Reef Platform Research Articles

Reefal ostracod assemblages from the Zanzibar Archipelago (Tanzania)

Abstract. Tropical reefs encompass tremendous biodiversity yet are imperiled by increasing natural and anthropogenic disturbances worldwide. Meiobenthic biota on coral reefs, for example, ostracods, may experience substantial diversity loss and compositional changes even before being examined. In this study, we investigated the reefal ostracod assemblages from the highly diverse and productive ecosystem in the Zanzibar Archipelago (Pemba, Zanzibar, and Mafia islands), Tanzania, to understand how their diversity and faunal structure vary in response to water depth, benthic community type, and human impacts. We characterized four distinct ostracod faunas associated with different benthic habitats, which were deep fore reefs, shallow fringing reefs, degraded fringing reefs, and algae-covered intertidal flats. We identified typical ostracod associations, i.e., Bairdiidae versus Loxoconchidae–Xestoleberididae, that showed affinities towards hard corals or algae on the reef platforms, respectively. The highest diversity was found on shallow fringing reefs where coral-affined and algae-affined taxa exhibited maximum overlap of their distributional ranges, while the sand flats, mangrove, and marginal reefs within the intertidal zone had much lower diversity with a high dominance of euryhaline taxa. Along the western coast of Zanzibar Island, coastal development likely resulted in a unique faunal composition and comparatively low diversity of ostracod assemblages among those in reefal habitats, in conjunction with overall reef ecosystem degradation. This study represents the first large-scale assessment of shallow-marine ostracods in the Zanzibar Archipelago. It lays a solid foundation for future research into the ecological significance of ostracods on coral reefs.

A dynamic subtropical coastal hotspot of benthic foraminifera in the Southeastern Mediterranean indicates early-stage tropicalization

Due to ongoing ocean warming, subtropical environments are becoming accessible to tropical species. Among these environments are the vermetid reefs of the Southeastern Mediterranean (SEM). In the last decades, these valuable coastal habitats witnessed the proliferation of numerous alien species of tropical origin. Among the meiofauna thriving on these reefs are benthic foraminifera, single cell marine organisms that make a significant contribution to global carbonate production. It has been widely recognized that benthic foraminifera, among other invasive species, thrive in the macroalgal cover, and it has been suggested that their populations are becoming a significant new source of sediment substrate. Here, we report on the first systematic assessment of the population size of the benthic foraminifera, allowing a comparison with data from the native tropical habitat of these species. Our study is based on a seasonal sampling of benthic foraminifera from confined sampling areas at four sites along the vermetid reef platforms of the Israeli SEM coast. Our survey reveals a patchy distribution of each species with peak population densities exceeding 100,000 specimens per m2, making the SEM a hotspot of benthic foraminifera, with population densities comparable to tropical coral reef environments. The assemblages of the SEM hotspot are dominated by cosmopolitan foraminiferal taxa and tropical invaders from the Indo-Pacific (e.g., Amphistegina lobifera, Pararotalia calcariformata, soritids, and Hauerina diversa). In contrast to foraminiferal hotspots in the tropics, which are completely dominated by larger symbiont-bearing taxa, the SEM hotspot stands out due to high abundances of non-symbiont-bearing species Textularia agglutinans and small miliolids. An intriguing observation is the significant heterogeneity in composition and density of foraminiferal assemblages between the vermetid reefs' southern and northern areas (Israel), indicating that the productivity of the dominant species are also modulated by local yet unknown environmental factors.

Numerical study of wave-induced flow and wave set-up on a platform reef with steep slope

A 2D numerical model was developed in this study to investigate the wave-induced flow and wave set-up on a platform reef with steep frontal slope. The model solves the Reynolds Averaged Navier-Stokes (RANS) equations, with the classical k−ε turbulence closure model. The inflow boundary was modified by adding fluid replenishment to balance the water level in the numerical flume to ensure long-term simulation in the platform reef system. The numerical model was first validated by systematically comparing the wave breaking, flow structure, and mean water level against laboratory measurements. The validated model was employed to further analyze the detailed flow field and reveal the influence of the reef morphology on the wave-induced flow and wave set-up. The complex distribution pattern of wave-induced flow and wave set-up on the reef flat were found to be related to the Froude number. The results also reveal that reef slope and reef roughness will not only change the wave breaking morphology but also affect the distribution pattern of wave-induced flow and wave set-up.

Peak Acceleration Distribution on the Surface of Reef Islands under the Action of Vertical Ground Motion

This paper adopts the fluid–structure coupling algorithm based on the acoustic fluid element, the fluid dynamic artificial boundary, and the consistent viscoelastic artificial boundary of solid media to establish a finite element model of the dynamic interaction of the reef-island–seawater system. Then, a numerical simulation of the seismic response of the reef-island site is carried out to study the seismic ground motion distribution patterns of the reef–seawater site and the reef-island–lagoon site. The innovation of this article is that the influence of reef–island topography and fluid–structure coupling is considered in the analysis when vertical ground motion is input. The results show that the slope angle of the bottom layer has a significant influence on the peak ground acceleration distribution and peak size on the island slope surface and the reef platform. For high-frequency input motion, a smaller reef platform width will induce a larger peak acceleration response on the reef platform. Seawater has a significant suppressive effect on vertical ground acceleration. The more high-frequency components of the input bedrock motion, the more obvious this suppression effect will be. The existence of the lagoon will amplify the maximum peak acceleration on the reef platform. According to the calculation results, lagoon terrain can amplify the maximum horizontal and vertical peak accelerations on the reef platform by about 19 and 6 times relative to the free-field results, respectively.

Meta‐Study of Carbonate Sediment Delivery Rates to Indo‐Pacific Coral Reef Islands

AbstractCoral reef islands are amongst the most vulnerable environments to sea‐level rise (SLR). Recent physical and numerical modeling studies have demonstrated that overwash processes may enable reef islands to keep up with SLR through island accretion. Sediment supply to these islands from the surrounding reef system is critical in understanding their morphodynamic adjustments, but is poorly constrained due to insufficient knowledge about sediment delivery rates. This paper provides the first estimation of sediment delivery rates to coral reef islands. Analysis of topographic and geochronological data from 28 coral reef islands indicates an average rate of sediment delivery of c. 0.1 m3 m−1 yr−1, but with substantial inter‐island variability. Comparison with carbonate sediment production rates from census‐based studies suggests that this represents one quarter of the amount of sediment produced on the reef platform. Results of this study are useful in future modeling studies for predicting morphodynamic adjustments of coral reef islands to SLR.

Annual and decadal surveys of molluscs on intertidal platform reefs in a warming marine environment in the southeastern Indian Ocean provide a baseline for monitoring for future environmental changes

ABSTRACT Few data sets are available to evaluate short- and long-term perturbations and impacts of increasing anthropogenic stresses and a warming marine environment in coastal regions near cities. The Perth, Western Australia shoreline has extensive, depauperate sandy beaches interrupted by small, biodiverse intertidal reef platforms. We present transect data on mollusc diversity and density on three Perth platforms recorded from 1983 to 1987, and compare it to 2007 and 2021 to analyse assemblage modifications in response to increased anthropogenic pressures. NMDS multivariate analysis showed a consistent drift in assemblage composition in 2007 and 2021, but this is was not statistically significant from the 1980s baseline (PERMANOVA, P = 0.336). There was no significant long-term decline (or increase) in molluscan species richness. Most species were uncommon but eight were recorded on at least 20 of the 21 surveys. The endemic Western Australian mussel Brachidontes ustulatus (Lamarck, 1819) accounted for 71.8% of all individuals. Total mollusc density varied primarily with changes in B. ustulatus. Tropical species were a minor component of the molluscs that did not change in diversity with warming temperatures. These data provide a long-term baseline for monitoring future anthropogenic changes.

Microfacies analysis and Depositional Basin Model of the Upper Cretaceous, Kurdistan Region, Northern Iraq

This study focuses on the Upper Cretaceous system in the Zagros fold and thrust belt in the Kurdistan Region / Iraq. The impression of this study is created by the variety of formations, facies change, and age of the Cretaceous formations, as well as the different consequences of the contact situation between the formations, particularly the Qamchuqa-Bekhme and Kometan-Shiranish contacts formations. Through investigation, evaluation, and combination of results from Lithostratigraphy and Microfacies of seven sections in various locations of KRI/ Iraq, new paleogeography of the Late Cretaceous of Northern Iraq is constrained. The microfacies of entire Late Cretaceous formations in different sections outcome 16 sub-microfacies types demonstrating open marine deep shelf environment of Shiranish Formation, the entire reef (back-reef, fore-reef, reef body) environment of Bekhme Formation and pelagic open marine ramp depositional environment of Kometan Formation. The output results of stratigraphy and microfacies of Late Cretaceous depositional successions in the Kurdistan Region are paleogeography maps reconstruction. During the Turonian to Early Santonian periods, the Kurdistan Region and northern Iraq were pre-subduction basins due to the presence of open marine outer ramp carbonate of the Kometan Formation in the east (Sulaimaniyah province exceeding Hizop area) and inner shelf of Tanuma and Khasib formations in the center and west (Erbil province). Major regression and unconformities covered the West of the study region during the Middle-Late Santonian; the result of this regression degraded the entire inner shelf facies that deposited during Turonian-Early Santonian periods. During the Early-Middle Campanian, a foreland basin emerged in the extreme northeastern and the Kurdistan Region, which was covered by open marine of the Shiranish Formation and reef platform of the Bekhme Formation. The Kurdistan Region was an active foreland basin margin in the Upper Campanian-Maastrichtian, with a greatly expanded Tanjero Formation, an Aqra reef in the north, and an open marine Shiranish Formation in the center and west.

Beta‐diversity within coral atolls: Terrestrial species turnover increases with cyclone frequencies

AbstractAimAtolls are a widely distributed, common type of tropical ecosystem, each consisting of an annular coral reef and up to several hundred individual islets sitting on the reef platform. The small land areas and low elevation render the terrestrial communities susceptible to local extinctions from overwash and inundation due to tropical cyclones. Such recurring catastrophic disturbances should be expected to drive strong priority effects and historical contingency in species assembly, thus also promoting species turnover within atolls. This stands contradictory to the received wisdom of atolls consisting of numerous uniform, replicated island systems. We tested the hypothesis that the individual islets within an atoll exhibit compositional turnover and that this species turnover within atolls is related to cyclone frequency.LocationAtolls worldwide.Time PeriodPresent.Taxa StudiedVascular plants, reptiles, birds.MethodsWe compiled a global dataset at the level of islets for species presence/absence within atolls. We obtained long‐term (80 year) tropical cyclone frequency data for each atoll from the NOAA hurricane database and used Bayesian regression to estimate the effects of tropical cyclone frequency on species turnover within atolls.ResultsWe consistently measured high within‐atoll species turnover (i.e., between the individual islets within an atoll) on atolls worldwide. The degree of within‐atoll species turnover increases with increasing tropical cyclone frequency. Atolls that are more frequently hit by tropical cyclones show higher species turnover between their islets than those that occur outside the tropical cyclone belt.Main ConclusionsTropical cyclones are a significant driver of the community assemblages on atolls. These catastrophic disturbances promote a heterogenous atoll landscape, which challenges perceptions that the islets within an atoll are identical replicates of the entire atoll terrestrial community. Biodiversity surveys undertaken at the islet‐level (alpha‐diversity) might therefore not be representative of the entire atoll metacommunity (gamma) diversity.

Numerical modelling of the physical response of coral reef Sandy island to sea level rise by considering seasonal patterns

Sea level rise (SLR) can increase the bathymetry of coral reef platforms, thereby increasing the potential for island erosion and flooding to damage coastal buildings. However, SLR is a threat to the coastal buildings of islands not only in this way, but also in the destruction of coastal buildings due to island morphology changes caused by SLR. Based on sediment, offshore wave, and tidal data from Yongxing Island (sandy island), Xuande Atoll, South China Sea, a process-based morphodynamic model, XBeach-G, was used to predict the morphological evolution of coral reef sandy island under future SLR based on the RCP8.5 scenario. As waves and tides in the South China Sea are distinctly seasonal, the influence of seasonal patterns was considered in the model. Sensitivity analysis of the effects of water level on the reef, offshore significant wave height, sediment porosity and sediment size on the morphological evolution of the islands was conducted. Numerical simulations indicate that under SLR and extreme wave action, the sandy islands eventually retreat landward and the foreshore height increases and the slope steepens and as the SLR rate increases, the island retreats more towards land and the foreshore height rise decreases. While this natural adjustment of the coral reef island increases the island's potential to offset future flood events and support future physical persistence, there is a negative impact on the safety of coastal buildings. This paper highlights the need to develop reasonable conservation policies based on the island's specific environment and conditions. Excessive use of artificial shorelines to protect islands can damage their natural adjustment to resist disasters such as flooding, while artificial defenses are necessary in the case of threats to coastal buildings or the disappearance of certain islands.

CLIMATE CHANGE IMPACTS ON REEF TOP ISLANDS

It is generally understood that climate change, particularly sea level rise, is a threat to communities and habitats on low lying tropical islands. However, the full nature of this threat is not well understood. These islands have been created by met-ocean forces on the reef platforms and represents the culmination of a dynamic balance between these forces and the supply of sediment over thousands of years. This paper assesses the impact of climate change on the reef top hydrodynamics and coastal morphology of reef top islands.

Does tidal phase determine successful plant colonisation of atoll lagoon islands in the Maldives?

Vegetation colonisation is an important process in the geomorphic development of atoll lagoon islands and understanding how plants establish may help model how these islands will respond to eustatic sea-level rise. The recent morphodynamics of two lagoon island forms, Maaodagalaa, a sand cay, and Mahutagalaa, a forested island, in Huvadhoo Atoll, Republic of the Maldives, provided an exceptional opportunity to examine processes of plant colonisation over annual to decadal time scales. Maaodagalaa sand cay migrated on the reef platform between June 2019 and February 2020 resulting in a new plant-free surface close to sea-level. During this period the new island surface was colonised by 10 species of marine-dispersed plant species. Seedlings were found across the highest surfaces formed in that period (0.62 m above local datum and 0.48 m above the highest tides recorded between 1987 and July 2020). An inundation event in February 2020 stranded debris up to 0.50 m above spring high tide, 15 m inland of the northern margin of the island. Our results indicate wave events, resulting from local, fetch-limited waves, in combination with spring high tides, can strand seeds of marine-dispersed plants on sand cays and around the margins of forested islands. Seedling survival is enhanced if seeds are stranded during peak perigean spring high tides during a local 7-month spring tide cycle. Lagoon islands in the Maldives are still being formed and colonised by plants despite local and ongoing sea-level rise.

New datings and elevations of a fossil reef in Lembetabe, southwest Madagascar: eustatic and tectonic implications

The study of geological sea-level proxies formed during previous interglacials is a common approach to assess how global sea level will evolve under warmer climate conditions. Over the last decades, technical advancements in both survey and geochronology have allowed improving our knowledge of past sea-level highstands. This is of prime importance to refine our understanding of processes contributing to sea-level changes, and ultimately to improve both local and global sea-level projections. Last Interglacial sea-level proxies in the Western Indian Ocean (and more specifically the island nation of Madagascar), have been less investigated than in other intertropical oceans over the last decades. As a result, paleo sea-level data in this region are less abundant and less precise than elsewhere. Here, we report the results of two field campaigns aimed at studying the site of Lembetabe, southwest Madagascar, where a fossil reef was first described by the researcher René Battistini more than 50 years ago. We estimate paleo relative sea level history in space and time from 15 new U-series ages from a fossil reef platform mapped with differential GNSS and drone photogrammetry. Our data suggest that, between 129 ka and 115 ka, paleo relative sea level at this location was about 3.4 ± 1.4 m above modern. Once corrected for glacial isostatic adjustment, we find that paleo global mean sea level did not exceed 3 m above modern. Only slight crustal subsidence would reconcile the peak Last Interglacial sea level measured at Lembetabe with the 5–10 m range reported in the literature.

Types and Distribution Characteristics of Callovian-Oxfordian Reservoir on the Right Bank of Amu Darya River in Turkmenistan

Given the characteristics of significant or enormous resources, a wide range, many reservoir types, and challenging exploration on the right bank of the Amu Darya River, we systematically studied the characteristics of its reservoirs. Based on the core description, thin sections in combination with logging and seismic facies characteristics as well as regional tectonic-sedimentary background, the Callovian-Oxfordian platform and the gentle slope sedimentary system in the Amu Darya Basin of Turkmenistan were summarized. Sedimentary subfacies such as evaporation platform, restricted platform, open platform, and platform margin reef are developed in block A and its west. In contrast, the upper slope, lower slope, and basin sedimentary subfacies are developed in the east of block A. On this basis, the main reservoir types on the right bank of the Amu Darya River are summarized, namely, porous reservoir, vuggy reservoir, fractured-porous reservoir, and fractured-vuggy reservoir. After describing the characteristics of various reservoirs in detail, the main controlling factors, development patterns, and distribution rules of the development of different reservoirs are summarized. Specifically, based on the platform reef, the porous reservoir and vuggy reservoirs are developed mainly on the concealed palaeouplift in the study area and are greatly influenced by atmospheric freshwater leaching and buried dissolution. Based on the dominant sedimentary facies, the fractured-porous reservoirs are mainly developed in the central areas on the right bank of the Amu Darya River. In the later period, hydrothermal fluid and hydrocarbon-generating acidic fluid can dissolve the reservoir through strike-slip faults and their associated fractures. Diagenetic fluid enters the reef through faults and associated fractures to form dissolved reservoir bodies of a certain scale. The fractured-vuggy reservoirs are mainly controlled by faults and dissolution and are mainly developed near the eastern thrust fault on the right bank of the Amu Darya River, effectively guiding the direction for further exploration and development in this area.

Production and accumulation of reef framework by calcifying corals and macroalgae on a remote Indian Ocean cay

Abstract. Coral reefs face increasing pressures in response to unprecedented rates of environmental change at present. The coral reef physical framework is formed through the production of calcium carbonate (CaCO3) and maintained by marine organisms, primarily hermatypic corals, and calcifying algae. The northern part of Western Australia, known as the Kimberley, has largely escaped land-based anthropogenic impacts and this study provides important metabolic data on reef-building organisms from an undisturbed set of marine habitats. From the reef platform of Browse Island, located on the mid-shelf just inside the 200 m isobath off the Kimberley coast, specimens of the dominant coral (six species) and algal (five species) taxa were collected and incubated ex situ in light and dark shipboard experimental mesocosms for 4 h to measure rates of calcification and production patterns of oxygen. During experimental light and dark incubations, all algae were net autotrophic producing 6 to 111 mmolO2m-2d-1. In contrast, most corals were net consumers of O2 with average net fluxes ranging from −42 to 47 mmolO2m-2d-1. The net change in pH was generally negative for corals and calcifying algae (−0.01 to −0.08 h−1). Resulting net calcification rates (1.9 to 9.9 gCaCO3m-2d-1) for corals and calcifying algae (Halimeda and Galaxura) were all positive and were strongly correlated with net O2 production. In intertidal habitats around Browse Island, estimated relative contributions of coral and Halimeda to the reef production of CaCO3 were similar at around 600 to 840 gm-2yr-1. The low reef platform had very low coral cover of < 3 % which made a smaller contribution to calcification of ∼ 240 gCaCO3m-2yr-1. Calcification on the subtidal reef slope was predominantly from corals, producing ∼ 1540 gCaCO3m-2yr-1, twice that of Halimeda. These data provide the first measures of community metabolism from the offshore reef systems of the Kimberley. The relative contributions of the main reef builders, in these undisturbed areas, to net community metabolism and CaCO3 production is important to understand exclusively climate-driven negative effects on tropical reefs.

Responses of intertidal invertebrates to rising sea surface temperatures in the southeastern Indian Ocean

IntroductionThe west coast of Western Australia (WA) is a global hotspot for increasing sea surface temperatures and marine heatwaves.MethodsWe used visual survey transects to compare mollusc and echinoderm populations on three coastal intertidal platform reefs on the Perth shoreline with two intertidal platforms at the west end of Rottnest Island (32°S) which are under the influence of the southward flowing Leeuwin Current.ResultsIn 1983, temperate species dominated Perth coastal molluscan diversity, but the tropical mussel Brachidontes sculptus dominated density. Species richness on coastal platforms remained constant in 2007 and 2021, but total densities were lower in 2007 as B. sculptus declined; partial recovery occurred on coastal platforms in 2021. Tropical species were a significant component of mollusc diversity and density at Rottnest Island in 1982 and 2007. Total mollusc density declined by 98% at the island sites of Radar Reef and 86% at Cape Vlamingh and total echinoderm density by 52% and 88% respectively from 2007 to 2021; species diversity also declined sharply.DiscussionTropical species have moved southward in WA subtidal environments, but tropical, temperate and WA endemic species all suffered losses of biodiversity and catastrophic declines in density of 90% or more on the two Rottnest Island intertidal platforms. Data presented here provide a sound basis for exploring the possible causes of the catastrophic mortality at the west end of Rottnest Island and monitoring for recovery.

Lagoon infilling by coral reef sand aprons as a proxy for carbonate sediment productivity

AbstractSand aprons are distinctive landforms that offer important insights into sedimentary dynamics for reef platform development. Here we link temporal and spatial scales of 21 sand aprons in the southern Great Barrier Reef to understand their Holocene formation and evolution in response to relative sea-level changes, the depth of the Pleistocene base, and contemporary morphodynamics. Our results show that lagoon infilling is a function of reef size and is a self-limiting process controlled by hydrodynamics and relative sea-level changes. Lagoon infilling does not depend on the type of reef or degree of exposure to waves, but it could reflect past wave climates. Our carbonate productivity estimates based on lagoon infilling are remarkably similar to those inferred from habitat classification. Finally, we hypothesize that current carbonate productivity has slowed because of the effects of climate change.

Daily timing of low tide drives seasonality in intertidal emersion mortality risk

Sea level exerts a fundamental influence on the intertidal zone, where organisms are subject to immersion and emersion at varying timescales and frequencies. While emersed, intertidal organisms are exposed to atmospheric stressors which show marked diurnal and seasonal variability, therefore the daily and seasonal timing of low water is a key determinant of survival and growth in this zone. Using the example of shallow coral reefs, the coincidence of emersion with selected stressors was investigated for eight locations around the Australian coastline. Hourly water levels (1992 – 2016) from a high-resolution sea level hindcast (http://sealevelx.ems.uwa.edu.au), were linked to maximum surface solar radiation data from the Copernicus ERA5 atmospheric model and minimum atmospheric temperature observations from the Australian Bureau of Meteorology to identify seasonal patterns and historical occurrence of coral emersion mortality risk. Local tidal characteristics were found to dictate the time of day when low water, and therefore emersion mortality risk occurs, varying on a seasonal and regional basis. In general, risk was found to be greatest during the Austral spring when mean sea levels are lowest and a phase change in solar tidal constituents occurs. For all Great Barrier Reef sites, low tide occurs close to midday during winter and midnight in the summer, which may be fundamental factor supporting the historical bio-geographical development of the reef. Interannual variability in emersion mortality risk was mostly driven by non-tidal factors, particularly along the West Coast where El Niño events are associated with lower mean sea levels. This paper highlights the importance of considering emersion history when assessing intertidal environments, including shallow coral reef platform habitats, where critical low water events intrinsically influence coral health and cover. The study addresses a fundamental knowledge gap in both the field of water level science and intertidal biology in relation to the daily timing of low tide, which varies predictably on a seasonal and regional basis.

Commercial fishery assessment of Malaysian water offshore structure

To have a better understanding of the impact of the PETRONAS oil and gas platform on commercial fisheries activities, Universiti Malaysia Terengganu (UMT) examined two approaches which are data collection from satellite and data collection from fishermen and anglers. By profiling the anglers who utilize reefed oil and gas structures for fishing, it can determine if the design and location of the reef platforms will benefit or negatively impacts those anglers and fisherman. Furthermore, this assessment will be contributing to the knowledge regarding the value of offshore oil and gas platforms as fisheries resources. Collectively, the apparent fishing activity data included, combined with the findings in the reefing viability index will help to inform PETRONAS’s future decommissioning decisions and may help determine if the design and proposed locations for future rigs-to-reefs candidates would benefit commercial fishing groups, further qualifying them as appropriate artificial reef candidates. The method applied in this study is approaching by using a data satellite known as Google’s Global Fishing Watch technology, which is one of the applications to measure commercial fishing efforts around the globe. The apparent commercial fishing effort around the selected twelve PETRONAS platforms was analyzed from January 2012 to December 2018. Using the data collection from fishermen which is the total estimation of commercial fish value cost (in Malaysia ringgit, MYR [RM]) in Peninsular Malaysia Asset, Sabah Asset, and Sarawak Operation region. The data were extracted every month from 2016 to 2018 from the National Oceanic and Atmospheric Administration database. Most of the selected platforms that show a high frequency of vessels around the year are platform KP-A, platform BG-A and platform PL-B. The estimated values of commercial fishes varied between platforms, with ranged from RM 10,209.92 to RM 89,023.78. Thus, platforms with high commercial fish value are selected for reefing in-situ and will serve multi-purposes and benefit the locals as well as the country. The current study has successfully assessed the potential reefing area of the Malaysian offshore environment with greater representativeness and this paper focused on its potential as a new fishing ground.

Wave Hydrodynamics across Steep Platform Reefs: A Laboratory Study

This paper presents a laboratory study on wave transmission across steep platform reefs, aiming at furthering knowledge of wave hydrodynamics and establishing empirical formulations of spectral wave parameters across the reef flat. The ultimate aim of this study is to determine the design wave load to design fixed offshore structures on coral reefs flat. The process of wave transmission across the underground coral strip with a large front slope has been studied on a physical model in the wave trough with nearly 300 experimental cases combined from 05 underground band models and many random wave scenarios and scenarios at different submerged depths. Experimental results show that the abrupt transition in depth from deep water ahead to relatively shallow water in the reef is responsible for the difference in the wave at the top of the strip compared to the wave on the normal beach, especially regarding the statistical distribution of the wave height. Breaking waves at the abrupt transition have deviated the wave height distribution curve from the deep-water (Rayleigh) theoretical distributions and even the existing shallow-water distributions, including the effect of breaking waves. Two sets of empirical formulations of the spectral wave parameters (Hm0 and Tm-1, 0) are eventually derived for the surf zone and the region behind the surf zone, respectively. These local wave parameters can be used as inputs to a wave height distribution model for determining other design characteristic wave heights on steep platform reefs. Doi: 10.28991/CEJ-2022-08-08-015 Full Text: PDF

Variability of Shoreline Water Levels on an Atoll in the Maldives

AbstractMany atolls in tropical oceans are under the threat of sea‐level rise. Mitigation planning requires reliable estimates of increasing wave runup in response to sea‐level rise and changes in wave climate. However, shoreline dynamics on reef‐lined coasts, particularly the effect of low‐frequency waves, are not yet fully understood. Herein, we present a field study on the southernmost atoll of the Maldives to clarify the variability of shoreline water levels driven by waves and tides. The atoll is located just south of the equator and has a moderate wave climate; it is occasionally impacted by long‐distance swells from the South Indian Ocean. Wave evolution on a reef‐lined coast was observed in situ for a period of seven months. The shallow water depth tightly constrains reef‐flat waves under normal wave conditions; however, this constraint is broken under extreme conditions by low‐frequency waves generated from long‐period swells. Particularly, very low‐frequency waves (<0.005 Hz) are amplified via wave resonance over the reef platform and significantly contribute to shoreline motions. Our observation of shoreline motions confirmed that, under energetic conditions, extreme wave run‐ups are characterized by swell run‐ups riding on crests of low‐frequency waves. A combined approach of observations and analytical modeling revealed the long‐term variability of shoreline water levels, highlighting the stochastic nature of extreme water levels influenced by a joint probability of far‐field swells and high tides. Our results suggested that extreme shoreline water levels would increase at a significantly higher rate than sea‐level rise in the future.