Outer Reef Flats Research Articles

The global biogeography of reef morphology

AbstractAimThe Caribbean and Indo‐Pacific are separate biogeographical realms with distinct biogeographical and evolutionary histories, a 10‐fold difference in biodiversity, and highly disparate sea‐level histories. Since reef morphology often reflects interactions between biological activity and biogeographical history, including sea levels, the widths of shallow coral reef habitats are likely to differ markedly between realms, with ramifications for numerous ecosystem functions. Our goal, therefore, was to assess the impact of global‐scale biogeographical and evolutionary histories on coral reef habitats. Specifically, are Indo‐Pacific reefs wider than their Caribbean counterparts?LocationGlobal.Time PeriodModern.Major Taxa StudiedCoral reefs.MethodsWe used the Allen Coral Atlas, a global reef mapping system (3 m pixel resolution), to examine 3765 transects, 3 km long and 1 km apart, on 60 reefs across the two realms, quantifying shallow reef habitat widths (Inner and Outer Reef Flat, and Reef Crest) using ArcGIS.ResultsShallow reef habitat widths were strikingly similar between the Caribbean and Indo‐Pacific. Estimated modal widths diverged by just 37 m; means by just 122 m. Although shallow reef zones appeared to be wider in the Indo‐Pacific, habitat widths on atolls were almost identical across realms (means varying by less than 8 m).Main ConclusionsOur remote sensing approach provides a global description of the biogeography of coral reefs as biogenic structures. Furthermore, we can assess the relative importance of realm‐wide differences in coral diversity and sea‐level history on reef growth. The striking similarity of reef widths across realms suggests that reef growth (net reef accretion) is largely independent of coral diversity, or sea‐level history, and that other factors may have played a major role in constraining shallow reef widths. These factors may include geomorphology (e.g. antecedent topography and historical accommodation space) and, once at sea level, self‐limiting local hydrodynamics.

Microplastics in the Coral Reef Systems from Xisha Islands of South China Sea.

The impacts of microplastics on coral reefs are gaining attention due to findings that microplastics affect coral health. This work investigated the distribution and characteristics of microplastics in the seawater, fish, and corals in 3 atolls from the Xisha Islands of South China Sea. In the seawater samples, microplastics were detected in the outer reef slopes, reef flats, and lagoons with abundances ranging from 0.2 to 11.2, 1.0 to 12.2, and 1.0 to 45.2 items L-1, respectively. Microplastic abundance was 0-12.0 items individual-1 (0-4.7 items g-1) in fish and 1.0-44.0 items individual-1(0.02-1.3 items g-1) in coral. The predominant shape and polymer of microplastics in seawater, fish, and coral were fibrous rayon and polyethylene terephthalate (PET). Microplastic sizes primarily ranged from 20-330 μm in both the seawater and fish, while there were relatively more 1-5 mm microplastics in the corals. The shape, size, color, and polymer type distribution patterns of microplastics in seawater more closely resembled those in fish gills than those in fish gastrointestinal tracts or coral samples. This study shows that microplastics are abundant in these coral reef systems and they are captured by fish or "trapped" by corals.

Recovery of coral cover in records spanning 44 yr for reefs in Kāne‘ohe Bay, Oa‘hu, Hawai‘i

Published and unpublished long-term studies are assembled to examine trends in coral cover and the dependence of change in coral cover on the initial coral cover in Kāne‘ohe Bay over the last 44 yr. Each study showed there had been periods of increase in coral cover in the bay and showed that the rate of change in cover has been inversely dependent on the initial cover at a site. When coral cover is high on upper reef slopes, the fragile structure of reefs in this sheltered bay often collapses, resulting in a decrease in coral cover. The rate of change in coral cover was also inversely dependent on cover in one of the two studies that included analysis of reef-flat corals; the cause of decrease in cover in this habitat is thought to be attributable to particularly low sea levels in Hawai‘i in the late 1990s and 2009–2010. The inverse relationship between initial coral cover and change in cover, and the intersections of the regression lines of these variables with the x-axis at intermediate values of coral cover, is indicative of resilience in this ecosystem over the last 44 yr. In the 1970s, the invasive macroscopic green alga Dictyosphaeria cavernosa covered a high percentage of coral habitat and commonly displaced corals from the reef slope and outer reef flats; the change was cited as an example of a phase shift on a reef. This alga has virtually disappeared from the bay, thus increasing the space available to corals; its disappearance is coincident with the increase in coral cover. Other species of macroalgae, including alien species, have not replaced D. cavernosa as major space competitors. The increase in coral cover and virtual disappearance of D. cavernosa constitute an example of a phase-shift reversal.

Formation and sedimentary evolution characteristics of Yongshu Atoll in the South China Sea Islands

An on-site comparison is made on the reef body shape and landform of the Yongshu Atoll in the South China Sea. Accordingly, the reef top of the Yongshu Atoll in the transverse direction is divided into six facies zones. In the longitudinal direction, the developmental model of the Yongshu Atoll has a stepwise, tower-like structure. The results of engineering geological drilling show that since the Miocene the Yongshu Reef development has passed through 9 sedimentary cycles which merge into 6 large evolutionary stages (separated by 5 hiatuses of the reef body). This implies that the whole development and evolution process has undergone 6 periods of great sea level rises and 5 periods of sea level drops. The resedimentation of the reef body after several large hiatuses changed from lagoon facies to inner and outer reef-flat facies. These cyclical characteristics reflect the development process of the coral reef when the sea level was gradually rising in the interglacial stage. During different sedimentary evolution stages, the coral reef growth, sediment accumulation and topographic relief were all different, so that significant differences between facies zones emerged.

Fifty years of changes in reef flat habitats of the Grand Récif of Toliara (SW Madagascar) and the impact of gleaning

The Grand Recif of Toliara (GRT) in Madagascar is a large (33 km2) barrier reef system of the SW Indian Ocean that had been well investigated in the 1960s and early 1970s. A massive degradation of the reef has been reported since at least the early 1980s, just a few years after research activities ceased in the area. Examination of historical aerial photographs and modern high-resolution remote sensing images confirms a continuous loss of coral habitat on GRT outer reef flats between 1962 and 2011, with an average loss of 65 % and a range of 37–79 % loss during this 50-year period. The usual suspects of coral community declines (cyclones, bleaching and sedimentation) may have contributed to the demise of the GRT. However, an independent study (Salimo 1997) suggests that the chronic pressure of fisherman gleaning on reef flats with destructive tools is the main driver of the observed changes. Salimo’s reported level of frequentation (6.8 fishermen per day and per km−2) and rates of destruction per fisherman (7.7 m2 of coral habitat h−1) yield a cumulated overall loss in agreement with the image-based rates of habitat loss. The GRT is unlikely to recover because this chronic stress is unlikely to decrease in the near future. Indeed, the GRT daily provides subsistence fishery resources for local Vezo people and to agriculturalist or pastoralist ethnic groups who have turned to exploiting coastal resources due to increasing aridity and dwindling agricultural and livestock production.

Increased sea level promotes coral cover on shallow reef flats in the Andaman Sea, eastern Indian Ocean

Sea level in the Indian Ocean is subject to considerable temporal and spatial variabilities. During the period 1960–2009 at Phuket, Thailand, in the NE Indian Ocean, mean sea level increased by 2.7 mm y−1. Regular monitoring of coral cover on fringing reef flats at Phuket since 1979 revealed a sensitive response of this habitat to both transient sea-level depressions and sea-level elevation. Since 1987 when more frequent sampling began, coral cover was positively correlated with the mean sea level experienced over the preceding months. Changing mean sea level explained a high proportion of the observed variation in cover, with overall increasing sea levels and a lack of negative sea-level anomalies promoting cover on the outer reef flats. Concomitantly, there have been no changes in reef community structure or any apparent shifts in zonation patterns across the reef. While future benefits of continued increases in mean sea level on reef flats in the region will be constrained by the frequency and intensity of sea-level depressions associated with the Indian Ocean Dipole, and bleaching events, the overall picture for these shallow reefs is a positive one as they respond to increasing sea level and show rapid recovery from environmental disturbances.

High-precision U-series dating of very young cyclone-transported coral reef blocks from Heron and Wistari reefs, southern Great Barrier Reef, Australia

Numerous high-energy-transported coral reef blocks occur on the outer reef flats of Heron and Wistari reefs at the southern end of the Great Barrier Reef. In a pilot study, five samples from Heron and one from Wistari were collected for high-precision U-series dating by thermal ionization mass spectrometry. Five of the six dated corals yield U-series dates that suggest these corals were killed and transported during the last century. Without precise knowledge of cyclone impacts in the area, it is not possible to unequivocally link these reef blocks to specific cyclones. Nevertheless, this pilot study demonstrates that cyclone-transported coral reef blocks over the past 300 years can be precisely dated to an age uncertainty of ±1–4 years. This opens up the opportunity for high-age-resolution reconstruction of cyclone history and high-precision assessment of the trend and frequency of cyclone occurrences over the past millennium, as well as for a better understanding of anthropogenic impact on cyclone activity.

Serial patterns of biodiversity change in corals across shallow reef flats in Ko Phuket, Thailand, due to the effects of local (sedimentation) and regional (climatic) perturbations

A 17-year monitoring programme of reef flats at Ko Phuket, Thailand afforded an opportunity to evaluate both univariate and multivariate measures of environmental stress in an assessment of change on coral reef ecosystems. The sites at Ko Phuket suffered the effects of dredging in 1986–1987 and then anomalously low sea levels in 1997–1998 as a result of climate-related events in the Indian Ocean. Univariate measures of species diversity and taxonomic distinctness (Δ*) increased across the reef flat, reflecting the greater effects of physical stresses on the inner reef flats (compared with outer reef flats) at all sites, with more congeneric species present on the inner reef and more confamilial species on the outer reef. Multivariate measures showed a clear breakdown in seriation patterns at all sites during the dredging in 1987 and in 1998, as a result of earlier negative sea-level anomalies. Recovery from environmental disturbances was obvious within 12 months in each case. The domination of the reefs by massive coral species, which are physiologically adapted to intertidal living and which display partial rather than total colony mortality, may be a factor contributing to the apparent resilience of the reef flats together with continued recruitment and survival of juvenile corals during adverse environmental conditions. Elevated sea temperatures and extensive bleaching of corals in 1991, 1995, and 1998 had no effect upon coral community measures, with many corals recovering their zooxanthellae numbers within 3–5 months of the bleaching events.

Effects of herbivory, nutrient levels, and introduced algae on the distribution and abundance of the invasive macroalga Dictyosphaeria cavernosa in Kaneohe Bay, Hawaii

Since the 1960s, and possibly earlier, the macroalga Dictyosphaeria cavernosa has overgrown and displaced corals on reef slopes and outer reef flats in Kaneohe Bay, Oahu. This shift in reef community composition is generally attributed to nutrient enrichment resulting from sewage discharge. Following the diversion of most of the sewage effluent in 1977–1978, it was expected that D. cavernosa growth would become nutrient-limited and its abundance would consequently decline, but the alga remains abundant in much of the bay. One explanation for its persistence is that nutrients are once again high enough to support the alga's growth. An alternative explanation is that there has been a reduction in grazing intensity in the bay. In this study we resurveyed the distribution and abundance of D. cavernosa at 120 reef slope sites originally surveyed in 1969. We conducted additional surveys to estimate the biomass of herbivores and the areal coverage of D. cavernosa and other macroalgae on reef slopes and flats. Field experiments were used to determine spatial and temporal patterns of grazing intensity on and growth rates of D. cavernosa and the introduced macroalga Acanthophora spicifera. Laboratory experiments were used to examine preferences among herbivores for some of the most abundant macroalgae on Kaneohe Bay reefs. Twenty years after sewage diversion, D. cavernosa cover on reef slopes has decreased substantially in southern Kaneohe Bay, the site of most of the historical sewage discharge. D. cavernosa cover has changed less in other regions, remaining high in the central bay and low in the north bay. D. cavernosa thalli protected by grazer exclusion cages sustained positive growth rates on reef slopes and flats throughout the bay. Reduced nutrient concentrations may have caused a reduction in D. cavernosa growth rates, and a consequent reduction in D. cavernosa abundance in the south bay shortly after sewage diversion. Measurements of grazing intensity and surveys of herbivorous fish abundance suggest that the continued abundance of D. cavernosa is the result of a reduction in grazing intensity. Reduced grazing intensity on D. cavernosa may in turn be the result of a historical reduction in herbivore biomass or the establishment of several introduced macroalgae on reef flats. The introduced species are preferred by herbivorous fishes over D. cavernosa, as indicated by preference tests. The hypothesis that reduced grazing pressure on D. cavernosa is related to the establishment of introduced species is supported by the observation that D. cavernosa cover is highest on reef slopes where the cover of preferred introduced macroalgae on the adjacent outer reef flat is also high. Conversely, D. cavernosa cover is low or zero on reef slopes where the cover of introduced macroalgae on the adjacent reef flat is low or zero

Dynamics of fish communities on Reunion fringing reefs, Indian Ocean. I. Patterns of spatial distribution

This work describes the distribution of fish communitites, including species richness, abundance of individuals and trophic structures on the four main fringing reefs of Reunion Island. Patterns are quantified on several spatial scales: an intra-reef scale and an interreefs scale which includes within-reefs, between-reefs, and between-reefs and zones scales. The total ichthyofauna (217 species) is structured into three main communities — back-reef, inner reef flat and outer reef flat communities — which are influenced by the wave exposure and topographic relief. These communities are organized in a spatial continuum rather than into three totally separate communities. Herbivorous (mainly acanthurids) and omnivorous fishes (mainly pomacentrids) are the most numerous on the inner and outer reef flats. Diurnal carnivorous (mainly mullids) and herbivorous fishes (mainly schooling juveniles) are the dominant fishes in the back-reef zones. The low abundance of carnivorous fishes on the reef flat, irrespective of diet, is most likely due to alterations of benthic communities (e.g. coral damage, increase in algae populations) as a result of anthropogenic activities. The ichthyofauna of each of the four reefs is organized into the same three fish communities. There is a north-south gradient of decreasing distribution of species and abundance of individuals in these communities. However, fish communities are more clearly separated on the northern reefs compared with those of the southern reefs. This pattern is probably due to differences in coral vitality, habitat complexity and width of the reefs. Hydrodynamic conditions, more pronounced in the south of the island, might also lead to a higher flux of recruitment of fish larvae on the northern reefs.

Reproductive biology of the holothurians from the major communities of the New Caledonian Lagoon

The reproductive biology of nine species of aspidochirote holothurians from the major communities of the New Caledonian lagoon has been analysed. The data were obtained from monthly sampling from 1978 to 1982 of their populations on reef slopes and passes for Holothuria nobilis, H. fuscogilva, H. fuscopunctata and Thelenota ananas, in the inner lagoon for Stichopus variegatus, H. scabra and H. scabra versicolor and on reef flats for Actinopyga mauritiana, A. echinites and H. atra. The morphology and anatomy of the gonads and their annual cycle are described. The reproductive cycles, composed of the five phases of gonad growth, maturing, spawning, post-spawning and resting, were determined from the changes in the sexual stages, the gonad indices and the percentage of indeterminate sex. Three main reproductive patterns are recognized, annual cycles with spawning occurring during the warm season or during the cool season and a semiannual cycle. Other population parameters, such as the size of the mature oocytes, the mean size of the individuals at first sexual maturity, and absolute and relative fecundities were calculated and related to the mean size of the species, the taxonomy and the ecology. The reproductive strategies of the species show a gradient that is probably related to the stability of the biotopes, with smaller species living in the more intrable outer reef flats and larger species in deeper lagoon floors.

Impact of hurricane Firinga on fish community structure on fringing reefs of Reunion Island, S.W. Indian Ocean

The reef flats of Saint-Pierre and Saint-Leu (Reunion Island, Indian Ocean) suffered badly from hurricane Firinga on 29 January 1989. The high degree of silting due to increased run-off killed the coral colonies. Fish communities were surveyed at four periods following the hurricane (March and September 1989, March and September 1990). An increase in both species richness (31 to 47 spp. per census) and abundance (169 to 265 individuals per census) of fishes was observed with time, along with changes in their trophic structure. This positive succession may be linked to the disappearance of the silt layer from reef flats since September 1989. Nevertheless, there were differences in fluctuations and trophic structure of the fish community between back-, inner- and outer-reef flats. Finally, differences in recovery between the two reefs are related to the overall environmental degradation of the Island, chiefly by human perturbation, prior to the hurricane.

Experimental analysis of burrow defense in Echinometra mathaei (De Blainville) on Indo-West Pacific reef flat

Burrow defense of the rock-boring echinoid Echinometra mathaei (De Blainville) was investigated in reef-flat habitats at Guam (Mariana Islands). Outer reef-flat inhabitants more commonly defend resident burrows than do inner reef-flat inhabitants. Urchins transplanted from the outer reef flat to the inner reef flat do not alter defensive tendencies, and individuals transplanted from inner reef-flat regions to the outer reef flat do not defend burrows. Recolonization rates of empty burrows in inner reef-flat regions are five times higher than in outer reef-flat regions. Residents of the outer reef flat are unable to successfully defend burrows against larger intruders. These behavioral differences support suggestions that Indo-Pacific Echinometra are represented by at least two species.

Scleractinian Coral Communities of Two Inshore High Island Fringing Reefs at Magnetic Island, North Queensland

The characteristics of the scleractinian coral communities on 2 fringing reefs of an inshore high island were compared, using data from surveys by the line transect method. The communities are divided into zones on the basis of numbers of species per transect, species diversity indices, percentage coral cover and numerical classification of transects. The Geoffrey Bay community is divided into a Reef Flat Zone dominated by Goniastrea aspera, Platygyra sinensis and Montipora ramosa and a Reef Slope Zone dominated by arborescent and tabulate Acropora species. The Cockle Bay community is divided into an Inner Reef Flat Zone dominated by Montipora rarnosa, an Outer Reef Flat Zone dominated by Platygyra sinensisand Symphyllia recta, and a Reef Slope Zone dominated by Goniopora tenuidens. Differences in community structure, species composition and distributions of abundant species of the zones of the 2 reefs, can be attributed to prevailing environmental conditions. In particular, differences in exposure to wave action and sedimentation between the reefs are reflected in smaller numbers of species, lower coral cover, lower values of the Shannon Diversity Index and selection of silt resistant species on the less exposed reef in Cockle Bay.

On the spatial distribution, feeding and reproduction of the vermetid gastropod Dendropoma maximum

D. maximum is a dominant species of outer reef flats in the Red Sea, reaching densities of about 22/m2 and biomass of 15.8 g dry tissue/m2. A few individuals attached to loose rocks are found inside the breaker zone but they may have been dislodged by heavy seas from the outer reef flat. D. maximum feeds from a mucus net which is spread by wave action over the substratum. Hauling the net occurs at approximately 13 minute intervals throughout the 24 hours and lasts about two minutes. Neighbours with overlapping nets stimulate each other to haul and reduce feeding efficiency. The net is grasped by a pair of lateral jaws, tugged free of the substratum by rotation of the body and ingested by a zipper‐like action of the lateral and marginal radula teeth. The robust, central and lateral teeth become worn, possibly while channelling out the substratum to accommodate new shell. Defaecation occurs about 2.4 times an hour, amounting to 10450 kcal/m2/y. Females may brood simultaneously at least 11 egg capsules at various stages of development, which are suspended by stalks from the roof of the shell and pass through a dorsal slit in the mantle. Each capsule contains–500 embryos which develop into larvae with simple, coiled shells 0.33 mm in diameter. There is no planktonic phase. Adult shells amount to 2.5 kg/m2 on the outer reef flat, while dead shells are often occupied by blennies. Although D. maximum is not a specialized filter feeder, the highly developed ciliary mechanisms suggest that filtering may be an auxiliary feeding method.