Reef Surface Research Articles

Comparative analysis of biofilm bacterial communities developed on different artificial reef materials.

Artificial reefs play a vital role in restoring and creating new habitats for marine species by providing suitable substrates, especially in areas where natural substrates have been degraded or lost due to declining water quality, destructive fishing practices, and coral diseases. Artificial reef restoration aimed at coral larval settlement is gaining prominence and initially depends on the development of biofilms on reef surfaces. In this study, we hypothesized that different artificial reef materials selectively influence the composition of biofilm bacterial communities, which in turn affected coral larval settlement and the overall success of coral rehabilitation efforts. To test this hypothesis, we evaluated the impact of six different reef made materials (porcelain, granite, coral-skeleton, calcium-carbonate, shell-cement, and cement) on the development of biofilm bacterial communities and their potential to support coral larval settlement. The biofilm bacterial communities were developed on different artificial reef materials and studied using 16S rRNA gene amplicon sequencing and analysis. The bacterial species richness and evenness were significantly (P<0.05) low in the seawater, while these values were high in the reef materials. At the phylum level, the biofilm bacterial composition of all materials and seawater was majorly composed of Pseudomonadota, Cyanobacteria, and Bacteroidetes, however, significantly (P<0.05) low Bacteroidetes were found in the seawater. At the genus level, Thalassomonas, Glaciecola, Halomicronema, Lewinella, Hyphomonas, Thalassospira, Polaribacter, and Tenacibaculum were significantly (P<0.05) low in the coral-skeleton and seawater, compared to the other reef materials. The genera Pseudoaltermonas and Thalassomonas (considered potential inducers of coral larval settlement) were highly abundant in the shell-cement biofilm, while low values were found in the biofilm of the other materials. The biofilm bacterial community composition can be selective for different substrate materials, such as shell-cement exhibited higher abundances of bacteria known to facilitate coral larval settlement, highlighting their potential in enhancing restoration outcomes.

Island change framework defines dominant modes of atoll island dynamics in response to environmental change

Climatic change threatens the persistence of atoll islands and the cultural and ecosystem services they support. However, adaptation and ecosystem management are constrained by lack of knowledge of island-specific transformations. We present an empirically-based island change framework that characterises the physical trajectory of islands, based on high-resolution shoreline analysis on 509 atoll islands in the central Pacific over the past half-century. Using changes in island size and position we identify seven distinct styles of island transformation in the Pacific, including contraction (21.4%), stability (46.1%) and expansion (32.4%), and show that 40% of islands are currently mobile on reef surfaces. Results challenge the framing of islands as erosional, which misrepresents island behaviour and constrains understanding of island futures. The island change framework highlights a broader set of island-specific management considerations, and opportunities, that scale with the style and rate of island change, and provides an empirical basis to inform management.

Adjusting Manning coefficients to simulate tsunami propagation over porous coral reef

This study investigates the effect of porous coral reef on the tsunami propagation in terms of experimental and numerical modelling. It aims at quantifying the influence of several input parameters on the wave attenuation and at adjusting Manning coefficients to reproduce experimental results. The density and the surface of individual reefs are fixed as well as the width and length of the coral barrier. Results show that the reef height is the most sensitive parameter. This latter affects the tsunami propagation with an attenuation of the first wave reaching 15 % compared to the case with a smooth reef. Wave breaking occurs on the reef flat for each test but, as expected, its location depends greatly on the reservoir depths difference. Numerical simulations show that the Manning coefficient must be adjusted both by considering the coral reef height and the spatial grid resolution. It varies from 0.01 (for lowest reef with highest grid resolution) to 0.058 (for higher reefs with coarsest grid resolution).

Effects of Environmental and Climatic Changes on Coral Reef Islands.

Coral reef islands are low-lying, wave-deposited sedimentary landforms. Using an eco-morphodynamic framework, this review examines the sensitivity of islands to climatic and environmental change. Reef island formation and morphological dynamics are directly controlled by nearshore wave processes and ecologically mediated sediment supply. The review highlights that reef islands are intrinsically dynamic landforms, able to adjust their morphology (size, shape, and location) on reef surfaces in response to changes in these processes. A suite of ecological and oceanographic processes also indirectly impact hydrodynamic and sediment processes and thereby regulate morphological change, though the temporal scales and magnitudes of impacts on islands vary, leading to divergent morphodynamic outcomes. Climatic change will modify the direct and indirect processes, causing complex positive and negative outcomes on islands. Understanding this complexity is critical to improve predictive capabilities for island physical change and resolve the timescales of change and lag times for impacts to be expressed in island systems.

2DH Numerical Study of Solitary Wave Processes around an Idealized Reef-Fringed Island

In order to better understand the role of coral reefs around an isolated island in mitigating tsunami hazards, this study performed a horizontally two-dimensional (2DH) numerical study of tsunami-like solitary wave propagation and run-up around an idealized reef-fringed island. The shock-capturing Boussinesq wave model, the FUNWAVE-TVD is used in the present study and well-validated with existing experimental data for its robustness in predicting 2DH solitary wave processes around an island. Based on the validated model, the typical solitary propagation process around the reef-fringed island and the effects of morphological and hydrodynamic parameters on the maximum run-up heights were systematically investigated. It is found that coral reefs can effectively reduce maximum run-up heights around an isolated island. The reef flat’s water depth, reef flat width, and reef surface roughness are the main factors affecting maximum run-up heights around an island, while the fore-reef slope has little impact. For the idealized reef-fringed island in this study, sea-level rise will cause coral reefs to lose their protective capability on the lee side, and the presence of coral reefs may even enhance tsunami hazards around an island when the reef flat width is very narrow or coral bleaching happens.

Impact of aragonite coral powder on hydration, strength, and rheology of cement paste

Understanding the intricate bonding mechanism between coral reef surfaces and C–S–H is crucial for advancing large-scale engineering applications. This study conducts a comprehensive analysis of the impact of coral powder (CP) on cement paste. Initially, zeta potential experiments were performed to probe ion adsorption from the cement pore solution at the CP interface. We propose a novel model to quantify the adsorption of Ca2+ by CP. Subsequently, the study delves into the morphological properties of C–S–H nucleation and growth at this interface and formulates an equation elucidating the CP and Ca2+ adsorption relationship. Then, the crack extension behavior of cementitious materials was scrutinized. Finally, an examination of CP effects on the rheological properties of fresh cement pastes is presented. The findings indicate that CP particles exhibit a notably weaker adsorption affinity with Ca2+ compared to their interaction with SO42−. Compared to limestone (LP), CP exhibited a 32.1 % reduction in Ca2+ adsorption in Ca(OH)2 solutions, 44.2 % reduction in Ca(OH)2 + 10 mmol/L K2SO4, and 40.6 % reduction in Ca(OH)2 + 50 mmol/L K2SO4 solutions. Higher zeta potential values correspond to stronger Ca2+ adsorption and broader surface coverage of Ca2+. The inferior surface characteristics of CP hinder C–S–H nucleation and growth, leading to a low CH orientation index in cement paste containing CP. These pastes exhibit cracks more frequently between CP and hydration products. An attractive force is observed between CP and cement particles, decreasing cement paste flowability.

Location and reef size drive oyster reef restoration success

Optimizing habitat restoration success requires understanding how restoration location and design enhance the persistence and function of a restored habitat. Particular attention to the configuration of structure and its interaction with landscape‐scale processes is critical for enhancing the habitat value of restored areas. We monitored six subtidal restored oyster reefs in Pamlico Sound, North Carolina, United States, to identify how oyster demographics responded to initial habitat characteristics (e.g. reef area, volume, vertical relief, and perimeter‐to‐area ratio) and how changes in habitat characteristics over time altered suitability for oysters. Changes in reef habitat were measured by repeated mapping using bathymetric and side‐scanning sonar systems. A 2‐year time series of oyster demographic data quantified oyster response to habitat changes over time. All reefs provided habitat for the settlement and growth of oysters. Within 2 years of restoration, relative differences in oyster recruitment and survival emerged and were related to variations in reef location and two‐dimensional habitat characteristics among reefs, namely reef area and perimeter‐to‐area ratio. Larger reefs that were less fragmented resisted burial from sedimentation and enhanced oyster densities and biomass relative to smaller, more fragmented reefs that became heavily sedimented and failed to support oyster recruitment and survival. Positive feedback mechanisms between habitat characteristics and oyster recruitment success were established within 1 year of restoration and were likely driven by landscape‐scale processes such as sediment dynamics and larval supply. To improve restoration success, we recommend creating larger reef surfaces with low perimeter‐to‐area ratios in areas that promote habitat persistence.

Spatiotemporal variability in the structure and diversity of understory faunal assemblages associated with the kelp Eisenia cokeri (Laminariales) in Peru

Kelp species function as foundation organisms in coastal marine ecosystems, where they alter environmental conditions and promote local biodiversity by providing complex biogenic habitat for an array of associated organisms. The structure and functioning of kelp forest ecosystems in some regions, such as along the Peruvian coastline, remain critically understudied. We quantified the structure and diversity of faunal assemblages within both holdfast and understory reef habitats within Eisenia cokeri forests. We sampled both habitat types within four subtidal kelp forests on multiple occasions between 2016 and 2020, and quantified fauna at a fine taxonomic level (mostly species). We recorded a total of ~ 55,000 individuals representing 183 taxa across the study, with holdfast assemblages typically exhibiting higher richness, abundance and biomass values compared with understory reef-associated assemblages. Holdfast assemblages were structurally and functionally dissimilar to those on reef surfaces and were less variable and consistent across sites and sampling events. Even so, assemblages associated with both habitat types varied significantly between sites and sampling events, with variation in upwelling strength, ocean currents, and grazing pressure among potential drivers of this ecological variability. Overall, E. cokeri supports diverse and abundant holdfast assemblages and functions as a foundation organism in Peru. Given that no other habitat-forming kelp species persist at the low latitudes of E. cokeri in mid-to-north Peru, the lack of functional redundancy suggests that effective management and conservation of this species is vital for wider ecosystem processes and biodiversity maintenance.

Control of Sediment Grainsize on Lead (Pb) Content in the Reef Sediment Systems: A Case Study of Panjang Island, Banten Bay, Indonesia

Sediments are known to accumulate pollutants from terrestrial and coastal waters, and can be used as an indicator to monitor metal pollution in the biosphere and the effects of anthropogenic events in the environment. Eight samples of reef surface sediments were collected from the north and east coasts of Panjang Island. Panjang Island, located to the north of Banten Bay, was chosen for this study because of its proximity to many industrial areas. Nevertheless, the island supports a natural ecosystem, including coral reefs, seagrass, and mangroves. The samples were analyzed for the grainsize, component analysis, and Pb concentrationusing AAS flame method. The result indicated Pb concentration in the grain-supported surface sediment in Panjang Island is varied. The detected Pb concentrations were 0-28.68 mg/kg in dry weight. The study indicated that the different concentration of Pb value is mainly controlled by the different of sediment grainsize. The highest Pb accumulation occurs in the areas with very fine sediment grainsize, which have better adsorption capabilities for heavy metals. This study also suggests that the medium sand fraction may accumulate different Pb values. The result indicated that, according to the classification of ANZECC/ARMCANZ, all sediment samples in the study area is in the low risk of toxicant level. Additionally, the SQG-Q (Sediment Quality Guidelines) quotient analysis indicates that Pb concentration in the study area posed a low-moderate impact to adverse biological effect.

Imprint of relative sea level histories on Last Interglacial coral preservation

SUMMARY Fossil corals are commonly used to reconstruct Last Interglacial (∼125 ka, LIG) sea level. Sea level reconstructions assume the water depth at which the coral lived, called the ‘relative water depth’. However, relative water depth varies in time and space due to coral reef growth in response to relative sea level (RSL) changes. RSL changes can also erode coral reefs, exposing older reef surfaces with different relative water depths. We use a simplified numerical model of coral evolution to investigate how sea level history systematically influences the preservation of corals in the Bahamas and western Australia, regions which house &amp;gt;100 LIG coral fossils. We construct global ice histories spanning the uncertainty of LIG global mean sea level (GMSL) and predict RSL with a glacial isostatic adjustment model. We then simulate coral evolution since 132 ka. We show that preserved elevations and relative water depths of modelled LIG corals are sensitive to the magnitude, timing and number of GMSL highstand(s). In our simulations, the influence of coral growth and erosion (i.e. the ‘growth effect’) can have an impact on RSL reconstructions that is comparable to glacial isostatic adjustment. Thus, without explicitly accounting for the growth effect, additional uncertainty is introduced into sea level reconstructions. Our results suggest the growth effect is most pronounced in western Australia due to Holocene erosion, but also plays a role in the Bahamas, where LIG RSL rose rapidly due to the collapsing peripheral bulge associated with Laurentide Ice Sheet retreat. Despite the coral model's simplicity, our study highlights the utility of process-based RSL reconstructions.

Reduced small-scale structural complexity on sponge-dominated areas of Indo-Pacific coral reefs

Corals provide a complex 3D framework that offers habitat to diverse coral reef fauna. However, future reefs are likely to experience reduced coral abundance. Sponges have been proposed as one potential winner on future coral reefs, but little is known of how they contribute to reef 3D structure. Given the ecological importance of structural complexity, it is critical to understand how changes in the abundance of structure-building organisms will affect the three-dimensional properties of coral reefs. To investigate the potentially important functional role of coral reef sponges as providers of structural complexity, we compared the structural complexity of coral- and sponge-dominated areas of an Indonesian coral reef, using 3D photogrammetry at a 4 m2 spatial scale. Structural complexity of 31 4 m2 quadrats was expressed as rugosity indicating reef contour complexity (R), vector dispersion indicating heterogeneity of angles between reef surfaces (1/k), and fractal dimension indicating geometrical complexity at five different spatial scales between 1 and 120 cm (D1–5). Quadrats were identified as high- or low-complexity using hierarchical clustering based on the complexity metrics. At high structural complexity, coral- and sponge-dominated quadrats were similar in terms of R and 1/k. However, smallest-scale refuge spaces (1–5 cm) were more abundant in coral-dominated quadrats, whereas larger scale refuge spaces (30–60 cm) were more abundant in sponge-dominated quadrats. Branching and massive corals contributed the most to structural complexity in coral-dominated quadrats, and barrel sponges in sponge-dominated quadrats. We show that smaller-scale refugia (1–5 cm) are reduced on sponge-dominated reefs at the spatial scale considered here (4 m2), with potential negative implications for smaller reef fauna.

Mapping reef terrain roughness and benthic habitat complexity in the Pamegaran Island, Kepulauan Seribu National Marine Park

The formation of coral cays in the Kepulauan Seribu Marine Parks, Jakarta Special Capital Region, is the result of patch reef evolution which linked to the geology of sediment accretion, including the ecology of coral growth and competition of benthic community. This study aims to map the level of reef complexity in Pamegaran Island using Benthic Terrain Modeler. Field observation to monitor benthic communities in four stations at 3- and 5- meter depth was combined with SPOT-7 image processing to evaluate in situ rugosity, 3-dimensional bathymetry and benthic habitat classification. The results of this study revealed low degree of terrain complexity as results of habitat degradation and dominant benthic class of sand. Classification of reef benthic habitat resulted in five classes, namely sand (Sd= 126.65 ha), seagrass (Sg= 35.29 ha), bare rubble (Rb= 32.68 ha), reefs dominated by dead corals (DC=3.16 ha) and dead coral with algae (DA= 20.50 ha); with 67.21% score of overall accuracy (OA). In situ rugosity at 5 m was higher than 3 m, as result of higher coverage of live corals particularly in the east site. Similar profile was shown for rugosity index, which reflects ratio between surface to planar area and seafloor gradient along the reef slope. Finally, positive correlation between reef surface and in situ rugosity implies the importance of live coral communities in supporting complex terrain and benthic habitat in the patch reef environment.

Defining multi-scale surface roughness of a coral reef using a high-resolution LiDAR digital elevation model

A decline in coral reef surface roughness may indicate that a coral reef ecosystem is approaching functional collapse. This is because surface roughness underpins many critical ecosystem metrics, such as, live coral cover and high fish biomass. Yet, we understand very little about how surface roughness changes at ecosystem scales due to the limited spatial extents of coral reef ecological surveys. Here, we investigate the surface roughness of a coral reef across multiple spatial scales on a high-resolution Light Detection and Ranging (LiDAR) derived digital elevation model (DEM, 0.25 m cells). We found that complex features dominated regions with high roughness (e.g., spur and groove systems on the reef slope) and increased the average roughness of geomorphic zones that are otherwise flat and featureless (e.g., lagoonal patch reefs in the sandy deep lagoon). Each geomorphic zone had a unique signature with high values of roughness usually expressed over finer spatial scales and, conversely, lower values of roughness observed over broader spatial scales. We allocated each geomorphic zone into one of four roughness equivalent habitats (REHs 1–4) that share similar surface roughness properties but are not necessarily geographically contiguous regions. These results identify fine scale features and geomorphic zones that are important for providing roughness to coral reef systems, such as spur and groove systems and the fore-reef slope respectively. They also suggest that morphodynamic processes operating at broader spatial scales influence the physical structure of coral reef ecosystems. The quantification of coral reef surface roughness is becoming increasingly important due to the wider availability of high (<1 m) and hyper (<0.1 m) resolution DEMs. Our approach and findings here can be used to provide greater surface textural information for common coral reef geomorphic zones and aid future management and research efforts, such as, monitoring of coral reef ecosystem response to environmental change and reef restoration and adaptation programs.

Characterizing canopy complexity of natural and restored intertidal oyster reefs (Crassostrea virginica) with a novel laser‐scanning method

The structural complexity of oyster reef canopy plays a major role in promoting biodiversity, balancing the sediment budget, and modulating hydrodynamics in estuarine systems. Although oyster canopy structure is both spatially and temporally heterogeneous, oyster canopies are generally characterized using simple first‐order quantities, like oyster density, which may lack the ability to sufficiently parameterize reef roughness. In this study, a novel laser‐scan approach was used to map the surface of intact reference and restored reefs (restoration age: 1–4 years) during low tide, when the oyster canopy was fully exposed. Measurements were used to estimate hydrodynamically relevant roughness characteristics over the entire reef surface (&gt;140 m2; 0.50 m resolution), providing estimates of the canopy height (hc), standard deviation (), rugosity index (R), and fractal dimension (D). Average canopy heights ranged from 3.6 to 4.9 cm, with canopy height standard deviations between 1.4 and 2.0 cm. Mean rugosity indices and fractal dimensions were relatively low on the youngest (1 year) restored reef (R = 1.28;D = 2.67), with substantial increases observed for more mature reef canopies (4 years:R = 1.56;D = 2.71). Structural complexity was consistently greater on reef margins than in reef interiors. Increases in complexity were linked to restoration age, with older reefs exhibiting more complex oyster canopies. The highest fractal dimension was observed on the intact reference reef, highlighting the importance of sustained reef growth for maintaining higher‐order structural complexity. Results provide spatially explicit surface roughness characterizations for healthy, intertidal oyster reefs, with applications in both restoration science and natural and nature‐based feature design.

Active elements based on composite material with additives of biogenic substances for biological treatment systems of seawater

The increasing anthropogenic pollution of the Black Sea is one of the urgent environmental problems. The currently proposed seawater biological treatment systems using attached hydrobionts (the main part of which are various types of algae) cultivated on the surface of artificial reefs have a number of limitations related to the complexity of maintenance of such reefs and the variability of the concentration of nutrients necessary to ensure the vital activity of hydrobionts. The objectives of the study were to develop a composite material containing additives of biogenic substances of prolonged action and compact active elements based on it, from which seawater biological treatment systems can be formed in certain water areas characterized by increased intake of anthropogenic pollutants (marinas, bays, ports). The results of the experiments have shown that the synthesized highly porous composite material based on an environmentally friendly thermoplastic polymer (polyethylene) and additives of a number of biogenic substances of inorganic origin embedded in its structure can be considered as a promising artificial carrier that provides optimal conditions for the cultivation of marine microalgae. Due to the high concentration of microalgae in the biofilm, which is formed on its porous surface, this carrier is able to effectively purify seawater from ammonium ions at four times their maximum permissible concentration (hereinafter referred to as MPC): the degree of purification was 1.5–2 times higher than the same indicator of the seawater self-purification process; at the same time, the self-purification process during the experiment it did not provide a decrease in the concentration of ammonium ions to the MPC. The developed composite material can be recommended as a basis for the production of block-type active elements having the shape of a cube with dimensions of 0.5x0.5x0.5 m and a relatively small mass (about 35 kg). Seawater biological treatment systems can be organized by installing the required number of such elements in selected areas of the water area; maintenance of such systems (cleaning, replacement of elements) should not be problematic due to the compactness and small weight of the elements.

Drone lidar-derived surface complexity metrics as indicators of intertidal oyster reef condition

Eastern oysters (Crassostrea virginica) generate structurally complex reef systems that offer diverse ecosystem services. However, there is limited understanding of how reef structure translates into reef condition. This knowledge gap might be better addressed if oyster reef structure could be more rapidly assessed. Conventional in situ monitoring techniques are often time-intensive, invasive, and do not provide spatially continuous information on the reef structure. Unoccupied Aircraft Systems (UAS), commonly referred to as drones, equipped with optical sensors can rapidly and non-invasively map intertidal oyster reef surfaces. We demonstrate how a digital surface model from UAS-based light detection and ranging (lidar) can enable very high-resolution characterization and monitoring of intertidal oyster reef surface morphology. Generalized linear models (GLMs) identified relationships between in situ live oyster counts and surface complexity metrics derived from digital surface models produced from lidar point clouds. Statistically significant relationships between surface complexity metrics (e.g., gray level co-occurrence features, volume to area ratio, skewness of elevation) and live oyster counts suggest that surface complexity provides useful proxies for reef condition. Advancing the application of remote sensing to intertidal oyster reefs can help identify reefs that are prone to degradation and inform conservation and restoration strategies.

Effects of the reef roughness on the harbor oscillations induced by low-frequency waves

Low-frequency waves can be captured and amplified within harbors and present a prevalent inducing factor of harbor oscillations. Effects of the rough reef surface and the possible inhomogeneity of the roughness on the oscillating behaviors within an elongated harbor on reef topography are investigated based on numerical simulations. The reef roughness is applied via hydraulic roughness coefficient within the well-recognized empirical range and the inhomogeneity is arranged following the probable location of the coral covers and the possible engineering inventions. With wavelet transformation analysis and bispectral analysis for inspecting energy distribution of oscillating components and their interactions, the lowest oscillation mode is found to be more influenced by the rough extents of the reef surface whereas higher modes, by the location of roughness. The friction induced by rough surface in the vicinity of reef crest is revealed to be effective in reducing the oscillating behaviors by acting on the generation of low-frequency waves on the reef by the incident short ones. In addition to the damping effects of the reef roughness, the transition from smooth reef surface to rougher one may also lead to evolutions of coupling between different oscillating components and consequently affect oscillating energy distributions.

Xenia umbellata (Octocorallia): A novel model organism for studying octocoral regeneration ability

Climate change is leading to phase shifts in coral reefs worldwide. In many biogeographic regions, octocorals are now becoming the most abundant benthic components, due to their environmental resilience and ability to rapidly colonize reef surfaces. Regeneration abilities and asexual reproduction are highly important for this ability and probably contribute to the successful spread of certain octocorals, including invasive species. Regeneration, however, has been little investigated in octocorals. To achieve a deeper understanding of octocoral regeneration, we employed Xenia umbellata, a common octocoral in the Red Sea, as a novel experimental model for laboratory studies. Using single-polyp modules, we investigated its regeneration ability and polyp asexual reproduction (budding). Excised polyps successfully reattached to tissue-culture plates within 2-3 days and started budding within 10 days. Amputation of the oral disc led to full regeneration within 7-10 days, with budding continuing throughout this period. Moreover, amputated tentacles developed into polyps within 21 days, demonstrating an unusual capacity for whole-body regeneration. The regeneration abilities of this species imply high totipotency of all polyp parts and are likely important for its life cycle. Further research using this model is expected to enhance the ecological and molecular understanding of octocoral development and provide insights into phase shifts currently occurring in coral reefs. Our study also suggests that X. umbellata has potential as a model organism for integrative studies on regeneration, physiology, developmental biology, and more, encouraging its adoption as a novel colonial cnidarian model organism.

Stories told by corals, algae, and sea-urchins in a Mesoamerican coral reef: degradation trumps succession.

Understanding the mechanisms that allow the permanence of coral reefs and the constancy of their characteristics is necessary to alleviate the effects of chronic environmental changes. After a disturbance, healthy coral reefs display trajectories that allow regaining coral cover and the establishment of framework building corals. Through a comparative approach, in a patch reef partially affected by a ship grounding, we analyzed the successional trajectories in affected and unaffected sectors. Fleshy algae (which do not promote the recruitment of corals) dominated the reef surface irrespective of the impact of the ship grounding incident. Acropora species had near-zero contributions to community structure, whereas non-framework building corals like Porites sp. had a slightly higher recruitment. Cover of coral and calcareous crustose algae decreased over time, and neither the latter nor adult coral colonies had any effect on the occurrence probabilities of small corals. Sea urchin (Diadema antillarum) densities were generally low, and thus unlikely to contribute to reverting algal dominance. The successional trajectories of the community in the impacted and non-impacted sectors of the coral patch reef agree with the inhibition successional model, leading to the development of a degraded state dominated by fleshy algae. It is probable that the stability and resilience of this degraded state are high due to the ability of fleshy algae to monopolize space, along with low coral recovery potential.

Microscale evolution of reefal microbialites

Reefal microbialites are crusts and infills of microbialites developed within primary cavities of late Quaternary coralgal reef frameworks. In spite of many previous studies, factors controlling mesofabric transitions, particularly from laminated to digitate microbialites but as well as the microscale formation process in relation to microbial communities, have not yet been fully understood. Furthermore, there are still controversies regarding age differences between reef framework growth and microbialite formation. The authors have discussed the macroscale evolution of primary cavities as well as the microscale evolution of reefal microbialites obtained in a middle Holocene reef core drilled off Okinawa Island, south-west Japan, based on petrographic observations using light and scanning electron microscopy with energy dispersive X-ray spectroscopy and radiocarbon (14C) ages of corals and reefal microbialites. The authors’ petrographic observations show that coralgal reef framework growth was followed by encrusting foraminifers and metazoans, coevally with macro-bioerosion and microbioerosion of coralgal frameworks, then finally infilled with an intraskeletal and boring-filling microbialite (IBFM) and encrusted by reefal microbial crusts (RMCs), including structureless and digitate microbialites. The observed macroscale evolution was probably driven in response to progressive changes to reduced light and water circulation in the primary cavities associated with middle Holocene transgression and reef formation. The microfabric of reefal microbialites consists of primary in situ precipitated peloids (spherical micritic clots), voids (primary pores) and allochthonous detrital grains (bioclastic and siliciclastic) fallen from upper reef surfaces. IBFM geopetally infilled inside bioeroded cavities soon after the bioerosion of coralgal frameworks. RMCs have a two-layered microfabric succession, which is composed of the alternation of dense peloidal aggregates and laterally aligned voids in the lower part and the mixing of peloidal aggregates and irregularly and vertically developed voids in the upper part. The authors speculate that the repeated occurrences of laterally aligned voids in the lower part may imply the intermittent pauses of peloidal formation, while peloids accumulated on the irregular outer surfaces may result in the formation of vertically developed voids and digitate surfaces in the upper part. These peloids are probably formed by coccoid sulfate-reducing bacteria in anoxic environments developed by the degradation of bacterial biofilms and organic matters, allowing sulfate reduction and increasing alkalinity, therefore carbonate precipitation. The two-layered microfabric succession generally observed in RMCs may reflect spatial-temporal oxic/anoxic variations in interstitial water and the degree of sulfate ion supply by the gradual closing of primary cavities.