A coral reef refuge in the Red Sea

The stability and persistence of coral reefs in the decades to come is uncertain due to global warming and repeated bleaching events that will lead to reduced resilience of these ecological and socio‐economically important ecosystems. Identifying key refugia is potentially important for future conservation actions. We suggest that the Gulf of Aqaba (GoA) (Red Sea) may serve as a reef refugium due to a unique suite of environmental conditions. Our hypothesis is based on experimental detection of an exceptionally high bleaching threshold of northern Red Sea corals and on the potential dispersal of coral planulae larvae through a selective thermal barrier estimated using an ocean model. We propose that millennia of natural selection in the form of a thermal barrier at the southernmost end of the Red Sea have selected coral genotypes that are less susceptible to thermal stress in the northern Red Sea, delaying bleaching events in the GoA by at least a century.     

Direct evaluation of macroalgal removal by herbivorous coral reef fishes

Few studies have examined the relative functional impacts of individual herbivorous fish species on coral reef ecosystem processes in the Indo-Pacific. This study assessed the potential grazing impact of individual species within an inshore herbivorous reef fish assemblage on the central Great Barrier Reef (GBR), by determining which fish species were able to remove particular macroalgal species. Transplanted multiple-choice algal assays and remote stationary underwater digital video cameras were used to quantify the impact of local herbivorous reef fish species on 12 species of macroalgae. Macroalgal removal by the fishes was rapid. Within 3 h of exposure to herbivorous reef fishes there was significant evidence of intense grazing. After 12 h of exposure, 10 of the 12 macroalgal species had decreased to less than 15% of their original mass. Chlorodesmis fastigiata (Chlorophyta) and Galaxaura sp. (Rhodophyta) showed significantly less susceptibility to herbivorous reef fish grazing than all other macroalgae, even after 24 h exposure. Six herbivorous and/or nominally herbivorous reef fish species were identified as the dominant grazers of macroalgae: Siganus doliatus, Siganus canaliculatus, Chlorurus microrhinos, Hipposcarus longiceps, Scarus rivulatus and Pomacanthus sexstriatus. The siganid S. doliatus fed heavily on Hypnea sp., while S. canaliculatus fed intensively on Sargassum sp. Variation in macroalgal susceptibility was not clearly correlated with morphological and/or chemical defenses that have been previously suggested as deterrents against herbivory. Nevertheless, the results stress the potential importance of individual herbivorous reef fish species in removing macroalgae from coral reefs.     

Varying responses of herbivorous and invertebrate-feeding fishes to macroalgal reduction on a coral reef

 The consequences of macroalgal overgrowth on reef fishes and means to reverse this condition have been little explored. An experimental reduction of macroalgae was conducted at a site in the Watamu Marine National Park in Kenya, where a documented increase in macroalgal cover has occurred over the last nine years. In four experimental 10 m by 10 m plots, macroalgae were greatly reduced (fleshy algal cover reduced by 84%) by scrubbing and shearing, while four similar plots acted as controls. The numerical abundance in all fish groups except wrasses and macroalgal-feeding parrotfishes (species in the genera Calotomus and Leptoscarus) increased in experimental algal reduction plots. Algal (Sargassum) and seagrass (Thalassia) assays, susceptible to scraping and excavating parrotfishes, were bitten more frequently in the algal reduction plots one month after the manipulation. Further, surgeonfish (Acanthurus leucosternon and A. nigrofuscus) foraging intensity increased in these algal reduction plots. The abundance of triggerfishes increased significantly in experimental plots relative to control plots, but densities remained low, and an index of sea urchin predation using tethered juvenile and adult Echinometra mathaei showed no differences between treatments following macroalgal reduction. Dominance of reefs by macrofleshy algae appears to reduce the abundance of fishes, mostly herbivores and their rates of herbivory, but also other groups such as predators of invertebrates (triggerfishes, butterflyfishes and angelfishes). Accepted: 2 February 1999     

Augmented coral reef monitoring using a stationary reef monitoring system

Coral reef monitoring is a reliable tool to assess the effect of climate change as corals are sensitive to increases in water temperatures between 30 °C and 35 °C resulting in bleaching - a whitening process when the corals lose their color and the reefs begin to die. Existing satellite-based monitoring products facilitate coral bleaching monitoring over large spatial scales, but their use in predicting local scale stress that influences the bleaching severity across reefs is limited. In this paper, we describe a Stationary Reef Monitoring System (SRMS) that monitors the time evolution of coral reefs through the photography of nearby coral clusters. Simultaneously, the SRMS measures and records environmental parameters such as temperature, solar irradiance (PAR), and salinity in the waters surrounding the coral colonies. When deployed in the sea, the SRMS detected a 0.1–0.4 °C variability in temperature between the in situ and satellite datasets. The SRMS uses color photography along with quantitative data on environmental parameters to monitor the health of corals and eliminates the need for physical/visual verification of coral health by a diver. By this approach, one can determine the stress thresholds of corals and identify the vulnerable and resilient reefs so as to prioritize conservation efforts.     

Current rectification in a tropical coral reef system

Acoustic Doppler current profiles (ADCP) and conductivity-temperature-depth (CTD) profiles were recorded in the Veracruz Reef System (VRS) to elucidate the effect of topography on current circulation and zooplankton distribution. Measurements showed the baroclinic behavior of the tropical coral reef system with vertical temperature and salinity gradients of 0.4°C m⁻¹ and 0.5 psu m⁻¹, respectively. Under sustained southerly wind conditions, a cyclonic eddy generated by subtidal and tidal current rectification was observed between the two groups of coral colonies studied. Current rectification was attributed to the shallowness of the coral colonies and to a small cape close to Anton Lizardo Village. The cyclonic eddy, with low surface temperature, produced an increment in the vertically integrated acoustic scattering volume (biovolume), suggesting that this is a high-productivity area.     

Corals from an Early Jurassic coral reef in British Columbia: refuge on an oceanic island reef

An Early Jurassic (Sinemurian) reef in the Telkawa Range, British Columbia Canada, yields coral species previously known from Morocco, Great Britain, Italy, Peru, and Chile. The principal constructional coral, Phacelostylophyllum rugosum (Laube), known from the Upper Triassic Dolomite Alps in northern Italy, is a holdover species. This coral survived the mass extinctions of the end-Triassic without leaving any other Jurassic records outside Canada. Other corals from the Telkwa reef include Stylophyllopsis victoriae (Duncan) and Actinastraea minima Beauvais known from Jurassic rocks of the Tethys. Closely related corals, Phacelostylophyllum chocolatensis (Wells) and Actinastraea plana (Duncan), are from southern Peru. The paleogeographic Occurrence of the Canadian reef in the volcanic terrane of Stikinia supports the contention that volcanic islands in distant outposts of the ancient Pacific served as refugia. In the aftermath of the end-Triassic reef decimation affecting the Tethys, corals and reef-building activities continued on ancient islands of the ancestral Pacific. The Hispanic Corridor, connecting the western Tethys with the western Pacific, may have played an important role during Sinemurian time. ***Reef; corals, Triassic, Jurassic, extinctions, paleobiogeography.     

Damselfish territories as a refuge for macroalgae on coral reefs

Herbivory is widely accepted as a key process determining the benthic community structure and resilience of coral reefs. Recent studies have mostly focused on the importance of roving herbivorous fishes in ecosystem processes. Here, we examine the role of territorial damselfish in shaping patterns of macroalgal distribution based on benthic surveys and macroalgal bioassays. The territory composition and effect of resident damselfish on the removal of Sargassum bioassays were quantified for six species of damselfish on Lizard Island, a mid-shelf reef in the northern Great Barrier Reef (GBR). The functional composition of algal communities within territories varied markedly among species. The territories of four species (Dischistodus perspicillatus, Dischistodus pseudochrysopoecilus, Plectroglyphidodon lacrymatus, and Stegastes nigricans) were characterized by algal turfs, while the territories of two species (Dischistodus prosopotaenia and Hemiglyphidodon plagiometopon) were characterized by foliose and leathery brown macroalgae. Sargassum, a generally rare alga on mid-shelf reefs, was a particularly common alga within D. prosopotaenia territories on the leeward side of the island but absent within their territories on the windward side of the island. D. prosopotaenia was the only species to retain the transplanted Sargassum, with only a minimal reduction in Sargassum biomass (1.1%) being recorded within their territories at both leeward and windward sites over a 24-h period. In contrast, reductions in Sargassum biomass were high in areas adjacent to D. prosopotaenia territories (83.8%), and within and adjacent to the territories of the five remaining damselfish species (76.2–92.5%). Overall, only one of the six damselfish species provided a refuge for leathery brown macroalgae and may facilitate the development of this macroalgae on mid-shelf reefs of the GBR.     

High macroalgal cover and low coral recruitment undermines the potential resilience of the world's southernmost coral reef assemblages

Coral reefs are under increasing pressure from anthropogenic and climate-induced stressors. The ability of reefs to reassemble and regenerate after disturbances (i.e., resilience) is largely dependent on the capacity of herbivores to prevent macroalgal expansion, and the replenishment of coral populations through larval recruitment. Currently there is a paucity of this information for higher latitude, subtropical reefs. To assess the potential resilience of the benthic reef assemblages of Lord Howe Island (31°32'S, 159°04'E), the worlds' southernmost coral reef, we quantified the benthic composition, densities of juvenile corals (as a proxy for coral recruitment), and herbivorous fish communities. Despite some variation among habitats and sites, benthic communities were dominated by live scleractinian corals (mean cover 37.4%) and fleshy macroalgae (20.9%). Live coral cover was higher than in most other subtropical reefs and directly comparable to lower latitude tropical reefs. Juvenile coral densities (0.8 ind.m(-2)), however, were 5-200 times lower than those reported for tropical reefs. Overall, macroalgal cover was negatively related to the cover of live coral and the density of juvenile corals, but displayed no relationship with herbivorous fish biomass. The biomass of herbivorous fishes was relatively low (204 kg.ha(-1)), and in marked contrast to tropical reefs was dominated by macroalgal browsing species (84.1%) with relatively few grazing species. Despite their extremely low biomass, grazing fishes were positively related to both the density of juvenile corals and the cover of bare substrata, suggesting that they may enhance the recruitment of corals through the provision of suitable settlement sites. Although Lord Howe Islands' reefs are currently coral-dominated, the high macroalgal cover, coupled with limited coral recruitment and low coral growth rates suggest these reefs may be extremely susceptible to future disturbances.     

DNA barcoding as a tool for coral reef conservation

DNA Barcoding (DBC) is a method for taxonomic identification of animals that is based entirely on the 5′ portion of the mitochondrial gene, cytochrome oxidase subunit I (COI-5). It can be especially useful for identification of larval forms or incomplete specimens lacking diagnostic morphological characters. DBC can also facilitate the discovery of species and in defining “molecular taxonomic units” in problematic groups. However, DBC is not a panacea for coral reef taxonomy. In two of the most ecologically important groups on coral reefs, the Anthozoa and Porifera, COI-5 sequences have diverged too little to be diagnostic for all species. Other problems for DBC include paraphyly in mitochondrial gene trees and lack of differentiation between hybrids and their maternal ancestors. DBC also depends on the availability of databases of COI-5 sequences, which are still in early stages of development. A global effort to barcode all fish species has demonstrated the importance of large-scale coordination and is yielding promising results. Whether or not COI-5 by itself is sufficient for species assignments has become a contentious question; it is generally advantageous to use sequences from multiple loci.     

Seasonal calcification of the coral Acropora digitifera from a subtropical marginal Okinawa reef under ocean acidification

Coral Reefs - Coral calcification is affected by the decrease in aragonite saturation state (Ωarag) caused by ocean acidification (OA). However, OA effects are modulated by other environmental...     

Marine plants in a coral reef ecosystem

Based on a review of the literature and our own data, marine plants are shown to play an important role in the formation and sustaining of coral reefs, which are one of coastal tropical ecosystems of the World Ocean. The important ecosystem roles of marine plants include primary production and recycling of organic matter, the construction of the hard base of a coral reef, the fixation of dissolved molecular nitrogen in sea-water, the formation of an initial link in the food chain, the provision of a habitat for marine animals, and the protection of the reef against destructive wave action.     

Model of a coral reef ecosystem

The results of modelling of a coral reef ecosystem at French Frigate Shoals and independent field measures of benthic primary productivity indicate relatively good agreement between food required by consumer trophic levels and organic carbon produced by primary producers. Based upon the high internal predation necessary for the model to match primary production estimates, we reason that the ecosystem is primarily regulated from the top down by forces of predation and that primary production appears to be controlled by nutrients, rate limits, and the distribution of space and habitat. In spite of relatively high primary productivity, potential yield at the top of the food chain is low because of high internal predation and high trophic complexity (6 trophic levels). Fishery yield might be maximized by harvesting low on the food chain particularly if top carnivores can be cropped to release predator pressure on selected prey. Agreement between field measures of metabolism and model (ECOPATH) results provides reasonable confidence that the model can be used as one tool for resource management.     

Nitrification on a coral reef.

We report that the algal pavement just behind the reef crest at Enewetak Atoll produces nitrate at measurable rates. In situ and in vitro incubations with N-Serve indicate that the autotrophic pathway involving two separate organisms is effective in this oxidation of ammonia to nitrate. Significant nitrification is indicated throughout the reef environment; Nitrobacter agilis has specifically been identified as at least one of the organisms responsible for the terminal oxidation of nitrite to nitrate.     

The role of turtles as coral reef macroherbivores

Herbivory is widely accepted as a vital function on coral reefs. To date, the majority of studies examining herbivory in coral reef environments have focused on the roles of fishes and/or urchins, with relatively few studies considering the potential role of macroherbivores in reef processes. Here, we introduce evidence that highlights the potential role of marine turtles as herbivores on coral reefs. While conducting experimental habitat manipulations to assess the roles of herbivorous reef fishes we observed green turtles (Chelonia mydas) and hawksbill turtles (Eretmochelys imbricata) showing responses that were remarkably similar to those of herbivorous fishes. Reducing the sediment load of the epilithic algal matrix on a coral reef resulted in a forty-fold increase in grazing by green turtles. Hawksbill turtles were also observed to browse transplanted thalli of the macroalga Sargassum swartzii in a coral reef environment. These responses not only show strong parallels to herbivorous reef fishes, but also highlight that marine turtles actively, and intentionally, remove algae from coral reefs. When considering the size and potential historical abundance of marine turtles we suggest that these potentially valuable herbivores may have been lost from many coral reefs before their true importance was understood.     

Density-associated recruitment mediates coral population dynamics on a coral reef

Theory suggests that density-associated processes can modulate community resilience following declines in population size. Here, we demonstrate density-associated processes in two scleractinian populations on the outer reef of Moorea, French Polynesia, that are rapidly increasing in size following the effects of two catastrophic disturbances. Between 2006 and 2010, predation by the corallivorous crown-of-thorns sea star reduced coral cover by 93 %; in 2010, the dead coral skeletons were removed by a cyclone, and in 2011 and 2012, high coral recruitment initiated population recovery. Coral recruitment was associated with coral cover, but the relationship differed between two coral genera that are almost exclusively broadcast spawners in Moorea. Acroporids recruited at low densities, and the density of recruits was positively associated with cover of Acropora, whereas pocilloporids recruited at high densities, and densities of their recruits were negatively associated with cover of Pocillopora. Together, our results suggest that associations between adult cover and density of both juveniles and recruits can mediate rapid coral community recovery after large disturbances. The difference between taxa in sign of the relationships between recruit density and coral cover indicate that they reflect contrasting mechanisms with the potential to mediate temporal shifts in taxonomic composition of coral communities.     

Bioindication in coral reef ecosystems

The concept of bioindication in the sense of the use of organisms for detecting environmental stress has been employed in coral reef conservation and management for the past several years. Important tools are coral growth rates and various community parameters, notably hard coral cover. The present need is the optimal coordination of international efforts for the earliest possible institution of an effective monitoring system.     

Mesophotic depths as refuge areas for fishery-targeted species on coral reefs

Coral reefs are subjected to unprecedented levels of disturbance with population growth and climate change combining to reduce standing coral cover and stocks of reef fishes. Most of the damage is concentrated in shallow waters (<30 m deep) where humans can comfortably operate and where physical disturbances are most disruptive to marine organisms. Yet coral reefs can extend to depths exceeding 100 m, potentially offering refuge from the threats facing shallower reefs. We deployed baited remote underwater stereo-video systems (stereo-BRUVs) at depths of 10–90 m around the southern Mariana Islands to investigate whether fish species targeted by fishing in the shallows may be accruing benefits from being at depth. We show that biomass, abundance and species richness of fishery-targeted species increased from shallow reef areas to a depth of 60 m, whereas at greater depths, a lack of live coral habitat corresponded to lower numbers of fish. The majority of targeted species were found to have distributions that ranged from shallow depths (10 m) to depths of at least 70 m, emphasising that habitat, not depth, is the limiting factor in their vertical distribution. While the gradient of abundance and biomass versus depth was steepest for predatory species, the first species usually targeted by fishing, we also found that fishery-targeted herbivores prevailed in similar biomass and species richness to 60 m. Compared to shallow marine protected areas, there was clearly greater biomass of fishery-targeted species accrued in mesophotic depths. Particularly some species typically harvested by depth-limited fishing methods (e.g., spearfishing), such as the endangered humphead wrasse Cheilinus undulatus, were found in greater abundance on deeper reefs. We conclude that mesophotic depths provide essential fish habitat and refuge for fishery-targeted species, representing crucial zones for fishery management and research into the resilience of disturbed coral reef ecosystems.     

Echinoid grazing as a structuring force in coral communities: Whole reef manipulations

To examine the effects of echinoid grazing on shallow water coral community structure, the entire Diadema antillarum Philippi population (> 3000 individuals) was eliminated from a patch reef in Discovery Bay, Jamaica W.I. in January 1974. All remaining regular echinoids, notably including Echinometra viridis A. Agassiz (> 7000 individuals), were removed from one-half of the same reef 6 months later.Where Echinometra was present, its grazing was highly patchy, even at densities of up to 50/m². On the other hand, the effects of grazing by Diadema were more uniformly distributed at equivalent or lower densities.Percent-cover of adult corals was enhanced in the presence of Echinometra viridis, however, it was greatly reduced in the absence of all echinoid grazing as a result of massive overgrowth by algae. Success of coral recruitment (density of coral spat) increased in the absence of Diadema due to the alleviation of biological disturbance imposed by this urchin's feeding activities. The presence of Echinometra viridis, even at the high densities naturally present on the reef, did not deter coral settlement.Coral diversity was highest in the presence of all echinoids, intermediate in the presence of E. viridis (without Diadema), and lowest in the absence of all echinoids. This was due to the disproportionately successful recruitment and resultant high relative abundance of Agaricia at low echinoid densities. Favia, Millepora, Helioseris and Acropora all exhibited universally low recruitment levels. Predation by Diadema and Echinometra is responsible for damaging larger coral colonies, particularly Porites.As echinoid species composition and density was altered, the species composition of benthic algae and associated invertebrates shifted. In this way, the array of species competing for space with coral spat shifted as did their competitive relationships. Major epifaunal competitors included spirorbid polychaetes, various foraminifera (particularly Gypsina), and sponges. Algal competitors included filamentous chlorophytes, corallines (e.g. Jania), and numerous other red algae; the species composition of algal competitors shifted through time. Competitive success in corals varied as echinoid density changed and was species-specific with respect to coral genus. Interspecific competition between coral spat (i.e., interspecific aggression via extracoelenteric digestion) was negligible under all treatments.The presence of Echinometra viridis in high densities (in the absence of Diadema) created conditions where growth and fusion in Agaricia spat were optimized. Under these conditions, fusion allows a young coral colony to attain a large enough size to survive damage incurred from grazing or from competitive overgrowth, decreasing the probability of whole-colony mortality during the early stages of development.     

Perspectives in coral reef hydrodynamics

Knowledge of skeletogenesis in scleractinian corals is central to reconstructing past ocean and climate histories, assessing and counteracting future climate and ocean acidification impacts upon coral reefs, and determining the taxonomy and evolutionary path of the Scleractinia. To better understand skeletogenesis and mineralogy in extant scleractinian corals, we have investigated the nature of the initial calcium carbonate skeleton deposited by newly settling coral recruits. Settling Acropora millepora larvae were sampled daily for 10 days from initial attachment, and the carbonate mineralogy of their newly deposited skeletons was investigated. Bulk analyses using Raman and infrared spectroscopic methods revealed that the skeletons were predominantly comprised of aragonite, with no evidence of calcite or an amorphous precursor phase, although presence of the latter cannot be discounted. Sensitive selected area electron diffraction analyses of sub-micron areas of skeletal regions further consolidated these data. These findings help to address the uncertainty surrounding reported differences in carbonate mineralogy between larval and adult extant coral skeletons by indicating that skeletons of new coral recruits share the same aragonitic mineralogy as those of their mature counterparts. In this respect, we can expect that skeletogenesis in both larval and mature growth stages of scleractinian corals will be similarly affected by ocean acidification and predicted environmental changes.     

Photoinhibition of photosynthesis on a coral reef

Photoinhibition of macroalgae in the epilithic algal community (KAC) of coral reefs was studied using chlorophyll fluorescence techniques at One Tree Island, Great Barrier Reef, Australia. F_v/F_m (variable to maximum fluorescence, darkened samples) of shallow macroalgae declined by 50% on fine summer and winter days, recovering in late afternoon. Within a species, thalli from low-light habitats were more photoinhibited (2h at 1400μimol m^?2 s^?1) than those from high-light habitats. The sensitivity of Lobophora variegata (Phacophyta) and Chlorodesmis fastigiata (Chlorophyta) increased with depth (1 versus 20 m). However, shallow Halimeda tuna (Chlorophyta) plants growing between corals were more photoinhibited than those from deep, open areas. Photoinhibition and recovery were depth- and species-specific. Shallow Lobophora and Chlorodesmis maintained a greater degree of Q _A oxidation during photoinhibition. In deep thalli, reduced effective quantum yield of open photosystem II centres reflected lower proportions and excitation capture efficiencies of open centres. In Lobophora, zeaxanthin formation accompanied non-photochemical fluorescence quenching (NPQ), but in Chlorodesmis NPQ was limited and no zeaxanthin or antherxanthin formed. Higher photosynthetic efficiency in the lower storey of the EAC may compensate for photoinhibition in the upper storey, thereby reconciling photoinhibition of individual thalli with previous observations of no net inhibition of community productivity.