations in coral reefs. Submergence of coral reefs by sea level rise of up to one meter results in a loss of their breakwater function and in atoll islands submergence of national land. “+2 °C world” is the threshold to maintain coral reefs. Factors of global warming and responses of coral reefs are coupled to form feedback loops, which enhance or stabilize the changes within a system.

Coupled records of Sr/Ca and oxygen isotope ratios (δ18O) of coral skeletons have been used to produce quantitative estimates of paleo-sea surface temperature (SST) and δ18O of surface seawater that can in some cases be converted to sea surface salinity (SSS). Two fossil corals from Kikai Island in the subtropical northwestern Pacific, a location affected by East Asian summer and winter monsoons, were analyzed to investigate differences between mid-Holocene and present-day SST and SSS. At 6180 cal yr BP, SSTs were roughly the same as today, both in summer and winter; δ18Oseawater and SSS values were higher both in summer (+ 0.5‰, +1.1 psu) and in winter (+ 0.2‰, + 0.6 psu) than modern values. At 7010 cal yr BP, SSTs were slightly cooler both in summer and winter (−0.8 and −0.6 °C), whereas δ18Oseawater and SSS had higher values in summer (+ 0.3‰, + 0.6 psu) and in winter (+ 0.8‰, + 1.9 psu) than present-day values. These results are consistent with other marine records for the mid-Holocene of the low and midlatitudes in the northwestern Pacific. Such regional conditions indicate that the East Asian summer and winter monsoons were more intense in the mid-Holocene, which was likely a function of the mid-Holocene insolation regime.

A coral radiocarbon (Δ14C) investigation with a high time-resolution is crucial for reconstructing secular and seasonal Δ14C changes in the surface seawater which potentially reflect ocean circulations and dynamic ocean-atmosphere interactions. The Δ14C values of a modern coral (Porites sp.) from Kikai Island, southern Japan, in the subtropical northwestern Pacific, were determined for the period of 1991-1998 at a monthly resolution. A coral Δ14C time series for the 8 yr indicated seasonal cycles superimposed on a secular decreasing trend of 3.8 per yr. The seasonal amplitude of the coral Δ14C was about 18 on the average, and the minimum Δ14C was observed in late spring and summer. The Δ14C changes were tentatively explained by horizontal oceanic advections around Kikai Island or over the wide range of the equatorial and sub-equatorial Pacific.

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.

Larval dispersal and recruitment are important in determining adult coral distribution; however, few studies have been made of coral larval dispersal. This study examined the larval behavior, survivorship competency periods and settlement of two brooding corals, Heliopora coerulea and Pocillopora damicornis, in relation to different potential larval dispersal patterns. We also examined the lipid content of H. coerulea as a means of flotation and a source of energy. Planulae of H. coerulea were on average 3.7 mm in length, lacked zooxanthellae, and were mostly benthic, probably because of restricted movement and low lipid content (54% by dry weight). Planulae of P. damicornis were on average 1.0 mm in length, had zooxanthellae and swam actively. The competency period of H. coerulea was shorter (30 days) than that of P. damicornis (100 days). Forty percent of H. coerulea planulae crawled onto the substrata within 1 h of release, and 47% settled within 6 h. By contrast, fewer than 10% of P. damicornis planulae crawled onto the substrata within the first hour and 25% settled within 6 h of release. The planulae of H. coerulea may have a narrower dispersal range than those of P. damicornis, settling and recruiting near parent colonies. Thus, brooding corals exhibit variations in larval dispersal patterns, which are characterized by their position in the water column and competency periods.

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.

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.

[1] Low-frequency variations in the seasonally phase-locked signal of the Indian Ocean Dipole (IOD) and the Pacific El Nino/Southern Oscillation (ENSO) recorded in 115-year coralδ18O variability from Kenyan coast, the tropical western Indian Ocean, were investigated. A comparison of the monthly coral δ18O corresponding to IOD and ENSO years shows that Kenyan coral distinctly records the East African Short Rain anomaly related to the IOD variability in January, a few months after the Short Rain peak due to oceanographic condition. On the other hand, the ENSO-induced signals do not appear clearly as the positive sea surface temperature (SST) and rainfall anomalies in the monthly coral record. Moreover, annual mean coralδ18O and ENSO show only a weak coherence at the periodicity of 4 to 5 years. These results support the suggestion that the IOD is the dominant climate mode rather than ENSO in the Kenyan coast. The coral records indicate that the negative IOD- like anomalously cold SST condition in the western Indian Ocean precedes the evolution of the Pacific El Nino by one year. The anomalously cold SST condition was prominent in the late 19th century, but weakened in the 20th century. This retreat of the cold SST condition due to warming of the western tropical Indian Ocean may influence the nature of the Pacific ENSO.

[1] A 115-year coral record from Kenya has been found to preserve the history of rainfall anomalies in East Africa in relation to global warming-induced Indian Ocean Dipole (IOD) variability. The coral IOD index demonstrates a dominantly decadal periodicity in the early part of the 20th century. This low-frequency IOD occurred more frequently before 1924 with mostly quasi-biennial ranging from 18 months to 3 years events since 1960. The mode shift has also coincided with an intensified coupling with Indian summer monsoon rainfall. We suggest that a warming of the western Indian Ocean, which has attenuated and replaced the El Nino/Southern Oscillation effect over the Indian Ocean, has driven the observed shift.

[1] The Indian Ocean Dipole (IOD), which governs climate variability not only over the Indian Ocean but also globally, produces precipitation anomalies in the East African short rain period from October to November. These precipitation anomalies were recorded in annual bands from a coral core obtained in the Malindi Marine Park, Kenya. A signature of the IOD in precipitation was detected in the coral core by luminescence intensity under UV light and by oxygen isotope values dated to January, a few months after the short rain period. Anomalies of the oxygen isotope values correlated well with high and low precipitation, which were related to positive and negative IOD indices, respectively. This study suggests that the coral annual band record is effective for reconstructing IOD events and should be able to reconstruct the IOD record prior to instrumental observations.

[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.

[i] Changes in sea surface salinity (SSS) in the western Pacific warm pool (WPWP) provide information on the variability of the El Nino Southern Oscillation (ENSO) because SSS reflects the balance between evaporation and precipitation. We reconstructed the long-term variability of SSS at the northwestern edge of the WPWP from oxygen isotope analysis of annual coral bands. The estimated SSS corresponded well with in situ observations, demonstrating that isotopic records from coral can be used as a paleosalinometer. Estimated SSS between 1954 and 2000 peaked during the El Ninos of 1972-73, 1982-83, and 1997-98. These SSS peaks were caused by anomalously low precipitation accompanying the eastward shift in the WPWP. Citation: Iijima, H., H. Kayanne, M. Morimoto, and O. Abe (2005), Interannual sea surface salinity changes in the western Pacific from 1954 to 2000 based on coral isotope analysis.

[1] Seasonal sea surface salinity (SSS) records can be of great value in reconstructing El Nino/Southern Oscillation (ENSO) variability in the equatorial Western and Central Pacific, a region where ENSO-related evaporation and precipitation (E-P) changes are dramatic. The δ18O values of coral skeleton (δ18Ocoral) reflect seawater δ18O (δ18Oseawater) changes, which in tropical oceans are also generally influenced by E-P changes. Therefore, δ18Ocoral is a good indicator of rainfall and ENSO variability. We present biweekly data from Palau, in the northern part of the Western Pacific Warm Pool, for the period 1998–2000, which indicate a strong quantitative relationship between δ18Ocoral, sea surface temperature, δ18Oseawater and SSS. The coral skeletal δ18O values documented the SSS changes after the 1997–98 El Nino; therefore, δ18Ocoral can be used to estimate paleosalinity changes. However, the slope of the δ18Oseawater - SSS relationship cannot be assumed to be constant throughout the tropics, making site-specific calibrations is essential.

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.

Coral cays are low-lying Island which is formed by the gravel on the coral reef, in this position slightly higher than a field. In past research, there were reported that formed and disappeared by short-term of high waves. But many parts of the mechanism are unknown because of difficulty of observation data acquisition. In this study, we investigated the mechanism for topographic change by laboratory model experiment and field surveys on Barasu Island near the north side of Iriomote Island. In order to understand the topography change mechanism of coral reef cays, the field survey of Barasu Island and model experiment were conducted. Topographic change of deposition, erosion, and movement was repeated by the strong external force at the time of a typhoon, And, it turned out that the move direction of Barasu Island is qualitatively in agreement with the excellence direction of the external force. Moreover, although it was erosive in the external force of one direction, when the external force of two directions was received, it checked that Barasu Island was expanded.

By using a step-wise heating method in conjunction with static vacuum mass spectrometry, we have measured at high temporal resolution (one month) the nitrogen isotopic ratio of a coral skeletal sample collected from Palau in the northern part of the Western Pacific. The δ15N value from the 900°C combustion step varies significantly, from +1.1‰ to +10.9‰ for the estimated time period from July 1995 to January 1998. The higher values occurred during winter and the lower values in summer. The δ18O and δ13C values of the skeleton were also analyzed for the same period, using conventional methods. The ranges of δ18O and δ13C variations are from -6.1‰ to -5.4‰ and from -1.6‰ to -0.9‰, respectively, which are significantly smaller than the δ15N variation. The δ13C change is possibly attributable to kinetic isotope effects, as deduced from a negative correlation between the δ13C value and mean sea surface temperature (SST) in the region. On the other hand, the δ15N variation could be derived from seasonal change of the relative ratio of two components utilized by symbiotic algae: a 15N-depleted component from N2 fixation and 15N-enriched nitrate from the tropical oligotrophic open ocean.