Back-reef sand aprons are conspicuous and dynamic sedimentary features in coral reef systems. The development of these features influences the evolution and defines the maturity of coral reefs. ...However, the hydrodynamic processes that drive changes on sand aprons are poorly understood with only a few studies directly assessing sediment entrainment and transport. Current and wave conditions on a back-reef sand apron were measured during this study and a digital elevation model was developed through topographic and bathymetric surveying of the sand apron, reef flats and lagoon. The current and wave processes that may entrain and transport sediment were assessed using second order small amplitude (Stokes) wave theory and Shields equations. The morphodynamic interactions between current flow and geomorphology were also examined. The results showed that sediment transport occurs under modal hydrodynamic conditions with waves the main force entraining sediment rather than average currents. A morphodynamic relationship between current flow and geomorphology was also observed with current flow primarily towards the lagoon in shallow areas of the sand apron and deeper channel-like areas directing current off the sand apron towards the lagoon or the reef crest. These results show that the short-term mutual interaction of hydrodynamics and geomorphology in coral reefs can result in morphodynamic equilibrium.
•Back-reef sand aprons are important evolutionary features in coral reef systems.•Sediment transport occurred under typical hydrodynamic conditions due to waves.•Differentials between flood and ebb velocities were found.•A morphodynamic relationship between current flow and geomorphology was observed.
Some of the most profound effects of climate change on ecological communities are due to alterations in species interactions rather than direct physiological effects of changing environmental ...conditions. Empirical evidence of historical changes in species interactions within climate-impacted communities is, however, rare and difficult to obtain. Here, we demonstrate the recent disappearance of key habitat-forming kelp forests from a warming tropical–temperate transition zone in eastern Australia. Using a 10-y video dataset encompassing a 0.6 °C warming period, we show how herbivory increased as kelp gradually declined and then disappeared. Concurrently, fish communities from sites where kelp was originally abundant but subsequently disappeared became increasingly dominated by tropical herbivores. Feeding assays identified two key tropical/subtropical herbivores that consumed transplanted kelp within hours at these sites. There was also a distinct increase in the abundance of fishes that consume epilithic algae, and much higher bite rates by this group at sites without kelp, suggesting a key role for these fishes in maintaining reefs in kelp-free states by removing kelp recruits. Changes in kelp abundance showed no direct relationship to seawater temperatures over the decade and were also unrelated to other measured abiotic factors (nutrients and storms). Our results show that warming-mediated increases in fish herbivory pose a significant threat to kelp-dominated ecosystems in Australia and, potentially, globally.
Habitat interactions play key roles in regulating biodiversity and ecosystem functions. This is particularly important in aquatic ecosystems, where the flow of water facilitates exchanges of energy ...and matter. Oyster reefs, a highly degraded habitat globally and a key focus for restoration efforts, can reduce water movement and facilitate the deposition of particles around them, affecting nutrient cycling in surrounding sediments. The effects of these reefs on sediment infauna taxonomic and functional biodiversity, however, remain unknown. We sampled sediments at increasing distances from reefs at three estuaries to evaluate the relationships between proximity to oyster reefs and composition and functionality of infaunal communities and explored the potential mechanisms behind those relationships. Sediments close to oyster reefs had consistently greater amounts of labile organic matter, which in turn was positively related to the number of taxa and total abundance of infauna. Also, the functional traits of infauna, such as bioturbation and feeding modes, were related to proximity to reefs, but they were variable between estuaries indicating the importance of background estuarine environmental conditions. These results suggest that habitat linkages between oyster reefs and sediments are important in regulating taxonomic biodiversity, while functional biodiversity seems to be driven by processes operating at larger scales. Given burgeoning restoration initiatives worldwide, particularly those of oyster reefs, incorporating seascape interactions can help inform recovery of biodiversity and functions beyond the target habitat at the seascape level, which is often overlooked.
Measurements of geomorphic change in the intertidal zones of coral reefs are made using a variety of remote sensing and in-situ techniques, where variations in the coverage and spatial-temporal ...precision achieved are directly related to the cost of data acquisition. We present a novel, low-cost technique for measuring high-resolution changes in reef environments based on Kite Aerial Photography (KAP) and photogrammetry/structure-from-motion post-processing. KAP images are used to measure fine-scale changes in intertidal topography and sediment texture characteristics, including rubble particle size, of a coral shingle cay at One Tree Island, Great Barrier Reef in the context of storm activity. Validation using Real Time Kinematic DGPS demonstrates the ability to measure topographic elevation with an error of 5.53cm (RMSE) and a spatial resolution of 5cm per point, an accuracy/resolution that is superior to airborne LiDAR and equivalent to terrestrial LiDAR, but at a fraction of the equipment cost.
•Kite Aerial Photography (KAP) was used to study changes on a coral shingle cay.•Methods developed for measuring both topographic and sediment texture changes•Changes quantified in context of storm activity at One Tree Island, GBR Australia•KAP/Structure-from-motion provided a low-cost and accurate means of studying change.•First time these techniques have been applied for measuring change in coral reefs.
This paper investigates the mechanics of sediment transport on a subtidal sand apron located on a coral reef environment. In this environment 100% of the sediment is carbonate bioclasts generated in ...situ. The sand apron is located on the back reef and only affected by waves during high tides. It is commonly accepted in the literature that sand aprons are features that prograde lagoonwards and that most of the progradation occurs during high-energy events. Measurements of water depths, waves, currents and near bed suspended sediment concentrations (all at 10Hz) on the sand apron were undertaken over a nine day intensive field campaign over both spring and neap tides; waves and tides were also measured in the lagoon. The topography and bathymetry of the sand apron were measured and mixing depth was obtained on three transects using depth of disturbance rods. We found that sediment transport on sand aprons is not solely restricted to high-energy events but occurs on a daily basis during spring tides. The main factor controlling the sediment transport was the water depth above the bed, with depths of 2–2.3m allowing waves to promote the most sediment transport. This corresponds to a depth over the reef crest of 1.6–1.9m. The second most important control was waves; transport was observed when Hs on the apron was 0.1m or greater. In contrast, current magnitude was not a controlling mechanism for sediment entrainment but did affect sediment transport. The morphology of the sand apron was shown to affect the direction of currents with the currents also expected to influence the morphology of the sand apron. The currents measured during this field campaign were aligned with a shallow channel in the sand apron. Mixing depths were small (<2.5cm) yet they were larger than the values predicted by empirical formulae for gentle siliciclastic ocean beaches.
•Sediment transport on a sand apron occurs under spring tides with low-energy waves.•Maximum suspension occurs with water depths above bed between 2 and 2.3m.•Current intensity is not crucial in suspending sediment.•On sand aprons ebb flow dominates in channels and flood flow dominates over shoals.
Abstract
Coral reef islands can support large legal maritime zones (i.e. ocean spaces where States have rights and responsibilities) and are of international and geopolitical importance. This review ...focuses on low-lying coral reef islands supplied with sediments derived from skeletons and shells of calcifying organisms. For coral islands, the outer ‘low-water line’ of the reef can be used as the legal ‘baseline’ to establish maritime zones. Coral islands and the reefs that support them are experiencing the effects of rising and warming seas, increased storminess and ocean acidification. Coral reefs, their islands and associated maritime zones support millions of people, including those in Small Island Developing States (SIDS). SIDS communities are arguably the least responsible for climate change but are at the forefront of its impacts so ensuring their continued wellbeing is a global responsibility. Securing the future of coral reefs and islands is dependent on reducing global climate threats and emissions, improving local management, and investing in restoration and adaption research. It is uncertain if coral islands will persist into the future, and on what timelines. This raises questions such as, where coral islands support maritime zones, what are the legal implications of island instability or loss? This review focuses on the bio-physical interactions of coral islands and associated reefs in the face of changing climates, and implications for legal maritime zones and SIDS.
The Great Barrier Reef (GBR) is the largest coral reef system on earth, with ecological and scientific importance for the world and economic and iconic value for Australia. However, the ...characterisation of its offshore wave climate remains challenging because of its remoteness and large dimensions. Here, we present a detailed analysis of the offshore wave climate of the GBR, unveiling the details of both modal conditions and extreme events. We used a calibrated satellite radar altimeter dataset (spanning from 1985 to 2018) to quantify wave climate, assess the influence of climate drivers, and analyse the wave conditions generated by tropical cyclones at three main regions of the GBR (northern, central, and southern). Our results indicate average significant wave heights of 1.6 m, 1.5 m, and 1.7 m for the northern, central, and southern GBR, respectively. The modal wave climate exhibits substantial seasonality, particularly in the northern region with dry season wave heights up to twofold larger than during wet season. The northern and central wave climates show decreasing wave height and wave energy trends over the last 33 yrs, whilst the southern region remains stable. Consistent with prior studies, we found that the wave climate in the southern region is modulated by the El Niño-Southern Oscillation and the southern annular mode, with influence additionally extending to the central region. Analysis of the extreme waves generated by tropical cyclones revealed they generate large, long period waves, frequently above 7 m, resulting in wave power up to 32-fold higher than median conditions.
Rubble islands are dynamic sedimentary features present on reef platforms that evolve under a variety of morphodynamic processes and controlling mechanisms. They provide valuable inhabitable land for ...small island nations, critical habitat for numerous species, and are threatened by climate change. Aiming to investigate the controlling mechanisms dictating the evolution of One Tree Island (OTI), a rubble island in the Southern Great Barrier Reef, we combined different remotely-sensed data across varying timescales with wave data extracted from satellite altimetry and cyclone activity. Our findings show that (1) OTI had expanded by 7% between 1978 and 2019, (2) significant gross planform decadal adjustments were governed by the amount, intensity, proximity, and relative position of cyclones as well as El Niño Southern Oscillation (ENSO) phases, and (3) the mechanisms of island growth involve rubble spits delivering and redistributing rubble to the island through alongshore sediment transport and wave overtopping. Frequent short-term monitoring of the island and further research coupling variations in the different factors driving island change (i.e., sediment availability, reef-wave interactions, and extreme events) are needed to shed light on the future trajectory of OTI and other rubble islands under a climate change scenario.
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