Hard Coral Cover Research Articles

Spatial Variation in Upper Limits of Coral Cover on the Great Barrier Reef

ABSTRACTAimIdentifying the maximum coral cover that a coral community can sustain (i.e., its ‘upper limit’) is important for predicting community dynamics and improving management strategies. Here, we quantify the relationship between estimated upper limits and key environmental factors on coral reefs: hard substrate availability, temperature and water clarity.LocationGreat Barrier Reef (GBR), Australia (over 1400 km).Time Period1990 to 2022.Major Taxa StudiedScleractinian corals.MethodsWe used 32 years of data on coral cover around reef perimeters. Each reef was divided into four wave‐exposure habitats depending on prevailing wind conditions. For each site, we determined if hard coral cover had reached a plateau or upper limit. Next, we extracted existing estimates of hard substrate availability, modelled water temperature and Secchi depth. Then, we quantified the relationship between these environmental variables and the upper limits.ResultsWe found varying upper limits across the GBR, with a median of 33% coral cover and only 17% of the estimated upper limits exceeded 50% coral cover. Upper limits increased towards the southern reefs. Our results show that upper limits increased with increasing hard substrate availability and decreased with temperature and, to a lesser extent, with water clarity.Main ConclusionsThe upper limits estimated in this study are much lower than what is commonly assumed when modelling ecological dynamics, most likely resulting in predicted recovery rates being inappropriately high. Although hard substrate ultimately restricted upper limits, there are mechanisms constraining the proportion of hard substrate that is covered by hard corals. The negative relationship between temperature and upper limits cannot be explained by changes in macroalgal abundance but may be related to changes in species composition. The quantitative relationships between the upper limits of coral cover and environmental variables will provide critical information to prioritise sites for management interventions.

Differential spatio-temporal responses of Red Sea coral reef benthic communities to a mass bleaching event

Understanding how coral reefs respond to disturbances is fundamental to assessing their resistance and resilience, particularly in the context of climate change. Due to the escalating frequency and intensity of coral bleaching events, it is essential to evaluate spatio-temporal responses of coral reef communities to disentangle the mechanisms underlying ecological changes. Here, we used benthic data collected from 59 reefs in the Red Sea over five years (2014–2019), a period that encompasses the 2015/2016 mass bleaching event. Reefs were located within three different geographic regions with different environmental settings: north (Duba; Al Wajh), central (Jeddah; Thuwal), and south (Al Lith; Farasan Banks; Farasan Islands). Coral community responses were region-specific, with communities in the south being more promptly affected than those in the northern and central regions, with hard and soft coral cover dropping drastically in several reefs from around > 40% to < 5% two years after bleaching. Coral bleaching effects were particularly evident in the decrease of cover in branching corals. Overall, we documented a shift towards a dominance of macroalgae, turf algae, and crustose coralline algae (CCA). Using remote sensing data, we analyzed sea surface temperature (SST) regimes at the study sites to infer potential drivers of changes in benthic composition. Both SST and Degree Heating Weeks (DHW) only partially aligned with the responses of benthic communities, highlighting the need for more accurate predictors of coral bleaching in the Red Sea. In times of intense coastal development along Saudi Arabia’s Red Sea coast, our study provides crucial baseline information on developments in coral reef community composition, as well as to guide decision-making, namely restoration efforts.

Long-term dynamics of hard coral cover across Indonesia

Long-term dynamics of hard coral cover across Indonesia

Depth, hard coral, and turf cover as predictors of micro-scale spatial distribution of fishes in a subtropical rocky reef

Depth, hard coral, and turf cover as predictors of micro-scale spatial distribution of fishes in a subtropical rocky reef

Complex coral reefs offer hope for management in a Marine Protected Area in Zanzibar

Management of Marine Protected Areas can be supported by a detailed understanding of ecosystem state. We assessed coral reefs within Pemba Channel Conservation Area in 2019 and evaluated changes over the previous decade. In 2019, hard coral cover ranged from 3.5 % to 58.4 % across sites, while coral recruitment was exceptionally high in places (14 recruits/m2). The highest total fish biomass was ∼ 500 kg/ha, which is low compared to forereefs elsewhere in the Western Indian Ocean. Between 2009 and 2019, benthic substrate shifted to higher soft coral and turf algae cover, and mean fish density significantly decreased. This suggests that overfishing is a major threat and some reefs will require restoration to facilitate recovery. However, Pemba's unique geomorphology might offer refuge for corals from future warming and habitat for large, endangered fishes such as groupers. This could elevate the global significance of Pemba's reefs and should motivate investment in management and conservation of these resources.

Increasing disturbance frequency undermines coral reef recovery

AbstractClimate‐driven alterations to disturbance regimes are increasingly disrupting patterns of recovery in many biomes. Here, we examine the impact of disturbance and subsequent level of recovery in live hard coral cover on the Great Barrier Reef (GBR) across the last three decades. We demonstrate that a preexisting pattern of infrequent disturbances of limited spatial extent has changed to larger and more frequent disturbances, dominated by marine heatwaves and severe tropical cyclones. We detected an increase in the impact (measured as coral loss) across 265 individual disturbance impacts on 131 reefs in a 36‐year dataset (1985–2022). Additionally, the number of survey reefs impacted by disturbance has increased each decade from 6% in the 1980s to 44% in the 2010s, as has the frequency of mass coral bleaching across the GBR, which has increased between 19% and 28% per year, and cyclones (3%–5% per year), resulting in less time for recovery. Of the 265 disturbance impacts we recorded, complete recovery to the highest levels of coral cover recorded earlier in this study (the “historical benchmark”) occurred only 62 (23%) times. Of the 23% of disturbance impacts that resulted in complete recovery to historical benchmarks, 34/62 recovered to their benchmark in 2021 or 2022. Complete recovery was more likely when the historical benchmark was &lt;25% live hard coral cover. The lack of recovery was attributed to recovery time windows becoming shorter due to increases in the frequency of cyclones and of thermal stress events that result in mass coral bleaching episodes. These results confirm that climate change is contributing to ecosystem‐wide changes in the ability of coral reefs to recover.

Environmental Health Impacts on The Sustainability of The Coral Reef Ecosystem in Gede Island, Indonesia

Gede Island, located north of Rembang Regency in Central Java, Indonesia, hosts coral reefs in poor environmental health. This study aimed to assess the impact of environmental health on various dimensions of sustainability to guide stakeholder decision-making. A descriptive quantitative method was employed. Observations of coral reef variables, including type and percentage of cover, were conducted using the Point Intercept Transect (PIT) method. The sustainability status of management practices was analysed using the Multidimensional Scaling (MDS) method with Rapfish (Rapid Assessment Techniques for Fisheries) 2013 version R software. The results identified 16 genera from three sampling points: Station 1 had 30.35% hard coral cover, Station 2 had 1%, and Station 3 had 12.94%. The sustainability status of Gede Island's coral reefs was evaluated across five attributes: ecological (41.61%), social (48.05%), economic (40.08%), institutional (26.36%), and technological (57.03%). Among these, only the technological attribute was classified as moderately sustainable, while the others were deemed less sustainable. A high sustainability status offers significant potential and benefits for the local community, such as opportunities for ecotourism and fisheries sustainability.

Distribution of Coral Life Forms Based on Sea Current Velocity in Kei Besar Island, Maluku, and Mare Island, North Maluku

Abstract Diverse life forms found on coral reefs indicate resilience to environmental change, sustaining marine biodiversity and ensuring the stability of the coastal community’s ecosystems. This study investigates the relationship between sea currents and the life forms of coral reefs in Indonesia and focuses on two primary locations: Kei Besar Island, Maluku, and Mare Island in North Maluku. Sea current data from the current real-time global forecasting and coral cover data were analysed to assess the influence of current velocities on the percentage of hard coral cover and the morphological composition of corals. The dominant bottom substrate in both locations is hard coral, with varying percentages of cover. Thirteen distinct life forms of coral were identified in both study sites, with massive and sub-massive forms being the most prevalent on Kei Besar Island (41%) having average current velocities of 0.159 m/s, and branching forms (40%) being dominant on Mare Island with average current velocities were 0.103 m/s. The massive form (CM) is strongly positively (negatively) correlated with faster (slower) currents, with a correlation coefficient of 0.81 (-0.69). On the other hand, several life form of fragile coral reefs particularly Acropora branching (ACB); Acropora sub-massive (ACS); and coral Tubipora (CTU) were found to be positively (negatively) correlated with slower (faster) currents, with a correlation coefficient of 0.55 (-0.63); 0.77 (-0.75); and 0.58 (-0.67).

Changing dynamics of Great Barrier Reef hard coral cover in the Anthropocene

Changing dynamics of Great Barrier Reef hard coral cover in the Anthropocene

Responses of the coral reef cryptobiome to environmental gradients in the Red Sea.

An essential component of the coral reef animal diversity is the species hidden in crevices within the reef matrix, referred to as the cryptobiome. These organisms play an important role in nutrient cycling and provide an abundant food source for higher trophic levels, yet they have been largely overlooked. Here, we analyzed the distribution patterns of the mobile cryptobiome (>2000 μm) along the latitudinal gradient of the Saudi Arabian coast of the Red Sea. Analysis was conducted based on 54 Autonomous Reef Monitoring Structures. We retrieved a total of 5273 organisms, from which 2583 DNA sequences from the mitochondrially encoded cytochrome c oxidase I were generated through sanger sequencing. We found that the cryptobiome community is variable over short geographical distances within the basin. Regression tree models identified sea surface temperature (SST), percentage cover of hard coral and turf algae as determinant for the number of operational taxonomic units present per Autonomous Reef Monitoring Structures (ARMS). Our results also show that the community structure of the cryptobiome is associated with the energy available (measured as photosynthetic active radiation), sea surface temperature, and nearby reef habitat characteristics (namely hard corals, turf and macroalgae). Given that temperature and reef benthic characteristics affect the cryptobiome, current scenarios of intensive climate change are likely to modify this fundamental biological component of coral reef functioning. However, the trajectory of change is unknow and can be site specific, as for example, diversity is expected to increase above SST of 28.5°C, and with decreasing hard coral and turf cover. This study provides a baseline of the cryptobenthic community prior to major coastal developments in the Red Sea to be used for future biodiversity studies and monitoring projects. It can also contribute to better understand patterns of reef biodiversity in a period where Marine Protected Areas are being discussed in the region.

A quantitative assessment of the status of benthic communities on US Atlantic coral reefs using a novel standardized approach

As coral reefs decline globally, the need for an objective approach to quantify the status and trends of corals has become increasingly important. Empirical data on predisturbance conditions are rare, and integrating data from multiple and disparate survey designs and methods can be analytically challenging. Our goal was to conduct a holistic, data-driven evaluation of the status of corals and benthic communities in US Atlantic coral reef jurisdictions: Florida, Flower Garden Banks, Puerto Rico, and the US Virgin Islands. A quantitative approach based upon standardized data was used to compare the change in multiple indicators of coral condition (hard coral, macroalgae, and crustose coralline algae cover, coral density, and old mortality) from historic to current conditions in each geographic region. For each indicator, historic, reference baseline conditions from long-term monitoring data or literature data were first identified, reviewed, and classified on a categorical scale from Very Good to Critical by regional experts to account for condition changes that pre-dated current monitoring data. A reference-centering approach then allowed for categorization of statistical changes from historic to current conditions on the same scale to produce results that could be communicated to a broad audience. Our findings show continued declines for multiple indicators in all regions except Flower Garden Banks, illustrate particularly dire declines from regions that had been impacted by Stony Coral Tissue Loss Disease at the most recent monitoring included in this study, and demonstrate the increasingly critical need for effective coral reef conservation.

Depth variation in benthic community response to repeated marine heatwaves on remote Central Indian Ocean reefs.

Coral reefs are increasingly impacted by climate-induced warming events. However, there is limited empirical evidence on the variation in the response of shallow coral reef communities to thermal stress across depths. Here, we assess depth-dependent changes in coral reef benthic communities following successive marine heatwaves from 2015 to 2017 across a 5-25 m depth gradient in the remote Chagos Archipelago, Central Indian Ocean. Our analyses show an overall decline in hard and soft coral cover and an increase in crustose coralline algae, sponge and reef pavement following successive marine heatwaves on the remote reef system. Our findings indicate that the changes in benthic communities in response to elevated seawater temperatures varied across depths. We found greater changes in benthic group cover at shallow depths (5-15 m) compared with deeper zones (15-25 m). The loss of hard coral cover was better predicted by initial thermal stress, while the loss of soft coral was associated with repeated thermal stress following successive warming events. Our study shows that benthic communities extending to 25 m depth were impacted by successive marine heatwaves, supporting concerns about the resilience of shallow coral reef communities to increasingly severe climate-driven warming events.

Unidentified fish sounds as indicators of coral reef health and comparison to other acoustic methods

As biodiversity hotspots, coral reefs are rich with sound cues. Monitoring these soundscapes for changes is essential as coral reefs decline around the world rapidly. Despite this, acoustic metrics that reliably represent reef health are still debated, and ground-truthing of methods are limited. We sought to investigate the occurrence and prevalence of fish sounds in relation to reef health, providing a foundation to compare assessment methods. We first quantified fish call rates for three U.S. Virgin Islands reefs exhibiting different community assemblages, by manually annotating fish calls across 8 days per site. Call rates were then compared with traditional visual surveys, and several acoustic methods commonly used in underwater soundscape research. Manually detected fish call rates successfully differentiated between the three reefs, capturing variation in crepuscular activity levels and predicting hard coral cover, fish abundance, and fish species richness. Meanwhile, most acoustic indices failed to parse out fine distinctions among the three sites, although sound pressure level showed the greatest correlation to call rates. Development of methods to improve the utilization of unknown fish sounds, such as automatic detection and classification tools, supports global sound library efforts and is essential to implement large-scale passive acoustic monitoring of coral reefs.

Unidentified fish sounds as indicators of coral reef health and comparison to other acoustic methods

The global decline of coral reefs is a major contributor to the global biodiversity crisis and requires improved monitoring at these critically important habitats. Non-invasive passive acoustic assessments may address this need, leveraging the rich variety and spatiotemporal variability of biological sounds present in coral reef environments and offering near-continuous temporal coverage. Despite this, acoustic metrics that reliably represent coral reef health are still debated, and ground-truthing of methods is limited. Here we investigated how the prevalence of low frequency biotic sounds (without species information) relates to coral reef health, providing a foundation from which one can compare assessment methods. We first quantified call rates of these low frequency sounds for three reefs exhibiting different community assemblages around St. John, U.S. Virgin Islands, by manually annotating presumed fish noises for 1 min every 30 min across 8 days for each site. Annotated days were selected at key points across lunar cycles. These call rates were then compared with traditional visual surveys, and several acoustic methods and indices commonly used in underwater soundscape research. We found that, overall, manually detected fish call rates successfully differentiated between the three reefs, capturing variation in crepuscular activity levels–a pattern consistent with previous work that highlights the importance of diel choruses. Moreover, fish vocal rates were predictors of hard coral cover, fish abundance, and fish species richness, while most acoustic indices failed to parse out fine distinctions among the three sites. Some, such as the Acoustic Complexity Index, failed to reveal any expected differences between sites or times of day, while the Bioacoustic Index could only identify the most acoustically active reef, otherwise having weak correlations to visual metrics. Of the indices tested, root-mean-squared sound pressure level and Acoustic Entropy, both calculated in the low frequency fish band (50–1,200 Hz), showed the strongest association with visual health measures. These findings present an important step toward using soundscape cues for reef health assessments. The limited generalizability of acoustic indices across different locations emphasizes the need for caution in their application. Therefore, it is crucial to improve methods utilizing fish sounds, such as automatic fish call detectors that are able to generalize well to new soundscapes.

Meta-analysis reveals weak associations between reef fishes and corals.

Habitat associations underpin species ecologies in high-diversity systems. Within tropical, shallow water coral reefs, the relationship between fishes and corals is arguably the most iconic and highly scrutinized. A strong relationship between fishes and reef-building hard corals is often assumed, a belief supported by studies that document the decline of reef fishes following coral loss. However, the extent of this relationship is often unclear, as evidenced by conflicting reports. Here we assess the strength of this ecological association by relying on literature that has surveyed both fishes and corals synchronously. We quantitatively synthesize 723 bivariate correlation coefficients (from 66 papers), published over 38 years, that relate fish metrics (abundance, biomass and species richness) with the percentage of hard coral cover. Remarkably, despite extensive variation, the pattern of association on a global scale reveals a predominantly positive, albeit weak (|r| < 0.4), correlation. Even for commonly hypothesized drivers of fish-coral associations, fish family and trophic group, associations were consistently weak. These findings question our assumptions regarding the strength and ubiquity of fish-coral associations, and caution against assuming a direct and omnipresent relationship between these two iconic animal groups.

Method for Calculating the Coral Health Index (Case Study: Mapur Island, Bintan, Indonesia)

This study evaluates the health and condition of coral reefs in the Mapur Island - Bintan Conservation Area, located in the Riau Islands Province, Indonesia. The research was conducted at 5 site locations using the Underwater Photo Transect (UPT) method, with a focus on hard coral cover and the Coral Health Index (CHI). Coral lifeform data were analyzed using Coral Point Count with Excel extensions (CPCe) software. The results revealed significant variability in coral cover, ranging from 14.47% to 59.27%, with an average of 39.36%, placing the coral reefs in the “Moderate” category. However, despite high hard coral cover at some sites, coral health varied, with certain locations exhibiting significant disease prevalence. Among the 5 sites surveyed for CHI, four were classified as “healthy,” while one were categorized “diseased.” Statistical analyses, including Welch’s t-test and Bayesian inference, indicated no significant difference between the CHI and hard coral cover, suggesting that high coral cover does not necessarily imply a healthier reef. Diseases such as Pink- Blotch Syndrome, Black Band Disease, and White Syndrome were prevalent across several sites, highlighting the influence of environmental stressors on coral health. The study underscores the importance of regular coral health monitoring and the need for conservation efforts to mitigate the impact of coral diseases and climate change. These findings provide crucial insights into the status of coral reefs in the Mapur Island - Bintan Conservation Area and contribute to broader efforts to preserve marine biodiversity.

Benthic and fish community composition on mesophotic reefs in Grand Cayman.

Mesophotic Coral Ecosystems (MCEs) represent unique ecological habitats that range from 30 to 150 m deep, harbouring phylogenetically distinct species and offering refuge for many taxa during times of environmental stress. Yet owing to inaccessibility of ecosystems at these depths, most MCEs remain unexplored, with quantifications of ecological communities in these habitats lacking across many regions. Here, using open- and closed-circuit technical diving, we quantified benthic and fish community composition at four mesophotic reef sites (45 m depth) in Grand Cayman. We show significant differences in benthic community composition over a small spatial scale driven by disparate coverage of sponges, crustose coralline algae, and sand/rubble, yet consistent patterns of macroalgal dominance representing >50% coverage at each site and low hard coral cover at an average of 2.4%. Reef fish species richness, biomass, and density was consistent across sites, however the relative contribution of individual species to community composition differed significantly. Macrocarnivores were found to be the dominant contributors to biomass, with invertivores the most speciose, and omnivores and planktivores at the highest densities, consistent with previous descriptions of mesophotic fish assemblages in other regions. Similarly, the low hard coral cover and high macroalgae and sponge cover of the benthic communities also appear ecologically similar to several described mesophotic reefs yet is not uniform across the Caribbean. The ecological organisation of Grand Cayman's MCEs may result from a variety of factors such as isolation from other major land masses, geology, local geography, and anthropogenic activity at both the local and global scale and highlight the importance of continued exploration and documentation of MCE communities.

Evaluation of Coral Reef Health Status in the Fish Sanctuary of San Esteban, Ilocos Sur

Studies on corals are lacking in Ilocos Sur’s marine protected areas (MPAs), hindering management effort development. This study evaluated the health status of coral reefs in the San Esteban Fish Sanctuary. Hard coral cover, hard coral diversity in terms of Taxonomic Amalgamation Units (TAUs), and coral reef health status in terms of live coral coverage, algae coverage, sand coverage, and coral mortality index were measured in two sampling stations inside an over-a-decade-old fish sanctuary that does not have previous data on its coral reef health status. The Underwater Photo Transect (UPT) method was employed to take image samples subjected to the Coral Point Count with Excel extensions (CPCe) software for analysis. The study revealed that the coral reef in San Esteban Fish Sanctuary is in good condition with a hard coral cover of 30.0%, falls under Hard Coral Cover Category C, and a hard coral diversity of 16.8TAUs that falls under Diversity Category D. The fish sanctuary is also in a healthy condition based on excellent live coral coverage, algae coverage, sand coverage, and low coral mortality index. The two stations are more or less the same in terms of the aforementioned parameters. Overall, it is recommended to continuously strengthen the enforcement of the fish sanctuary to protect its coral reefs, measure other parameters that might impact and indicate coral reef health status, employ evaluations inside and outside MPAs in the province, and use high-resolution cameras during image sampling for more efficient analysis.

Bleaching threatens positive carbonate budgets on Bahraini reefs

Bleaching events impact coral reef functionality and carbonate budget dynamics, which is reflected in reduced reef framework accretion, hindering reef ability to keep pace with sea-level rise projections. Reefs in the Arabian Gulf exist in harsh environmental conditions with seasonal temperatures ranging between 16 and 36 °C. Despite the high thermal thresholds (~ 35 °C) of corals in this region, extensive bleaching and high coral mortality have been reported regionally in 2017. We quantify reef carbonate budgets at a near shore and an offshore reef site in Bahrain pre-bleaching in May 2017 and post-bleaching in May 2018 to assess the impacts of the 2017 severe bleaching event on the budgetary state. Results indicate an overall decrease in hard coral cover from 14.2 ± 5.5 to 8.5 ± 1.4% and a decline in the net carbonate budget state from 3.6 ± 2.2 to 0.3 ± 0.3 kg CaCO3 m−2 year−1 at the shallow nearshore Fasht Al Adhm reef indicating a shift from a positive budgetary state to net neutral, while the deeper offshore Reef Bul Thamah has increased positively from 3.7 ± 1.2 to 4.2 ± 0.6 kg CaCO3 m−2 year−1. We attribute the decline in the nearshore reef to the bleaching event which took place between July and October 2017, resulting in high coral mortality rates and subsequent reduced framework carbonate production. Predicted warming trends present a threat to the structural integrity of shallow Bahraini reefs, compromising their ability to keep pace with future sea-level rise projections.

Evaluating the effects of climate change and chemical, physical, and biological stressors on nearshore coral reefs: A case study in the Great Barrier Reef, Australia.

An understanding of the combined effects of climate change (CC) and other anthropogenic stressors, such as chemical exposures, is essential for improving ecological risk assessments of vulnerable ecosystems. In the Great Barrier Reef, coral reefs are under increasingly severe duress from increasing ocean temperatures, acidification, and cyclone intensities associated with CC. In addition to these stressors, inshore reef systems, such as the Mackay-Whitsunday coastal zone, are being impacted by other anthropogenic stressors, including chemical, nutrient, and sediment exposures related to more intense rainfall events that increase the catchment runoff of contaminated waters. To illustrate an approach for incorporating CC into ecological risk assessment frameworks, we developed an adverse outcome pathway network to conceptually delineate the effects of climate variables and photosystem II herbicide (diuron) exposures on scleractinian corals. This informed the development of a Bayesian network (BN) to quantitatively compare the effects of historical (1975-2005) and future projected climate on inshore hard coral bleaching, mortality, and cover. This BN demonstrated how risk may be predicted for multiple physical and biological stressors, including temperature, ocean acidification, cyclones, sediments, macroalgae competition, and crown of thorns starfish predation, as well as chemical stressors such as nitrogen and herbicides. Climate scenarios included an ensemble of 16 downscaled models encompassing current and future conditions based on multiple emission scenarios for two 30-year periods. It was found that both climate-related and catchment-related stressors pose a risk to these inshore reef systems, with projected increases in coral bleaching and coral mortality under all future climate scenarios. This modeling exercise can support the identification of risk drivers for the prioritization of management interventions to build future resilient reefs. Integr Environ Assess Manag 2024;20:401-418. © 2023 Norwegian Institute for Water Research and The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC). This article has been contributed to by U.S. Government employees and their work is in the public domain in the USA.