Reef Crest Research Articles

Vertical and horizontal distributions of coral‐reef fish larvae in open water immediately prior to reef colonization

To explore the vertical and horizontal distributions of fish larvae near the end of their pelagic period, six light traps were set up over four lunar months at different depths (sub-surface, midwater and bottom) and different habitat types (reef slope: 50 m horizontal distance from the reef crest; frontier zone: 110 m horizontal distance; sandy zone: 200 m horizontal distance) on the outer reef slope of Moorea Island, French Polynesia. The highest captures were in sub-surface traps on the reef slope and the frontier zone, and in bottom traps on the sandy zone and the frontier zone. It is hypothesized that fish larvae move towards the surface near the reef slope to avoid reef-based planktivores and to get into a favourable position for surfing over the reef crest.

Pair formation in the herbivorous rabbitfish Siganusdoliatus

This study investigated the basis of pair formation in the abundant herbivorous rabbitfish Siganus doliatus on Orpheus Island, Great Barrier Reef. Pair formation was the most common social system in S. doliatus, with 67.4% of all individuals occurring in pairs. Pairs were stable (i.e. individuals remained with the same partner throughout the study) and pair members were found within 5 m of each other 82.9% of the time. Of the examined pairs, 25% were homosexual resulting in a proportion of heterosexual pairs (75%) that was significantly lower than expected if pairs were formed solely for reproductive reasons. Therefore, although reproduction appears to be the main driver of pair formation in S. doliatus, other factors are likely to influence this behaviour. The high density of individuals on the reef crest (5.7 ± 0 .9 individuals 200 m(-2); mean ± s.e.) and extensively overlapping home ranges of pairs indicated that the defence of territories plays no role in pair formation. Instead, it appears that pair formation in S. doliatus is driven, in part, by other, non-reproductive, ecological factors. It is suggested that pair formation allows for increased vigilance against predation and enables S. doliatus to execute a novel feeding behaviour.

Influence of turbidity regime on larval settlement patterns of stony coral Pocillopora damicornis in Polhena reef, Sri Lanka

Coral population structure is determined mainly by the larval settlement and recruitment pattern of sclerectenian corals. This pattern is mainly influenced by various environmental parameters such as temperature, pH, salinity, depth, current velocity and direction, tidal variations and turbidity. Experiments reported here examine the impacts of turbidity on the spatio-temporal distribution patterns of larval settlement of Indo-Pacific coral, Pocillopora damicornis at Polhena reef of Southern Sri Lanka. The experiment was conducted in four sites. Depending on the topography and the characteristics of the reef substrate each site was further divided in to three locations as reef crest, mid reef flat and reef lagoon. Terracotta tiles were attached to quantify larval settlement rates. Tiles were retrieved in monthly intervals and observed for P. damicornis larval settlements. Turbidity measurements were taken from the same locations. Larval settlement and turbidity showed a significant temporal (P DOI: http://dx.doi.org/10.4038/sljas.v15i0.5484 Sri Lanka J. Aquat. Sci. 15 (2010): p.45-60

Estimating submarine groundwater discharge in a South Pacific coral reef lagoon using different radioisotope and geophysical approaches

The multi-scale and temporally variable nature of submarine groundwater discharge (SGD) has led to various ways of characterizing SGD fluxes. In this study, qualitative and quantitative methods were applied to describe SGD in a South Pacific coral reef lagoon and to determine the limitations and advantages of these ways of characterizing SGD fluxes. Synoptic and continuous monitoring of 222Rn and salinity were used over nested spatial and temporal scales and, electrical resistivity (ER) imaging was used to infer surface and groundwater salinity distributions throughout the lagoon. Moored deployments were used to estimate the lagoon residence time using ratios between radium isotopes. Spatial surveys indicated between a five and ten-fold decrease in 222Rn away from the beach face. The magnitude of the decrease was dependent on the survey scale. However, only a high resolution medium scale survey was able to detect a zone of higher 222Rn water towards the reef crest. The high 222Rn concentrations towards the reef crest corresponded to areas of higher ER and lower temperatures which indicated a point source SGD approximately 350m off the beach. Concentrations of 222Rn and salinity during the spatial surveys and time series at each location were affected by the level of seawater recharge, the terrestrial hydraulic gradient, point source SGD, and the concentration of 222Rn laden creek water. Radioisotope measurements were used in a steady-state model, a non-steady state model and an exit point model to calculate SGD rates into the lagoon. The steady-state model resulted in the lowest estimate of 7835m3∙day−1. The non-steady state model, using the near-shore and mid-lagoon 24hour time series 222Rn concentrations, gave SGD volumes of 58422m3day−1 and 28259m3day−1, respectively. The exit point model indicated a rate of 31278m3day−1. The areal-averaged SGD fluxes from the three models ranged between 0.2 and 1.8cmday−1. The suitability of each model for quantifying fluxes rates depended on a range of factors including the presence or absence of a dominant or local seepage zone, temporal variability, spatial heterogeneity, and the degree of mixing of low 222Rn offshore water. This study highlighted the need to employ different methods and sampling strategies to qualitatively and quantitatively characterise SGD fluxes occurring over multiple spatial and temporal scales.

In Situ Coral Reef Oxygen Metabolism: An Eddy Correlation Study

Quantitative studies of coral reefs are challenged by the three-dimensional hard structure of reefs and the high spatial variability and temporal dynamics of their metabolism. We used the non-invasive eddy correlation technique to examine respiration and photosynthesis rates, through O2 fluxes, from reef crests and reef slopes in the Florida Keys, USA. We assessed how the photosynthesis and respiration of different reef habitats is controlled by light and hydrodynamics. Numerous fluxes (over a 0.25 h period) were as high as 4500 mmol O2 m−2 d−1, which can only be explained by efficient light utilization by the phototrophic community and the complex canopy structure of the reef, having a many-fold larger surface area than its horizontal projection. Over diel cycles, the reef crest was net autotrophic, whereas on the reef slope oxygen production and respiration were balanced. The autotrophic nature of the shallow reef crests implies that the export of organics is an important source of primary production for the larger area. Net oxygen production on the reef crest was proportional to the light intensity, up to 1750 µmol photons m−2 s−1 and decreased thereafter as respiration was stimulated by high current velocities coincident with peak light levels. Nighttime respiration rates were also stimulated by the current velocity, through enhanced ventilation of the porous framework of the reef. Respiration rates were the highest directly after sunset, and then decreased during the night suggesting that highly labile photosynthates produced during the day fueled early-night respiration. The reef framework was also important to the acquisition of nutrients as the ambient nitrogen stock in the water had sufficient capacity to support these high production rates across the entire reef width. These direct measurements of complex reefs systems yielded high metabolic rates and dynamics that can only be determined through in situ, high temporal resolution measurements.

Seasonally changing habitat use patterns among roving herbivorous fishes in the southern Red Sea: the role of temperature and algal community structure

Coral reefs are characterized by intense herbivory. Spatial patterns in herbivory—particularly along the depth gradient—influence the distribution and abundance of algae. Depth gradients in herbivorous reef fishes are generally assumed to be temporally stable, but this assumption has rarely been questioned. Here, we use underwater visual census and herbivore exclusion experiments to study the community composition and temporal patterns in habitat use by roving herbivorous fishes in an environment characterized by profound seasonal changes in algal biomass and distribution and extreme summer temperatures. Among the 18 species of roving herbivores recorded, parrotfishes were dominant in species richness and biomass, while regional endemic species represented 77 % of the total biomass. During most of the year, roving herbivores aggregate in the shallow reef zones and their biomass declines with depth. The herbivore community on the reef flat is distinct from that in deeper zones. The former is characterized by Siganus rivulatus, Acanthurus gahhm andHipposcarus harid, while the deeper reef zones are characterized by S. ferrugineus, Chlorurus sordidus and Ctenochaetus striatus. In summer, the distinct community structures among reef zones are lost as reef flat herbivores tend to exploit deeper reef zones and some reef crest species venture on to the reef flat. This summer change in herbivore distribution is also reflected in reduced turf biomass and increased yield to herbivores in the deeper reef zones. Habitat use is related to the feeding mode such that browsers dominate the reef flat and scrapers the reef crest, while the seasonal changes correspond to changes in availability of targeted algal resources. These seasonal changes appear to be driven by the extreme temperatures in summer, reaching 36 °C on the shallow reef flat.

Effects of reef roughness on wave setup and surf zone currents

ABSTRACT Franklin, G.L., Marino-Tapia, I. and Torres-Freyermuth, A., 2013. Effects of reef roughness on wave setup and surf zone currents. Circulation in reef systems controls an important number of key processes, such as the transport and dispersion of larvae, nutrients, and sediments. In fringing reef systems, where a shallow lagoon backs onto the reef crest, circulation appears to be dominated by wave breaking. Despite the identified importance of wave driven flows within reef systems, there is a lack of detailed knowledge on these processes within the surf zone of reef crests and the effects reef roughness has on these flows. A numerical study, using two-dimensional (2DV) simulations, was carried out in order to improve the understanding of wave-driven flows in the surf zone of a fringing reef. The model used is Cornell Breaking Wave and Structures (COBRAS), which solves the Reynolds-Averaged Navier-Stokes (RANS) equations with a turbulence closure scheme. The effect of different reef roughness on wav...

Particulate nutrient fluxes over a fringing coral reef: Source‐sink dynamics inferred from carbon to nitrogen ratios and stable isotopes

We examined spatial and temporal variations in particulate organic matter (POM) dynamics over a fringing coral reef (Ningaloo Reef) in Western Australia during the austral autumn and spring. Total POM concentrations generally did not differ between seasons or reef zones, but the composition of POM, in terms of carbon isotope ratios (δ13C‐POM), carbon to nitrogen ratios (C : N), and fatty acids, changed consistently in water flowing across the reef. Both δ13C‐POM and C : N increased from the fore reef to the reef flat and lagoon, −23.0‰ to −20.1‰ and 7.31 to 8.34, respectively. Average rates of net POM uptake by the reef community were highest over the reef crest (4 to 30 mmol N m−2 d−1 and 6 to 130 mmol C m−2 d−1), with a Bayesian isotope model confirming independent measurements of high uptake rates of allochthonous POM (oceanic phyto‐ and zooplankton). In contrast, over the reef flat, net release of POM was observed (−4 to −5 mmol N m−2 d−1 and −50 mmol C m−2 d−1), with gross release rates (estimated as −6 to −8 mmol N m−2 d−1 and −30 to −90 mmol C m−2 d−1) indicating that the release of autochthonous POM may be of similar magnitude to allochthonous uptake. Examining POM dynamics in terms of gross fluxes reinforces the dependence of coral reef systems on oceanographic processes for allochthonous POM supply, as well as highlighting the potential for autochthonous POM production to supply nutrients to benthic and pelagic communities downstream.

Depth transect of an Upper Triassic (Rhaetian) reef from Gosau, Austria: Microfacies and community ecology

In the Late Triassic (~235–201.3Ma) scleractinian corals and hypercalcified sponges built large, diverse reef ecosystems, the most famous of which are the Dachstein reefs. This study presents a depth transect along an Upper Triassic (lower Rhaetian) Dachstein reef from the Gosausee margin of the Dachsteingebirge (Gosau, Austria). The Gosausee microbial-sponge-coral reef is a complete barrier reef with an almost continuous fore reef to lagoon transect preserved (a very rare occurrence for the Triassic), and thus provides a window into depth zonation of Dachstein-type reef facies and biotic succession. The Gosausee reef facies exhibit strong depth control and five classic reef facies or zones are identified: the fore reef, reef front, reef crest, back reef, and lagoon facies. Thin, rare microbial fabrics and a high abundance of fine-grained, mud-rich skeletal wackestones (transported reef debris) characterize the deepest fore reef. As the reef shallows, muddy sediments decrease in abundance and are replaced by microbial fabrics, corals, and cements. Abundant sponges, microbial crusts, and thick, marine cements typify the reef crest, whereas microbialite-coated phaceloid corals are dominant in the back reef facies. Heavily cemented oncoids or microbial-sponge bindstones are characteristic of the lagoon. Based on their compositional and biotic similarities, the Gosausee reef was likely part of the same barrier reef systems as the source reef for the Gosaukamm reef breccia (one of the classic Norian–Rhaetian Dachstein reefs). The reef zones of the Gosausee margin can be used to interpret the depth or reef zone of less well preserved reef fragments, can inform models of community ecology and niche utilization in the Late Triassic, and highlight the need for additional research into the environmental factors that controlled biotic distribution in Upper Triassic reefs.

Evolution of Coral Rubble Deposits on a Reef Platform as Detected by Remote Sensing

An investigation into the evolution of coral rubble deposits on a coral reef platform is assessed using high-resolution remote sensing data and geospatial analysis. Digital change detection analysis techniques are applied to One Tree Reef in the southern Great Barrier Reef by analysing aerial photographs and satellite images captured between 1964 and 2009. Two main types of rubble deposits were identified: (1) rubble flats that are featureless mass accumulations of coral rubble; and, (2) rubble spits that are shore-normal linear features. While both deposits prograde in a lagoon-ward direction, rubble spits move faster (~2 m/yr) than rubble flats (~0.5 m/yr). The volume of rubble, the underlying substrate, the energy regime, and storm frequency control the rate of progradation. Rubble flat occurrence is restricted to the high-energy (windward) margin of the coral reef platform, while rubble spits are distributed reef wide, both in modal high energy and modal low energy regions of the reef. Rubble spit deposition is considered to be a result of enlarged spur and groove morphology of the forereef, whereby wave energy is focused through the enlarged groove formations causing the preferential deposition of coral rubble in particular zones of the adjacent reef flat. One last control is thought to be the elevation of the reef crest whereby lower areas are more prone to rubble flat development. A vertical and ocean-ward accumulation of rubble is occurring on the windward margin of the reef leading to a build-up and build-out of the reef, governing the expansion of the reef footprint. This study shows for the first time the evolution of a coral reef rubble flat and rubble spits over decadal time scales as detected through remotely sensed images spanning 45 years.

Numerical modeling of low-frequency wave dynamics over a fringing coral reef

Low-frequency (infragravity) wave dynamics on a fringing coral reef were investigated using the numerical model XBeach (Roelvink et al., 2009). First, the skill of the model was evaluated in one- and two-dimensions based on its predictions of short waves (0.04–0.2Hz), infragravity waves (0.004–0.04Hz) and water level measurements (tidal and wave setup) obtained during a 2009 field study at Ningaloo Reef in Western Australia. The model calibration was sensitive to friction coefficients for short waves and current/infragravity bed friction, which were assumed independent in this model study. Although the one-dimensional cross-shore model captured the gradients in the dominant hydrodynamic processes at the site, a high current/IG bed friction coefficient was required. This resulted in an overestimation and a phase lag between the observed and predicted wave setup signal. In the two-dimensional model, a lower (more realistic) current/infragravity wave friction coefficient was required to achieve optimum performance due to the presence of significant reef and lagoon mean flows in the model, which led to reduced setup across the reef. The infragravity waves were found to propagate from the surf zone across the reef in a dominantly cross-shore direction towards the shore, but with substantial frictional damping. The infragravity waves were strongly modulated also over the reef by tidal depth variations, primarily due to the variability in frictional dissipation rates when the total water depth over the reef varied. Two mean wave-driven circulation cells were observed in the study area, with cross-shore flow becoming more alongshore-dominated before exiting the system via the two channels in the reef. The results reveal that short waves dominated bottom stresses on the forereef and near the reef crest; however, inside the lagoon, infragravity waves become increasingly dominant, accounting up to 50% of the combined bottom stresses.

MODELLING INFRAGRAVITY WAVES AND CURRENTS ACROSS A FRINGING CORAL REEF

Low-frequency (infragravity) wave dynamics on a fringing coral reef were investigated using the numerical model XBeach (Roelvink et al, 2009). First, the skill of the one-dimensional model was evaluated based on its predictions of short waves (0.04-0.2 Hz), infragravity waves (0.004-0.04 Hz) and water level measurements (tidal and wave setup) obtained during a 2009 field study at Ningaloo Reef in Western Australia. The model calibration was sensitive to friction coefficients for short waves and current / infragravity bed friction, which were assumed independent in this model study. The infragravity waves were found to be generated primarily in the surf zone through the breakpoint generation mechanism rather than through offshore forcing. The infragravity waves were strongly also modulated over the reef by tidal depth variations, primarily due to the variability in frictional dissipation rates when the total water depth over the reef varied. The results reveal that short waves dominated bottom stresses on the fore reef and near the reef crest; however, inside the lagoon, infragravity waves become increasingly dominant, accounting up to 50% of the combined bottom stresses

Lysmata jundalini, a new peppermint shrimp (Decapoda, Caridea, Hippolytidae) from the Western Atlantic

A new peppermint shrimp species, Lysmata jundalini sp. nov., is described based on five specimens collected in shallowsubtidal waters on Enrique Reef at the University of Puerto Rico, Mayagüez Isla, Magueyes Laboratories. Lysmata jund-alini sp. nov. was identified from fresh material collected at the reef crest and back reef among coral rubble in June 2005and April 2009. The new species is most closely related to the Atlantic Lysmata intermedia and eastern Pacific L. holthu-isi. It can be readily distinguished from all those in the genus Lysmata by its color pattern, the presence of a well developedaccessory branch, the number of free vs. fused segments of the accessory branch, the number of carpal segments of the second pereiopod and well developed pterygostomian tooth.

The dynamics of infragravity wave transformation over a fringing reef

A 3 week field study was conducted to investigate the dynamics of low‐frequency (infragravity) wave motions over a fringing reef at Ningaloo Reef, Western Australia. Short‐period wave motions (0.04–0.2 Hz) were observed to dissipate on the reef crest beyond which infragravity wave motions (0.004–0.04 Hz) gradually dominated toward the lagoon. However, both the short waves and the infragravity waves were relatively small (both <0.3 m) on the reef flat owing to the shallow water depth (<2 m). The results revealed that the surf zone generation of free infragravity wave motions on the steep (∼1:20) fore‐reef slope was dominated by breakpoint forcing (as opposed to shoaling bound waves), which was also supported by detailed numerical simulations of the generation process. This is consistent with theory suggesting the efficiency of the breakpoint forcing mechanism should be high in this steep‐slope regime. Shoreward propagating infragravity waves traveled across the reef but were damped by bottom friction dissipation; however, this was at a rate much smaller than experienced by the residual short waves. With these rates of frictional dissipation also strongly dependent on the water depth over the reef, the infragravity wave heights increased at higher water levels and hence were strongly modulated by the tide. Due to the strong dissipation of infragravity waves over this wide and shallow reef that is hydraulically rough, any seaward propagating infragravity waves that reflected at the shoreline were small, leading to the dominance of progressive (shoreward propagating) infragravity wave motions throughout the reef and lagoon.

LOW FREQUENCY WAVE RESONANCE IN FRINGING REEF ENVIRONMENTS

Low frequency wave resonance has been postulated to enhance damage to coral reef protected coastlines during storm events. This paper uses the numerical model XBeach to examine the dynamics that contribute to resonance that have been previously observed on a fringing reef on Guam during tropical storm Man-Yi (Péquignet et al., 2009). The methods to identify resonance in numerical (or field data) are comprehensively reviewed with three indicators of resonance proposed based upon data obtained at two locations in the model domain: 1. The water surface elevation must be highly coherent, 2. The phase difference must (closely) correspond to 0° or 180°, and 3. Amplification of the signal must be observed between the reef crest and the shoreline. XBeach simulations demonstrated that resonance could be reproduced under 'normal' wave conditions, but only when bottom friction was minimal and hence values that were atypically low for coral reefs. However, under tropical storm Man-Yi conditions, resonance was reproduced with reasonable bottom friction values. A sensitivity analysis demonstrated that, although the frequency associated with resonance was not affected by the choice of bottom friction coefficients, the magnitude of the amplification was significantly affected. Ongoing research is being undertaken to investigate the resonant response for a wider variety of reef morphologies and incident wave forcing conditions.

Sediment suppresses herbivory across a coral reef depth gradient

Sediments are a ubiquitous feature of all coral reefs, yet our understanding of how they affect complex ecological processes on coral reefs is limited. Sediment in algal turfs has been shown to suppress herbivory by coral reef fishes on high-sediment, low-herbivory reef flats. Here, we investigate the role of sediment in suppressing herbivory across a depth gradient (reef base, crest and flat) by observing fish feeding following benthic sediment reductions. We found that sediment suppresses herbivory across all reef zones. Even slight reductions on the reef crest, which has 35 times less sediment than the reef flat, resulted in over 1800 more herbivore bites (h(-1) m(-2)). The Acanthuridae (surgeonfishes) were responsible for over 80 per cent of all bites observed, and on the reef crest and flat took over 1500 more bites (h(-1) m(-2)) when sediment load was reduced. These findings highlight the role of natural sediment loads in shaping coral reef herbivory and suggest that changes in benthic sediment loads could directly impair reef resilience.

MODELLING WAVE TRANSFORMATION ACROSS A FRINGING REEF USING SWASH

This paper presents the application of the open source non-hydrostatic wave-flow model SWASH to wave propagation over a fringing reef, and the results are discussed and compared with observations obtained from a laboratory experiment subjected to various incident wave conditions. This study focus not only on wave breaking, bottom friction, and wave-induced setup and runup, but also on the generation and propagation of infragravity waves beyond the reef crest. Present simulations demonstrate the overall predictive capabilities of the model for a typical coral reef with steep slopes and extended reef flats.

Ecological succession evidence in an Upper Jurassic coral reef system (Izwarn section, High Atlas, Morocco)

Late Jurassic coral reefs are present in the Western High Atlas of Morocco along the eastern paleomargin of the young Atlantic Ocean. The Izwarn section shows two reef units made of scleractinian corals and microbialites, included in a progradational shallowing-upward succession. Changes in reefal composition and structure observed in a ca. 50cm-thick interval at the base of the reef unit 1 are interpreted to represent the first well-documented ecological succession in Late Jurassic reefs. Bioclastic deposits, sparse and poorly diversified corals (five genera) preserved in life position, and finally a more diversified coral assemblage (11 genera) building a reef framework with stromatolitic crusts reflect pioneer, colonization and diversification stages, respectively. A specific depth window, where light was reduced but sufficient for a catch-up growth mode and below the erosive action of waves, probably at the lower slope of a fore-reef area, has made possible the development and the record of this ecological succession. With a more diversified coral assemblage (16 genera), the reef unit 2 reflects shallower and well-agitated waters of a fore-reef area. Overlaying this second reef unit, the upper part of the section reveals back-reef deposits made of reworked reef elements, whereas a putative reef crest area is not recorded.

Spatial variation in mechanical properties of coral reef substrate and implications for coral colony integrity

The physical structure of coral reefs plays a critical role as a barrier to storm waves and tsunamis and as a habitat for living reef-building and reef-associated organisms. However, the mechanical properties of reef substrate (i.e. the non-living benthos) are largely unknown, despite the fact that substrate properties may ultimately determine where organisms can persist. We used a geo-mechanical technique to measure substrate material density and strength over a reef hydrodynamic gradient. Contrary to expectation, we found a weak relationship between substrate strength and wave-induced water flow: flow rates decline sharply at the reef crest, whereas substrate properties are relatively constant over much of the reef before declining by almost an order of magnitude at the reef back. These gradients generate a novel hump-shaped pattern in resistance to mechanical disturbances for live corals, where colonies closer to the back reef are prone to dislodgement because of poorly cemented substrate. Our results help explain an intermediate zone of higher taxonomic and morphological diversity bounded by lower diversity exposed reef crest and unstable reef back zones.

Skeletal extension and calcification of reef-building corals in the central Indian Ocean

Skeletal extension, density and calcification rate of 12 scleractinian coral species comprising 7 morphological groups were examined on the reef crest of Vabbinfaru platform, Maldives. Growth rates of coral specimens were measured over the period of February 2010–March 2011 using alizarin staining and direct measurements. Skeletal extension rate was highly variable between coral species. Colony morphology was a major control on the skeletal extension and calcification of coral specimens. Growth rates of Acropora and Porites corals were comparable to existing data recorded for Caribbean and Indo-Pacific reef provinces. Skeletal density was less variable between species and was typically consistent among morphological groups. Findings represent the first estimates of coral growth in the central Indian Ocean region and add to the limited growth studies available that have examined a broad range of coral growth morphologies in other reef-building regions.