Large Porites Research Articles

Long-term survival of large relocated

In August 2009, eight large Porites corals were relocated prior to dredging works at Cocos (Keeling) Islands. In March 2020, relocated colonies were found to have survived and grown, demonstrating that large Porites colonies growing on unconsolidated substrates can be relocated successfully if moved swiftly to a similar habitat to their source.

Large Porites microatoll found by aerial survey at Sesoko Island, Okinawa, Japan

Large Porites microatoll found by aerial survey at Sesoko Island, Okinawa, Japan

A modern Sr/Ca‐δ18O‐sea surface temperature calibration for Isopora corals on the Great Barrier Reef

AbstractIsopora (Acroporidae) is a genus of often encrusting, branching to submassive corals that are common in many shallow habitats on modern and fossil Indo‐West Pacific reefs. Although abundant, Isopora is largely absent from paleoceanographic literature. The scarcity of large Porites and the abundance of Isopora retrieved from the Great Barrier Reef (GBR) on Integrated Ocean Drilling Program Expedition 325 focused paleoceanographic attention on Isopora. Here we provide the first independent high‐resolution calibration of both Sr/Ca and δ18O for temperature analyses based on Isopora and demonstrate its consistency with Porites records. We developed modern skeletal Sr/Ca‐ and δ18O‐sea surface temperature (SST) calibrations based on five modern Isopora colonies from Heron Island in the southern GBR. Pairing the coral Sr/Ca record with monthly SST data yielded Sr/Ca‐SST sensitivities from −0.061 ± 0.004 (centered) to −0.083 ± 0.007 (raw) mmol/mol °C−1 based on reduced major axis regressions. These sensitivities are in the middle of the range of published Porites values and overlap most published values for Isopora, −0.075 ± 0.011 to −0.065 ± 0.011 mmol/mol °C−1. The δ18O‐SST sensitivities range from −0.184 ± 0.014 (centered) to −0.185 ± 0.014 (raw) ‰ °C−1, assuming that all seasonal variation in δ18O was due to SST. These δ18O‐SST sensitivities are smaller than the widely accepted value of −0.23‰ °C−1 for biogenic aragonite but are at the upper end of high‐resolution Porites‐defined sensitivities that are consistently less than the aforementioned established value. Our results validate the use of Isopora as an alternative source of paleoceanographic records in habitats where large massive Porites are scarce or absent.

New records and molecular characterization of Acrozoanthus (Cnidaria: Anthozoa: Hexacorallia) and its endosymbionts (Symbiodinium spp.) from Taiwan

During a recent survey of the zoanthid (Cnidaria: Anthozoa) fauna of Taiwan, specimens resembling Acrozoanthus australiae (family Zoanthidae) were found at Kenting and Green Island, Taiwan, attached to eunicid worm tubes growing out from under large Porites coral colonies in coral reef environments. As this species had previously been described only from eunicid worm tubes in mud flats in Australia and Indonesia, and no studies had specifically examined its phylogenetic position, molecular and morphological examinations were conducted to determine the identity of the Taiwan specimens, and its phylogenetic relationships with other Zoanthidae genera and species. At the same time, endosymbiotic Symbiodinium types within specimens were also investigated. Results from the phylogenetic analyses of sequences of three DNA markers [cytochrome oxidase subunit I, mitochondrial 16S ribosomal DNA, internal transcribed spacer 2 of ribosomal DNA (ITS-2)] strongly suggested that the Taiwan specimens were identical with A. australiae. Based on endosymbiont ITS-2 sequences, these colonies were in symbiosis with Symbiodinium clade D1a (= S. trenchii), theorized to be adapted to both comparatively cold and hot marine environments. Furthermore, phylogenetic analyses from all three zoanthid DNA markers suggest that Acrozoanthus may be within the closely related genus Zoanthus. This study demonstrates the overall lack of data on zoanthid species’ distributions, and it is recommended the diversity of zoanthids within the nearby Coral Triangle be investigated to link Indo-Australian zoanthid data with information from Japan and the northwest Pacific.

Historic 1771 Meiwa tsunami confirmed by high‐resolution U/Th dating of massive Porites coral boulders at Ishigaki Island in the Ryukyus, Japan

The chronicles of the Ryukyu Kingdom describe at least four coastal inundation events at Ishigaki Island during the seventeenth through nineteenth centuries: the 1771 Meiwa earthquake tsunami, which killed 12,000 people; another earthquake tsunami (1687); and two events with unrecorded causes (1625 and 1714). We used a high‐resolution U/Th method to date well‐preserved surface parts of massive Porites spp. coral head boulders that had been cast ashore and found that the ages of two boulders correspond to the 1771 event. One large Porites boulder (diameter > 4 m) was dated to 1625, suggesting that that event was also a large inundation event. Calibrated 14C ages, based on a newly estimated local value for the radiocarbon marine reservoir effect, clustered around 1625 and 1771, suggesting that these events were larger than the others. Our study demonstrated that Porites boulders have enormous potential value for studies of paleoinundation events such as tsunamis and extreme storms.

The aftermath of coral bleaching on a Maldivian reef—a quantitative study

Since the bleaching event of 1998, the development of the reef flat and upper reef slope on a Maldivian reef (the Komandoo house reef; Komandoo Island, Lhaviyani atoll or Fadiffolu atoll) is under detailed observation. We quantitatively recorded specific losses, re-colonisation by coral larvae on transects on the reef flat and on dead Acropora tables at the reef slope and regeneration of partly damaged large Porites and Diploastrea—colonies over the period from 1999 to 2004. The detrimental effects on the reef structure by bioerosion and hydrodynamics, as well as the overall status of the reef community were qualitatively assessed. Recruitment soon after the bleaching was more pronounced than in the following years, Pavona varians being a main constituent. The temporal re-colonisation pattern points at an emergency spawning of local Scleractinia just prior to the bleaching, whereas a sharp decrease of young settlers in 2001 and 2002 confirms a reduction of fertile colonies. The dominant species in the coral community shifted from acroporids and pocilloporids to agariciids. The skeletal deposition of recovering Diploastrea heliopora was equivalent to that before the bleaching, but much less than that of neighbouring Porites lobata colonies. The slow and scattered formation of new reef substance, which would structurally strengthen the reef, is however outweighed by the collapse of dead protruding and spacious colonies (e.g., Acropora tables). Six years after the bleaching, the formerly three-dimensional structure of the reef flat and upper reef slope presents as a levelled field of rubble, only partly consolidated by incrusting corals. Considering the recurrence of bleaching events (1987, 1998) and the results of the present study, one may assume a cascading deterioration of the status of the reef for the future.

Diagenetic effects on the distribution of uranium in live and Holocene corals from the Gulf of Aqaba

We investigated the effects of diagenetic alteration (dissolution, secondary aragonite precipitation and pore filling) on the distribution of U in live and Holocene coral skeletons. For this, we drilled into large Porites lutea coral-heads growing in the Nature Reserve Reef (NRR), northern Gulf of Aqaba, a site close to the Marine Biology Laboratory, Elat, Israel, and sampled the core material and porewater from the drill-hole. In addition, we sampled Holocene corals and beachrock aragonite cements from a pit opened in a reef buried under the laboratory grounds. We measured the concentration and isotopic composition of U in the coral skeletal aragonite, aragonite cements, coral porewater and open NRR and Gulf of Aqaba waters. Uranium concentration in secondary aragonite filling the skeletal pores is significantly higher than in primary biogenic aragonite (17.3 ± 0.6 compared to 11.9 ± 0.3 nmol · g −1, respectively). This concentration difference reflects the closed system incorporation of uranyl tri-carbonate into biogenic aragonite with a U/Ca bulk distribution coefficient (K D) of unity, versus the open system incorporation into secondary aragonite with K D of 2.4. The implication of this result is that continuous precipitation of secondary aragonite over ∼1000 yr of reef submergence would reduce the coral porosity by 5% and can produce an apparent lowering of the calculated U/Ca - SST by ∼1°C and apparent age rejuvenation effect of 7%, with no measurable effect on the calculated initial U isotopic composition. All modern and some Holocene corals (with and without aragonite cement) from Elat yielded uniform δ 234U = 144 ± 5, similar to the Gulf of Aqaba and modern ocean values. Elevated δ 234U values of ∼180 were measured only in mid-Holocene corals (∼5000 yr) from the buried reef. The values can reflect the interaction of the coral skeleton with 234U-enriched ground-seawater that washes the adjacent granitic basement rocks. We conclude that pore filling by secondary aragonite during reef submergence can produce small but measurable effects on the U/Ca thermometry and the U-Th ages. This emphasizes the critical importance of using pristine corals where the original mineralogy and porosity are preserved in paleooceanographic tracing and dating.

Storm cycles in the last millennium recorded in Yongshu Reef, southern South China Sea

Large storm-relocated Porites coral blocks are widespread on the reef flats of Nansha area, southern South China Sea. Detailed investigations of coral reef ecology, geomorphology and sedimentation on Yongshu Reef indicate that such storm-relocated blocks originated from large Porites lutea corals growing on the spurs within the reef-front living coral zone. Because the coral reef has experienced sustained subsidence and reef development during the Holocene, dead corals were continuously covered by newly growing coral colonies. For this reason, the coral blocks must have been relocated by storms from the living sites and therefore the ages of these storm-relocated corals should approximate the times when the storms occurred. Rapid emplacement of these blocks is also evidenced by the lack of coral overgrowth, encrustation or subtidal alteration. U-series dating of the storm-relocated blocks as well as of in situ reef flat corals suggests that, during the last 1000 years, at least six strong storms occurred in 1064±30, 1210±5–1201±4, 1336±9, 1443±9, 1685±8–1680±6, 1872±15 AD, respectively, with an average 160-year cycle (110–240 years). The last storm, which occurred in 1872±15 AD, also led to mortality of the reef flat corals dated at ∼130 years ago. Thus, the storm had significant impacts on coral reef ecology and morphology.

Coral mortality and recovery in response to increasing temperature in the southern Arabian Gulf

Analysis of a new data set for sea surface temperature shows that the seawater temperature rise in 1998 in the southern Arabian Gulf was the largest since 1870; it also showed that there is an underlying rising trend of sea surface temperature across the whole of the measured area spanning 5 degrees longitude and 3 degrees latitude. Over the last 50 years, sea surface temperature rise was about 0.2°C per decade, though this has accelerated to about 0.45°C per decade in the last 20 years. The pattern in temperature values in each 1 degree ‘cell’ are used to show that, at any one time, temperature rises towards the east of this area, which supports the concept of a general anti-clockwise circulation in the southeastern half of the Arabian Gulf. Spatial and temporal patterns in sea surface temperature rises also show that the mean temperature field at any location is similar to that which existed in the one degree cell immediately to its east approximately 30 years earlier.Coral cover in shallow water (<3 m depth) was <1 percent immediately following the 1998 event, due to near total mortality of vast areas of shallow Acropora (stagshorn) corals. Deeper than 3 m there was increasing survival of the large Porites (boulder) corals, survival increasing with depth. Coral survival improved towards the east, though overall, less than twenty coral species were found altogether. Juvenile recruitment also increased towards the east, and at any one site, recruitment increased with increasing depth. The identity of new juveniles was mainly faviid corals, not the Acropora or Porites which dominated these reefs before the 1998 event. This suggests that there may be a phase shift to a different stable state in terms of coral communities. We also speculate that, because the Arabian Gulf contains an extreme warm-tolerant subset of the Indian Ocean coral fauna, the changes seen in the Gulf may reflect future patterns across a greater part of the Indian Ocean.

Several centuries of variation in skeletal extension, density and calcification in massive Porites colonies from the Great Barrier Reef: A proxy for seawater temperature and a background of variability against which to identify unnatural change

Annual variations in skeletal density were measured by gamma densitometry in 35 cores removed from large Porites colonies growing at sites throughout the Great Barrier Reef (GBR). Density variations along each core provided data for average annual density and annual extension. These were used to estimate average annual calcification. Records ranged from 49 to 507 years in length. The period common to all colonies was 1934–1982. Annual growth data were averaged over periods of at least 5 years to avoid problems associated with dating of records and with measurement techniques. This also made some allowance for distortion of environmental information during coral growth and for intrusion into the environmental signal of information associated with skeletal architecture. The period common to the 10 longest cores was 1746–1982. Instrumental records of sea surface temperature (SST) are available for the GBR back to 1906. Annual calcification, averaged across these 10 cores, was significantly related to SST. Thus, average annual calcification for these 10 colonies provides a proxy for SST variations on the GBR back to the 18th century. Interpretation of evidence of a recent decline in calcification of Porites of the GBR is tempered by (1) evidence of similar declines and recoveries over the past several centuries and (2) evidence that coral calcification on the GBR has been above the long-term average for most of this century and the recent decline may be a return to more normal conditions.