Coral-algal Phase Shifts Research Articles

Degradation of coral reef ecosystems: Mathematical-dynamical modeling approach

Abstract The review delves into the frontier of degradation of coral reef ecosystems, a pressing urgent issue to be addressed in marine ecological environments, and provides a critical assessment of current advances and future perspective on the topic. This study highlights the recent developments in coral bleaching, coral-algal phase shifts, coral disease, and natural enemy bloom, all of which are thoroughly documented and expounded. Special attention focus on the studies employing mathematical-dynamical modeling approaches. This review study also offers some valuable insights, illuminating ideas, and clear perspective into some current, emerging, and promising research directions for future studies, particularly those based on mathematical-biological modeling approaches. These approaches are anticipated to offer a theoretical foundation and provide crucial decision-making reference for the protection, restoration, and management of coral reef ecosystems.

The role of sea hares as significant algal herbivores on the Southern Great Barrier Reef

Sea hares are cryptic, herbivorous sea slugs present on many coral reefs worldwide. Although they are known to consume high quantities of macroalgae, they are rarely considered or quantified in assessing algal suppression. Their role as algal suppressors is not well understood, therefore this study sought to quantify sea hare herbivory on the abundant red algal genus Laurencia. The sea hares Aplysia dactylomela, Dolabella auricularia and Dolabella sp. were collected and used in herbivory trials in aquaria at Heron Island on the Southern Great Barrier Reef to quantify their rate of consumption. A. dactylomela consumed 76.9 g partially dried weight of Laurencia per 24-h, which was significantly more than the other species, with D. auricularia consuming an average of 22.5 g and Dolabella sp. consuming 37.4 g partially dried weight. Consumption of Laurencia by A. dactylomela increased with body size, with the exception of the smallest animals, which ate up to 12.5 times their body weight. D. auricularia consumed significantly more algae than was lost in control tanks. Although we observed Dolabella sp. feeding on Laurencia, the amount consumed was not significantly different to loss of algae in control tanks. The rate of herbivory on Laurencia by A. dactylomela is comparable or greater than that of more commonly studied reef herbivores such as fishes and urchins, which suggests that they may contribute to algal suppression on coral reefs and further studies are warranted to evaluate their possible role in coral-algal phase shift dynamics.

The effects of atmospheric nitrogen deposition in coral-algal phase shifts on remote coral reefs

Remote seawater has been considered a potential refuge for corals in the face of anthropogenic disturbances. However, these remote areas may receive increased atmospheric N deposition, and the ecological consequences remain unclear. This field survey revealed coral-algal phase shifts in the mid-north of the South China Sea. These shifts were observed in 44%, 13.6%, and 26.5% of the sampled reef sites at depths of 1-4 m, 5-8 m, and 10-15 m, respectively. Over 50% of sections in the deeper depths hosted fewer corals compared to shallower areas, coinciding with a higher abundance of macroalgae in the deeper layers. Furthermore, based on long-term observation of atmospheric N flux, laboratory experiments were conducted to explore the cause of coral declines. The results indicate that N supply efficiently promoted macroalgae growth. The saturation of N absorption by macroalgae occurred within 2 weeks, leading to nutrient accumulation in seawater, especially nitrate, which had a direct impact on corals. While moderate N fluxes appeared to mitigate coral bleaching, high N fluxes, even with a balanced P supply or medium level of nutrients with an imbalanced N/P ratio, can both increase the susceptibility of corals to heat bleaching. This study explains the coral-algal phase shift in remote and relatively deep seawater and improves understanding of the cause-and-effect relationship between atmospheric N deposition and coral reef decline.

Evidence of Coral Diseases, Phase Shift, and Stressors in the Atolls of Lakshadweep Islands, Arabian Sea—With Geographical Notes on Their Occurrence within the Indian EEZ and Contiguous International Waters

Photographic evidence of some important coral diseases (black band disease, black disease/Terpios hoshinota, white syndrome, pink line syndrome, pink spots, invertebrate galls, skeletal growth anomalies, tissue loss), coral competing sponges, and coral–algal phase shifts (competitive overgrowth of the seaweed Caulerpa spp. over corals and competitive scleractinian interactions such as with Halimeda spp.) have been collected during field observations in a few atolls within the Lakshadweep archipelago, Arabian Sea. Further, earlier reports of similar diseases and other stressors within the Indian Exclusive Economic Zone (EEZ) and its contiguous international waters, including the reefs of the Maldives and Sri Lanka, are highlighted and their distributional ranges are shown.

The Pattern of Herbivorous Fish Assemblages in The In Western and Eastern Outermost Island Indonesia

The herbivorous fishes have been considered as a critical functional group and have capability maintaining coral reef resilience and avoiding coral-algal phase-shifts. The present condition shown, almost in tropical reef location, alga has dominated coral, even in the small outer island. The requirement to conduct comprehensive basic research in studying the patterns and composition of herbivorous fish, especially on the small outer islands. Twelve coral reef sites in eastern Indonesia (Liki Islands) and western Indonesia (Natuna Island) used as a research location for comparing the structure patterns of herbivorous fish communities (diversity, density, and body size) using the Underwater Visual Census (UVC) method. There was different pattern of herbivorous fishes families in Liki Island and Natuna Islands, where Acanthuridae is dominant in eastern Indonesia (Liki Islands), including Ctenochaetus striatus (41,00 ± 11,72 se) individuals/350m2, A. maculiceps (23,33 ± 13,61 se) individuals/350m2, Naso hexacanthus (18,67 ± 6,34 se) individuals/350m2 while Scaridae is dominant in western Indonesia (Natuna island), including Scarus rivulatus (31,67 ± 10,61 se) individuals/350m2, Chlorurus sordidus (30,00 ± 8,52 se) individuals/350m2 and Scarus quoyi (19,00 ± 9,73 se) individuals/350m2. Based on herbivore fishes composition Liki Island has a higher density and biomass compared to Natuna Island.Keywords: herbivore, fish, coral, small outer island, Indonesia

High summer temperatures amplify functional differences between coral- and algae-dominated reef communities.

Shifts from coral to algal dominance are expected to increase in tropical coral reefs as a result of anthropogenic disturbances. The consequences for key ecosystem functions such as primary productivity, calcification, and nutrient recycling are poorly understood, particularly under changing environmental conditions. We used a novel in situ incubation approach to compare functions of coral‐ and algae‐dominated communities in the central Red Sea bimonthly over an entire year. In situ gross and net community primary productivity, calcification, dissolved organic carbon fluxes, dissolved inorganic nitrogen fluxes, and their respective activation energies were quantified to describe the effects of seasonal changes. Overall, coral‐dominated communities exhibited 30% lower net productivity and 10 times higher calcification than algae‐dominated communities. Estimated activation energies indicated a higher thermal sensitivity of coral‐dominated communities. In these communities, net productivity and calcification were negatively correlated with temperature (>40% and >65% reduction, respectively, with +5°C increase from winter to summer), whereas carbon losses via respiration and dissolved organic carbon release more than doubled at higher temperatures. In contrast, algae‐dominated communities doubled net productivity in summer, while calcification and dissolved organic carbon fluxes were unaffected. These results suggest pronounced changes in community functioning associated with coral‐algal phase shifts. Algae‐dominated communities may outcompete coral‐dominated communities because of their higher productivity and carbon retention to support fast biomass accumulation while compromising the formation of important reef framework structures. Higher temperatures likely amplify these functional differences, indicating a high vulnerability of ecosystem functions of coral‐dominated communities to temperatures even below coral bleaching thresholds. Our results suggest that ocean warming may not only cause but also amplify coral–algal phase shifts in coral reefs.

Hydrodynamic conditions that favor the settlement of Diadema antillarum to a western Caribbean coral reef

The reduction in the Diadema antillarum Philippi, 1845 population across the Caribbean region in the 1980s had adverse ecological consequences, including coral-algal phase-shifts. Reduced larval supply, and subsequent low recruitment success, has hampered population recovery. To examine larval density and the influence of hydrodynamics on settlement of D. antillarum, monthly plankton tows were conducted from May 2014 to April 2015 in the lagoon and fore reef zones of a western Caribbean coral reef (Xcalak, Mexico). A moored Nortek acoustic waves and currents (AWAC-Nortek) oceanographic instrument recorded wave height, wave velocity, and suspended particles in the water column. To assess settlement of D. antillarum, collectors were placed along the lagoon and fore reef, and inspected monthly. Monthly samples had a mean density of 0.06 larvae per 15 m3 from June to August corresponding to two developmental stages. Five settlers with test diameter approximately 1 mm were collected from August to December. During August, Doppler profiles recorded via the AWAC-Nortek instrument showed the lowest wave height values (<0.3 m) and a low bottom shear stress of <0.15 N m–1. Acoustic signals indicated that the particle displacement was driven primarily by wave action and that currents played a secondary role. During settlement of D. antillarum, wave orbital velocity (Uw ) was below the threshold of motion (Uwr ), allowing deposition of particles at the seabed. These hydrodynamic conditions suggest a window of opportunity for larval transport and benthic settlement. Ours is the first field study documenting the mechanisms that link hydrodynamics to the settlement of D. antillarum to a Caribbean reef.

Paradigm Lost: Dynamic Nutrients and Missing Detritus on Coral Reefs

Some of the fundamental drivers of reef functioning are being rewritten through progress in understanding nutrient cycling. The sponge loop, described in 2013, revealed that some cryptic sponges can convert dissolved organic carbon (DOC) into detritus. In subsequent studies, researchers proposed that sponges and seaweeds could mutually reinforce their abundance by utilizing each other's waste products, thereby helping to suppress coral recovery. We build on these ideas, including the provision of a new but related hypothesis to explain the reinforcement of coral-algal phase shifts: Detritus and DOC resulting from fleshy algae reduce light penetration and increase the stress on corals, particularly in terms of calcification. We then consider the wider ecosystem implications of nutrient cycling and arrive at some novel conclusions. In particular, we propose that the combination of higher sponge biomass, which generates detritus, with the relative paucity of detritivory should generate a surplus of detritus on Caribbean reefs compared with that of the Indo-Pacific. New studies are needed to quantify the generality of the sponge loop and to track the fate of detritus to determine whether there is a surplus in the Caribbean. Although Charles Darwin identified enigmatic aspects of the nutritional status of coral reefs, this research field continues to surprise and redefine how we view the dynamics of this complex ecosystem.

Transient Grazing and the Dynamics of an Unanticipated Coral–Algal Phase Shift

Extreme disturbances often lead to community reorganisations, yet sometimes ecosystems unexpectedly fail to recover. Such surprising outcomes may pinpoint important yet overlooked mechanisms that drive ecosystems into undesirable states. Using long-term field observations, experimental manipulations and mechanistic modelling, we document the drivers of an unexpected phase shift from coral to macroalgal dominance following typhoon disturbance on reefs in Palau (Micronesia). After extensive coral mortality, an ephemeral bloom of a canopy-forming macroalga (Liagora sp.) provided physical refuge from herbivore grazing, resulting in the establishment of a secondary, understory macroalga (Lobophora spp.). After disappearance of Liagora canopies and resulting loss of grazing refuge, the Lobophora patches continued to expand and led to a macroalgal (Lobophora-) dominated state that has persisted for more than 2 years. We developed a mechanistic model of Lobophora patch dynamics parameterised with rates of growth measured in situ to simulate the observed proliferation of Lobophora under variable grazing refuges in space and time. Model simulations showed that short-term escapes from grazing were pivotal in allowing establishment of patches of Lobophora. Ephemeral grazing refuges created an opportunity to reach a cover above which Lobophora growth exceeds grazing, so that Lobophora could expand after disappearance of Liagora canopies. Critically, in the absence of grazing refuge, herbivore biomass was sufficient to prevent the establishment of Lobophora patches. Our model demonstrates that with rapid algal growth and low grazing, a relatively minor grazing refuge (6 month) is sufficient to escape herbivore control after extensive coral mortality, leading to unexpected recovery failure. Transient fluctuations in the intensity of control mechanisms, such as herbivore grazing, can have disproportionate and long-lasting effects on community structure. Overall, this study stresses that our perception of reef dynamics must integrate the time scales at which reefs can be sensitive to transient changes in mechanisms promoting coral dominance.

Identifying management challenges and implementation shortcomings of a new fishing refuge: Akumal reef, Mexico

No-take marine reserves (fish refuges -FR-in Mexico) eliminate fishing pressure for long periods in delimited areas, promoting fish reproduction and recruitment, recovery of fish community abundance and biomass, augment resistance and resilience in coral reef-associated marine ecosystems, and support local fishing-based economies. A FR was implemented on Akumal reef in 2015 limiting fishing for six years with the specific aim of increasing the biomass of commercial and sport-recreational interest fish species. However, no previous reference data were considered to establish an ecological baseline to quantify progress towards this aim, or to implement any other policy necessary to ensure compliance with this objective. To fill this gap, Akumal FR's current level of potential success was assessed (relating reef fish community characteristics and habitat components characteristics). FR's potential success was quantified as low due to the absence of 50% of listed commercial-sport fish species, critical biomass values for recorded species on the list, and generally low reef fish abundance and biomass values. In addition, current habitat condition is critical because of coral-algal phase shift, changes in coral community structure, and loss of reef structure and the associated reef's refuge function for fish species. Considering the performance of existing no-take marine reserves, Akumal low FR's potential success implies that a much longer no-take period (than the established six years), will be necessary to allow reef fish populations to recover biomass and attain functional levels within the ecosystem that could support on-going fishing impact. Based on this, a number of recommendations were established in order to strengthen the management provisions to allow the reef to attain a better condition, including an extension of the no-take period, enforcement of non-extractive uses, and simultaneous implementation of coastal management strategies.

Release of dissolved and particulate organic matter by the soft coral Lobophytum and subsequent microbial degradation

Understanding the release and remineralization of organic matter by benthic macroorganisms provides insight into nutrient cycling and microbial metabolism in coral reef environments. The release rate of particulate (POC) and dissolved organic carbon (DOC) by the soft coral Lobophytum crassum was quantified and subsequent bacterial growth rates determined in response to this resource, and compared with results from those of the common hard coral Acropora intermedia. The results of this study show that the soft coral released more DOC than POC into the surrounding seawater, similar to what was measured for the hard coral species. However, the soft coral-derived organic matter fostered a lower microbial growth rate with a lower growth efficiency compared to DOC and POC of hard corals, likely due to the lower C:N ratio of the organic matter derived from soft corals. These results suggest that soft coral exudates are relatively refractory compared to the mucus of hard corals. Possible phase shifts from hard to soft corals on degraded reefs may represent very different changes in microbial community dynamics and metabolism as compared to the widely studied coral-algal phase shifts.

A coral-algal phase shift in Mesoamerica not driven by changes in herbivorous fish abundance.

Coral-algal phase shifts in which coral cover declines to low levels and is replaced by algae have often been documented on coral reefs worldwide. This has motivated coral reef management responses that include restriction and regulation of fishing, e.g. herbivorous fish species. However, there is evidence that eutrophication and sedimentation can be at least as important as a reduction in herbivory in causing phase shifts. These threats arise from coastal development leading to increased nutrient and sediment loads, which stimulate algal growth and negatively impact corals respectively. Here, we first present results of a dynamic process-based model demonstrating that in addition to overharvesting of herbivorous fish, bottom-up processes have the potential to precipitate coral-algal phase shifts on Mesoamerican reefs. We then provide an empirical example that exemplifies this on coral reefs off Mahahual in Mexico, where a shift from coral to algal dominance occurred over 14 years, during which there was little change in herbivore biomass but considerable development of tourist infrastructure. Our results indicate that coastal development can compromise the resilience of coral reefs and that watershed and coastal zone management together with the maintenance of functional levels of fish herbivory are critical for the persistence of coral reefs in Mesoamerica.

Transitions in coral reef accretion rates linked to intrinsic ecological shifts on turbid-zone nearshore reefs

Abstract Nearshore coral communities within turbid settings are typically perceived to have limited reef-building capacity. However, several recent studies have reported reef growth over millennial time scales within such environments and have hypothesized that depth-variable community assemblages may act as equally important controls on reef growth as they do in clear-water settings. Here, we explicitly test this idea using a newly compiled chronostratigraphic record (31 cores, 142 radiometric dates) from seven proximal (but discrete) nearshore coral reefs located along the central Great Barrier Reef (Australia). Uniquely, these reefs span distinct stages of geomorphological maturity, as reflected in their elevations below sea level. Integrated age-depth and ecological data sets indicate that contemporary coral assemblage shifts, associated with changing light availability and wave exposure as reefs shallowed, coincided with transitions in accretion rates at equivalent core depths. Reef initiation followed a regional ∼1 m drop in sea level (1200–800 calibrated yr B.P.) which would have lowered the photic floor and exposed new substrate for coral recruitment by winnowing away fine seafloor sediments. We propose that a two-way feedback mechanism exists where past growth history influences current reef morphology and ecology, ultimately driving future reef accumulation and morphological change. These findings provide the first empirical evidence that nearshore reef growth trajectories are intrinsically driven by changes in coral community structure as reefs move toward sea level, a finding of direct significance for predicting the impacts of extrinsically driven ecological change (e.g., coral-algal phase shifts) on reef growth potential within the wider coastal zone on the Great Barrier Reef.

Trend in coral-algal phase shift in the Mandapam group of islands, Gulf of Mannar Marine Biosphere Reserve, India

The present study revealed proliferation of macro-algae modifying coral reef ecosystems in a different manner due to diseases and sedimentations in the Mandapam group of islands in the Gulf of Mannar. Benthic surveys were conducted with major attack of seven coral reefs diseases with high sedimentation rate, nine species of fleshy macro-algae (Turbinaria ornata, Turbinaria conaides, Caulerpa scalpelliformis, Caulerpa racemosa, Kappaphycus alvarezii, Padina gymnosphora, Sargassum wightii, Ulva reticulata and Calurpa lentillifera) proliferation against major corals life forms (Acropora branching, Acropora digitate, Acropora tabulate, coral massive, coral submassive, coral foliose and coral encrusting). The results confirm that diseased corals most favor to macro-algae growth (15.27%) rather than the sedimentation covered corals (8.24 %). In the degradation of coral life forms, massive corals were more highly damaged (7.05%) than any other forms. Within a short period of time (May to September), coral coverage shrank to 17.4% from 21.9%, macro-algae increased 23.51% and the average sedimentation rate attained 77.52 mg cm−2d−1 with persisting coral reef diseases of 17.59%. The Pearson correlation showed that the coral cover decreased with increasing macro-algae growth, which was statistically significant (r = −0.774, n = 100, P < 0.0005). The proliferation of the various macro-algae C. scalpellifrmis, T. ornata, C. racemosa, T. conaides, U. reticulata, S. wightii, K. alvarezii, P. gymnosphora and C. lentillifera increased with percentages of 6.0, 5.8, 5.7, 4.9, 4.2, 3.7, 2.7 and 1.9, respectively. If this trend continues, the next generation of new recruit corals will undoubtedly lead to a phase shift in Gulf of Mannar corals.

Progression of the Coral-Algal Phase Shift in the Caribbean: A Case Study in Bonaire, Dutch Caribbean

AbstractCoral reefs around the globe are subject to environmental and anthropogenic stressors that are causing habitat degradation and a decline in reef resilience. Past studies of Caribbean reefs document a decrease in coral cover with a simultaneous increase in algal cover after significant stress, disturbance, or coral mortality. The long-term shift from coral-dominated reefs to algae-dominated reefs is known as a coral-algal phase shift. This study assessed the progression of a coral-algal phase shift at a fringing reef around Bonaire, Dutch Caribbean, by comparing current coral and algal benthic cover to historical data, from 1997 to 2008, at a site nearby. Research was conducted over a 5-w period from September to October 2012. Twenty 10-m transects were filmed and analyzed through Coral Point Count with Excel extensions software to determine percent live coral and algal cover. Mean coral cover at the study site was 14.3%, and algal cover was 72.4%. In comparison to historical data, a significant increase in the algae-coral ratio indicated a progression of a coral-algal phase shift in Bonaire. This study contributes to the scientific knowledge of coral-algal phase shifts in the Bonaire reef ecosystem and the broader scientific reef conservation.

Typhoon-driven Coral-Algal Phase-shifts in Southern Negros Oriental, Philippines

An underwater visual census (UVC) conducted to assess the biophysical conditions of the two marine protected areas (Antulang Marine Protected Area and Andulay Marine Protected Area) in Siaton, Negros Oriental revealed slow recovery from severe damage resulting from Typhoon Sendong in December 2011, suggestive of “coral-algal phase-shift”. Live hard coral (LHC) cover was consistently poor in the survey sites (2.25±0.78 % in Antulang Marine Protected Area; 1.25±0.50 % in the Control Site; and 3.33±0.17 % in Andulay Marine Protected Area) following destruction by the typhoon and colonization of algae (between 37-53%), dominated by Bornetella oligospora. Mean total fish biomass (including fusiliers and schooling juvenile Caranx sexfasciatus) was highest in Antulang Marine Protected Area at 70.60±28.07 kg/500m2 (equivalent to 141.21±56.14 tonne/km2) while lowest in Andulay Marine Protected Area at 9.83±4.32 kg/500m2. Mean total fish density was highest in Antulang Marine Protected Area at 597.0±114.7 fish/500m2 followed by the Control Site with 361.7±159.0 fish/500 m2, while Andulay Marine Protected Area had 200.7±31.6 fish/500m2. Fish species composition was slightly altered, i.e. decline in coral-dependent species such as damselfishes. Keywords: coral reef, macroalgae, phase-shift, resilience, recovery, typhoon

Coral–algal phase shifts alter fish communities and reduce fisheries production

Anthropogenic stress has been shown to reduce coral coverage in ecosystems all over the world. A phase shift towards an algae-dominated system may accompany coral loss. In this case, the composition of the reef-associated fish assemblage will change and human communities relying on reef fisheries for income and food security may be negatively impacted. We present a case study based on the Raja Ampat Archipelago in Eastern Indonesia. Using a dynamic food web model, we simulate the loss of coral reefs with accompanied transition towards an algae-dominated state and quantify the likely change in fish populations and fisheries productivity. One set of simulations represents extreme scenarios, including 100% loss of coral. In this experiment, ecosystem changes are driven by coral loss itself and a degree of habitat dependency by reef fish is assumed. An alternative simulation is presented without assumed habitat dependency, where changes to the ecosystem are driven by historical observations of reef fish communities when coral is lost. The coral–algal phase shift results in reduced biodiversity and ecosystem maturity. Relative increases in the biomass of small-bodied fish species mean higher productivity on reefs overall, but much reduced landings of traditionally targeted species.

Natural Diet of Coral-Excavating Sponges Consists Mainly of Dissolved Organic Carbon (DOC)

Coral-excavating sponges are the most important bioeroders on Caribbean reefs and increase in abundance throughout the region. This increase is commonly attributed to a concomitant increase in food availability due to eutrophication and pollution. We therefore investigated the uptake of organic matter by the two coral-excavating sponges Siphonodictyon sp. and Cliona delitrix and tested whether they are capable of consuming dissolved organic carbon (DOC) as part of their diet. A device for simultaneous sampling of water inhaled and exhaled by the sponges was used to directly measure the removal of DOC and bacteria in situ. During a single passage through their filtration system 14% and 13% respectively of the total organic carbon (TOC) in the inhaled water was removed by the sponges. 82% (Siphonodictyon sp.; mean±SD; 13±17 μmol L−1) and 76% (C. delitrix; 10±12 μmol L−1) of the carbon removed was taken up in form of DOC, whereas the remainder was taken up in the form of particulate organic carbon (POC; bacteria and phytoplankton) despite high bacteria retention efficiency (72±15% and 87±10%). Siphonodictyon sp. and C. delitrix removed DOC at a rate of 461±773 and 354±562 μmol C h−1 respectively. Bacteria removal was 1.8±0.9×1010 and 1.7±0.6×1010 cells h−1, which equals a carbon uptake of 46.0±21.2 and 42.5±14.0 μmol C h−1 respectively. Therefore, DOC represents 83 and 81% of the TOC taken up by Siphonodictyon sp. and C. delitrix per hour. These findings suggest that similar to various reef sponges coral-excavating sponges also mainly rely on DOC to meet their carbon demand. We hypothesize that excavating sponges may also benefit from an increasing production of more labile algal-derived DOC (as compared to coral-derived DOC) on reefs as a result of the ongoing coral-algal phase shift.

Natural history of coral−algae competition across a gradient of human activity in the Line Islands

Competition between corals and benthic algae is prevalent on coral reefs worldwide and has the potential to influence the structure of the reef ben- thos. Human activities may influence the outcome of these interactions by favoring algae to become the superior competitor, and this type of change in com- petitive dynamics is a potential mechanism driving coralalgal phase shifts. Here we surveyed the types and outcomes of coral interactions with benthic algae in the Line Islands of the Central Pacific. Islands ranged from nearly pristine to heavily fished. We observed major differences in the dominant groups of algae interacting with corals between sites, and the outcomes of coralalgal interactions varied across reefs on the different islands. Corals were generally better competitors against crustose coralline algae regardless of location, and were superior competitors against turf algae on reefs surrounding uninhabited islands. On reefs surrounding inhabited islands, how- ever, turf algae were generally the superior competi- tors. When corals were broken down by size class, we found that the smallest and the largest coral colonies were the best competitors against algae; the former successfully fought off algae while being completely surrounded, and the latter generally avoided algal overgrowth by growing up above the benthos. Our data suggest that human disruption of the reef ecosys- tem may lead to a building pattern of competitive dis- advantage for corals against encroaching algae, par- ticularly turf algae, potentially initiating a transition towards algal dominance.

Ocean acidification and warming will lower coral reef resilience

Ocean warming and acidification from increasing levels of atmospheric CO2 represent major global threats to coral reefs, and are in many regions exacerbated by local-scale disturbances such as overfishing and nutrient enrichment. Our understanding of global threats and local-scale disturbances on reefs is growing, but their relative contribution to reef resilience and vulnerability in the future is unclear. Here, we analyse quantitatively how different combinations of CO2 and fishing pressure on herbivores will affect the ecological resilience of a simplified benthic reef community, as defined by its capacity to maintain and recover to coral-dominated states. We use a dynamic community model integrated with the growth and mortality responses for branching corals (Acropora) and fleshy macroalgae (Lobophora). We operationalize the resilience framework by parameterizing the response function for coral growth (calcification) by ocean acidification and warming, coral bleaching and mortality by warming, macroalgal mortality by herbivore grazing and macroalgal growth via nutrient loading. The model was run for changes in sea surface temperature and water chemistry predicted by the rise in atmospheric CO2 projected from the IPCC's fossil-fuel intensive A1FI scenario during this century. Results demonstrated that severe acidification and warming alone can lower reef resilience (via impairment of coral growth and increased coral mortality) even under high grazing intensity and low nutrients. Further, the threshold at which herbivore overfishing (reduced grazing) leads to a coral–algal phase shift was lowered by acidification and warming. These analyses support two important conclusions: Firstly, reefs already subjected to herbivore overfishing and nutrification are likely to be more vulnerable to increasing CO2. Secondly, under CO2 regimes above 450–500 ppm, management of local-scale disturbances will become critical to keeping reefs within an Acropora-rich domain.