Structure Of Coral Reefs Research Articles

Accuracy and Precision of Habitat Structural Complexity Metrics Derived from Underwater Photogrammetry

In tropical reef ecosystems corals are the key habitat builders providing most ecosystem structure, which influences coral reef biodiversity and resilience. Remote sensing applications have progressed significantly and photogrammetry together with application of structure from motion software is emerging as a leading technique to create three-dimensional (3D) models of corals and reefs from which biophysical properties of structural complexity can be quantified. This enables the addressing of a range of important marine research questions, such as what the role of habitat complexity is in driving key ecological processes (i.e., foraging). Yet, it is essential to assess the accuracy and precision of photogrammetric measurements to support their application in mapping, monitoring and quantifying coral reef form and structure. This study evaluated the precision (by repeated modeling) and accuracy (by comparison with laser reference models) of geometry and structural complexity metrics derived from photogrammetric 3D models of marine benthic habitat at two ecologically relevant spatial extents; individual coral colonies of a range of common morphologies and patches of reef area of 100s of square metres. Surface rugosity measurements were generally precise across all morphologies and spatial extents with average differences in the geometry of replicate models of 1–6 mm for coral colonies and 25 mm for the reef area. Precision decreased with complexity of the coral morphology, with metrics for small massive corals being the most precise (1% coefficient of variation (CV) in surface rugosity) and metrics for bottlebrush corals being the least precise (10% CV in surface rugosity). There was no indication however that precision was related to complexity for the patch-scale modelling. The 3D geometry of coral models differed by only 1–3 mm from laser reference models. However, high spatial variation in these differences around the model led to a consistent underestimation of surface rugosity values for all morphs of between 8% and 37%. This study highlights the utility of several off-the-shelf photogrammetry tools for the measurement of structural complexity across a range of scales relevant to ecologist and managers. It also provides important information on the accuracy and precision of these systems which should allow for their targeted use by non-experts in computer vision within these contexts.

David Stoddart OBE

David Stoddart <scp>OBE</scp>

Spatial variability in habitat structure and heterogenic coral reef fish assemblages inside a small-scale marine reserve after a coral mass mortality event

Coral reefs at the inner granitic islands in the Seychelles were heavily affected by the worldwide bleaching event in 1998, which led to subsequent coral mortality and widespread phase shifts to macroalgae dominated reefs. In this study, five sites within a small, but well enforced marine reserve at Cousin Island, were investigate using various methods to explore differences in coral habitat quality, coral recruitment, fish assemblages, key invertebrate grazers, and rugosity. The objective of the study was to collect a broad set of scientific data, which could be useful to describe linkage between coral reef and fish assemblages after a large-scale disturbance, as well as for future management decisions regarding marine resources, in terms of MPA protection and recovery abilities. The results showed high spatial variation in coral coverage between sites (from 1.5% to 43.2%), which were higher than previously reported, as well as high variation in dispersal of coral recruits. Furthermore, there were large heterogenic differences in fish densities and composition, which were directly linked to coral habitat quality, e.g. total fish abundance was 15 times higher on sites with high coral coverage in comparison to sites with low coral cover. In summary, this study demonstrates that coral reef habitat and fish assemblage may display high spatial variability and heterogenic differences after large-scale disturbances and suggests that potential recovery from coral mass mortality may occur in a non-linear and patchy procedure, which in turn may depend on underlying stocastical processes that affect coral recruitment and survivorship.

Can sea urchins beat the heat? Sea urchins, thermal tolerance and climate change.

The massive die-off of the long-spined sea urchin, Diadema antillarum, a significant reef grazer, in the mid 1980s was followed by phase shifts from coral dominated to macroalgae dominated reefs in the Caribbean. While Diadema populations have recovered in some reefs with concomitant increases in coral cover, the additional threat of increasing temperatures due to global climate change has not been investigated in adult sea urchins. In this study, I measured acute thermal tolerance of D. antillarum and that of a sympatric sea urchin not associated with coral cover, Echinometra lucunter, over winter, spring, and summer, thus exposing them to substantial natural thermal variation. Animals were taken from the wild and placed in laboratory tanks in room temperature water (∼22 °C) that was then heated at 0.16–0.3 °C min−1 and the righting behavior of individual sea urchins was recorded. I measured both the temperature at which the animal could no longer right itself (TLoR) and the righting time at temperatures below the TLoR. In all seasons, D. antillarum exhibited a higher mean TLoR than E. lucunter. The mean TLoR of each species increased with increasing environmental temperature revealing that both species acclimatize to seasonal changes in temperatures. The righting times of D. antillarum were much shorter than those of E. lucunter. The longer relative spine length of Diadema compared to that of Echinometra may contribute to their shorter righting times, but does not explain their higher TLoR. The thermal safety margin (the difference between the mean collection temperature and the mean TLoR) was between 3.07–3.66 °C for Echinometra and 3.79–5.67 °C for Diadema. While these thermal safety margins exceed present day temperatures, they are modest compared to those of temperate marine invertebrates. If sea temperatures increase more rapidly than can be accommodated by the sea urchins (either by genetic adaptation, phenotypic plasticity, or both), this will have important consequences for the structure of coral reefs.

Thresholds in Caribbean coral reefs: implications for ecosystem‐based fishery management

Summary Ecosystem‐based management of coral reef fisheries aims to sustainably deliver a diverse portfolio of ecosystem services. This goal can be undermined if the ecosystem shifts into a different state, with altered ecosystem functions and benefits to people. If levels of drivers that cause transitions between states are identified, management measures could be aimed at maintaining drivers below these levels to avoid ecosystem shifts. Analysing data from a large number of Caribbean coral reefs (N = 2001), suites of nonlinear thresholds were identified between metrics of coral reef processes and structure along a gradient of total fish biomass (a proxy for fishing pressure). Several metrics (macroalgal cover, invertivorous fishes and fish species richness) associated with coral‐dominated reefs exhibited thresholds at relatively high fish biomass levels (50–88% of unfished biomass). Other metrics (urchin biomass, ratio of macroalgal to coral cover, herbivorous fishes and coral cover) showed thresholds at lower fish biomass levels (28–37% of unfished biomass). Ratios of total fish biomass in fishing areas to closed areas (unfished biomass) in the Caribbean indicate that reefs may generally be at risk for change at ratios between 0·5 (coral dominated) and 0·3 (macroalgal dominated). Similar relationships were found for coral reefs in the Indian Ocean. While these results illustrate thresholds at the scale of the entire Caribbean, assessing local reefs is advisable because biomass levels vary within the region, and reef trajectories depend on past, present and future local conditions. Synthesis and applications. If the thresholds in this study are generalizable to scales relevant to management, it may be possible to produce sustainable yield while simultaneously maintaining coral‐dominated reefs by restricting fishing mortality to levels that result in biomass ratios near 0·5. Fishing down to biomass ratios near 0·3 may increase the risk of overfishing (resulting in lower long‐term yields) and transition to macroalgal‐dominated reefs. Thresholds offer a simple and powerful way for managers to operationalize precautionary ecosystem‐based fishery management by adaptively limiting fishing pressure in order to (i) maintain desirable coral reef conditions, (ii) establish a system‐specific target for generating pretty good yield and (iii) maintain sustainable multi‐species fishery yields.

Anticipative management for coral reef ecosystem services in the 21st century.

Under projections of global climate change and other stressors, significant changes in the ecology, structure and function of coral reefs are predicted. Current management strategies tend to look to the past to set goals, focusing on halting declines and restoring baseline conditions. Here, we explore a complementary approach to decision making that is based on the anticipation of future changes in ecosystem state, function and services. Reviewing the existing literature and utilizing a scenario planning approach, we explore how the structure of coral reef communities might change in the future in response to global climate change and overfishing. We incorporate uncertainties in our predictions by considering heterogeneity in reef types in relation to structural complexity and primary productivity. We examine 14 ecosystem services provided by reefs, and rate their sensitivity to a range of future scenarios and management options. Our predictions suggest that the efficacy of management is highly dependent on biophysical characteristics and reef state. Reserves are currently widely used and are predicted to remain effective for reefs with high structural complexity. However, when complexity is lost, maximizing service provision requires a broader portfolio of management approaches, including the provision of artificial complexity, coral restoration, fish aggregation devices and herbivore management. Increased use of such management tools will require capacity building and technique refinement and we therefore conclude that diversification of our management toolbox should be considered urgently to prepare for the challenges of managing reefs into the 21st century.

Benthic Crustacea on coral reefs: a quantitative survey

Benthic crustaceans are an important component of the coral reef fauna, yet our understanding of their ecological significance is incomplete. To determine the community structure, abundance, biomass and productivity of benthic Crustacea at Lizard Island, a mid-shelf reef on the Great Barrier Reef, Australia, we investigated 5 major microhabitats: dead coral, coral rubble, sand, epilithic algal matrix (EAM) and fine-branching live coral. Crustacean communities differed significantly among habitats, with the exception of dead coral and coral rubble. Dead coral was the most important microhabitat type in terms of crustacean abundance (7838 ± 628 ind. 100 cm-2, mean ± SE), biomass (0.75 ± 0.13 g m-2, wet weight) and estimated productivity (0.17 ± 0.043 g 100 cm-2 yr-1 ash free dry weight). These values were 2 to 3 orders of magnitude greater than those for the least important habitats (EAM and fine-branching live coral). Despite their abundance, the average crustacean body length was just 0.79 ± 0.32 mm, largely due to the dominance of harpacticoid copepods. In contrast, decapods exhibited very low abundances, but yielded the greatest biomass and productivity and were particularly abundant in dead coral and coral rubble. The results highlight the importance of small crustaceans and dead coral microhabitats as valuable contributors to the trophic structure of coral reefs.

Estado de los arrecifes de coral y comunidades de peces e invertebrados con interés comercial asociadas al Banco de Pedro, Jamaica

The coral reefs located off the north coast of the Jamaican mainland are some of the best and most studied reefs in the world. Coral reefs of Pedro Bank, Jamaica were assessed in March, 2012 as part of the KSLOF Global Reef Expedition using a modified Atlantic and Gulf Rapid Reef Assessment (AGRRA) protocol. The main objectives were to: 1) characterize the distribution, structure and health of coral reefs; and 2) evaluate the population status of commercially important reef fishes and invertebrates. This work was conducted to assist in characterizing coral reef habitats within and outside a proposed fishery reserve, and identify other possible conservation zones. Within 20 reefs, live coral cover ranged from 4.9% to 19.2%. Coral communities were dominated by small corals (esp. Agaricia, Porites and Siderastrea) although many sites had high abundances of large colonies of Montastraea annularis and M. faveolata, and these were generally in good condition. A single area, within the proposed fishery reserve, had extensive Acropora cervicornis thickets, and several shallow locations had small, but recovering A. palmata stands. Macroalgal cover at all sites was relatively low, with only three sites having greater than 30% cover; crustose coralline algae (CCA) was high, with eight sites exceeding 20% cover. Fish biomass at all sites near the Cays was low, with a dominance of herbivores (parrotfish and surgeonfish) and a near absence of groupers, snappers and other commercially important species. While parrotfish were the most abundant fish, these were all extremely small (mean size= 12cm; &lt;4% over 29cm), and they were dominated by red band parrotfish (Sparisoma aurofrenatum) followed by striped parrotfish (Scarus iseri). While coral communities remain in better condition than most coastal reefs in Jamaica, intense fishing pressure using fish traps (main target species: surgeonfish) and hookah/spear fishing (main target: parrotfish) is of grave concern to the future persistence of these reefs. The proposed fishery reserve encompasses some of the best coral reef habitat near the Cays, but this MPA should be expanded to encompass other habitats and MPAs should be considered for bank reefs at the northwestern end, as well as Banner Reef and Blowers Rock. Rev. Biol. Trop. 62 (Suppl. 3): 11-24. Epub 2014 September 01.

Octocoral Tissue Provides Protection from Declining Oceanic pH

Increase in anthropogenic pCO2 alters seawater chemistry and could lead to reduced calcification or skeleton dissolution of calcifiers and thereby weaken coral-reef structure. Studies have suggested that the complex and diverse responses in stony coral growth and calcification, as a result of elevated pCO2, can be explained by the extent to which their soft tissues cover the underlying skeleton. This study compared the effects of decreased pH on the microstructural features of both in hospite (within the colony) and isolated sclerites (in the absence of tissue protection) of the zooxanthellate reef-dwelling octocoral Ovabunda macrospiculata. Colonies and isolated sclerites were maintained under normal (8.2) and reduced (7.6 and 7.3) pH conditions for up to 42 days. Both in hospite and isolated sclerites were then examined under SEM and ESEM microscopy in order to detect any microstructural changes. No differences were found in the microstructure of the in hospite sclerites between the control and the pH treatments. In stark contrast, the isolated sclerites revealed dissolution damage related to the acidity of the water. These findings suggest a protective role of the octocoral tissue against adverse pH conditions, thus maintaining them unharmed at high pCO2. In light of the competition for space with the less resilient reef calcifiers, octocorals may thus have a significant advantage under greater than normal acidic conditions.

Complementarity of Rotating Video and Underwater Visual Census for Assessing Species Richness, Frequency and Density of Reef Fish on Coral Reef Slopes

Estimating diversity and abundance of fish species is fundamental for understanding community structure and dynamics of coral reefs. When designing a sampling protocol, one crucial step is the choice of the most suitable sampling technique which is a compromise between the questions addressed, the available means and the precision required. The objective of this study is to compare the ability to sample reef fish communities at the same locations using two techniques based on the same stationary point count method: one using Underwater Visual Census (UVC) and the other rotating video (STAVIRO). UVC and STAVIRO observations were carried out on the exact same 26 points on the reef slope of an intermediate reef and the associated inner barrier reefs. STAVIRO systems were always deployed 30 min to 1 hour after UVC and set exactly at the same place. Our study shows that; (i) fish community observations by UVC and STAVIRO differed significantly; (ii) species richness and density of large species were not significantly different between techniques; (iii) species richness and density of small species were higher for UVC; (iv) density of fished species was higher for STAVIRO and (v) only UVC detected significant differences in fish assemblage structure across reef type at the spatial scale studied. We recommend that the two techniques should be used in a complementary way to survey a large area within a short period of time. UVC may census reef fish within complex habitats or in very shallow areas such as reef flat whereas STAVIRO would enable carrying out a large number of stations focused on large and diver-averse species, particularly in the areas not covered by UVC due to time and depth constraints. This methodology would considerably increase the spatial coverage and replication level of fish monitoring surveys.

Measuring the effectiveness of a Caribbean oceanic island no-take zone with an asymmetrical BACI approach

No-take zones (NTZ) provide an effective tool for biodiversity conservation and fisheries management. Increases in abundance and biomass within NTZs, with spillover effects to adjacent areas, have been documented. However, most studies occurred at highly connected sites near regions that can replenish the NTZ. In this study we assess NTZ effects within the Mona Island marine protected area (MPA), located within the Mona Passage, a partial biogeographic barrier. We used the robust asymmetrical before-after-control-impact (BACI) design to evaluate changes in population size for fishery target species four years after designation. Data on fish abundance and size were collected within randomly placed belt transects and roving surveys. Permutational multivariate analysis of variance (PERMANOVA) and similarity percentages (SIMPER) were used to detect changes in the coral reef fish assemblage and structure. Responses in abundance and biomass were calculated at different levels of the assemblage grouped by body size or ontogenetic stage. Additionally, univariate models provided insight on the magnitude and direction of NTZ effects. Results indicate significant increases in locations both open and closed to fishing, predominantly in the abundance of small-sized species and early life stages suggesting a recruitment signal. Significant distinctive performance within two locations was observed, with one showing a higher contribution of medium and large-sized species over time. Mona Island's MPA is apparently achieving its goals of fish stock restoration despite its isolation, although more time, enforcement and monitoring are essential to confirm these trends.

Balancing of Ocean Acidification by Superoxide Redox Chemistry?

O acidification is typically caused by production of carbonic acid (H2CO3) through the dissolution of increasing atmospheric CO2, which adds to CO2 plus DIC (dissolved CO2, H2CO3, HCO3 −, and CO3 2−) produced in seawater by several processes including biological ones (primary production followed by respiration). Acidification can decrease the saturation states of carbonate minerals, which may considerably endanger the dynamics, structure and biodiversity of coral reefs and other marine calcifying organisms. The consequence is the decline in the early development stages of shellfish, coral reefs, or other marine calcifiers, with impacts on fertilization, sexual reproduction, cleavage, larval settlement, survival and growth, finally causing a substantial population decline. Decline in shellfish or coral reefs, which form the foundation of marine ecosystems, would markedly affect the whole food webs and marine population dynamics. Despite the apparent straightforwardness of such a scenario, the actual determination of the impact of atmospheric CO2 on seawater pH is a difficult task. This happens primarily because of the relatively small pH changes, unless very long time spans are taken into account. Moreover, pH modifications can also take place at a local scale because of several possible confounding factors, of which we will show the potential role of superoxide chemistry. Among additional causes for seawater pH variation, one is the acidification connected with eutrophication phenomena that have increased worldwide since the last few decades. Indeed, changes in land-use practices can induce the release of high amounts of nutrients and terrestrial organic matter (OM) to coastal seawaters, including dissolved OM (DOM) and particulate OM (POM). The transformation of organic P and N into phosphate and nitrate because of microbial or photochemical processes can cause a significant decrease of the alkalinity of coastal seawater and, therefore, a decrease in pH. Another possibility (although limited in space and time) is the elevated production of CO2 and DIC during harmful algal blooms, which can significantly alter the pH of seawater as long as they are operational. Eutrophication phenomena and algal blooms can both be enhanced by global warming, which causes an increase of surface seawater temperature and leads to a longer summer stratification period. In warm, sunlit surface seawater the photochemical and biological processes that are involved in the degradation of DOM and POM can be strongly enhanced, leading to increased production of CO2 and of anionic species that decrease the alkalinity of seawater. The third possible issue is connected with atmospheric acid rain, most notably involving HNO3 and H2SO4 that can directly lower the seawater pH. All such processes are usually limited to the coastal areas that are most affected by eutrophication phenomena, while acid rains are not expected to have a comparatively important impact on the pH of the open ocean. Within this context, an additional factor of potential importance is represented by the chemistry of superoxide. The radical ion O2 −• is produced by various sources including extracellular generation by heterotrophic bacteria that are commonly detected in lakes, soil, hydrothermal vents, marine sediments, estuaries, and oceans. Rates of superoxide production normalized to the proportion of metabolically active cells are found to vary between 0.02 ± 0.02 amol cell−1 hour−1 (mean ± standard error) and 19.4 ± 5.2 amol cell−1 hour−1. Such production rates could lead to an alteration of seawater pH because of H consumption upon dismutation of superoxide (2 O2 −• + 2H → H2O2 + O2). The latter process is catalyzed by redox-active metals such as Cu(I/II) and Fe(II/ III) and possibly (but still controversially) by organic compounds with redox-capable moieties. With a biological generation rate of O2 −• prudentially assumed to be 0.2 pmol dm−3 s−1, which is only 10% of that reported by Diaz et al., based on superoxide dismutation alone one can have a H consumption rate of ∼6 μmol dm−3 year−1. A simplified seawater model shows that the process would be able to compensate for an acidification rate of 0.1−0.3 pH units year−1 Viewpoint

Decadal-scale changes in the community structure of coral reefs of St. John, US Virgin Islands

Most coral reefs differ from those visited by explorers in the 15th century and described by ecologists in the 1950s, and reports of degraded reefs and hypotheses regarding the implications of the changes abound. Tests of these hypotheses require decadal-scale ecological analyses, yet data to support these efforts remain rare. In this study, 25 yr of time-series analyses from 3 habitats in St. John, US Virgin Islands, revealed changes in coral community structure that are spatially and temporally heterogeneous, only loosely coupled with local disturbances, and equivocal in terms of the coral community structure that can be projected from the recent past. In a near-shore habitat at 7 to 9 m depth, coral cover remained ~4% between 1992 and 2011, and variation in benthic community structure was driven mostly by Agaricia. In an Orbicella annularis (formerly Montastraea annularis)-dominated habitat at 9 m depth, coral cover declined from 45% in 1987 to 7% in 2011, and varied among decades. In a second O. annularis-dominated habitat at 14 m depth, coral cover increased from 32% in 1987 to 49% in 2002, but then declined to 29% in 2011. The density of small corals also changed between 1994 and 2011, beginning with a density (colonies per 0.25 m2) of 3.5 in 1992, rising to 4.9 in 2005, but declining to 3.0 in 2011. Each genus of small corals responded in dissimilar ways over time, with genera waxing and waning in abundance on multiple occasions and in asynchronous patterns. The reefs of St. John have changed markedly since 1987, but the present results show how adjacent habitats and sympatric coral taxa translate similar environmental signals into discordant community trajectories that suggest that these coral communities will not disappear in the short term.

Critical research needs for identifying future changes in Gulf coral reef ecosystems

Expert opinion was assessed to identify current knowledge gaps in determining future changes in Arabian/Persian Gulf (thereafter ‘Gulf’) coral reefs. Thirty-one participants submitted 71 research questions that were peer-assessed in terms of scientific importance (i.e., filled a knowledge gap and was a research priority) and efficiency in resource use (i.e., was highly feasible and ecologically broad). Ten research questions, in six major research areas, were highly important for both understanding Gulf coral reef ecosystems and also an efficient use of limited research resources. These questions mirrored global evaluations of the importance of understanding and evaluating biodiversity, determining the potential impacts of climate change, the role of anthropogenic impacts in structuring coral reef communities, and economically evaluating coral reef communities. These questions provide guidance for future research on coral reef ecosystems within the Gulf, and enhance the potential for assessment and management of future changes in this globally significant region.

Mixing of relict and modern tests of larger benthic foraminifera on the Great Barrier Reef shelf margin

The distribution of live, well preserved, and relict larger benthic foraminifera (LBF) in sediment samples collected off the shelf margin near Hydrographers Passage in the Great Barrier Reef, Australia, was studied to document taphonomic processes in this setting. In total, eight living species of LBF were found, mostly of the families Amphisteginidae and Nummulitidae, separated into a shallow (platform) assemblage and a deep (terrace) assemblage. The modern (well preserved) shells in the sediment revealed a good representation of this fauna, but differences in depth distribution were lost. The relict fauna is represented by a more diverse fauna, and is composed mostly of taxa occurring in much shallower conditions. We interpret this pattern as an initial downslope transport of the foraminifera from the shelf into deeper waters, and a mixing of foraminifera derived from nearby Pleistocene coral reef structures.

A new species of the genus Rhopalaea (Class: Ascidiacea) from the Red Sea

During regular surveys and collection of ascidians along the Red Sea coast of Israel, several specimens of an undescribed species of Rhopalaea were collected. Samples were collected by SCUBA from the natural coral reef and man-made structures at depths between 10 to 40m. This is the first species of the genus Rhopalaea described from the Red Sea, which is characterized by a transparent, uncolored gelatinous tunic with elongated attachment extensions, and is distinguished by its eight atrial lobes, thoracic muscle arrangement, and branchial sac structure.

Alteration of Coral Reef Community Structure Caused by Athropogenic Activity in Coastal Area of Giliketapang Island, Probolinggo, East Java, Indonesia

The aim of the research was to know the effect of anthropogenic activity around the coastal area of Giliketapang Island toward community structure of coral reefs. This study used belt transect method which consisted of 18 transects in three locations (harbour, near the forest, and transitionol areas) at 50-80 m from the coast, with a plot sized of 2×2m 2 . Abundance of coral reefs were observed based on their coverage. There were 11 families of coral reefs found. The percentage of coral damage decreased and the number of the species increased at the zones far from the coast. This also affected the coverage value of coral reefs. Coral coverage at the harbour and near the forest was 53. 06 % and 65. 36 % respectively, whereas in the transitional area was 38. 38%. Co-domination between Porites lobata and Chyphastrea microphthalma occurred at the harbour, while Poritesmayeri and Acropora spp. codominated in the location near the forest. Poritidae family dominated the transition area. Anthropogenic activities at transitional location caused severe damage toward coral reefs.

Preparing to manage coral reefs for ocean acidification: lessons from coral bleaching

Ocean acidification is a direct consequence of increasing atmospheric carbon dioxide concentrations and is expected to compromise the structure and function of coral reefs within this century. Research into the effects of ocean acidification on coral reefs has focused primarily on measuring and predicting changes in seawater carbon (C) chemistry and the biological and geochemical responses of reef organisms to such changes. To date, few ocean acidification studies have been designed to address conservation planning and management priorities. Here, we discuss how existing marine protected area design principles developed to address coral bleaching may be modified to address ocean acidification. We also identify five research priorities needed to incorporate ocean acidification into conservation planning and management: (1) establishing an ocean C chemistry baseline, (2) establishing ecological baselines, (3) determining species/habitat/community sensitivity to ocean acidification, (4) projecting changes in seawater carbonate chemistry, and (5) identifying potentially synergistic effects of multiple stressors.

Spatial variation in mechanical properties of coral reef substrate and implications for coral colony integrity

The physical structure of coral reefs plays a critical role as a barrier to storm waves and tsunamis and as a habitat for living reef-building and reef-associated organisms. However, the mechanical properties of reef substrate (i.e. the non-living benthos) are largely unknown, despite the fact that substrate properties may ultimately determine where organisms can persist. We used a geo-mechanical technique to measure substrate material density and strength over a reef hydrodynamic gradient. Contrary to expectation, we found a weak relationship between substrate strength and wave-induced water flow: flow rates decline sharply at the reef crest, whereas substrate properties are relatively constant over much of the reef before declining by almost an order of magnitude at the reef back. These gradients generate a novel hump-shaped pattern in resistance to mechanical disturbances for live corals, where colonies closer to the back reef are prone to dislodgement because of poorly cemented substrate. Our results help explain an intermediate zone of higher taxonomic and morphological diversity bounded by lower diversity exposed reef crest and unstable reef back zones.

Global coral disease prevalence associated with sea temperature anomalies and local factors

Coral diseases are taking an increasing toll on coral reef structure and biodiversity and are important indicators of declining health in the oceans. We implemented standardized coral disease surveys to pinpoint hotspots of coral disease, reveal vulnerable coral families and test hypotheses about climate drivers from 39 locations worldwide. We analyzed a 3 yr study of coral disease prevalence to identify links between disease and a range of covariates, including thermal anomalies (from satellite data), location and coral cover, using a Generalized Linear Mixed Model. Prevalence of unhealthy corals, i.e. those with signs of known diseases or with other signs of compromised health, exceeded 10% on many reefs and ranged to over 50% on some. Disease prevalence exceeded 10% on 20% of Caribbean reefs and 2.7% of Pacific reefs surveyed. Within the same coral families across oceans, prevalence of unhealthy colonies was higher and some diseases were more common at sites in the Caribbean than those in the Pacific. The effects of high disease prevalence are potentially extensive given that the most affected coral families, the acroporids, faviids and siderastreids, are among the major reef-builders at these sites. The poritids and agaricids stood out in the Caribbean as being the most resistant to disease, even though these families were abundant in our surveys. Regional warm temperature anomalies were strongly correlated with high disease prevalence. The levels of disease reported here will provide a much-needed local reference point against which to compare future change.