Macroalgal feedbacks and substrate properties maintain a coral reef regime shift

Inshore coral reef sediment and turf dynamics unaffected by canopy-forming macroalgae

Nearshore coral reefs face an increasing abundance of fleshy macroalgae, an indicator of degradation and threat to ecosystem functioning. Removal of macroalgae is proposed to assist coral recovery, though the ecological and physical impacts have not been studied. Nearshore reefs are also confronted with sedimentation stress, influencing reef dynamics including algal turfs, with flow-on impacts to coral recruitment, fish diets, and trophic cascades. In this study, the interplay between macroalgal canopies, sediment deposition and algal turf environments was investigated on the nearshore Great Barrier Reef. Removal of fleshy macroalgae over two years had no significant effect on the amount or composition of sediment deposited on proxy coral (SedPods) and algal turf (TurfPods) surfaces, nor was the height of algal turfs impacted. Deposition on TurfPods was greater with high-energy currents, likely due to retention of sediment within turfs. Therefore, macroalgae removal is unlikely to exacerbate nor alleviate sediment-related stress on benthic communities.

Decadal demographic shifts and size-dependent disturbance responses of corals in a subtropical warming hotspot

Long-term demographic studies at biogeographic transition zones can elucidate how body size mediates disturbance responses. Focusing on subtropical reefs in eastern Australia, we examine trends in the size-structure of corals with contrasting life-histories and zoogeographies surrounding the 2016 coral bleaching event (2010-2019) to determine their resilience and recovery capacity. We document demographic shifts, with disproportionate declines in the number of small corals and long-term persistence of larger corals. The incidence of bleaching (Pocillopora, Turbinaria) and partial mortality (Acropora, Pocillopora) increased with coral size, and bleached corals had greater risk of partial mortality. While endemic Pocillopora experienced marked declines, decadal stability of Turbinaria despite bleaching, coupled with abundance increase and bleaching resistance in Acropora indicate remarkable resilience of these taxa in the subtropics. Declines in the number of small corals and variable associations with environmental drivers indicate bottlenecks to recovery mediated by inhibitory effects of thermal extremes for Pocillopora (heat stress) and Acropora (heat and cold stress), and stimulatory effects of chlorophyll-a for Turbinaria. Although our study reveals signs of resilience, it foreshadows the vulnerability of subtropical corals to changing disturbance regimes that include marine heatwaves. Disparity in population dynamics suggest that subtropical reefs are ecologically distinct from tropical coral reefs.

Hierarchical settlement behaviours of coral larvae to common coralline algae

Natural regeneration of degraded reefs relies on the recruitment of larvae to restore populations. Intervention strategies are being developed to enhance this process through aquaculture production of coral larvae and their deployment as spat. Larval settlement relies on cues associated with crustose coralline algae (CCA) that are known to induce attachment and metamorphosis. To understand processes underpinning recruitment, we tested larval settlement responses of 15 coral species, to 15 species of CCA from the Great Barrier Reef (GBR). CCA in the family Lithophyllaceae were overall the best inducer across most coral species, with Titanoderma cf. tessellatum being the most effective species that induced at least 50% settlement in 14 of the coral species (mean 81%). Taxonomic level associations were found, with species of Porolithon inducing high settlement in the genus Acropora; while a previously understudied CCA, Sporolithon sp., was a strong inducer for the Lobophyllidae. Habitat-specific associations were detected, with CCA collected from similar light environment as the coral inducing higher levels of settlement. This study revealed the intimate relationships between coral larvae and CCA and provides optimal coral-algal species pairings that could be utilized to increase the success of larval settlement to generate healthy spat for reef restoration.

Macroalgal cover on coral reefs: Spatial and environmental predictors, and decadal trends in the Great Barrier Reef

Macroalgae are an important component of coral reef ecosystems. We identified spatial patterns, environmental drivers and long-term trends of total cover of upright fleshy and calcareous coral reef inhabiting macroalgae in the Great Barrier Reef. The spatial study comprised of one-off surveys of 1257 sites (latitude 11-24°S, coastal to offshore, 0-18 m depth), while the temporal trends analysis was based on 26 years of long-term monitoring data from 93 reefs. Environmental predictors were obtained from in situ data and from the coupled hydrodynamic-biochemical model eReefs. Macroalgae dominated the benthos (≥50% cover) on at least one site of 40.4% of surveyed inshore reefs. Spatially, macroalgal cover increased steeply towards the coast, with latitude away from the equator, and towards shallow (≤3 m) depth. Environmental conditions associated with macroalgal dominance were: high tidal range, wave exposure and irradiance, and low aragonite saturation state, Secchi depth, total alkalinity and temperature. Evidence of space competition between macroalgal cover and hard coral cover was restricted to shallow inshore sites. Temporally, macroalgal cover on inshore and mid-shelf reefs showed some fluctuations, but unlike hard corals they showed no systematic trends. Our extensive empirical data may serve to parameterize ecosystem models, and to refine reef condition indices based on macroalgal data for Pacific coral reefs.

Benthic composition changes on coral reefs at global scales

Globally, ecosystems are being reconfigured by a range of intensifying human-induced stressors. Coral reefs are at the forefront of this environmental transformation, and if we are to secure their key ecosystem functions and services, it is important to understand the likely configuration of future reefs. However, the composition and trajectory of global coral reef benthic communities is currently unclear. Here our global dataset of 24,468 observations spanning 22 years (1997-2018) revealed that particularly marked declines in coral cover occurred in the Western Atlantic and Central Pacific. The data also suggest that high macroalgal cover, widely regarded as the major degraded state on coral reefs, is a phenomenon largely restricted to the Western Atlantic. At a global scale, the raw data suggest decreased average (± standard error of the mean) hard coral cover from 36 ± 1.4% to 19 ± 0.4% (during a period delineated by the first global coral bleaching event (1998) until the end of the most recent event (2017)) was largely associated with increased low-lying algal cover such as algal turfs and crustose coralline algae. Enhanced understanding of reef change, typified by decreased hard coral cover and increased cover of low-lying algal communities, will be key to managing Anthropocene coral reefs.

Cascading effects of climate change on recreational marine flats fishes and fisheries

Tropical and subtropical coastal flats are shallow regions of the marine environment at the intersection of land and sea. These regions provide myriad ecological goods and services, including recreational fisheries focused on flats-inhabiting fishes such as bonefish, tarpon, and permit. The cascading effects of climate change have the potential to negatively impact coastal flats around the globe and to reduce their ecological and economic value. In this paper, we consider how the combined effects of climate change, including extremes in temperature and precipitation regimes, sea level rise, and changes in nutrient dynamics, are causing rapid and potentially permanent changes to the structure and function of tropical and subtropical flats ecosystems. We then apply the available science on recreationally targeted fishes to reveal how these changes can cascade through layers of biological organization-from individuals, to populations, to communities-and ultimately impact the coastal systems that depend on them. We identify critical gaps in knowledge related to the extent and severity of these effects, and how such gaps influence the effectiveness of conservation, management, policy, and grassroots stewardship efforts.

Priority effects in coral-macroalgae interactions can drive alternate community paths in the absence of top-down control

The outcomes of species interactions can vary greatly in time and space with the outcomes of some interactions determined by priority effects. On coral reefs, benthic algae rapidly colonize disturbed substrate. In the absence of top-down control from herbivorous fishes, these algae can inhibit the recruitment of reef-building corals, leading to a persistent phase shift to a macroalgae-dominated state. Yet, corals may also inhibit colonization by macroalgae, and thus the effects of herbivores on algal communities may be strongest following disturbances that reduce coral cover. Here, we report results from experiments conducted on the fore reef of Moorea, French Polynesia, where we: 1) tested the ability of macroalgae to invade coral-dominated and coral-depauperate communities under different levels of herbivory, 2) explored the ability of juvenile corals (Pocillopora spp.) to suppress macroalgae, and 3) quantified the direct and indirect effects of fish herbivores and corallivores on juvenile corals. We found that macroalgae proliferated when herbivory was low but only in recently disturbed communities where coral cover was also low. When coral cover was < 10%, macroalgae increased 20-fold within one year under reduced herbivory conditions relative to high herbivory controls. Yet, when coral cover was high (50%), macroalgae were suppressed irrespective of the level of herbivory despite ample space for algal colonization. Once established in communities with low herbivory and low coral cover, macroalgae suppressed recruitment of coral larvae, reducing the capacity for coral replenishment. However, when we experimentally established small juvenile corals (2 cm diameter) following a disturbance, juvenile corals inhibited macroalgae from invading local neighborhoods, even in the absence of herbivores, indicating a strong priority effect in macroalgae-coral interactions. Surprisingly, fishes that initially facilitated coral recruitment by controlling algae had a net negative effect on juvenile corals via predation. Corallivores reduced growth rates of corals exposed to fishes by ~ 30% relative to fish exclosures despite increased competition with macroalgae within the exclosures. These results highlight that different processes are important for structuring coral reef ecosystems at different successional stages and underscore the need to consider multiple ecological processes and historical contingencies to predict coral community dynamics.

The role of habitat in the facilitation of Ostreopsis spp. blooms

In recent decades, recurrent Ostreopsis spp. blooms have been recorded throughout the globe, causing public health issues and mass mortalities of invertebrates. Ostreopsis species are benthic and develop in shallow waters in close relation with a substrate, but possible substrate preferences are still ambiguous. Bloom develops on both living and dead substrates and several interacting biotic and abiotic factors acting at different spatial scales can potentially foster or regulate Ostreopsis spp. development. The objective of this review is to collect and summarize information on Ostreopsis spp. blooms related to the habitat at different spatial scales, in order to assess preferences and trends. References including Ostreopsis spp. samplings in the field were analysed in this review, as potentially including information about the micro- (substrate), meso‑ (community) and macrohabitat (ecosystem) related to Ostreopsis spp. blooms. The sampled substrate and the ecosystem where Ostreopsis spp. were collected were generally reported and described in the studies, while the description of the mesohabitat was rarely reported. Ostreopsis spp. were generally described as attached to biotic substrates and in particular, macroalgae, even in studies conducted in coral reefs, where macroalgae are generally not dominant (but they can be in case of coral reef degradation). In both temperate and tropical areas, Ostreopsis spp. were mostly sampled on algal species usually forming medium or low complexity communities (erect or turf-forming algae), often characteristic from post-regime shift scenarios, and rarely on canopy-forming species (such as fucoids and kelps). This literature review highlights the need of collecting more information about the mesohabitat where important Ostreopsis spp. blooms develop, as much as of the underlying mechanisms driving eventual differences on Ostreopsis spp. abundances. This knowledge would allow a better risk assessment of Ostreopsis spp. blooms, identifying areas at high risk on the base of the benthic habitats.

Coral reef benthic community changes in the Anthropocene: Biogeographic heterogeneity, overlooked configurations, and methodology

Non-random community changes are becoming more frequent in many ecosystems. In coral reefs, changes towards communities dominated by other than hard corals are increasing in frequency, with severe impacts on ecosystem functioning and provision of ecosystem services. Although new research suggests that a variety of alternative communities (i.e. not dominated by hard corals) exist, knowledge on the global diversity and functioning of alternative coral reef benthic communities, especially those not dominated by algae, remains scattered. In this systematic review and meta-analysis of 523 articles, we analyse the different coral reef benthic community changes reported to date and discuss the advantages and limitations of the methods used to study these changes. Furthermore, we used field cover data (1116 reefs from the ReefCheck database) to explore the biogeographic and latitudinal patterns in dominant benthic organisms. We found a mismatch between literature focus on coral-algal changes (over half of the studies analysed) and observed global natural patterns. We identified strong biogeographic patterns, with the largest and most biodiverse biogeographic regions (Western and Central Indo-Pacific) presenting previously overlooked soft-coral-dominated communities as the most abundant alternative community. Finally, we discuss the potential biases associated with methods that overlook ecologically important cryptobenthic communities and the potential of new technological advances in improving monitoring efforts. As coral reef communities inevitably and swiftly change under changing ocean conditions, there is an urgent need to better understand the distribution, dynamics as well as the ecological and societal impacts of these new communities.

First insights into coral recruit and juvenile abundances at remote Aldabra Atoll, Seychelles

Coral recruitment and successive growth are essential for post-disturbance reef recovery. As coral recruit and juvenile abundances vary across locations and under different environmental regimes, their assessment at remote, undisturbed reefs improves our understanding of early life stage dynamics of corals. Here, we first explored changes in coral juvenile abundance across three locations (lagoon, seaward west and east) at remote Aldabra Atoll (Seychelles) between 2015 and 2019, which spanned the 2015/16 global coral bleaching event. Secondly, we measured variation in coral recruit abundance on settlement tiles from two sites (lagoon, seaward reef) during August 2018-August 2019. Juvenile abundance decreased from 14.1 ± 1.2 to 7.4 ± 0.5 colonies m-2 (mean ± SE) during 2015-2016 and increased to 22.4 ± 1.2 colonies m-2 during 2016-2019. Whilst juvenile abundance increased two- to three-fold at the lagoonal and seaward western sites during 2016-2018 (from 7.7-8.3 to 17.3-24.7 colonies m-2), increases at the seaward eastern sites occurred later (2018-2019; from 5.8-6.9 to 16.6-24.1 colonies m-2). The composition of coral recruits on settlement tiles was dominated by Pocilloporidae (64-92% of all recruits), and recruit abundance was 7- to 47-fold higher inside than outside the lagoon. Recruit abundance was highest in October-December 2018 (2164 ± 453 recruits m-2) and lowest in June-August 2019 (240 ± 98 recruits m-2). As Acroporid recruit abundance corresponded to this trend, the results suggest that broadcast spawning occurred during October-December, when water temperature increased from 26 to 29°C. This study provides the first published record on coral recruit abundance in the Seychelles Outer Islands, indicates a rapid (2-3 years) increase of juvenile corals following a bleaching event, and provides crucial baseline data for future research on reef resilience and connectivity within the region.

Substrate stabilisation and small structures in coral restoration: State of knowledge, and considerations for management and implementation

Coral reef ecosystems are under increasing pressure from local and regional stressors and a changing climate. Current management focuses on reducing stressors to allow for natural recovery, but in many areas where coral reefs are damaged, natural recovery can be restricted, delayed or interrupted because of unstable, unconsolidated coral fragments, or rubble. Rubble fields are a natural component of coral reefs, but repeated or high-magnitude disturbances can prevent natural cementation and consolidation processes, so that coral recruits fail to survive. A suite of interventions have been used to target this issue globally, such as using mesh to stabilise rubble, removing the rubble to reveal hard substrate and deploying rocks or other hard substrates over the rubble to facilitate recruit survival. Small, modular structures can be used at multiple scales, with or without attached coral fragments, to create structural complexity and settlement surfaces. However, these can introduce foreign materials to the reef, and a limited understanding of natural recovery processes exists for the potential of this type of active intervention to successfully restore local coral reef structure. This review synthesises available knowledge about the ecological role of coral rubble, natural coral recolonisation and recovery rates and the potential benefits and risks associated with active interventions in this rapidly evolving field. Fundamental knowledge gaps include baseline levels of rubble, the structural complexity of reef habitats in space and time, natural rubble consolidation processes and the risks associated with each intervention method. Any restoration intervention needs to be underpinned by risk assessment, and the decision to repair rubble fields must arise from an understanding of when and where unconsolidated substrate and lack of structure impair natural reef recovery and ecological function. Monitoring is necessary to ascertain the success or failure of the intervention and impacts of potential risks, but there is a strong need to specify desired outcomes, the spatial and temporal context and indicators to be measured. With a focus on the Great Barrier Reef, we synthesise the techniques, successes and failures associated with rubble stabilisation and the use of small structures, review monitoring methods and indicators, and provide recommendations to ensure that we learn from past projects.

Early trajectories of benthic coral reef communities following the 2015/16 coral bleaching event at remote Aldabra Atoll, Seychelles

Documenting post-bleaching trajectories of coral reef communities is crucial to understand their resilience to climate change. We investigated reef community changes following the 2015/16 bleaching event at Aldabra Atoll, where direct human impact is minimal. We combined benthic data collected pre- (2014) and post-bleaching (2016–2019) at 12 sites across three locations (lagoon, 2 m depth; seaward west and east, 5 and 15 m depth) with water temperature measurements. While seaward reefs experienced relative hard coral reductions of 51–62%, lagoonal coral loss was lower (− 34%), probably due to three-fold higher daily water temperature variability there. Between 2016 and 2019, hard coral cover did not change on deep reefs which remained dominated by turf algae and Halimeda, but absolute cover on shallow reefs increased annually by 1.3% (east), 2.3% (west) and 3.0% (lagoon), reaching, respectively, 54%, 68% and 93% of the pre-bleaching cover in 2019. Full recovery at the shallow seaward locations may take at least five more years, but remains uncertain for the deeper reefs. The expected increase in frequency and severity of coral bleaching events is likely to make even rapid recovery as observed in Aldabra’s lagoon too slow to prevent long-term reef degradation, even at remote sites.

Long-term dynamics and drivers of coral and macroalgal cover on inshore reefs of the Great Barrier Reef Marine Park

Quantifying the role of biophysical and anthropogenic drivers of coral reef ecosystem processes can inform management strategies that aim to maintain or restore ecosystem structure and productivity. However, few studies have examined the combined effects of multiple drivers, partitioned their impacts, or established threshold values that may trigger shifts in benthic cover. Inshore fringing reefs of the Great Barrier Reef Marine Park (GBRMP) occur in high-sediment, high-nutrient environments and are under increasing pressure from multiple acute and chronic stressors. Despite world-leading management, including networks of no-take marine reserves, relative declines in hard coral cover of 40-50% have occurred in recent years, with localized but persistent shifts from coral to macroalgal dominance on some reefs. Here we use boosted regression tree analyses to test the relative importance of multiple biophysical drivers on coral and macroalgal cover using a long-term (12-18 yr) data set collected from reefs at four island groups. Coral and macroalgal cover were negatively correlated at all island groups, and particularly when macroalgal cover was above 20%. Although reefs at each island group had different disturbance-and-recovery histories, degree heating weeks (DHW) and routine wave exposure consistently emerged as common drivers of coral and macroalgal cover. In addition, different combinations of sea-surface temperature, nutrient and turbidity parameters, exposure to high turbidity (primary) floodwater, depth, grazing fish density, farming damselfish density, and management zoning variously contributed to changes in coral and macroalgal cover at each island group. Clear threshold values were apparent for multiple drivers including wave exposure, depth, and degree heating weeks for coral cover, and depth, degree heating weeks, chlorophyll a, and cyclone exposure for macroalgal cover, however, all threshold values were variable among island groups. Our findings demonstrate that inshore coral reef communities are typically structured by broadscale climatic perturbations, superimposed upon unique sets of local-scale drivers. Although rapidly escalating climate change impacts are the largest threat to coral reefs of the GBRMP and globally, our findings suggest that proactive management actions that effectively reduce chronic stressors at local scales should contribute to improved reef resistance and recovery potential following acute climatic disturbances.

Spatial variability in benthic assemblage composition in shallow and upper mesophotic coral ecosystems in the Philippines

Mesophotic coral ecosystems (MCEs) have received increasing attention in recent years in recognition of their unique biodiversity and also their potential importance as refuges from disturbance events. However, knowledge of the composition of MCEs and how they vary in space is lacking in many regions, particularly the Coral Triangle biodiversity hotspot. Here, we compared the benthic components and coral genera composition between shallow-water reefs (SWRs, 8-13 m depth) and upper MCEs (30-40 m) in four locations in the Philippines that are exposed to differing environmental conditions. Coral cover, abundance, and generic diversity were lower in MCEs than SWRs at three of the four locations. Benthic composition and coral generic composition also varied significantly among locations for both shallow and deep sites. Differences in benthic composition among sites was due primarily to variation in hard corals, macroalgae, sand and silt, while variation in coral assemblage was due to differences in abundance of encrusting Porites, branching Acropora, branching Seriatopora. Our results showed that the composition of MCE communities varied significantly from adjacent shallow reefs, but also among MCEs in differing geographic locations. Furthermore, our results suggest disturbances affecting shallow-water reefs, particularly sedimentation, also negatively impact MCEs, and that depth therefore provides no potential refuge from these disturbances. We recommend that conservation of MCEs consider spatial variability in community composition among sites, and urge further research to better understand the spatial variation in the composition of MCE communities in the Philippines.

Spatial and Temporal Variation in Fecundity of Acropora spp. in the Northern Great Barrier Reef

The amount of energy invested in sexual reproduction by scleractinian corals depends on their life history strategies (i.e., allocation of energy between growth, reproduction, and maintenance). However, energy allocated to reproduction will also be affected by the amount of energy acquired and prevailing environmental conditions. Coral fecundity is therefore likely to vary spatially, especially along marked gradients in environmental conditions. One of the foremost gradients in reef structure and environmental conditions occurs with distance from the coast, whereby inner-shelf or near shore reefs are generally subject to higher levels of nutrients, sediments and pollutants, which often adversely affect reef-building corals. This study quantified fecundity (oocytes per polyp) for three species, Acropora nasuta, A. spathulata, and A. hyacinthus, at six locations in the northern Great Barrier Reef (GBR), encompassing inner-, mid- and outer-shelf reefs. Replicate colonies were sampled at each location prior to the predicted date of spawning in 2013 and 2014. Both shelf position and year were important factors explaining variation in fecundity for each of the three coral species. Most notably, there were clear and consistent declines in the number of oocytes between 2013 and 2014, coinciding with the incidence of category 4 Cylone Ita in early 2014. Contrary to expectations, polyp-level fecundity was no lower (and in some cases substantially higher) on inner-shelf reefs, compared to conspecifics growing on mid-shelf or outer-shelf reefs. The observed patterns are much more complicated than anticipated, necessitating further research to understand differential population dynamics of corals on inner-shelf versus mid- and outer-shelf reefs.

Adapting to extreme environments: can coral reefs adapt to climate change?

Reef-building corals throughout the world have an annual value of tens of billions of dollars, yet they are being degraded at an increasing rate by many anthropogenic and environmental factors. Despite this, some reefs show resilience to such extreme environmental changes. This review shows how techniques in computational modelling, genetics, and transcriptomics are being used to unravel the complexity of coral reef ecosystems, to try and understand if they can adapt to new and extreme environments. Considering the ambitious climate targets of the Paris Agreement to limit global warming to 2°C, with aspirations of even 1.5°C, questions arise on how to achieve this. Geoengineering may be necessary if other avenues fail, although global governance issues need to play a key role. Development of large and effective coral refugia and marine protected areas is necessary if we are not to lose this vital resource for us all.

Acute drivers influence recent inshore Great Barrier Reef dynamics

Understanding the dynamics of habitat-forming organisms is fundamental to managing natural ecosystems. Most studies of coral reef dynamics have focused on clear-water systems though corals inhabit many turbid regions. Here, we illustrate the key drivers of an inshore coral reef ecosystem using 10 years of biological, environmental, and disturbance data. Tropical cyclones, crown-of-thorns starfish, and coral bleaching are recognized as the major drivers of coral loss at mid- and offshore reefs along the Great Barrier Reef (GBR). In comparison, little is known about what drives temporal trends at inshore reefs closer to major anthropogenic stress. We assessed coral cover dynamics using state-space models within six major inshore GBR catchments. An overall decline was detected in nearly half (46%) of the 15 reefs at two depths (30 sites), while the rest exhibited fluctuating (23%), static (17%), or positive (13%) trends. Inshore reefs responded similarly to their offshore counterparts, where contemporary trends were predominantly influenced by acute disturbance events. Storms emerged as the major driver affecting the inshore GBR, with the effects of other drivers such as disease, juvenile coral density, and macroalgal and turf per cent cover varying from one catchment to another. Flooding was also associated with negative trends in live coral cover in two southern catchments, but the mechanism remains unclear as it is not reflected in available metrics of water quality and may act through indirect pathways.

Coral reef degradation affects the potential for reef recovery after disturbance

The loss of coral cover is often accompanied by an increase of benthic algae, a decline in biodiversity and habitat complexity. However, it remains unclear how surrounding communities influence the trajectories of re-colonization between pulse disturbance events. Over a 12-month field experiment in the central Red Sea, we examined how healthy (hard-coral dominated) and degraded (algae-dominated) reef areas influence recruitment and succession patterns of benthic reef foundation communities on bare substrates. Crustose coralline algae and other calcifiers were important colonizers in the healthy reef area, promoting the accumulation of inorganic carbon. Contrary, substrates in the degraded area were predominantly colonized by turf algae, lowering the accumulation of inorganic carbon by 178%. While coral larvae settlement similarly occurred in both habitats, degraded areas showed 50% fewer recruits. Our findings suggest that in degraded reefs the replenishment of adult coral populations is reduced due to recruitment inhibition through limited habitat complexity and grazing pressure, thereby restraining reef recovery.