Reef Crest Research Articles

Geological faulting a possible trigger for brine-induced reef mortality at East Flower Garden Bank, NW Gulf of Mexico

Localized mortality within reef crest depressions in Flower Garden Banks National Marine Sanctuary in 2016 may have resulted from inundation by brine that erupted when reef rock collapsed into a solution cavern in the salt dome below the bank. This mechanism offers an alternative explanation of mortality that has previously been attributed to hypoxia caused by either offshore movement of brackish, organically rich flood waters, upwelling, or both. Faults at the mortality site, discovered previously, and field observations of water conditions and mortality patterns during the event suggest that salt dissolution, undermining, and collapse may have driven a brine eruption. Elsewhere on the bank, anoxic, sulfurous brine flows have been found adjacent to areas of seafloor collapse, producing localized, brine-induced stress and mortality, but evidence of acute, lethal brine eruptions caused by active faulting would be novel for this system.

Field measurements show rough fore reefs with spurs and grooves can dissipate more wave energy than the reef crest

Coral reefs are widely recognized as effective dissipaters of wave energy. Spurs and grooves (SAG) are common features of fore reefs worldwide and are thought to be particularly efficient at dissipating wave energy. However, very few studies have collected in-situ hydrodynamic data to verify this and understand SAG interactions with hydrodynamic forces. We present in-situ wave data from contrasting SAG sites at Moorea, French Polynesia, and One Tree Reef in the southern Great Barrier Reef, Australia. We measured high rates of wave energy dissipation (up to 0.1 kW/m2) across the SAG zone. Interestingly, under the modal conditions measured, the SAG zones often dissipated wave energy at higher rates than the adjacent reef crest/flat zone. Rates of dissipation were the greatest at sites with high live coral cover in mesotidal environments (i.e., One Tree Reef sites), suggesting the structural complexity of live corals may increase bed friction and that tidal currents may also contribute to dissipation. Correlations between measured dissipation, wave height and depth allowed us to suggest that the SAG zone dissipates more energy under high wave conditions at low tides, while the reef crest/flat dissipates more energy at high tides under small wave conditions. Further study is required to better understand and model the hydrodynamics of SAG zones and the important role they play in reef dynamics and coastal protection.

DNA-assisted inventory and community structure of benthic marine algae in Taiping Island (or Itu Aba Island), South China Sea

Taiping Island (also known as Itu Aba Island) is located in the middle of one of the most politically controversial land strips in the South China Sea, thereby imposing a great obstacle for the exploration of marine biodiversity in this area. The goals of this study were to improve our knowledge gap of biodiversity of benthic marine algae (including both seaweeds and cyanobacteria) and to provide basic herbarium and molecular references on their communities. With 199 molecular sequences assisted for species identification, we found 19 orders, 40 families, 68 genera and 121 species in Taiping Island, including those belonging to 34 Chlorophyta species, 9 Ochrophyta species, 62 Rhodophyta species, and 16 cyanobacteria species. Among them, six genera, 14 species were new records for the South China Sea and more than 70% species were considered as new records for Taiping Island. There were also many taxa which may be considered as undescribed species new to science. In this fringing reef island, the species compositions were significantly different between two types of habitats separated by a reef crest. In the Reef Flat Zone, the most common species encountered included members of Caulerpa, Dictyota, Galaxaura and Halimeda. In the Reef Slope Zone, the most common species included members of Corallinales and Peyssonneliales. The nuisance coral-competing seaweeds (Galaxaura divaricata and Ramicrusta texitilis) and the cyanobacterium (Moorea bouillonii) were reported for the first time in Taiping Island. Our expanded DNA barcoding data in Taiping Island provides a greater understanding of the benthic marine algal biodiversity in the South China Sea.

Communities of corals and crustose coralline algae of the Jardines de la Reina National Park, Cuba: global stressors and resilience (2001-2017)

This study was conducted in the Jardines de la Reina National Park, Cuba. The health of the communities of corals and crustose coralline algae was studied in the years 2001, 2012 and 2017. The probable effect of hurricanes and sea surface temperature on these communities was also assessed. The area was only affected by three hurricanes and a tropical storm from 2000 to 2017. Sea surface temperature showed an increasing trend (by 0.03°C). The highest percentage of old mortality was recorded in 2001 (74% on the fore reef and 53% on reef crests) and the lowest of recent mortality in 2012 (0.03% on the fore reef and 0.17% on reef crests). Coral cover increased on the fore reef by between 3% and 2% in 2017 in comparison with 2001 and 2012. On the reef crests, the highest cover percentage was in 2001 (14.8%). Unlike local stressors, it was determined that hurricanes and sea surface temperature have likely negatively affected the coral reefs, particularly on reef crests. Both habitats have shown resistance and/or recovery capacity from the impacts suffered after 2001, which suggests some level of resilience.

Hydrodynamic and biogeochemical evolution of a restored intertidal oyster (Crassostrea virginica) reef

Oyster reef restoration is increasingly used as a tool for restoring lost ecosystem services in degraded aquatic systems, but questions remain about the efficacy of the practice and when/if restored reefs may behave similarly to intact natural reefs. In this case study, field observations highlighted short- (<1 month post-restoration) and longer-term (30 months; 3 recruitment cycles) transformations in canopy, hydrodynamic, and biogeochemical characteristics of a restored intertidal oyster reef relative to nearby intact and degraded reefs. Within 12 months of restoration, live oyster density (326 oysters/m2), mean shell length (47 mm), and mean canopy height (76 mm) did not differ significantly from those observed on a reference reef. Lowering of the reef crest during restoration reestablished over-reef flow and periodic tidal inundation, improving hydraulic connectivity between the channel and the reef surface. This immediately restored much of the reef's hydrodynamic function and eliminated the irregular flow patterns observed on the previously degraded reef. Results showed that mean flow (channel-to-reef flow attenuation: 98% / 62%; within/above canopy) and velocity normalized turbulence (w'2¯/U2: 10−1/10−2; ϵ/U3: 100/10−2 m−1) characteristics were similar across the restored and reference reefs within 1 year of restoration, with temporal changes in mixing within the canopy attributed to increases in live oyster density. Nutrient pools (mean total carbon, total nitrogen) on reference and restored reefs had similar magnitudes within 1 year (C: 39 & 33 g/kg, N: 1.5 & 1.8 g/kg), while increases in DOC and NH4+ were correlated with the presence of live oysters. Most changes that occurred on the restored reef were linked to oyster recruitment and canopy growth, which modulated hydrodynamics through direct flow interactions and controlled sediment nutrient and organic matter content through waste deposition and burial.

Destructive outbreaks of the corallivorous starfish Acanthaster cf. solarisspare coral assemblages in the shallowest reef flat areas in New Caledonia

Abstract Outbreaks of the corallivorous starfish Acanthaster cf. solaris (COTS) are not fully understood, especially in the shallowest areas behind the reef crests where limited COTS impacts are often reported. Here, we investigated the outbreak occurrence and compared post-outbreak habitat impacts in reef flats vs. upper fore reefs at 13 sites in New Caledonia. In this Pacific territory so far considered marginally affected by COTS, we documented wide occurrences of aggregations, combining massive population increases (maximum 605 COTS per swim, abundances &amp;gt;100 COTS recorded in 46% of the sites) with limited spatiotemporal boundaries. Abundance levels met the highest values in the literature, but were restricted to relatively small, isolated mid-shelf reefs. Typical residence time was short (&amp;lt; two years), with at least one site where COTS seemingly disappeared within seven months. The spread of COTS population and subsequent impacts appeared self-contained, and never affected the reef flats (0.5–1.5 m). The transition to severely affected fore reefs was abrupt, with live coral cover divided by nine and morphological diversity halved within a very few meters (3–5 m). We suggest that these differential impacts likely result from harsh environmental conditions making shallow, coral-rich areas less suitable for COTS, thereby pushing predation pressure further downwards.

Sustained coral reef growth in the critical wave dissipation zone of a Maldivian atoll

Sea-level rise is expected to outpace the capacity of coral reefs to grow and maintain their wave protection function, exacerbating coastal flooding and erosion of adjacent shorelines and threatening coastal communities. Here we present a new method that yields highly-resolved direct measurements of contemporary reef accretion on a Maldivian atoll reef rim, the critical zone that induces wave breaking. Results incorporate the suite of physical and ecological processes that contribute to reef accumulation and show growth rates vary from 6.6 ± 12.5 mm.y−1 on the reef crest, and up to 3.1 ± 10.2 mm.y−1, and −0.5 ± 1.8 mm.yr−1 on the outer and central reef flat respectively. If these short-term results are maintained over decades, the reef crest could keep pace with current sea-level rise. Findings highlight the need to resolve contemporary reef accretion at the critical wave dissipation zone to improve predictions of future reef growth, and re-evaluate exposure of adjacent shorelines to coastal hazards.

Decadal status of Acanthaster planci (Linnaeus, 1758) along the coral reef habitat of Andaman and Nicobar Islands

The coral reef habitats have been facing insightful threats from natural and anthropogenic sources across the world; while natural threats are creating major ecological pressure with the maximum percentage of devastation to the entire ecosystems. The Acanthaster planci (L) or Crown-of-Thorns Sea star (CoTS) is known as potential natural predator of corals and can disrupt the entire reef ecosystem with their outbreaks within the much-stipulated time.&nbsp; CoTS are reported from all the reef areas of Andaman and Nicobar Islands with a mean density of 0.71/ hectare within the range of 1.08±0.33 to 0.40±0.18 per hectare between North &amp; Middle Andaman and Nicobar, respectively. The distributional pattern of CoTS signifies that the reef crest represents maximum occurrence in comparison with reef slop and reef top; while an inversely proportional relationship can be seen between the occurrence of individuals and depth gradients. Maximum association of the CoTS is recorded with acroporidae corals while corallivorous impacts of CoTS are also recorded with poritidae corals which are not much reported across the world. No observation on the population outbreaks of CoTS was made during the decadal study period from Andaman and Nicobar Islands while the density is intrinsically noted within the limit of the natural population level of CoTS. The presence of natural predators like CoTS in Andaman and Nicobar Islands used to take a leading role in the balancing of reef ecology which denotes the healthy reef ecosystem of this archipelago.

Experiment and RANS modeling of solitary wave impact on a vertical wall mounted on a reef flat

A series of two-dimensional flume experiments were carried out to study turbulent bore impact on the vertical wall mounted on a reef flat. Turbulent bores were generated by solitary waves propagating on typical fringing reef profiles with and without reef crest. The idealized reef model has a 1:4 face slope and a long reef flat, with an impermeable vertical wall at the end. The solitary wave height varies from 14.1% to 30.0% of the water depth to cover a range of nonlinear wave conditions. Datasets of free surface elevation and flow velocity along fringing reef profiles and pressure on the vertical wall are acquired. Corresponding RANS (Reynolds-averaged Navier–Stokes)-type simulations are performed utilizing the open-source CFD (Computational Fluid Dynamics) OpenFOAM (Open Field Operation and Manipulation) package with the SST k−ω turbulence closure solver and validated using experimental data. The characteristics of the main hydrodynamic and impact processes are examined in detail. The predictive capability of OpenFOAM and three existing formulas for calculating the maximum lateral force are evaluated using measured data. A new predictive equation for peak pressure along the vertical wall is also proposed to achieve better agreement with the experimental data.

Hidden Giants: The Story of Bolbometopon muricatum at Ningaloo Reef

Bolbometopon muricatum (bumphead parrotfish, Valenciennes, 1839) is a conspicuous, iconic and ecologically important coral reef fish species. B. muricatum plays an important role in the bioerosion of the reef framework and as a result has been described as both an ecosystem engineer and keystone species. Despite the complete absence of B. muricatum from 32 years of scientific surveys across the Ningaloo Reef World Heritage Area, we recorded a total of 155 individuals of B. muricatum across 63.2 ha of reef crest surveys, equating to mean density of 2.38 ind/ha. Our observations represent the first record of this iconic species in scientific surveys at Ningaloo and in combination with qualitative observations of B. muricatum by expert witnesses, indicate B. muricatum is likely to have been present in ecologically relevant densities since 2006. The densities of B. muricatum observed at northern Ningaloo in 2021 suggest this species is removing an estimated 13.42 tonnes/ha or 1.34 kg/m2 of calcium carbonate per year, which is broadly comparable with estimates of total parrotfish bioerosion across many reefs in the central Indian and Pacific Oceans. Although not currently afforded elevated conservation status within management plans, B. muricatum possess many life-history characteristics that make them vulnerable to overfishing and may justify consideration for increased protection within the world heritage listed Ningaloo Reef Marine Park.

Utilising Natural Attributes of Tropical Islands for Beach Protection

This study reveals the coastal protection benefits of small artificial reefs on tropical islands. A monitored case study involving field and computer modelling investigations, as well as construction of a 95 m long reef and 12,000 m3 of local sand nourishment in a tropical lagoon on the north-east coast of Mauritius, is presented. Monitoring showed that a large salient widened the beach by 50 m in one year. The salient has continued to grow slowly and has remained stable for four years, including during a cyclone. Only a simple and inexpensive artificial reef was needed in the shallow lagoon to rebalance the shoreline wave conditions, because most wave energy was lost by breaking further offshore on the natural reef. With rising sea levels, inshore reefs with nourishment can overcome increases in wave height, wave set-up and wave run-up at the shoreline, which are jointly responsible for erosion and the flooding of homes by erosion and over-topping. To find suitable nourishment sources, regional computer modelling identified the following dominant circulation patterns: currents both coming into the lagoon over the reef crest (driven by breaking wave energy) and exiting via relict river channels or zones of lower waves. Sand for nourishment may be extracted from the exit locations with reduced environmental impact, because net currents are driving sand out of the lagoon system into deeper water. These relict sands have the same grain size as the natural beach and are readily accessible.

Wave Transformation on a Fringing Reef System With Spur and Groove Structures

AbstractSpurs and Grooves (SAG) are coral forereef structures with their main axis normal to the reef crest. The dimensions of SAG vary considerably on a global scale. Despite being common, there are relatively few studies that relate SAG morphology with wave transformation processes. Measurements of wave transformation are presented from Xahuayxol, Quintana Roo, Mexico, across a SAG zone with larger spurs than in previous studies. An array of pressure sensors and current meters sampling at high frequencies were deployed over 8 days. Our results show that SAG structures are more important for wave transformation than previously reported since a high rate of wave energy flux dissipation (up to 0.077 kW/m2) was found and the corresponding wave dissipation friction factor, fw (1.01), was four times higher as compared to other reefs. We also found that the wave dissipation rate over the spurs can be up to three times higher than the adjacent grooves (0.098 vs. 0.028 kW/m2). This study demonstrates that SAG plays a very important role in wave dissipation over coral reefs.

Wave Peel Tracking: A New Approach for Assessing Surf Amenity and Analysis of Breaking Waves

The creation and protection of surfing breaks along populated coastlines have become a consideration for many councils and governments as surfing breaks are a major driver of tourism. To assess the surf amenity of surfing breaks, a quantitative and objective assessment method is required. A new wave peel tracking (WPT) method has been developed using a shore-based camera to assess surf amenity by measuring and quantifying potential surfing ride rate, length, duration, speed and direction on a wave-by-wave basis. The wave peel (or “curl” below the wave peak) is the optimal surfing region on a wave, and each wave peel track represents a surfable ride. Wave peel regions are identified, classified and tracked using traditional and machine learning-based computer vision techniques. The methodology is validated by comparing the rectified wave peel tracks with GPS-measured tracks from surfers in the wave peel regions. The WPT methodology is evaluated with data from a reef and adjacent natural beach at the Gold Coast, Australia. The reef produced longer ride lengths than the nearshore region and showed a consistent breaking location along the reef crest. Spatial maps of the wave peel tracks show the influence of tides on the wave breaking patterns and intensity. The WPT algorithm provides a robust, automated method for quantifying surf amenity to provide baseline data for surf break conservation. The methodology has potential uses to verify numerical modelling of surf breaks and to assess the impact of coastal development on surf breaks.

Large-scale variation in wave attenuation of oyster reef living shorelines and the influence of inundation duration.

One of the paramount goals of oyster reef living shorelines is to achieve sustained and adaptive coastal protection, which requires meeting ecological (i.e., develop a self-sustaining oyster population) and engineering (i.e., provide coastal defense) targets. In a large-scale comparison along the Atlantic and Gulf coasts of the United States, the efficacy of various designs of oyster reef living shorelines at providing wave attenuation was evaluated accounting for the ecological limitations of oysters with regard to inundation duration. A critical threshold for intertidal oyster reef establishment is 50% inundation duration. Living shorelines that spent less than one-half of the time (<50%) inundated were not considered suitable habitat for oysters, however, were effective at wave attenuation (68% reduction in wave height). Reefs that experienced >50% inundation were considered suitable habitat for oysters, but wave attenuation was similar to controls (no reef; ~5% reduction in wave height). Many of the oyster reef living shoreline approaches therefore failed to optimize the ecological and engineering goals. In both inundation regimes, wave transmission decreased with an increasing freeboard (difference between reef crest elevation and water level), supporting its importance in the wave attenuation capacity of oyster reef living shorelines. However, given that the reef crest elevation (and thus freeboard) should be determined by the inundation duration requirements of oysters, research needs to be refocused on understanding the implications of other reef parameters (e.g., width) for optimizing wave attenuation. A broader understanding of the reef characteristics and seascape contexts that result in effective coastal defense by oyster reefs is needed to inform appropriate design and implementation of oyster-based living shorelines globally.

Microplastics as a sedimentary component in reef systems: A case study from the Java Sea

AbstractMicroplastic pollution has been reported from coral reef systems all over the tropics. Exposure to microplastics has several negative impacts on coral health, such as bleaching, tissue necrosis, or an impairment of the coral’s immune system. Despite this potential risk for reef systems, the controlling processes for microplastics dispersion and accumulation in reef sediments are still largely under‐studied. Presented here is a study of microplastics (125 µm to 5 mm) distribution in two tropic atoll reef platforms in Kepulauan Seribu, Indonesia. Sediment samples were collected in different facies zones within the reef platform. Microplastics were concentrated using density floatation and characterized by light and scanning electron microscopy. Some particles were identified as polypropylene using micro‐Fourier transform infrared spectroscopy. All recovered microplastics were classified as secondary microplastics, likely derived from marine and local sources, with fibres as the most abundant type. Microplastics are showing similar transport and accumulation behaviour as fine siliciclastic grains. The abundance of microplastic is controlled by the proximity to the source area of larger plastic debris and hydrodynamic processes. Microplastics are not only present in low energy environments but also high energy settings such as the reef crest. Processes that contribute to accumulation in reef sediments are biofouling, interlocking and the creation of compound grains. Microplastics are present in sediment close to the seafloor (0 to 3.5 cm) but also at depths between 3.5 cm and 7.0 cm. Microplastic particles from below 3.5 cm are unlikely to be remobilized under modal weather conditions in the studied equatorial reefs. Subtidal reef sediment therefore can be regarded as a permanent sink for microplastics. The study shows that microplastics in coral reef environments deserve careful consideration since microplastics pose an additional threat to corals and their ability as framework builders in reef systems.

Coral Reef Restorations Can Be Optimized to Reduce Coastal Flooding Hazards

Coral reefs are effective natural coastal flood barriers that protect adjacent communities. Coral degradation compromises the coastal protection value of reefs while also reducing their other ecosystem services, making them a target for restoration. Here we provide a physics-based evaluation of how coral restoration can reduce coastal flooding for various types of reefs. Wave-driven flooding reduction is greatest for broader, shallower restorations on the upper fore reef and between the middle of the reef flat and the shoreline than for deeper locations on the fore reef or at the reef crest. These results indicate that to increase the coastal hazard risk reduction potential of reef restoration, more physically robust species of coral need to be outplanted to shallower, more energetic locations than more fragile, faster-growing species primarily being grown in coral nurseries. The optimization and quantification of coral reef restoration efforts to reduce coastal flooding may open hazard risk reduction funding for conservation purposes.

From the Reef to the Ocean: Revealing the Acoustic Range of the Biophony of a Coral Reef (Moorea Island, French Polynesia)

The ability of different marine species to use acoustic cues to locate reefs is known, but the maximal propagation distance of coral reef sounds is still unknown. Using drifting antennas (made of a floater and an autonomous recorder connected to a hydrophone), six transects were realized from the reef crest up to 10 km in the open ocean on Moorea island (French Polynesia). Benthic invertebrates were the major contributors to the ambient noise, producing acoustic mass phenomena (3.5–5.5 kHz) that could propagate at more than 90 km under flat/calm sea conditions and more than 50 km with an average wind regime of 6 knots. However, fish choruses, with frequencies mainly between 200 and 500 Hz would not propagate at distances greater than 2 km. These distances decreased with increasing wind or ship traffic. Using audiograms of different taxa, we estimated that fish post-larvae and invertebrates likely hear the reef at distances up to 0.5 km and some cetaceans would be able to detect reefs up to more than 17 km. These results are an empirically based validation from an example reef and are essential to understanding the effect of soundscape degradation on different zoological groups.

A Numerical Study of Wave‐Driven Mean Flows and Setup Dynamics at a Coral Reef‐Lagoon System

AbstractTwo‐dimensional mean wave‐driven flow and setup dynamics were investigated at a reef‐lagoon system at Ningaloo Reef, Western Australia, using the numerical wave‐flow model, SWASH. Phase‐resolved numerical simulations of the wave and flow fields, validated with highly detailed field observations (including &gt;10 sensors through the energetic surf zone), were used to quantify the main mechanisms that govern the mean momentum balances and resulting mean current and setup patterns, with particular attention to the role of nonlinear wave shapes. Momentum balances from the phase‐resolved model indicated that onshore flows near the reef crest were primarily driven by the wave force (dominated by radiation stress gradients) due to intense breaking, whereas the flow over the reef flat and inside the lagoon and channels was primarily driven by a pressure gradient. Wave setup inside the lagoon was primarily controlled by the wave force and bottom stress. The bottom stress reduced the setup on the reef flat and inside the lagoon. Excluding the bottom stress contribution in the setup balance resulted in an over prediction of the wave‐setup inside the lagoon by up to 200–370%. The bottom stress was found to be caused by the combined presence of onshore directed wave‐driven currents and (nonlinear) waves. Exclusion of the bottom stress contribution from nonlinear wave shapes led to an over prediction of the setup inside the lagoon by approximately 20–40%. The inclusion of the nonlinear wave shape contribution to the bottom stress term was found to be particularly relevant in reef regions that experience a net onshore mass flux over the reef crest.

Natural Intertidal Oyster Reef Growth Across Two Landscape Settings and Tidal Ranges

Intertidal oyster reefs are typically restored to offset the loss of reef-associated ecosystem services (e.g., improved water quality, shoreline stabilization, and fish habitat), but the scale of enhanced services is predicated on the health and growth of the restored reef. Previous work on young (<15 years) restored reefs showed the highest growth rates along the sides of reefs where they are aerially exposed 20–40% of the time, but much is still unknown about how those positions in the tidal frame change with landscape setting, tidal range, and reef maturity. This study compared the area of maximum growth among 12 natural intertidal reefs in coastal North Carolina that range between 1395 and 62 years old. The reefs include fringing and patch landscape settings in each of two estuaries with tidal ranges of 0.94 m and 1.51 m. Peak growth rates were similar among landscape and tidal settings and were faster than the rate of sea-level rise (SLR) indicating intertidal reef resilience to accelerating SLR. Flow baffling associated with fringing reefs and higher summer air temperatures in the southern estuary likely contributed to a lower position of the optimal growth zone, where growth rates are highest, in the tidal frame. Intertidal reef growth manifests differently across the range of aerial exposures at varying stages of maturity. Once reefs reached ~50 years old, the elevation of the reef crests equilibrated to ~60–70% aerial exposure and peak growth rates stabilized between 2 and 4 cm year−1 at ~50% aerial exposure. These results are a useful guide for identifying areas and cultch configurations that optimize reef growth rates, enhancing the probability for self-sustaining restored reefs.

Variation in the Size–Structure of Dominant Branching Coral Taxa (Acroporidae: Acropora) and (Pocilloporidae: Pocillopora) in New Ireland Province, Papua New Guinea1

The size–structure of coral populations can serve as an indicator of demographic structure and response to disturbance. This study examines variation in the size–structure of Acropora and Pocillopora colonies, two regionally dominant coral genera with contrasting life-history traits. The abundance and colony sizes of these genera were studied across reef flats, crests, and slopes, within marine protected areas (MPAs) in New Ireland Province, Papua New Guinea (PNG), an understudied region of the Coral Triangle. While a preponderance of small sized colonies have been previously detected in relatively undisturbed reefs, this study instead found negatively skewed size–structures for Pocillopora and Acropora distributions, reflecting the general persistence of larger and likely older colonies. Pocillopora size-distributions were uniformly more negatively skewed and more peaked than Acropora, whereas, Acropora colonies were significantly larger across all reef positions and nearly all sites, potentially reflecting their high growth rates and ‘competitive’ life-history traits. The size–structure (skewness and kurtosis) of both Pocillopora and Acropora did not significantly vary across reef positions or study sites, whereas mean-colony size decreased significantly with deeper reef positions, suggesting that colonies of all size-classes respond in unison to micro-site reef heterogeneity at the regional scale. Together, this study provides valuable insight into the size–structure of dominant coral taxa with contrasting life-history traits, which will serve as baseline data for future surveys of coral community responses to climate change in PNG.