Sedimentary deposits in coral reefs represent more than 50

The hydrodynamics of coral reefs control the transport and availability of sediments, nutrients, coral larvae and consequently, all life across coral reef systems and across all spatial and temporal scales. In this study, we investigate the hydrodynamics of a distinctive feature of forereef morphology known as spurs and grooves (SaG). Despite their presence across coral forereefs, there is limited in-situ hydrodynamic data of SaG, and no current data reported from the Great Barrier Reef (GBR). Here, we analyse a hydrodynamic dataset obtained in prevailing weather conditions with low wave energy over SaG at One Tree Island in the southern GBR, a platform reef with a large lagoon isolated from the open ocean during low tides. Our data show that, for the conditions measured, flow velocities in forereef SaG are controlled by tidal water levels, with greater flow velocities observed during low tides. Throughout the full tidal cycle, offshore flow occurs at the upper forereef groove, indicative of lagoonal outflow. Further investigation into the influence of these hydrodynamic conditions can assist in assessments of forereef morphological evolution.

Coral forereef hydrodynamics, driven by waves and currents, play a critical role in controlling the distribution of nutrients, sediments and corals across platform reefs. These processes drive the formation of reef systems and are critical to determining the future of coral reef environments in a changing global climate. Despite this, limited in-situ research from the Great Barrier Reef (GBR) into the interactions between spurs and grooves (SaG) and forereefs currents is available. Here we analyse the first dataset of current measurements from a coral forereef SaG system in the GBR and assess forereef currents adjacent to a large reef lagoon that is isolated from the open ocean during low tides. We find that under prevailing weather conditions, waves are low (Hs = 0.25 m) and oblique to forereef isobaths. Currents in SaG under low wave exposure exhibit high velocity (2 m/s) with a dominant offshore component driven by surf zone processes and lagoonal outflow. This is consistent with postulated mechanisms of constructional development of SaG whereby offshore flow transports sediments seaward of the forereef, providing a substrate for coral larvae and living coral fragments to attach. These in-situ observations provide the first evidence from the GBR linking prevalent hydrodynamic conditions to the morphological evolution of coral forereef spur and groove systems.

Sandy beaches in estuaries and bays (BEBs) are unique geomorphic features on the coasts of many of the world's megacities. They exist under various settings and act as dynamic interfaces that influence hydrodynamics, sediment transport, and shoreline stability. Here, we investigate various beach profiles, wave conditions, tidal regimes, and sediment characteristics across two estuarine settings with different tidal ranges: the mesotidal R & iacute;a de Vigo (Galicia, Spain) and the microtidal Pittwater estuary (Sydney, Australia) to understand the morphodynamic process governing changes in BEBs profile shapes. Spectral wave analysis indicates that the differences in estuarine geomorphology, fetch, tidal range, and exposure to offshore swell waves collectively drive the BEB morphodynamics and changes in beach profile shape. Our findings indicate that swell waves provide the dominant energy input in both the microtidal (52%) and mesotidal (90%) estuaries, while the relative contributions of wind and infragravity waves vary depending on bathymetry, fetch length, and geomorphological configurations. Overall, BEBs within the same estuary exhibit distinct wave signatures depending on their location relative to the estuary entrance, tidal regime, and exposure to offshore wave energy. This study shows that BEBs with greater exposure to swell have Convex cross-shore profiles and typically coarser, well-sorted sediments. On the other hand, wave sheltered BEBs typically have Concave profile morphology, fine and poorly sorted sediments. The relationship between wave signatures and BEB profile morphotypes demonstrated in this study enhances our understanding of the morphodynamic processes shaping these complex systems.

Tropical cyclones (TCs) are extreme storm events with the potential to cause significant damage to coral reef and island ecosystems. The evolution of coral rubble (shingle) islands within these ecosystems relies on the complex eco-morphodynamic relationship between the supply of biogenic sediment from the reef and subsequent transport by hydrodynamic forces. Storms have the potential to alter this relationship, posing a potential threat to rubble islands structure and stability with the forecasted increase of the intensity of storms in the Southwest Pacific with climate change. Traditional methods of monitoring the impacts of storm focus on long-term effects, often utilising field-based approaches. Here we use unpiloted aerial vehicles (UAVs) and remotely sensed data as a cost-effective method to provide high-resolution spatial data to understand short-term effects of tropical cyclones, and the resulting storm generated waves. We observed the impacts of TC Gabrielle (2023) on One Tree Reef within the Southern Great Barrier Reef and quantified the changes of One Tree Island (OTI), a well-developed rubble island (~5.9 Ha), and Two Tree Island (TTI), a developing rubble island (~0.1 Ha). At both islands, shoreline length decreased (-7.3% TTI, -0.5% OTI) and total rubble volume decreased (-1.8% TTI, -0.3% OTI). The rubble tracts attached to OTI prograded towards the island as result of the TC and distributed sediment along its shore. This study presents high-resolution data for remote rubble islands and incorporates a detailed volumetric analysis complementing traditional planimetric methods for the monitoring of remote islands.

Coral reefs rely on minimum thresholds of live hard coral cover to sustain key ecological and geomorphic functions, including habitat provisioning, carbonate production, and wave attenuation. While declining hard coral cover is widely recognised as a driver of reef degradation, the boundaries at which reefs transition between distinct functional states remain poorly defined-particularly across reefs with contrasting baseline conditions, where absolute cover values are not directly comparable. To address this gap, we quantified long-term changes in benthic composition from 1995 to 2022 across three eco-geomorphologically distinct reefs spanning the northern, central, and southern Great Barrier Reef. Hard coral cover was indexed relative to each reef's historical maximum, allowing reef state transitions to be parameterised using comparable thresholds. Three reef states were identified, driven predominantly by shifts in coral cover and corresponding changes in algal dominance, with secondary reef-specific reorganisation in coral morphological composition shaped by baseline community structure. 'Healthy' states were characterised by high structural complexity; 'coping' states by overall hard coral cover decline, with reef-specific shifts in dominant coral morphology; and 'struggling' states by near-complete coral loss and algal dominance. Indexed hard coral cover provides a practical benchmark-with transitions into 'coping' states occurring below ∼65% of historical maxima, and further declines below ∼35% associated with 'struggling' states. Regardless of baseline morphological composition, disturbance-driven transitions toward flatter, algal-dominated reef profiles represent a convergent trajectory of functional decline-one that is likely to accelerate as disturbance regimes intensify under climate change.

Climate change stressors such as ocean warming, acidification and deoxygenation are severely threatening coral reefs and the vital ecosystem services they provide. Corals found to survive in mangrove habitats that naturally possess stressful conditions, are being increasingly studied to investigate the impact of multiple co-occurring stressors on coral growth. However, the water quality within mangrove-coral habitats and how this changes with varying distance from the mangroves remains largely unknown. We used the Foraminifera in Reef Assessment and Monitoring Index (FORAM Index) to assess the suitability of environments for coral growth across a mangrove to reef gradient. Sediment samples were collected from five sites at varying proximity to the Low Isles mangroves in the northern Great Barrier Reef. Sites were located: amongst the inner mangroves, the mangrove fringe, the adjacent leeward and windward reef margins and at Opal Reef (control) 38 km away from the mangroves. Foraminiferal community assemblages were used to determine the environmental suitability for coral growth and recovery. The poorest seawater conditions for coral occurred at the inner mangrove site with water quality improving outside the mangroves. Leeward and windward margin sites differed in their suitability for coral growth despite being a similar distance from the mangroves, indicating that water conditions were not necessarily constrained by proximity to mangroves alone. These findings have important implications for studies investigating mangrove-coral habitats as refugia and for assessing linkages between coral reefs and mangroves - two highly threatened tropical ecosystems.

The long-term stability of coral reef islands and associated reef top sedimentary landforms requires the delivery of sediment from the forereef, but the rates and pathways of sediment delivery to these systems remain unclear. Spurs and grooves (SAGs) are ubiquitous geomorphic features fringing coral reefs, characterised by shore-normal coral ridges (spurs) separated by channels (grooves) with either bare substrate or a relatively low sediment infill. SAGs dissipate wave energy, facilitate offshore sediment transport and enhance nutrient exchange. Here we present the first evidence that SAG can also act as channels for onshore transport of rubble during high-energy events, contributing to maintaining reef islands and rubble-based ecosystems.

Beaches in estuaries and bays (BEBs) are common geomorphic features that provide habitats, recreational resources, and coastal infrastructure protection buffers. These BEBs are usually influenced by wind and swell waves, tides, riverine effects, and boat wakes. BEBs typically experience lower wave energy compared to open coast beaches due to different geomorphological and geological conditions. BEBs morphology depends on the relative percentage of the swell, wind, and infragravity waves including low and high-energy events (Hegge et al., 1996). It should also be noted that BEBs geomorphology is also controlled by the tidal-range conditions and asymmetry of tidal currents (Dronkers, 1986). The present study aims to identify the role of the variability of wave climate in the BEB's morphological evolution in micro and mesotidal environments. Our focus lies in determining the proportions of wind, ocean swell, and infragravity waves that collectively influence the wave characteristics of these BEBs.

Sandy beaches in estuaries and bays (BEBs) are common globally, and their existence relies on the complex interactions of regional and local waves, limited sediment input, tidal dynamics, and human interventions. BEB morphodynamics are poorly defined with only a few studies attempting to qualitatively relate different BEB profile shapes with relevant physical drivers. Here, we propose a novel Profile Morphotype Index (Gamma) to quantitatively classify the shape of beach profiles in BEBs into distinct morphotypes to allow an objective analysis of changes in beach morphodynamic states. For this study, beach profiles in three swash-aligned and one driftaligned BEBs in Gamay-Botany Bay (NSW, Australia) were measured periodically between 2016 and 2023, resulting in a total of 774 beach profile surveys. Based on this dataset, we defined equations that compute the profile shape assigning it to a specific morphotype. Ten unique morphotypes form the new morphometric index (Gamma) varying gradually between Concave (Gamma = -1.000) and Convex (Gamma = 1.000) profile shapes, which indicate more erosive and accretive beach states, respectively. Our results show that those BEBs closest to the estuary entrance favoured the Convex (Gamma = 1.000) morphotype, while BEBs farthest from the entrance and under erosive conditions were Mostly Concave (Gamma = -0.570 to -0.950) morphotypes. The morphometric index also captured changes in profile shape due to high-energy storm waves propagating into the estuary. This new morphotype index (Gamma) provides an objective classification of BEB profile shape allowing us to better predict beach state response to changes in coastal processes and hydrodynamics.

Coral reefs protect coastlines from inundation and flooding, servicing over 200 million people globally. Wave transformation has previously been studied on coral reef flats with limited focus on forereef zones where wave transformation is greatest during high-energy conditions. This study investigates the role of forereef spur and groove (SaG) morphology on wave energy dissipation and overtopping on coral reefs. Using XBeach on LiDAR-derived bathymetry, we reproduced dissipation rates comparable to SaG field studies. Our results emphasize accurate bathymetries’ role in forereef wave energy dissipation models by including morphological features (e.g., groove sinuosity, irregular forereef slopes) that control the mode of wave energy dissipation (frictional and breaking). We then investigated changes to wave energy dissipation and wave overtopping based on IPCC AR5 low and high emission scenarios (RCP2.6 and RCP8.5) and a total disaster scenario (TD) for the year 2100 considering changes to SaG morphology, wave power and relative sea-level rise. For RCP2.6, an increase in wave heights of 0.8 m and an increase in water level of 0.3 m resulted in a two-fold increase in dissipation rates. For RCP8.5 and TD, with no increase in incident wave height, dissipation rates were 29% and 395% lower than RCP2.6. This resulted in increased overtopping at the reef crest by 1.8 m and 2.7 m for RCP8.5 and TD scenarios, respectively, when compared to RCP2.6. Decreased dissipation rates and increased wave overtopping in forecasted climate conditions suggest the need for strategies to promote coral growth to facilitate high dissipation rates in the future.

Over 85 % of oyster reefs have been lost globally due to disease, overharvesting, global warming, and pollution. Consideration of the ecosystem services provided by healthy oyster reefs (e.g., coastal protection, water purification and carbon burial) has driven recent research and restoration efforts worldwide. However, hydrodynamic studies, specifically looking at the effects of different levels of wave exposure on the ecomorphodynamics of oyster reefs, are scarce. In this study, we consider oyster reefs in microtidal estuaries under different levels of relative wave exposure to determine how hydrodynamics may shape reef morphology and how reef morphology affects wave dissipation. We quantify oyster reef morphology through spatial analysis, using morphometrics and spatial density and relate these to the ability of oyster reefs to dissipate wave energy. Field campaigns were undertaken at three microtidal sites in southeast Australia with different hydrodynamic exposure and morphology: Gamay (Botany Bay), Port Hacking and Crookhaven River. We found that reef morphology and orientation is related to estuarine hydrodynamic conditions and thus we propose an ecomorphodynamic model with a continuum of morphologies from sparse reefs aligned perpendicular to the tidal currents and incoming waves (patch reefs), through broken up barriers semi -aligned or obliquely to the tidal flows (string reefs), to the total barrier that exists under the lowest hydrodynamic conditions (fringing reefs). The highest dissipative ability of locally generated wind waves occurred at Crookhaven (patch reef, 165 kW/m 2 ), and lowest at Gamay (string reef, 11.66 kW/m 2 ). Our results suggest that reef morphology, and orientation to currents and waves, influence wave dissipation, and that hydrodynamic conditions in turn influence reef morphology. These findings are important to inform future reef restoration under increasingly severe climate change conditions to optimise ecosystem services on restored oyster reefs.

There is a relative lack of research, targeted models and tools to manage beaches in estuaries and bays (BEBs). Many estuaries and bays have been highly modified and urbanised, for example port developments and coastal revetments. This paper outlines the complications and opportunities for conserving and managing BEBs in modified estuaries. To do this, we focus on eight diverse case studies from North and South America, Asia, Europe, Africa and Australia combined with the broader global literature. Our key findings are as follows: (1) BEBs are diverse and exist under a great variety of tide and wave conditions that differentiate them from open-coast beaches; (2) BEBs often lack statutory protection and many have already been sacrificed to development; (3) BEBs lack specific management tools and are often managed using tools developed for open-coast beaches; and (4) BEBs have the potential to become important in "nature-based" management solutions. We set the future research agenda for BEBs, which should include broadening research to include greater diversity of BEBs than in the past, standardising monitoring techniques, including the development of global databases using citizen science and developing specific management tools for BEBs. We must recognise BEBs as unique coastal features and develop the required fundamental knowledge and tools to effectively manage them, so they can continue providing their unique ecosystem services.

The bio-physical responses of low-lying coral islands to climate change are of concern. These islands exist across a broad range of bio-physical conditions, and vulnerabilities to rising and warming seas, ocean acidification and increased storminess. We propose a risk-based classification that scores 6 island eco-morphometric attributes and 6 bio-physical ocean/climate conditions from recent open-access data, to assign islands with respect to 5 risk classes (Very Low, Low, Moderate, High and Very High). The potential responses of 56 coral islands in Australia's jurisdiction (Coral Sea, NW Shelf and NE Indian Ocean) to climate change is considered with respect to their bio-physical attributes and eco-morphometrics. None of the islands were classed as Very Low risk, while 8 were classed as Low (14.3 %), 34 were Moderate (60.7 %), 11 were High (19.6 %), and 3 were Very High (5.4 %). Islands in the Very High risk class (located on the NW Shelf) are most vulnerable due to their small size (mean 10 Ha), low elevation (mean 2.6 m MSL), angular/elongated shape, unvegetated state, below average pH (mean 8.05), above average rates of sea-level rise (SLR; mean 4.6 mm/yr), isolation from other islands, and frequent tropical storms and marine heatwaves. In contrast, islands in the Low (and Very Low) risk class are less vulnerable due to their large size (mean 127 Ha), high elevation (mean 8.5 m MSL), sub-angular/round shape, vegetated state, near average pH (mean 8.06), near average SLR rates (mean 3.9 mm/yr), proximity to adjacent islands, and infrequent cyclones and marine heatwaves. Our method provides a risk matrix to assess coral island vulnerability to current climate change related risks and supports future research on the impacts of projected climate change scenarios. Findings have implications for communities living on coral islands, associated ecosystem services and coastal States that base their legal maritime zones on these islands.

Introduction Sand aprons are ubiquitous depositional sedimentary features that offer insights into the sediment dynamics of coral reef environments. Global studies found that the extent of sand aprons are not related to reef platform size and their widths are a function of environmental factors such as swell period and height, tidal amplitude, latitude, and exposure to wind and waves (Rankey and Garza-Perez 2012). Recent studies using numerical modelling have found that sand aprons in reef flats attain a critical water depth resulting in constant depth (Ortiz and Ashton 2019), and that lagoon infilling through sand apron progradation is a self-limiting process (Rankey 2021). Sand apron progradation is an eco-morphodynamic process and climate change, including intensification and increased frequency of marine heatwaves, ocean and coastal acidification, and changes in wave and tropical storm climates are triggering changes that need to be understood to inform sound management of coral reefs.This paper presents data on the Holocene evolution of the sand aprons on 21 offshore platform reefs located on the southern Great Barrier Reef and how it can be used to infer past wave climates. We then present the recent wave climate for the study area (Smith et al. 2022) and analyse sand apron evolution accordingly. Methods The sand aprons on 21 reefs located on the Capricorn Bunker Group (Southern Great Barrier Reef) were assessed from high-resolution satellite imagery, obtaining digital bathymetric models and digitizing the reef and lagoon contours. We then measured reef area and lagoon area to calculate the percentage of lagoon infilling. The wave climate for the study area was obtained from satellite altimetry using an open-source Python tool (Smith et al. 2020). Preliminary findings Our results showed that the most important factor for lagoon infilling was the size of the reef, with larger reefs typically appearing less infilled than smaller reefs. Wave incidence seemed to be unimportant: the three reefs with less than 50% infill were all medium-sized and exposed to incident waves while all six protected reefs had infilling above 50%. While some authors had pointed out at relative sea-level changes to explain current sand apron stability (Harris et al. 2015), our results show that the self-limiting nature of the sand apron progradation, combined with relative sea-level changes, is a better explanation for sand apron stability. In any case, the extent of the sand apron can be used to infer wave climate at the time of sand apron progradation. For example, for One Tree Reef, one can argue the sand apron could had stopped prograding 4 ka BP because of the self-limiting sediment transport and remained stable until the sea-level fell.The wave climate on the Southern Great Barrier Reef is characterized by significant wave heights (Hs) of 1.7 m and has been stable for the past 33 years (Smith et al. 2022). Future changes in the wave climate, storm frequency, increases in sea level and changes to sediment availability caused by anthropogenic climate change will modify the eco-morphodynamics of the sand aprons and the percentage infilling of the lagoons.

Beach nourishment is a soft engineering technique that is used to combat coastal erosion. To assess the efficacy of a beach nourishment program on the northwest coast of Lord Howe Island, remotely coordinated drone-based monitoring was undertaken at Lagoon Beach. Specifically, hypotheses were tested that beach nourishment could increase the dune height and the width of the beach where the sand was translocated but would not have any long-term impacts on other parts of the beach. During the beach nourishment program, sand was translocated from the north end to the south end of Lagoon Beach, where it was deposited over 2800 m2. Lagoon Beach was monitored using a time series of 3D orthomosaics (2019–2021) based on orthorectified drone imagery. The data were then analysed using a robust before-after-control-impact (BACI) experimental design. Initially, a fully automated drone mapping program and permanent ground control points were set up. After this, a local drone pilot facilitated automated drone mapping for the subsequent times of sampling and transferred data to mainland researchers. As well as being more cost-effective, this approach allowed data collection to continue during Island closures due to the COVID-19 pandemic. After sand translocation, the south end of Lagoon Beach had a lower dune with more vegetation and a more expansive beach with a gentler slope than the prior arrangement. Overall, drone monitoring demonstrated the efficacy of the beach nourishment program on Lord Howe Island and highlighted the capacity for drones to deliver cost-effective data in locations that were difficult for researchers to access.

Coral reef islands are under threat from warming and rising seas, ocean acidification and increased storminess. Coral islands are low-lying accumulations of sediment derived from the shells and skeletons of calcifying reef organisms. IPCC predications and COP26 highlight that the future of coral islands is not certain as islands are morphologically active. It is unclear what tipping points are causing negative impacts on some coral reef islands and not others. A better understanding of island stability and vulnerability is urgently needed. This has direct implications for over 200 million people that rely on reefs and their islands for their livelihoods (Ferrario et al., 2014). Further, coastal States such as Australia, USA, the Philippines and Small Island Developing States (SIDS) use coral reef islands as legal baselines to support their maritime jurisdictions (UNCLOS, 1982). We focus on 31 of Australia’s offshore coral reef islands on 10 reefs across the Coral Sea and NW Shelf. These islands are environmentally and geo-politically important as they support and extend Australia’s maritime jurisdiction. Island morphology (e.g., shoreline positions, areas, shapes) in historical aerial and satellite imagery (1977-now) were compared to ocean and climate data (e.g., storm tracks, NOAA Coral Watch). We used this data to identity tipping points that drive decadal changes in coral reef island stability.