Reef Rim Research Articles

Canopy-forming algae improve the colonization success of the vermetid reef-builder Dendropoma cristatum (Biondi 1859) on artificial substrates

Positive interactions among foundation species play a crucial role in achieving and maintaining a good state of the marine environment, enhancing the habitat stability, productivity, and the whole ecosystem functioning. In this regard, macroalgal canopies may affect the biotic and abiotic environmental features, improving the habitat quality for the establishment of other sedentary organisms. In this study, we investigated the effect of intertidal canopy-forming algae on the colonization success of the central-Mediterranean vermetid reef-builder Dendropoma cristatum (Biondi 1859) on artificial substrates produced for reef restoration purposes. An in situ experiment was carried out along the northwestern coast of Sicily during the breeding season of the vermetid snails, by using geopolymer concrete settlement discs with a topographic design to facilitate the vermetid settlement. The discs were placed on the seaward reef rim, underneath the macroalgal canopy, and on adjacent control bare reef areas. The canopy effect on the understory algal colonization and the environmental temperature at the vermetid settlement substrate were also surveyed. After 35 days of field exposure, the vermetid settlement increased by 1.7-fold on the discs underneath the macroalgal canopy compared with that on the bare settlement discs. Moreover, the understory algae showed a higher homogeneity and a lower percent cover on the settlement discs underneath the macroalgal canopy. The peaks of temperature were higher on the bare reef, and the macroalgal canopy also reduced temperature variability under maximum sun irradiance during the diurnal low tides. The vermetid settlement was positively correlated to the canopy cover, which, directly or indirectly, improved the success of colonization of the reef-builder snails on artificial substrates. The deployment of settlement discs where canopy-forming algae are naturally present may facilitate the gardening of vermetid clusters that may be translocated to restock the reef-builder density at degraded areas.

Holocene Depositional History of Low-Lying Reef-Rim Carbonate Islets of Fakarava Atoll, Northwest Tuamotu, Central South Pacific

Assumptions about the fate of low-lying coral reef islands (atolls) facing global warming are poorly constrained, due to insufficient information on their depositional history. Based on the U/Th dating of 48 coral clasts, the chronostratigraphic analysis of excavated sections through rim islets (motu) at the windward and leeward sides of Fakarava Atoll (Tuamotu, French Polynesia) reveal that the deposition of coral detritus started approximately 2000 years ago. Most of these deposits lie on conglomerate pavements or reef flat surfaces, and are about 4500 to 3000 years old. The islet expansion at the windward sites seems to have operated coevally across the reef rim, from the ocean-facing shore lagoonwards. Meanwhile, well-developed, continuous, elongated, vegetated islets mostly occur along the windward, northeast to southeast coast, and isolated islets, vegetated or not, associated with the dense networks of conglomerates, are common on the leeward, partly submerged, western rim. Islet accretion on the windward rim sides is believed to have been mainly triggered by winter storms and occasional cyclonic events, whilst the leeward atoll parts were most likely shaped by distant-source swells from mid to high latitudes. The projections of the accelerated sea level rise in the future suggest that the long-term islet stability at Fakarava could be altered because the islets have accreted under the conditions of the falling sea level.

Sedimentary evolution and sequence stratigraphy of Ediacaran high‐grade phosphorite–dolomite–shale successions of the Bocaina Formation (Corumbá Group), Central Brazil: Implications for the Neoproterozoic phosphogenic event

AbstractExtensive phosphorite deposition is observed in the Neoproterozoic after a prolonged hiatus during most of the Mesoproterozoic era. This event is thought to represent an important record of major palaeoenvironmental, palaeoceanographic and biotic changes that shaped Neoproterozoic ecosystems, suggesting close relationships between phosphogenesis and the preservation of key Ediacaran biotas. However, high‐grade Ediacaran phosphorite deposits are relatively uncommon, diminishing the opportunity to test current phosphate mineralization–deposition models and their implications for Neoproterozoic research. In this scenario, widespread Ediacaran phosphorite–dolomite–shale successions of the Bocaina Formation (Corumbá Group – Central Brazil) are poorly explored in international literature. Nevertheless, recent advances in phosphate exploration gave access to continuous drill core sections and freshly opened mine pits, revealing an unprecedented record of complex phosphatic successions featuring the occurrence of Ediacaran microfossils assigned to the Doushantuo–Pertatataka assemblages. This work seeks to constrain main lithofacies, sequence stratigraphy and depositional settings from these phosphatic successions in order to analyse the sedimentary evolution of the unit under the current Neoproterozoic phosphorite research framework. These results indicate that the Bocaina Formation records secular sustained phosphate deposition. These deposits are related to unprecedented, microbialite reef rim phosphorites deposited during a lower accretionary rimmed platform stage, followed by the deposition of Doushantuo‐like, whole platform phosphorites associated with a later, drowned platform stage, therefore, reinforcing the evidence for the operation of strong allogeneic controls on phosphate mineralization–concentration. In addition, this study concludes that fossiliferous Ediacaran phosphatic deposits such as the Bocaina Formation are important to understanding Neoproterozoic phosphogenic events, because they may record the transition from a Precambrian to Phanerozoic‐like phosphogenesis associated with the instauration of the Ediacaran–Cambrian phosphatic taphonomic window. This evidence hints that the growing dataset from the Bocaina Formation may bring new, exciting perspectives for Neoproterozoic research as a whole.

Patch reefs as sources of long and continuous skeletal records of horizontal reef expansion in Dongsha Atoll, South China Sea

Atolls are annular midocean reefs with various topographical features. The reef rim of an atoll encloses a central lagoon and lagoonal patch reefs therein. Two hypotheses explaining the origins of patch reefs in atoll lagoons have been developed: the karst hypothesis and the growth hypothesis, which emphasize the role of uneven erosion and uneven timing of reef accretion (especially horizontal expansion), respectively, in the formation and growth of patch reefs. In this study, we investigated trends in the locations, depths, ages, sizes, and other features of flat-top and pinnacle-top reefs in the Dongsha Atoll in the South China Sea by analyzing bathymetric LiDAR images and performing U-Th dating on samples we collected from various reefs in the atoll. Three of our main findings were consistent with the growth hypothesis: (1) the differences in ages of the centers of different flat-top patch reefs reflected the different times at which each patch reef reached the surface and started to expand horizontally; (2) the top surfaces of individual flat-top patch reefs had old centers and young peripheries, reflecting horizontal growth; and (3) all the pinnacle-top reefs we examined had more live corals on their tips than on the surrounding bases. We observed no positive correlation between the ages of the centers and the diameters of flat-top patch reefs; that is, larger reefs did not necessarily have older centers, indicating that various complex factors affect the expansion and age patterns of patch reefs. Overall, the findings of this study provide support for the growth hypothesis and for the use of flat-top patch reefs as sources of easily accessible long-term coral skeletal records. They may therefore serve as a reference for future studies on asymmetric reef expansion and other relevant topics.

Paleo-sea levels, Late-Holocene evolution, and a new interpretation of the boulders at the Rocas Atoll, southwestern Equatorial Atlantic

Rocas Atoll is a remote place and the only atoll in the southwestern Atlantic Ocean. This study presents new paleo-sea level reconstructions and a new interpretation on former ones. The new data indicate that sea level was higher at the Late-Holocene than it is at present and that the hydrodynamic factors overlap with sea-level changes, precluding more precise paleo-sea level reconstructions. According to previous works, the Rocas Atoll presents one of the most conspicuous paleo-sea level indicators, represented by reef remains. But, in this paper, these supposedly reef remains are reinterpreted as algal-reef boulders detached from the atoll reef-front by high-wave energy events and deposited over the reef rim, where they remain detached or are cemented and, sometimes, mushroom shaped. It is considered that the bigger boulders may have been deposited in their current position by the cumulative effects of past energetic wave events. Furthermore, the use of beachrocks as paleo-sea level indicators in former studies is discussed, leading to the conclusion that they actually indicate beach progradation and lateral accretion resulting from a positive sand budget. Finally, a schematic morphological evolution of the atoll during the Late-Holocene is proposed. At the atoll, leveling with RTK and imagery with laser scanner were collected, and eleven radiocarbon ages were measured by accelerator mass spectrometry (AMS).

Reef-rim structure and building history, Rangiroa, an uplifted Atoll, French Polynesia: The role of morphotectonics and extreme marine hazard events

Reconstructing the responses of low-lying, atoll-rim islands to future environmental changes due to global warming requires a robust knowledge of the building history of reef rims through time. In the northwest of Tuamotu Archipelago, atolls have experienced opposite vertical motions (uplift versus subsidence) in response to differential tectonic constraints undergone by the underlying oceanic crust. As a result, during Pleistocene high sea levels, some atolls remained emerged whereas others remained flooded. Based on two electrical resistivity tomography profiles across the northern rim-island of the uplifted Rangiroa Atoll and on U/Th dating of corals clasts collected from unconsolidated sand-rubble deposits within two wells through the same island area, the internal structure of the atoll-rim and its mode and timing of building are tentatively reconstructed. The irregularly shaped, upper boundary of the basal resistive unit is interpreted as residual, pinnacle-shaped reliefs of Miocene age. At the seaward rim side, the uppermost highly resistive layers, an about 5 m-thick, were assumed to be reef-related, partly consolidated material emplaced during the Holocene. Thicker resistive layers that infill cavities in the Miocene basement, in the central part of the atoll rim, refer to the fresh water lense in porous, more or less consolidated material (phosphorites?) presumably emplaced during the Mio-Pliocene. In northern Rangiroa, the age of the oldest coral clasts trapped into the atoll-rim islands ranges between about 11,000 and 8000 calendar years when sea level was approximately 60 to 20 m below its present position. This suggests that coral-clast supply from the adjacent forereef slopes was triggered by extreme wave-surge events (presumably, tsunamis generated by island flank failures). The earliest island initiation phase is thought to start by approximately 6000 cal. yr and to be achieved by about 3000 cal. yr. By constrast, on the nearby subsiding Takapoto Atoll, the rim island began to initiate not earlier than 2500 cal. yr. At Rangiroa, morphotectonics is assumed to have played a major role in controlling the internal rim architecture and, associated to extreme marine hazard events, to have promoted the earlier settlement of low-lying, rim islands through an earlier infilling of accommodation space as compared to nearby subsiding atolls.

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.

Tide and wave driven flow across the rim reef of the atoll of Raroia (Tuamotu, French Polynesia)

The currents flowing across the rim of the atoll of Raroia were investigated with a 1 year-long dataset of wave, water level and currents. Offshore waves break on the edge of the reef outside the atoll's rim and drive current into the lagoon, through the shallow hoa that cut across the rim. The additional water volume generated by this wave driven flow induces an elevation of water level throughout the atoll's lagoon and is evacuated back into the open ocean through a deep reef pass. The water level inside the atoll is also driven by astronomical tides, which enter the lagoon thought the reef pass, after undergoing a ~50% decrease in amplitude and a ~4 hour lag. Using a simple parametric model with three calibrated coefficients, we show that currents across the atoll's rim can be estimated as a function of the offshore wave conditions and the water level difference between the ocean and the lagoon.

Brooded coral offspring physiology depends on the combined effects of parental press and pulse thermal history.

Reef-building corals respond to the temporal integration of both pulse events (i.e., heat waves) and press thermal history (i.e., local environment) via physiological changes, with ecological consequences. We used a "press-pulse-press" experimental framework to expose the brooding coral Porites astreoides to various thermal histories to understand the physiological response of temporal dynamics within and across generations. We collected adult colonies from two reefs (outer Rim reef and inner Patch reef) in Bermuda with naturally contrasting thermal regimes as our initial "press" scenario, followed by a 21-day ex situ "pulse" thermal stress of 30.4°C during larval brooding, and a "press" year-long adult reciprocal transplant between the original sites. Higher endosymbiont density and holobiont protein was found in corals originating from the lower thermal variability site (Rim) compared to the higher thermal variability site (Patch). The thermal pulse event drove significant declines in photosynthesis, endosymbiont density, and chlorophyll a, with bleaching phenotype convergence for adults from both histories. Following the reciprocal transplant, photosynthesis was higher in previously heated corals, indicating recovery from the thermal pulse. The effect of origin (initial press) modulated the response to transplant site for endosymbiont density and chlorophyll a, suggesting contrasting acclimation strategies. Higher respiration and photosynthetic rates were found in corals originating from the Rim site, indicating greater energy available for reproduction, supported by larger larvae released from Rim corals post-transplantation. Notably, parental exposure to the pulse thermal event resulted in increased offspring plasticity when parents were transplanted to foreign sites, highlighting the legacy of the pulse event and the importance of the environment during recovery in contributing to cross-generational or developmental plasticity. Together, these findings provide novel insight into the role of historical disturbance events in driving differential outcomes within and across generations, which is of critical importance in forecasting reef futures.

Large-scale margin collapses along a partly drowned, isolated carbonate platform (Lansdowne Bank, SW Pacific Ocean)

The Lansdowne Bank is a partly drowned, isolated carbonate platform of around 4000 km2 located 300 km west of New Caledonia, in the SW Pacific Ocean, in water depths of 20 to 100 m. New multibeam bathymetric data, high resolution seismic reflection profiles and sediment gravity cores have been acquired on the bank top and adjacent slopes. This dataset reveals an almost continuous 4 km wide outer reef rim located in ca. 50 m water depth, surrounding a gently deepening inner platform, reaching up to 100 m water depth. The bank is bordered by very steep slopes showing numerous erosional morphologies such as canyons, channels and gullies. Along with these bypass features, spectacular bank margin collapses and slope failures are evidenced by up to 20 km-wide bank edge and intraslope failure scars, respectively, resulting in a typical “scalloped” geometry of the bank margin. These failure scars can lead to a complete collapse of the outer reef rim and impact subsequent reef development. Bank margin collapses are evidenced by hectometer to kilometer-scale blocks and debris shed on the slope, likely emplaced by rock fall/avalanching processes originating from the brittle failure of early cemented bank edge and upper slope sediments. In turn, failures triggered on the un-cemented mud-prone middle to lower slopes likely generate more cohesive, submarine debris flows that could be at the origin of erosive morphologies within the debris fields. Estimated individual failure volumes can reach up to 3 km3. Quaternary sea-level lowstands, that would have led to platform exposure, fracturing and karstification, and the development of an erosional sea cliff, as well as subsequent rising sea-level are believed to play a significant role in mass wasting event emplacement, yet “bottom up” submarine processes such as the upslope propagation of bypass morphologies by retrogressive headward erosion cannot be ruled out. In terms of geomorphic and stratigraphic constraints, the documented bank margin collapses affect a terrace located in 70 m water depth around the bank, which, depending on its age and origin, could provide a minimum age for collapse events. Finally, considering the shallow water depth of failure headscarps, the volumes of material involved in the slides as well as their vicinity to the nearby main island of New Caledonia, numerical simulations of the tsunamigenic potential of submarine slides have been performed. They showed that these slides would have been able to produce a meter-scale wave that would reach the northern coast of the island in less than an hour.

Lateral, Vertical, and Temporal Variability of Seawater Carbonate Chemistry at Hog Reef, Bermuda

Spatial and temporal carbonate chemistry variability on coral reefs is influenced by a combination of seawater hydrodynamics, geomorphology, and biogeochemical processes, though their relative influence varies by site. It is often assumed that the water column above most reefs is well-mixed with small to no gradients outside of the benthic boundary layer. However, few studies to date have explored the processes and properties controlling these multi-dimensional gradients. Here, we investigated the lateral, vertical, and temporal variability of seawater carbonate chemistry on a Bermudan rim reef using a combination of spatial seawater chemistry surveys and autonomous in situ sensors. Instruments were deployed at Hog Reef measuring current flow, seawater temperature, salinity, pHT, pCO2, dissolved oxygen (DO), and total alkalinity (TA) on the benthos, and temperature, salinity, DO, and pCO2 at the surface. Water samples from spatial surveys were collected from surface and bottom depths at 13 stations covering ∼3 km2 across 4 days. High frequency temporal variability in carbonate chemistry was driven by a combination of diel light and mixed semi-diurnal tidal cycles on the reef. Daytime gradients in DO between the surface and the benthos suggested significant water column production contributing to distinct diel trends in pHT, pCO2, and DO, but not TA. We hypothesize these differences reflect the differential effect of biogeochemical processes important in both the water column and benthos (organic carbon production/respiration) vs. processes mainly occurring on the benthos (calcium carbonate production/dissolution). Locally at Hog Reef, the relative magnitude of the diel variability of organic carbon production/respiration was 1.4–4.6 times larger than that of calcium carbonate production/dissolution, though estimates of net organic carbon production and calcification based on inshore-offshore chemical gradients revealed net heterotrophy (−118 ± 51 mmol m–2 day–1) and net calcification (150 ± 37 mmol CaCO3 m–2 day–1). These results reflect the important roles of time and space in assessing reef biogeochemical processes. The spatial variability in carbonate chemistry parameters was larger laterally than vertically and was generally observed in conjunction with depth gradients, but varied between sampling events, depending on time of day and modifications due to current flow.

Coral calcification responses to the North Atlantic Oscillation and coral bleaching in Bermuda.

The North Atlantic Oscillation (NAO) has been hypothesized to drive interannual variability in Bermudan coral extension rates and reef-scale calcification through the provisioning of nutritional pulses associated with negative NAO winters. However, the direct influence of the NAO on Bermudan coral calcification rates remains to be determined and may vary between species and reef sites owing to implicit differences in coral life history strategies and environmental gradients across the Bermuda reef platform. In this study, we investigated the connection between negative NAO winters and Bermudan Diploria labyrinthiformis, Pseudodiploria strigosa, and Orbicella franksi coral calcification rates across rim reef, lagoon, and nearshore reef sites. Linear mixed effects modeling detected an inverse correlation between D. labyrinthiformis calcification rates and the winter NAO index, with higher rates associated with increasingly negative NAO winters. Conversely, there were no detectable correlations between P. strigosa or O. franksi calcification rates and the winter NAO index suggesting that coral calcification responses associated with negative NAO winters could be species-specific. The correlation between coral calcification rates and winter NAO index was significantly more negative at the outer rim of the reef (Hog Reef) compared to a nearshore reef site (Whalebone Bay), possibly indicating differential influence of the NAO as a function of the distance from the reef edge. Furthermore, a negative calcification anomaly was observed in 100% of D. labyrinthiformis cores in association with the 1988 coral bleaching event with a subsequent positive calcification anomaly in 1989 indicating a post-bleaching recovery in calcification rates. These results highlight the importance of assessing variable interannual coral calcification responses between species and across inshore-offshore gradients to interannual atmospheric modes such as the NAO, thermal stress events, and potential interactions between ocean warming and availability of coral nutrition to improve projections for future coral calcification rates under climate change.

Modelling reef hydrodynamics and sediment mobility under sea level rise in atoll reef island systems

Abstract Low-lying coral reef islands will be significantly impacted by future sea-level rise (SLR). It is generally expected that SLR will destabilise reef islands because increasing reef submergence allows larger waves, and therefore greater energy transmission, across reef flats. However, the potential impacts of SLR on reef flat sediment transport and sediment delivery to island shorelines are poorly understood. Here, we use the currents of removal approach (coupling two-dimensional wave modelling with settling velocity data from 186 benthic sediment samples) to model shifts in both reef hydrodynamics and benthic sediment transport under different scenarios of mean reef submergence (MRS = +0 m, +0.5 m, +1 m) at two atoll rim reef sites in the Maldives. Under contemporary conditions (MRS = +0 m), we found that benthic sediment transport is likely occurring, consistent with active reef-to-island sediment connectivity. Under conditions of increased MRS, shifts in wave velocities, and in turn sediment potential mobility, were both non-linear and non-uniform. Significant between-site differences were found in the magnitude of projected shifts in sediment mobility under scenarios of increased MRS, which implies that morphological responses to increased MRS are likely to be diverse, even over local scales. Under increased MRS, the largest increases in sediment mobility were projected on the inner reef flat, whereas lagoonal zones remained as sinks for sediment deposition. We thus hypothesize that while reef islands will persist as sedimentary landforms under projected rates of MRS, lagoonward reef island migration is likely to occur. Findings have implications for predicting the future adaptive capacity of atoll nations. Our results highlight the need for national-scale vulnerability assessments to incorporate (1) potential increases in island mobility; (2) the importance of allowing natural sediment transport processes to occur (unhindered by human constructions); and (3) intra-regional diversity in reef system geomorphic responses to sea level rise.

Facies model on the modern isolated carbonate platform in the Xisha Archipelago, South China Sea

The Xuande Atoll, with a discontinuous reef rim caused by platform drowning, is a typical ellipse isolated platform on the northern slope of the South China Sea. Based on the data of drilling holes, the analysis of surface sediments and high resolution acoustic data, six sedimentary facies are identified in the Xuande Atoll, including reef outer slope, reef front, reef flat, aeolianite sand island, back-reef slope and lagoon. Facies map shows that the elevated reef-rimmed margins with islands are more noticeable on the windward of the platform. With an elevated and discontinuous reef-rimmed margin as well as coarse-grained sediment in the interior platform, Xuande can serve as a case of a leaky bucket atoll which is characterized by initially drowning scenarios. The facies architecture of the Xuande Atoll is evidently influenced by complex and interacting parameters including antecedent topography, sea level, wind-driven currents, water depth and tectonic subsidence. Different controlling factors exist in the drowning of the windward and leeward sides of the platform margin. The study of modern depositional settings in the Xuande Atoll will provide insight on predictions for ancient carbonate provinces.

Assessing Morphologic Controls on Atoll Island Alongshore Sediment Transport Gradients Due to Future Sea-Level Rise

Atoll islands’ alongshore sediment transport gradients depend on how island and reef morphology affect incident wave energy. It is unclear, though, how potential atoll morphologic configurations influence shoreline erosion and/or accretion patterns, and how these relationships will respond to future sea-level rise (SLR). Schematic atoll models with varying morphologies were used to evaluate the relative control of individual morphological parameters on alongshore transport gradients. Incident wave transformations were simulated using a physics-based numerical model and alongshore erosion and accretion was calculated using empirical formulae. The magnitude of the transport gradients increased with SLR: initial erosion or accretion patterns intensified. Modeled morphologic parameters that significantly influenced alongshore transport were the atoll diameter, reef flat width, reef flat depth, and island width. Modeled atolls with comparably small diameters, narrow and deep reef flats with narrow islands displayed greater magnitudes of erosion and/or accretion, especially with SLR. Windward island shorelines are projected to accrete toward the island’s longitudinal ends and lagoon due to SLR, whereas leeward islands erode along lagoon shorelines and extend toward the island ends. Oblique island, oriented parallel to the incident deepwater wave direction, shorelines are forecast to build out leeward along the reef rim and toward the lagoon while eroding along regions exposed to direct wave attack. These findings make it possible to evaluate the relative risk of alongshore erosion/accretion on atolls due to SLR in a rapid, first-order analysis.

Facies, depositional environments, and anatomy of the Subis build-up in Sarawak, Malaysia: implications on other Miocene isolated carbonate build-ups

Isolated carbonate build-ups of Miocene age are important hydrocarbon reservoirs in SE Asia and globally. The architecture of the build-up rim and fore-reef talus is poorly understood. The Subis build-up is one of the cycle I–II carbonate outcrops exposed along the coastal Sarawak in an area of 5 × 6 km2, with an exposed thickness of 390 m. This provides an opportunity to document the composition and architecture of rim and talus deposits in an isolated build-up as an analogue for similar gas-bearing structures located offshore Central Luconia, in Malaysia, which were neither drilled nor cored so far. This paper documents sedimentological characteristics, lateral and vertical facies relationships, of the marginal sections of a build-up in a large quarry exposing the Subis Limestone. The Subis Limestone is composed of massive and branching corals, coralline red algae, benthic foraminifera, bivalves, gastropods, echinoids, and occasionally bryozoans and sponges. Eight microfacies types and four environments were defined to describe the Subis Limestone: outer talus, inner talus, reef rim, and lagoon. Nine genera of benthic foraminifera were interpreted, confirming an early Miocene age (cycle II) of the succession. Three major backstepping events were observed at the build-up, where they developed inward towards the center of the build-up.

LABORATORY EXPERIMENTS OF WAVE OVERTOPPING OF REVETMENT STRUCTURES IN REEF ENVIRONMENTS

Reefs offer natural protection to many coastal island nations through the physical processes of wave breaking on the reef rim and thus, reducing the wave energy that reaches the shoreline. However, both mean and infragravity water levels build up over the reef due to wave breaking on the reef top that can lead to coastal inundation and serious damage (Gourlay 1996a; Gourlay 1996b; Blacka et al. 2015). To protect the community around the coastal region, the construction of defensive structures, such as revetments or sea dikes may be required. The present study is focused on wave overtopping discharge on smooth revetments within the reef environment, which is not fully understood. Classical formulas that estimate overtopping of revetments (see Van der Meer et al. 2016 and references therein) are based on waves breaking in deep to shallow water either in front of the structure or directly on the structure. Recently, Altomare (2016) updated Van Gent’s (1999) formula of overtopping discharge of smooth sea dikes on shallow foreshores to include very shallow foreshores. The updated equation estimates dimensionless over-topping as a function of the full spectrum wave properties at the toe of the structure and an equivalent slope term. As the reef environment can often be described as a very flat and shallow lagoon seaward of the shoreline, the Altomare formula can potentially be adopted to predict the overtopping discharge on smooth revetments in reef environments, but this has yet to be tested.

Hydrodynamics of a Tidally Forced Coral Reef Atoll

AbstractThe hydrodynamics of a tidally forced semi‐enclosed coral reef atoll (North Scott) at the edge of the continental shelf of northwestern Australia were investigated by combining field observations and numerical modeling. The observations revealed that the spring tidal range outside the atoll reaches 4 m, and as the water level drops below mean sea level, the reef rim surrounding the shallow (~10–15 m) lagoon becomes exposed. During this time, the lagoon can only exchange with the open ocean through two narrow channels, resulting in highly asymmetric water levels and velocities that were most pronounced during spring tide. On average, the ebb tide duration was ~2 hr longer than the flood, with rapid flood velocities in the channel reaching 2 m/s. We applied an unstructured grid model Delft3D‐Flexible Mesh to simulate the atoll hydrodynamics and were able to replicate the asymmetric water levels and complex velocities in the lagoon. The results revealed that at higher tidal stages, a dominant momentum balance exists between the pressure gradient (established by the propagation of the tide on the shelf) and the local flow acceleration of water throughout the interior of the atoll. At lower tidal stages, which coincided with a reversal of the offshore tidal pressure gradient, the lagoon became isolated from offshore dynamics and all momentum terms were negligible. This resulted in a tidally averaged residual westward flow within the lagoon that drove an asymmetric flushing pattern within the atoll, which we propose would be a common flushing mechanism within other tide‐dominated atolls worldwide.

Microplastic and charred microplastic in the Faafu Atoll, Maldives

Microplastics are recognized as a growing threat for the marine environment that may even affect areas generally considered pristine. In this work we surveyed the microplastic contamination in the Faafu Atoll (Maldives, Indian Ocean) across twelve sampling station, located either inside or outside the reef rim. Sediments and seawater samples were collected. Despite the remoteness of the atoll, the scarce local population and low touristic annual afflux, the detected average abundance were 0.32 ± 0.15 particles/m3 in the surface water and 22.8 ± 10.5 particles/m2 in the beach sediments. Polymers identified through Fourier-Transform Infrared spectroscopy were mostly polyethylene, polypropylene, polystyrene, polyvinylchloride, polyethyleneterephtalate, and polyamide. Elastomeric residues and charred microparticles were also found. In particular, the charred microparticles were prevalently located nearby the inhabited island and they might be considered a peculiarity of the area, related to local practice of burning plastic waste at the shoreline.

Coral Reef Carbonate Chemistry Variability at Different Functional Scales

There is a growing recognition for the need to understand how seawater carbonate chemistry over coral reef environments will change in a high-CO2 world to better assess the impacts of ocean acidification on these valuable ecosystems. Coral reefs modify overlying water column chemistry through biogeochemical processes reflected in thesuch as net community organic carbon production (NCP) and calcification (NCC). However, the relative importance and influence of these processes on seawater carbonate chemistry vary across multiple functional scales (defined here as space, time, and benthic community composition), and have not been fully constrained. Here, we use Bermuda as a case study to assess 1) spatiotemporal variability in physical and chemical parameters along a depth gradient at a rim reef location, 2) the spatial variability of total alkalinity (TA) and dissolved inorganic carbon (DIC) over distinct benthic habitats to infer NCC:NCP ratios (< several km2; rim reef vs seagrass and calcium carbonate (CaCO3) sediments) on diel timescales, and 3) compare how TA-DIC relationships and NCC:NCP vary as we expand functional scales from local habitats to the entire reef platform (10’s of km2) on seasonal to interannual timescales. Our results demonstrate that TA-DIC relationships were strongly driven by local benthic metabolism and community composition over diel cycles. However, as the spatial scale expanded to the reef platform, the TA-DIC relationship reflected processes that were integrated over larger spatiotemporal scales, with effects of NCC becoming increasingly more important over NCP. This study demonstrates the importance of considering drivers across multiple functional scales to constrain carbonate chemistry variability over coral reefs.