Coral Reef Platforms Research Articles

Field observations of wave refraction and propagation pathways on coral reef platforms

ABSTRACTField experiments were conducted to investigate the refraction and propagation of ocean waves across two coral reef platforms in the Maldives, central Indian Ocean. A total of seven pressure sensors were deployed on each reef to quantify temporal and spatial variations in wave characteristics across the platform surfaces. Directional wave properties were calculated from high frequency (2 Hz) wave and current records obtained at two locations on each reef and corroborate theoretically predicted propagation pathways derived from an analytical wave refraction model. Results demonstrate that reef geometry critically controls the refraction and propagation behaviour of incident swell across the reef structures. Differences in the magnitude of refraction (approximately 57° and 14°) observed on each reef can be attributed to variations in platform shape and orientation to incident waves. Results demonstrate that reef flat wave patterns define the segmentation of platform surfaces into distinctive high and low wave energy zones. Furthermore, wave focussing has been identified as a major mechanism controlling the transformation of wave energy across the reefs. Results provide the first field‐based validation of wave refraction and convergence on coral reefs and have significant implications for sedimentation processes and the formation of platform deposits. Reef configurations which promote marked wave convergence are more likely to retain sediment on the reef surface, whereas platforms that induce less refraction and changes in the direction of incident waves have a higher potential for the off‐reef evacuation of sediment over leeward reef margins. Results of wave measurements substantiate such projections and provide a first order explanation for the existence and absence of a coral cay on the two study reefs. The study presents empirical evidence of wave refraction and convergence on coral reefs and establishes a baseline for future investigations of hydrodynamic process controls on platform sedimentation and island formation. Copyright © 2012 John Wiley & Sons, Ltd.

Analytical modelling of wave refraction and convergence on coral reef platforms: Implications for island formation and stability

An analytical model was constructed to simulate the refraction of waves on coral reef platforms comprising an idealised bathymetry of uniform depth and vertical reef faces. Model simulations were conducted to investigate the influence of key parameters such as reef shape and depth as well as wave period on the propagation behaviour of incident waves. Results of the refraction analysis demonstrate that different reef shapes produce characteristic patterns of wave convergence on reef surfaces. The location and stability of focal zones and hence wave convergence is largely controlled by the shape of platforms. Platform configuration further controls the distribution of wave energy across platform surfaces and determines the influence of incident wave forcing on different reef sections. Results have significant implications for sedimentation processes and hence the formation and stability of islands on reef platforms. Wave propagation patterns define sediment transport vectors and subsequently control the transport and deposition of different sized material. Platforms which promote marked wave convergence behaviour, such as elliptical and circular reefs, are more likely to retain sediment on reef surfaces, whereas narrow linear structures have a higher potential for the off-reef evacuation of sediment over leeward reef margins and the subsequent infill of deeper lagoonal areas. The study provides a physical basis for future investigations of reef hydrodynamics and platform sedimentation processes.

Black reefs: iron-induced phase shifts on coral reefs.

The Line Islands are calcium carbonate coral reef platforms located in iron-poor regions of the central Pacific. Natural terrestrial run-off of iron is non-existent and aerial deposition is extremely low. However, a number of ship groundings have occurred on these atolls. The reefs surrounding the shipwreck debris are characterized by high benthic cover of turf algae, macroalgae, cyanobacterial mats and corallimorphs, as well as particulate-laden, cloudy water. These sites also have very low coral and crustose coralline algal cover and are call black reefs because of the dark-colored benthic community and reduced clarity of the overlying water column. Here we use a combination of benthic surveys, chemistry, metagenomics and microcosms to investigate if and how shipwrecks initiate and maintain black reefs. Comparative surveys show that the live coral cover was reduced from 40 to 60% to <10% on black reefs on Millennium, Tabuaeran and Kingman. These three sites are relatively large (>0.75 km(2)). The phase shift occurs rapidly; the Kingman black reef formed within 3 years of the ship grounding. Iron concentrations in algae tissue from the Millennium black reef site were six times higher than in algae collected from reference sites. Metagenomic sequencing of the Millennium Atoll black reef-associated microbial community was enriched in iron-associated virulence genes and known pathogens. Microcosm experiments showed that corals were killed by black reef rubble through microbial activity. Together these results demonstrate that shipwrecks and their associated iron pose significant threats to coral reefs in iron-limited regions.

“Stones from the dangerous winds”: reef platform mega-clasts in the tropical Pacific Islands

Tsunamis and intense tropical storms compete as viable mechanisms for catastrophic marine inundations of coastlines, and so interpretation of deposits laid down by palaeo-events is often problematic. Here, examples are presented from the tropical South Pacific Islands to demonstrate how indigenous knowledge may provide vital clues supporting sedimentological investigation and thereby helping to explain the origin of giant boulders emplaced on top of coral reef platforms.

Forcing of resonant modes on a fringing reef during tropical storm Man‐Yi

Open basin resonant modes have been observed at tidal frequencies on coastal shelves, but their excitation on coral reef platforms has only been suggested. The topography of and water depth over most fringing reefs correspond to resonant periods that are the order of tens of minutes and fall outside of the energetic part of the wave spectrum. During tropical storm Man‐Yi, low frequency near resonant oscillations dominated the variance of the sea surface elevation at the shoreline of Ipan reef, Guam. The excitation of the resonant modes resulted both from a large increase in water level over the reef due to wave setup, which increased the resonant frequencies, and the occurrence of wave group forcing with time scales commensurate with these increased resonant frequencies.

Phosphorus speciation in the sediment and mass balance for the central region of the Great Barrier Reef continental shelf (Australia)

Solid phase P speciation has been determined in sediments from a transect across the central section of the continental shelf and slope of the Great Barrier Reef (GBR) lagoon. This region is characterized by a gradient of riverine aluminosilicate clay and silt nearshore, seawards of which biogenic carbonate sediment predominates. Phosphorus speciation results show large variations along this transect. Organic P and authigenic (apatite) P are the major chemical forms of phosphorus in the central GBR continental shelf sediments. Post-depositional reorganization of P was also observed, converting organic P and iron bound P (Fe–P) to authigenic (apatite) P. Phosphorus burial rate was estimated from measurements of total P concentration and excess 210Pb sediment mass accumulation rates. Burial efficiency varies significantly over the shelf. Inshore areas showed significant P remobilization from sediments to the water column (up to ∼50%). The mid and the outer shelf showed little evidence for remobilization (except for coral reef platform sediments), with more of the sediment P being in the less reactive authigenic apatite phases. An appreciable fraction of this non-labile authigenic apatite phase was identified as fish bone. P sources and sinks over the central part of the GBR shelf were quantified using a mass balance approach. This showed that Coral Sea shelf edge upwelling events are essential to satisfy the large P nutrient demand of the whole GBR lagoon. P inputs due to upwelling events were greater than those contributed by local rivers over an average year.

Scaling water motion on coral reefs: from regional to organismal scales

To live successfully in wave-swept habitats, plants and animals must be able to survive, consume resources, and reproduce in the presence of incessant, variable and often unpredictable levels of water motion at a range of scales. However, there is a relatively poor understanding of water motion in natural habitats at the scales necessary to determine its potential physiological and ecological consequences. Using an historic record of hourly wind conditions, a depth profile and two rigorously tested oceanographic models, 37-years of hourly wave driven water motion were hindcast spatially on a typical subtidal coral reef platform (maximum horizontal displacement, velocity and acceleration per wave cycle). For larger waves, those likely to constitute ecological disturbances, around 95% of the wave’s height that is lost over the whole reef occurs within the first 50 m of the crest. The field-validated model of spatiotemporal variation in water motion provided a framework for quantitatively predicting several physiological and ecological effects of wave motion, such as nutrient and gas fluxes and mortality rates from hydrodynamic disturbances. It also suggested a sharp ecological transition between a crest habitat in which disturbance-mediated coexistence mechanisms are important, and a flat habitat in which they are much less important.

Recent Developments on a Nearshore, Terrigenous-Influenced Reef: Low Isles Reef, Australia

Abstract Low Isles Reef is the most southerly located of 46 coral reef platforms unique to the inner shelf of the northern Great Barrier Reef Province, Australia, which support both sea grass and mangrove growth. Such reefs develop in areas that are influenced by river flood plumes and where interreef sediments are dominated by terrigenous mud. Low Isles Reef has long been a popular tourist destination. Informal reports of decreasing visibility, a decline in scleractinian corals, and increases in soft coral and macroalgae have sparked speculation that agricultural activities in coastal catchments are affecting the reef. Comparison of the modern surface of Low Isles Reef with historical surveys and photographs dating back to 1928 allows quantification of modern sedimentary processes, rates of change, and factors influencing reef development. Results indicate that changes on Low Isles Reef are related to remobilization of coarse sediment during storm events and gradual shoreline retreat associated with risi...

Spatial and temporal variations in wave characteristics across a reef platform, Warraber Island, Torres Strait, Australia

A field experiment was conducted at Warraber Island, Torres Strait, Australia to investigate spatial and temporal variations in wave characteristics and energy across a mesotidal coral reef platform. Measurements of water depth were obtained using five pressure sensors deployed across a 2.7-km section of reef flat from July 3–5, 2001. The reef surface was uneven and consisted of an outer reef flat, a central reef flat depression, an inner reef ramp, a palaeo-reef surface and the shoreline. Water levels decreased landward across the platform with tide ranges at the shoreline being almost 50% lower than at the outer reef flat. Rising and falling tides were characterised by a bimodal energy distribution with both short-period (0–3 s) and wind (3–8 s) waves present. Higher water levels were dominated by wind waves. The highest waves occurred at high tides associated with nocturnal tidal cycles with Hs decreasing from 0.5 to 0.2 m from the outer reef flat to the shoreline. Wave energy at swell (8–20 s) and infragravity (>20 s) frequencies was negligible across the reef platform although there was evidence of wave groups at higher water levels.Reef geometry and changes in water level determine the magnitude of wave energy on the reef platform. Up to 85–95% of incident wave energy is attenuated by the central reef flat depression at high and low tide, respectively, and strong linear relationships exist between Hs and h at all locations. Both wave height and wave type are strongly depth dependent. Critical reef rim depths required to produce Hs of a given size vary spatially across the reef rim due to variations in reef topography. A distinct depth related threshold exists at which short-period and wind wave dominance reverses. Over a 14-day spring-neap tidal cycle, the time of occurrence of wave action diminishes across the reef platform to the shoreline. Larger waves (Hs=0.2 m) occur for only 9% of time at the outer reef flat and for less than 0.5% over the remaining reef platform. This implies that on mesotidal reef platforms, sediment entrainment and transport are severely constrained under normal wave energy conditions and significant change is likely restricted to extreme events.

Holocene sediments of Wistari Reef: towards a global quantification of coral reef related neritic sedimentation in the Holocene

Wistari Reef, within the southern Great Barrier Reef, is a shallow coral reef platform featuring a very clearly defined leeward accretionary wedge of carbonate sediments. The total global area of shallowly submerged coral reef has been quantified as 255 000 km 2. The question then becomes: ‘What additional area of sediment of significant thickness is associated with the measured shallow reef areas?’ At Wistari Reef, the leeward sedimentary wedge has an area and a thickness that are roughly equal to the Holocene sediments that have accumulated on the platform. Several important observations can be made from these data. Firstly, the area of significant neritic carbonate sedimentation (>1 m/ka) associated with coral reefs is near 500 000 km 2. Secondly, the production rate of neritic carbonates at Wistari Reef is almost 50% less than the accumulation rate needed to obtain the volume of Holocene reef sediments observed. This implies that both production and accumulation neritic carbonate must have been more than a factor of two higher in the early to mid Holocene.

The structure of the Easter Platform, Houtman Abrolhos reefs: Pleistocene foundations and Holocene reef growth

Seismic reflection profiles have been used to expand an evolutionary model developed from a drilling and dating program of the Easter Platform, one of three shelf-edge coral reef platforms on the western margin of Australia. The Houtman Abrolhos reefs are surrounded by an open shelf which is mantled by cool-water carbonate sediments, and exist in a temperate-tropical transition zone, in conditions which are close to the limits for coral reef development. The Easter Platform has distinct windward and leeward reefs, and a lagoon divided by a central platform consisting of several small islands. Reef limestones of the central platform are calcretised, and are Last Interglacial in age, whilst the windward and leeward reefs, and associated “blue-hole” and reticulate reef terrains, are Holocene features. Seismic investigations indicate that Holocene reef growth was strongly asymmetrical about the antecedent Last Interglacial reef platform. The windward reef front is dominated by relict reef morphology, and relatively thin (ca. 9 m or less) Holocene reefs have recolonised the windward margin of the antecedent platform at the site of the present-day windward reef. Most Holocene reef growth took place on the lee-side of the antecedent platform, and colonised an essentially flat surface at SL −40 m, which has been seismically identified as the Last Interglacial surface. Up to 40 m of interpreted Holocene reef overlies this monotonous surface, throughout the leeward reef and associated “blue-hole” and reticulate reef terrains, demonstrating that the antecendent surface has had little influence on the processes and morphology of Holocene reef construction. In addition to the Last Interglacial reflector, three deeper subparallel reflectors at SL −46 m, −52 m and −60 m, interpreted to represent calcrete horizons, are present beneath the leeward lagoon and leeward reef terrains, and are traceable to beneath the adjacent shelf surface. The unconformity-bound sequences so defined are thin (6–8 m) and blanket-like and are likely to be shelfal equivalents of pre-Holocene reef buildup phases found within the central platform.

Hydraulic roughness of coral reef platforms

This paper describes the derivation of the hydraulic roughness of a large, fixed bed, coral reef platform from field measurements. These measurements were made in near ideal conditions, where oscillatory waves propagated in water of constant depth, such that the only active wave transformation process present was that of wave bed friction damping. The derived roughness value enables relative roughness and friction factor values to be determined on this, or any other similar reef, for any combination of wave and water depth conditions ( T,H,h). The paper discusses the influence a low magnitude current may have had on the results, and finds it to be negligible. Some comparisons with movable beds are also made.

Microatoll microbialites of Lake Clifton, Western Australia: Morphological analogues ofCryptozoön proliferum Hall, the first formally-named stromatolite

The Linnaean nameCryptozoon proliferum Hall was proposed in 1883 for a previously undescribed life-form preserved in spectacular exposures of Cambrian limestones in New York State, USA. It is now recognised that these are exposures of stromatolitic microbialites, laminated organosedimentary structures formed from interaction between a benthic microbial community (BMC) and the environment. Microbialites are neither fossil organisms nor trace fossils. These complex structures are the products of dissipative, self-organising systems involving a BMC, the external environment and the accreting microbialite. Functionally analogous BMCs of different species compositions may build similar structures in similar environments in quite separate periods. The type exposures ofCryptozoon proliferum show objects composed of complex, concentric rings, up to a metre in diameter, that have grown laterally without any restriction other than that provided by neighbouring structures. They are not the relicts of domes truncated by penecontemporaneous erosion or Pleistocene glaciation, but depositional forms in which upward growth was restricted. Analogous modern structures occur on a reef platform along the north east shore of hyposaline Lake Clifton, Western Australia. These are tabular thrombolitic microbialites that vary lakeward across the reef platform from low, compound structures to discrete, concentric structures up to 50 cm high. The Lake Clifton forms are, in turn, morphological analogues of microatolls found on coral reef platforms. Coral microatolls are coral colonies with flat, dead tops and living perimeters in which upward growth is constrained by the sea surface. In shallow water they form circular rims of laterally growing coral around a dead centre. In deeper water they form coral heads that develop flat tops on reaching sea level. It is concluded that both the tabular microbialites of Lake Clifton and the type exposures ofCryptozoon proliferum are analogous to coral microatolls in both form and origin-structures that have been able to grow laterally, but in which upward growth is restricted by subaerial exposure. Similar microatoll microbialites have been described from other modern environments, including Great Salt Lake, Utah, USA and Stocking Island, Exuma Cays, Bahamas. Ancient examples may include some of the “tufa” deposits of the Basal Purbeck Formation in Dorset, UK, as well as the coalesced domal bioherms of the Upper Cambrian Arrinthrunga Formation of the Georgina Basin, Central Australia, and the “washbowl” structures described from the Batsfjord Formation of the Varanger Peninsula, north Norway. Progress towards a reliable interpretation of ancient microbialites depends on an understanding of modern environments in which analogous structures are forming. This study of microatolls has demonstrated that other sessile life forms may create colonial ecomorphs that, used cautiously, can serve as analogues for understanding the factors controlling the growth and form of microbialites. The surprising lack of pre-Pleistocene examples of microatolls recorded to date has simply been due to their lack of recognition in the geological record. They occur in sequences from the Proterozoic onwards, and provide powerful environmental indicators of ancient reef platforms on which biological growth was adjusted to contemporary sea level.

Mediterranean Miocene Carbonates: Facies Models and Diagenesis: ABSTRACT

Miocene carbonates can bridge the gap between Holocene and older carbonate sequences, thus enhancing understanding of depositional and diagenetic patterns. Miocene carbonates can bridge this gap because of their similarity to Holocene counterparts and the ease of using these carbonates to reconstruct tectonic, paleogeographic, and paleoclimatic settings. In the Mediterranean, the Miocene provides a superb set of exposures and a wide variety of facies models in different geologic settings. Mediterranean Miocene carbonates contain three major types of platform facies: coral reefs, macroforaminifer-rodolithic carbonates (rodalgal facies), and molluscan-bryozoan-foraminifer calcarenites (foramol facies). A combination of interrelated factors (e.g., water depth, temperature, and nutrients) control the distribution and lateral vertical transitions of these platform types. The rodalgal facies is widespread and occurs as a transition between the coral reef platform and the foramol platform. Modern carbonate sedimentation in the Mediterranean provides instructive analogies for many varieties of foramol and rodalgal facies. The most extensive porosity type is a combination of secondary intergranular and moldic porosity with chalky microporosity, locally in association with minor primary integranular porosity. This porosity is normally associated with dolomitization and is interpreted as having originated in intermediate burial environments. Miocene coral reefs were particularly abundant and well developed in the late Miocene, before and during the Messinian salinity crises and basinal evaporite deposition. These events implied drastic variations in sea level, water chemistry, and nutrients, and coincided with high sedimentation rates in coastal areas. To survive these adverse conditions, coral reefs grew very fast, with spacially successful colonial morphologies and intense carbonate cementation. Many coral reef sections present marked cyclicity with repeated intercalations of exceptionally well-developed stromatolitic carbonates. Present outcrops record details comparable to Quaternary reefs, as well as details of the facies geometry of the different reef complexes and their responses to Miocene sea level oscillations. End_of_Article - Last_Page 1440------------

Distribution of seagrasses and associated juvenile commercial penaeid prawns in north-eastern Queensland waters

Thirteen species of seagrass and six species of juvenile penaeid prawn of commercial importance were found in seagrass beds in waters between Cape York and Cairns. The pioneering seagrass species, Halodule uninemis (Forsk.) Aschers, and Halophila ovalis (R. Br.) Hook. f., were present in almost all coastal areas. Two seagrass species often dominant in the tropics, Enhalus acoroides (L.f.) Royle and Thalassia hemprichii (Ehrenb.) Aschers., were uncommon. This is likely to have resulted from environmental stress during periods of reduced salinity and high turbidity associated with seasonal summer rainfall. Halophila decipiens Ostenfeld was the only species found in depths &gt; 11 m. Three depth-related zones of species are described. With the exception of E. aroroides, most seagrasses were &lt;20 cm in height. Seagrass biomass was greatest in 2-6 m of water. Maximum dry weight of above-ground plant material and maximum leaf area from a square metre of bottom was 99.6 g and 4.5 m2, respectively, at the Flinders Group of islands. The distribution of prawns and seagrasses did not show a trend with latitude. Penaeus esculentus Haswell and Metapenaeus endeavouri (Schmitt), the most numerous penaeid prawn species in commercial trawl catches, were also the most numerous juvenile prawns in seagrass beds. Juvenile P. longistylus Kubo were found for the first time in northern Queensland seagrass beds. Although juveniles of this species appeared to occupy only a narrow habitat range, these habitats were found in coastal bays, in estuaries and on coral-reef platforms where conditions were suitable. Size-class data suggest that reef prawns and coastal prawns are parts of the same population. The timing of the life cycle of the major commercial prawn species differed from that found previously in the Gulf of Carpentaria.

Microhabitat factors affecting abundance and diversity of Conus on coral reefs.

Frequency and per cent cover of particular substratum types are shown to be important factors influencing abundance and diversity of mobile, predatory benthic invertebrates on spatially complex coral reef platforms. The most favorable portions of reefs for the gastropod Conus have <20% cover of algal-bound sand and <20% cover of living coral. The former microhabitat provides diurnal shelter for smaller Conus, harbors dense prey populations, and is typically interspersed with sand- and rubble-filled depressions in which many of the gastropods shelter during the day. In Micronesia and Australian Great Barrier Reef censuses, 39 1x10-m transects on favorable habitat supported an average of 7 Conus of 3 species. In striking contrast, portions of reef with >20% living coral are most unfavorable for Conus. Here density averaged 0.3 individual/10 m2, in 17 transects. Living coral harbors few suitable prey organisms, and contact with it elicits a strong avoidance response by Conus. Comparison of subtidal reef and intertidal bench habitats indicates that species diversity and population density are inversely related, but within both habitat types these attributes of Conus assemblages are positively associated.

Geology and Origin of the Florida Keys

The Florida Keys form a crescentic chain of small limestone islands which extend from near Miami to Key West, a distance of about 150 miles. They are made of two main formations of Pleistocene age—the Key Largo Limestone and the Miami Limestone. The former, named and described by Sanford, is an elevated coral reef rock, and the latter, also described by Sanford, is an oolitic limestone in this area. The Key Largo Limestone is the surface rock of the Upper Keys, and the Miami Limestone covers the Lower Keys. A contact found at Big Pine Key shows that the oolitic limestone overlaps the Key Largo, and core borings show that the Key Largo underlies the oolitic cover for the entire area of the Lower Keys. The Key Largo, therefore, extends for the total length of the Keys. A core placed at Dry Tortugas, 70 miles west of Key West, encountered it at 30 feet below sea level. The Key Largo has a maximum thickness of over 200 feet. The fact that the Keys are located about 5 miles from the seaward edge of the Florida coral reef platform creates an interesting problem. They may be (1) remnants of an ancient outer reef similar to that found today bordering the living coral reef tract, or (2) they may have become established as a line of patch reefs in the back reef zone of a pre-existing platform which extended seaward some distance, possibly as far as the present platform. It is believed that the answer to this problem can be found through an understanding of what was happening seaward and leeward of the old reef at the time its corals were flourishing. Consequently, the composition and structure of the reef platform seaward of the Keys and the geologic history of the rocks of the Florida mainland immediately behind the Keys were studied. Information thus obtained plus observations on the composition, especially the coral content, of the Keys themselves have led the authors to believe that the Keys were formed as a line of patch reefs in a back reef area which was bordered on its seaward edge by an outer reef which has since been lowered, chiefly by erosion, and covered by more recent material.

Diurnal changes in oxygen content of the water over the coral reef platform at Heron I

Heron I., in the Australian Great Barrier Reef, is a small sand cay situated near the western end of a reef 6.5 miles long. The area investigated was south-west of the cay and approximately level, with extensive areas of living coral in the seaward half. Larger algal species were common near the cay but were not in evidence in the areas of active coral growth. Oxygen levels were determined without sampling using a membrane-enclosed polarographic probe. Results are presented as profiles across the platform and also as time sequences in the main draining channel. Some deep water results are included. Oxygen production and consumption were associated primarily with areas of rich coral growth and hence larger algae seemed unimportant in the overall oxygen exchange. Low tide oxygen levels in less than 1 ft of water ranged from 2.1 mg O2/l for a spring tide after midnight to more than 10.8 mg O2/l (the limit of the instrument) in the early afternoon. High tide levels in more than 6 ft of water ranged from 9.0 mg O2/l in the early afternoon to 6.4 mg O2/l 2hr after sunset. High tide readings were not taken late at night. There is some evidence that water already reduced or enriched in oxygen content on the previous tide was returned in significant quantities to the reef platform.

Primary Organic Productivity of a Hawaiian Coral Reef1

Primary organic productivity was determined by measuring changes in oxygen concentration of sea water flowing over a fringing coral reef at Kapaa, Kauai, Hawaii. Prevalence of a rather strong, unidirectional current facilitated use of this method.Determinations were made both in summer (July) and winter (November). The reef is autotrophic, as were two previously studied atoll reefs in the Marshall Islands. Gross primary productivity is about 2,900 g C/m2/yr. This is in impressive agreement with results obtained on the atoll reefs, which are the only previous measurements of coral reef productivity.All three results are considerably higher than determinations of productivity of open ocean waters. The authors attribute this difference to photosynthesis by benthic algae on the coral reef platform.