Showing papers in "Coral Reefs in 2008"

Ocean acidification and calcifying reef organisms: a mesocosm investigation

Abstract: A long-term (10 months) controlled experiment was conducted to test the impact of increased partial pressure of carbon dioxide (pCO2) on common calcifying coral reef organisms. The experiment was conducted in replicate continuous flow coral reef mesocosms flushed with unfiltered sea water from Kaneohe Bay, Oahu, Hawaii. Mesocosms were located in full sunlight and experienced diurnal and seasonal fluctuations in temperature and sea water chemistry characteristic of the adjacent reef flat. Treatment mesocosms were manipulated to simulate an increase in pCO2 to levels expected in this century [midday pCO2 levels exceeding control mesocosms by 365 ± 130 μatm (mean ± sd)]. Acidification had a profound impact on the development and growth of crustose coralline algae (CCA) populations. During the experiment, CCA developed 25% cover in the control mesocosms and only 4% in the acidified mesocosms, representing an 86% relative reduction. Free-living associations of CCA known as rhodoliths living in the control mesocosms grew at a rate of 0.6 g buoyant weight year−1 while those in the acidified experimental treatment decreased in weight at a rate of 0.9 g buoyant weight year−1, representing a 250% difference. CCA play an important role in the growth and stabilization of carbonate reefs, so future changes of this magnitude could greatly impact coral reefs throughout the world. Coral calcification decreased between 15% and 20% under acidified conditions. Linear extension decreased by 14% under acidified conditions in one experiment. Larvae of the coral Pocillopora damicornis were able to recruit under the acidified conditions. In addition, there was no significant difference in production of gametes by the coral Montipora capitata after 6 months of exposure to the treatments.

Capturing the cornerstones of coral reef resilience: linking theory to practice

Abstract: Coral reefs can undergo unexpected and dramatic changes in community composition, so called phase shifts. This can have profound consequences for ecosystem services upon which human welfare depends. Understanding of this behavior is in many aspects still in its infancy. Resilience has been argued to provide insurance against unforeseen ecosystem responses in the face of environmental change, and has become a prime goal for the management of coral reefs. However, diverse definitions of resilience can be found in the literature, making its meaning ambiguous. Several studies have used the term as a theoretical framework and concern regarding its practical applicability has been raised. Consequently, operationalizing theory to make resilience observable is an important task, particularly for policy makers and managers dealing with pressing environmental problems. Ultimately this requires some type of empirical assessments, something that has proven difficult due to the multidimensional nature of the concept. Biodiversity, spatial heterogeneity, and connectivity have been proposed as cornerstones of resilience as they may provide insurance against ecological uncertainty. The aim of this article is to provide an overview of the divergent uses of the concept and to propose empirical indicators of the cornerstones of coral reef resilience. These indicators include functional group approaches, the ratios of “good” and “bad” colonizers of space, measurements of spatial heterogeneity, and estimates of potential space availability against grazing capacity. The essence of these operational indicators of resilience is to use them as predictive tools to recognize vulnerability before disturbance occurs that may lead to abrupt phase shifts. Moving toward operationalizing resilience theory is imperative to the successful management of coral reefs in an increasingly disturbed and human-dominated environment.

Cross-shelf variation in the role of parrotfishes on the Great Barrier Reef

Abstract: Herbivorous fishes are a key functional group on coral reefs. These fishes are central to the capacity of reefs to resist phase shifts and regenerate after disturbance. Despite this importance few studies have quantified the direct impact of these fishes on coral reefs. In this study the roles of parrotfishes, a ubiquitous group of herbivorous fishes, were examined on reefs in the northern Great Barrier Reef. The distribution of 24 species of parrotfish was quantified on three reefs in each of three cross-shelf regions. Functional roles (grazing, erosion, coral predation and sediment reworking) were calculated as the product of fish density, bite area or volume, bite rate, and the proportion of bites taken from various substrata. Inner-shelf reefs supported high densities but low biomass of parrotfishes, with high rates of grazing and sediment reworking. In contrast, outer-shelf reefs were characterised by low densities and high biomass of parrotfish, with high rates of erosion and coral predation. Mid-shelf reefs displayed moderate levels of all roles examined. The majority of this variation in functional roles was attributable to just two species. Despite being rare, Bolbometopon muricatum, the largest parrotfish species, accounted for 87.5% of the erosion and 99.5% of the coral predation on outer-shelf reefs. B. muricatum displayed little evidence of selectivity of feeding, with most substrata being consumed in proportion to their availability. In contrast, the high density of Scarus rivulatus accounted for over 70% of the total grazing and sediment reworking on inner-shelf reefs. This marked variation in the roles of parrotfishes across the continental shelf suggests that each shelf system is shaped by fundamentally different processes.

Survival dynamics of scleractinian coral larvae and implications for dispersal

Abstract: Survival of pelagic marine larvae is an important determinant of dispersal potential. Despite this, few estimates of larval survival are available. For scleractinian corals, few studies of larval survival are long enough to provide accurate estimates of longevity. Moreover, changes in mortality rates during larval life, expected on theoretical grounds, have implications for the degree of connectivity among reefs and have not been quantified for any coral species. This study quantified the survival of larvae from five broadcast-spawning scleractinian corals (Acropora latistella, Favia pallida, Pectinia paeonia, Goniastrea aspera, and Montastraea magnistellata) to estimate larval longevity, and to test for changes in mortality rates as larvae age. Maximum lifespans ranged from 195 to 244 d. These longevities substantially exceed those documented previously for coral larvae that lack zooxanthellae, and they exceed predictions based on metabolic rates prevailing early in larval life. In addition, larval mortality rates exhibited strong patterns of variation throughout the larval stage. Three periods were identified in four species: high initial rates of mortality; followed by a low, approximately constant rate of mortality; and finally, progressively increasing mortality after approximately 100 d. The lifetimes observed in this study suggest that the potential for long-distance dispersal may be substantially greater than previously thought. Indeed, detection of increasing mortality rates late in life suggests that energy reserves do not reach critically low levels until approximately 100 d after spawning. Conversely, increased mortality rates early in life decrease the likelihood that larvae transported away from their natal reef will survive to reach nearby reefs, and thus decrease connectivity at regional scales. These results show how variation in larval survivorship with age may help to explain the seeming paradox of high genetic structure at metapopulation scales, coupled with the maintenance of extensive geographic ranges observed in many coral species.

Coral calcification responds to seawater acidification: a working hypothesis towards a physiological mechanism

Abstract: The decrease in the saturation state of seawater, Ω, following seawater acidification, is believed to be the main factor leading to a decrease in the calcification of marine organisms. To provide a physiological explanation for this phenomenon, the effect of seawater acidification was studied on the calcification and photosynthesis of the scleractinian tropical coral Stylophora pistillata. Coral nubbins were incubated for 8 days at three different pH (7.6, 8.0, and 8.2). To differentiate between the effects of the various components of the carbonate chemistry (pH, CO32−, HCO3−, CO2, Ω), tanks were also maintained under similar pH, but with 2-mM HCO3− added to the seawater. The addition of 2-mM bicarbonate significantly increased the photosynthesis in S. pistillata, suggesting carbon-limited conditions. Conversely, photosynthesis was insensitive to changes in pH and pCO2. Seawater acidification decreased coral calcification by ca. 0.1-mg CaCO3 g−1 d−1 for a decrease of 0.1 pH units. This correlation suggested that seawater acidification affected coral calcification by decreasing the availability of the CO32− substrate for calcification. However, the decrease in coral calcification could also be attributed either to a decrease in extra- or intracellular pH or to a change in the buffering capacity of the medium, impairing supply of CO32− from HCO3−.

Bleaching increases likelihood of disease on Acropora palmata (Lamarck) in Hawksnest Bay, St John, US Virgin Islands

Abstract: Anomalously high water temperatures may enhance the likelihood of coral disease outbreaks by increasing the abundance or virulence of pathogens, or by increasing host susceptibility. This study tested the compromised-host hypothesis, and documented the relationship between disease and temperature, through monthly monitoring of Acropora palmata colonies from May 2004 to December 2006, in Hawksnest Bay, St John, US Virgin Islands (USVI). Disease prevalence and the rate of change in prevalence showed a positive linear relationship with water temperature and rate of change in water temperature, respectively, but only in 2005 during prolonged periods of elevated temperature. Both bleached and unbleached colonies showed a positive relationship between disease prevalence and temperature in 2005, but the average area of disease-associated mortality increased only for bleached corals, indicating host susceptibility, rather than temperature per se, influenced disease severity on A. palmata.

Carbonate budgets and reef production states: a geomorphic perspective on the ecological phase-shift concept

Abstract: Recent, region-scale estimates suggest that high levels of coral cover loss have occurred in both the Caribbean and Indo-PaciWc reef provinces (Gardner et al. 2003; Bruno and Selig 2007), and that some 60–70% of coral communities globally are directly threatened by anthropogenic activities (Wilkinson 1996; Hoegh-Guldberg 1999; Goreau et al. 2000). The ecological changes that have resulted from these disturbances have been widely discussed in terms of coral reef ‘health’ and resilience (Hughes et al. 2003; Bellwood et al. 2004) and often aligned with the ecological phase-shift concept originally outlined by Done (1992). A phase shift, in the context of a coral reef, has been deWned as a transition in the ecological state of the reef to conditions of low coral cover and persistent high Xeshy macroalgal cover (Done 1992; McManus and Polsenberg 2004). Whilst episodic natural disturbance events can be important drivers of such ecological transitions, a variety of both direct and indirect anthropogenic disturbances are also widely implicated (Done 1999). Such disturbances might modify natural reef processes, either by altering the ecological balance within individual reef systems (Jackson 1997; Jackson et al. 2001) or the environmental conditions under which reef growth occurs (Kleypas et al. 1999). Whilst earlier concerns over passive reef ‘drowning’ in response to greatly accelerated sea-level rise (e.g. Buddemeier and Smith 1988) have not been maintained (Spencer 1995), there remains considerable concern as to how more modest rates of sea-level rise, alongside climate change-related shifts in chronic stress from changing ocean temperatures and ocean chemistry, may lead to damage acute events occurring at ever more frequent intervals (e.g. Hoegh-Guldberg et al. 2007). The implications for reefs: as geomorphic structures, in terms of the extent of reef framework development and in terms of net carbonate accumulation (Edinger et al. 2000); the geomorphic integrity of reef associated sedimentary landforms; and the ecological eVects, in terms of changes in reef community structures (Chadwick-Furman 1996; Harvell et al. 2002; Hughes et al. 2003; Hoegh-Guldberg et al. 2007) remain, however, unclear. In the scientiWc literature, much of the discussion about the eVects of such ecological and environmental change has focused on quantifying the impacts upon coral communities, especially in terms of monitoring changes in coral cover and diversity, and in coral community structure. Central to many such discussions has been the issue of variations in the relative abundance of corals and macroalgae Communicated by Geology Editor Dr Bernhard Riegl

Mass extinctions and ocean acidification: biological constraints on geological dilemmas

Abstract: The five mass extinction events that the earth has so far experienced have impacted coral reefs as much or more than any other major ecosystem. Each has left the Earth without living reefs for at least four million years, intervals so great that they are commonly referred to as ‘reef gaps’ (geological intervals where there are no remnants of what might have been living reefs). The causes attributed to each mass extinction are reviewed and summarised. When these causes and the reef gaps that follow them are examined in the light of the biology of extant corals and their Pleistocene history, most can be discarded. Causes are divided into (1) those which are independent of the carbon cycle: direct physical destruction from bolides, ‘nuclear winters’ induced by dust clouds, sea-level changes, loss of area during sea-level regressions, loss of biodiversity, low and high temperatures, salinity, diseases and toxins and extraterrestrial events and (2) those linked to the carbon cycle: acid rain, hydrogen sulphide, oxygen and anoxia, methane, carbon dioxide, changes in ocean chemistry and pH. By process of elimination, primary causes of mass extinctions are linked in various ways to the carbon cycle in general and ocean chemistry in particular with clear association with atmospheric carbon dioxide levels. The prospect of ocean acidification is potentially the most serious of all predicted outcomes of anthropogenic carbon dioxide increase. This study concludes that acidification has the potential to trigger a sixth mass extinction event and to do so independently of anthropogenic extinctions that are currently taking place.

The non-linear relationship between body size and function in parrotfishes

Abstract: Parrotfishes are a group of herbivores that play an important functional role in structuring benthic communities on coral reefs. Increasingly, these fish are being targeted by fishermen, and resultant declines in biomass and abundance may have severe consequences for the dynamics and regeneration of coral reefs. However, the impact of overfishing extends beyond declining fish stocks. It can also lead to demographic changes within species populations where mean body size is reduced. The effect of reduced mean body size on population dynamics is well described in literature but virtually no information exists on how this may influence important ecological functions. The study investigated how one important function, scraping (i.e., the capacity to remove algae and open up bare substratum for coral larval settlement), by three common species of parrotfishes (Scarus niger, Chlorurus sordidus, and Chlorurus strongylocephalus) on coral reefs at Zanzibar (Tanzania) was influenced by the size of individual fishes. There was a non-linear relationship between body size and scraping function for all species examined, and impact through scraping was also found to increase markedly when fish reached a size of 15–20 cm. Thus, coral reefs which have a high abundance and biomass of parrotfish may nonetheless be functionally impaired if dominated by small-sized individuals. Reductions in mean body size within parrotfish populations could, therefore, have functional impacts on coral reefs that previously have been overlooked.

Effect of aragonite saturation state on settlement and post-settlement growth of Porites astreoides larvae

Abstract: In response to the increases in pCO2 projected in the 21st century, adult coral growth and calcification are expected to decrease significantly. However, no published studies have investigated the effect of elevated pCO2 on earlier life history stages of corals. Porites astreoides larvae were collected from reefs in Key Largo, Florida, USA, settled and reared in controlled saturation state seawater. Three saturation states were obtained, using 1 M HCl additions, corresponding to present (380 ppm) and projected pCO2 scenarios for the years 2065 (560 ppm) and 2100 (720 ppm). The effect of saturation state on settlement and post-settlement growth was evaluated. Saturation state had no significant effect on percent settlement; however, skeletal extension rate was positively correlated with saturation state, with ~50% and 78% reductions in growth at the mid and high pCO2 treatments compared to controls, respectively.

Elevated temperature restricts growth potential of the coral reef fish Acanthochromis polyacanthus

Abstract: In order to test the effect of temperature variation on the growth of a common coral-reef fish, Acanthochromis polyacanthus, juveniles, sub-adults and adults were reared on either high or low food rations at temperatures corresponding to the long-term (14 year) minimum, average and maximum summer sea-surface temperatures (26, 28 and 31°C respectively) at Orpheus Island, Great Barrier Reef, Australia. Both temperature and food supply affected the growth of juvenile and adult A. polyacanthus. Individuals grew more on high food rations, but growth declined with increasing temperature. Importantly, at 31°C, the growth of juveniles and adults on the high food ration was nearly identical to growth on the low food ration. This indicates that the capacity for growth is severely limited at higher ocean temperatures that are predicted to become the average for Orpheus Island within the next 100 years as a result of rapid climate change.

Remote video bioassays reveal the potential feeding impact of the rabbitfish Siganus canaliculatus (f: Siganidae) on an inner-shelf reef of the Great Barrier Reef

Abstract: Herbivores are widely acknowledged as key elements maintaining the health and resilience of terrestrial and aquatic ecosystems. Understanding and quantifying the impact of herbivores in ecosystems are fundamental to our ability to manage these systems. The traditional method of quantifying the impact of herbivorous fishes on coral reefs has been to use transplanted pieces of seagrass or algae as "bioassays". However, these experiments leave a key question unanswered, namely: Which species are responsible for the impact being quantified? This study revisits the use of bioassays and tested the assumption that the visual abundance of species reflects their role in the removal of assay material. Using remote video cameras to film removal of assay material on an inner-shelf reef of the Great Barrier Reef, the species responsible for assay-based herbivory were identified. The video footage revealed that Siganus canaliculatus, a species not previously recorded at the study site, was primarily responsible for removal of macroalgal biomass. The average percentage decrease in thallus length of whole plants of Sargassum at the reef crest was 54 ± 8.9% (mean ± SE), and 50.4 ± 9.8% for individually presented Sargassum strands (for a 4.5-h deployment). Of the 14,656 bites taken from Sargassum plants and strands across all reef zones, nearly half (6,784 bites or 46%) were taken by S. canaliculatus, with the majority of the remainder attributable to Siganus doliatus. However, multiple regression analysis demonstrated that only the bites of S. canaliculatus were removing macroalgal biomass. The results indicate that, even with detailed observations, the species of herbivore that may be responsible for maintaining benthic community structure can go unnoticed. Some of our fundamental ideas of the relative importance of individual species in ecosystem processes may be in need of re-evaluation.

Predation on the invasive red lionfish, Pterois volitans (Pisces: Scorpaenidae), by native groupers in the Bahamas

Abstract: On 26 January 2008, a tiger grouper, Mycteroperca tigris (472-mm standard length [SL]), was caught off New Providence (25 04.6¢¢N, 77 20.6¢¢W), Bahamas and found to contain a single red lionfish, Pterois volitans (61-mm SL) in its stomach. This observation was considered an anomaly given both the venomous nature of lionfish, and their relatively recent introduction to the Bahamas (Snyder and Burgess 2007). Anecdotal evidence provided by fishers, however, suggested that native grouper species were preying on red lionfish with some regularity. Subsequently, five Nassau groupers, Epinephelus striatus, caught off Eleuthera Island (25 10.0¢¢N, 76 14.0¢¢W) at an approximate depth of 14 m on 5 March 2008, were dissected. Two of the stomachs contained red lionfish. The first grouper (477-mm SL) contained a partially digested lionfish, identifiable only by the morphology and multiplicity of the remaining fin rays. The second slightly larger grouper (482-mm SL) contained a red lionfish of 137-mm SL which was in almost pristine condition (Fig. 1). The successful invasion and establishment of the piscivorous red lionfish in western Atlantic waters (Fig. 2) (Whitfield et al. 2002; Snyder and Burgess 2007) have lead to concerns over its potential impact on native fish biotas. To our knowledge, this is the first documented evidence of introduced red lionfish being preyed upon by native species within their novel range.

Decadal trends in a coral community and evidence of changed disturbance regime

Abstract: A 23 year data set (1981-2003 inclusive) and the spatially explicit individual-based model ''Compete � '' were used to investigate the implications of changing dis- turbance frequency on cover and taxonomic composition of a shallow coral community at Lizard Island, Australia. Near-vertical in situ stereo-photography was used to esti- mate rates of coral growth, mortality, recruitment and outcomes of pair-wise competitive interactions for 17 physiognomic groups of hard and soft corals. These data were used to parameterise the model, and to quantify impacts of three acute disturbance events that caused sig- nificant coral mortality: 1982—a combination of coral bleaching and Crown-of-Thorns starfish; 1990—cyclone waves; and 1996—Crown-of-Thorns starfish. Predicted coral community trajectories were not sensitive to the outcomes of competitive interactions (probably because average coral cover was only 32% and there was strong vertical separation among established corals) or to major changes in recruitment rates. The model trajectory of coral cover matched the observed trajectory accurately until the 1996 disturbance, but only if all coral mortality was con- fined to the 3 years of acute disturbance. Beyond that date (1997-2003), when the observed community failed to recover, it was necessary to introduce annual chronic background mortality to obtain a good match between modelled and observed coral cover. This qualitative switch in the model may reflect actual loss of resilience in the real community. Simulated over a century, an 8 year distur- bance frequency most closely reproduced the mean community composition observed in the field prior to major disturbance events. Shorter intervals between dis- turbances led to reduced presence of the dominant hard coral groups, and a gradual increase in the slow growing, more resilient soft corals, while longer intervals (up to 16 years) resulted in monopolization by the fastest growing table coral, Acropora hyacinthus.

Epidemiology of skeletal eroding band on the Great Barrier Reef and the role of injury in the initiation of this widespread coral disease

Abstract: Skeletal eroding band (SEB), which manifests as dense aggregations of the ciliate Halofolliculina corallasia, was the first coral disease described from the Indo-Pacific. Little is known about its etiology or impact. This study describes the distribution, prevalence and host range of SEB on a 500 km extent of the Great Barrier Reef (GBR), together with in situ rates of progression and infection following experimental injury. SEB occurred on 90–100% of reefs surveyed (n=18) in each of 3 years, demonstrating that SEB is widely distributed and persistent. SEB had the highest prevalence of any disease, affecting approximately 2% of 283,486 scleractinians and hydrocorals surveyed. Its host range was large, affecting 12 families and at least 82 scleractinian species, as well as the hydrocoral, Millepora. Corals in the families Pocilloporidae and Acroporidae were most susceptible, the former being up to five times more susceptible than other families. Progressive tissue loss was recorded on 95% of Acropora muricata colonies monitored (n = 18), with rates of SEB progression averaging ∼2 mm/day. Injury experiments demonstrated that H. corallasia, the putative pathogen of SEB, readily colonised recently exposed coral skeleton in the absence of a vector, but did not colonise intact coral tissue. Invading ciliates failed to form band-like aggregations associated with progressive tissue loss on any of three coral species tested experimentally, suggesting that, while H. corallasia readily colonises recently exposed coral skeleton, it may not be sufficient in itself to cause tissue mortality. Interactions with additional agents or factors, increasing ciliate virulence or lowering disease resistance of coral hosts may be required before halofolliculinid infections become associated with tissue loss.

Recruitment failure in Florida Keys Acropora palmata, a threatened Caribbean coral

Abstract: Recovery of Acropora palmata from its currently imperiled status depends on recruitment, a process which is poorly documented in existing Caribbean coral population studies. A. palmata is thought to be well adapted to proliferate through the recruitment of fragments resulting from physical disturbances, such as moderate intensity hurricanes. This study monitored fifteen 150 m2 fixed study plots on the upper Florida Keys fore-reef for asexual and sexual recruitment from 2004 to 2007. Between July and October 2005, 4 hurricanes passed by the Florida Keys, producing wind speeds on the reef tract of 23 to 33 m s−1. Surveys following the hurricanes documented an average loss of 52% estimated live tissue area within the study plots. The percentage of “branching” colonies in the population decreased from 67% to 42% while “remnant” colonies (isolated patches of tissue on standing skeleton) increased from 11% to 27%. Although some detached branches remained as loose fragments, more than 70% of the 380 fragments observed in the study plots were dead or rapidly losing tissue 3 weeks after Hurricane Dennis. Over the course of the study, only 27 fragments became attached to the substrate to form successful asexual recruits. Meanwhile, of the 18 new, small encrusting colonies that were observed in the study, only 2 were not attributable to asexual origin (i.e., remnant tissue from colonies or fragments previously observed) and are therefore possible sexual recruits. In summary, the 2005 hurricane season resulted in substantial loss of A. palmata from the upper Florida Keys fore-reef from a combination of physical removal and subsequent disease-like tissue mortality, and yielded few recruits of either sexual or asexual origin. Furthermore, the asexual and sexual fecundity of the remaining population is compromised for the near future due to the lack of branches (i.e., “asexual fecundity”) and overall loss of live tissue.

Effects of suspended sediments, dissolved inorganic nutrients and salinity on fertilisation and embryo development in the coral Acropora millepora (Ehrenberg, 1834)

Abstract: Exposure of coral reefs to river plumes carrying increasing loads of nutrients and sediments is a pressing issue for coral reefs around the world including the Great Barrier Reef (GBR). Laboratory experiments were conducted to investigate the effects of changes in inorganic nutrients (nitrate, ammonium and phosphate), salinity and various types of suspended sediments in isolation and in combination on rates of fertilisation and early embryonic development of the scleractinian coral Acropora millepora. Dose–response experiments showed that fertilisation declined significantly with increasing sediments and decreasing salinity, while inorganic nutrients at up to 20 μM nitrate or ammonium and 4 μM phosphate had no significant effect on fertilisation. Suspended sediments of ≥100 mg l−1 and salinity of 30 ppt reduced fertilisation by >50%. Developmental abnormality occurred in 100% of embryos at 30 ppt salinity, and no fertilisation occurred at ≤28 ppt. Another experiment tested interactions between sediment, salinity and nutrients and showed that fertilisation was significantly reduced when nutrients and low concentrations of sediments co-occurred, although both on their own had no effect on fertilisation rates. Similarly, while slightly reduced salinity on its own had no effect, fertilisation was reduced when it coincided with elevated levels of sediments or nutrients. Both these interactions were synergistic. A third experiment showed that sediments with different geophysical and nutrient properties had differential effects on fertilisation, possibly related to sediment and nutrient properties. The findings highlight the complex nature of the effects of changing water quality on coral health, particularly stressing the significance of water quality during coral spawning time.

Predictability of reef fish diversity and abundance using remote sensing data in Diego Garcia (Chagos Archipelago)

Abstract: The diversity, abundance and distribution of reef fish are related to heterogeneity and physical complexity of benthic habitat However, the field effort required to evaluate these aspects of the benthos in situ, at the scale of entire reefscapes, is greatly constrained by logistical and resource limitations With moderate ground truthing, both substratum type and seabed topography are amenable to monitoring using satellite data Here, remote sensing imagery was used to resolve the bathymetry and benthic character of a reef system in Diego Garcia (British Indian Ocean Territory) Replicate fish counts were made at seven measurement stations across the study area using visual census Monte Carlo simulation revealed that species richness and abundance of several guilds and size groupings of reef fish appraised in situ were correlated with the satellite-derived seabed parameters over areas of seafloor as large as 5,030 m2 The study suggests that satellite remote sensing is capable of predicting habitat complexity at a scale relevant to fish Furthermore, as larger size classes of fish were better predicted with the satellite habitat complexity data, this technique could be used to predict fish stocks and identify potential sites for marine protected areas where intensive field surveys are not practical

Resilience of coral communities on an isolated system of reefs following catastrophic mass-bleaching

Abstract: As a result of climate change, sea-water temperatures around the world are expected to increase, potentially causing more frequent and severe episodes of coral bleaching. In this study, the impact of elevated water temperatures at an isolated system of reefs was assessed by quantifying the changes in benthic communities over almost 10 years. Mass-coral bleaching in 1998 dramatically altered the community structure of the reefs, including a >80% relative decrease in the cover of hard and soft corals and a twofold increase in the cover of algae, but which did not include macroalgae. The magnitude of the impact varied among the different sites according to their initial cover and community structure, largely due to the differing susceptibilities of the dominant groups of hard corals. Subsequent increase in the cover of these groups varied according to their life history traits, such as modes of reproduction and rates of growth. Additionally, the increase in cover was strongly correlated with the magnitude of the impact at the different sites, suggesting that recovery was driven by processes acting over local scales. Six years after the bleaching, the hard corals had returned to approximately 40% of their pre-bleaching cover, but there was little change in the cover of soft corals, and the structure of most hard coral communities remained very different to that prior to the bleaching. These data provides insights into the degree to which coral communities are resilient to catastrophic disturbances, when they are isolated from other reef systems but not exposed to some of the chronic stressors affecting many reefs around the world.

Linkages between coral assemblages and coral proxies of terrestrial exposure along a cross-shelf gradient on the southern Great Barrier Reef

Abstract: Coral core records, combined with measurements of coral community structure, were used to assess the long-term impact of multiple environmental stressors on reef assemblages along an environmental gradient. Multiple proxies (luminescent lines, Ba/Ca, δ15N) that reflect different environmental conditions (freshwater discharge, sediment delivery to the nearshore, nutrient availability and transformations) were measured in Porites coral cores collected from nearshore reefs at increasing distance from the intensively agricultural region of Mackay (Queensland, Australia). The corals provide a record (1968–2002) of the frequency and intensity of exposure to terrestrial runoff and fertilizer-derived nitrogen and were used to assess how the present-day coral community composition may have been influenced by flood-related disturbance. Reefs closest to the mainland (5–32 km offshore) were characterized by low hard coral cover (≤10%), with no significant differences among locations. Distinct annual luminescent lines and elevated Ba/Ca values (4.98 ± 0.63 μmol mol−1; mean ± SD) in the most inshore corals (Round Top Island; 5 km offshore) indicated chronic, sub-annual exposure to freshwater and resuspended terrestrial sediment that may have historically prevented reef formation. By contrast, corals from Keswick Island (32 km offshore) indicated episodic, high-magnitude exposure to Pioneer River discharge during extreme flood events (e.g., 1974, 1991), with strongly luminescent lines and substantially enriched coral skeletal δ15N (12–14‰). The reef assemblages at Keswick and St. Bees islands were categorically different from all other locations, with high fleshy macroalgal cover (80.1 ± 7.2% and 62.7 ± 7.1%, respective mean ± SE) overgrowing dead reef matrix. Coral records from Scawfell Island (51 km offshore) indicated little exposure to Pioneer catchment influence: all locations from Scawfell and further offshore had total hard and soft coral cover comparable to largely undisturbed nearshore to middle shelf reefs of the southern Great Barrier Reef.

Predation risk assessment by olfactory and visual cues in a coral reef fish

Abstract: Assessment of predation risk is vital for the success of an individual. Primary cues for the assessment include visual and olfactory stimuli, but the relative importance of these sources of information for risk assessment has seldom been assessed for marine fishes. This study examined the importance of visual and chemical cues in assessing risk for the star goby, Asterropteryx semipunctatus. Visual and chemical cue intensities were used that were indicative of a high threat situation. The behavioural response elicited by both the visual cues of a predator (the rock cod, Cephalopholis boenak) and the chemical alarm cues from conspecifics were similar in magnitude, with responses including a decrease in feeding strikes and moves. A bobbing behaviour was exhibited when the predator was visible and not when only exposed to the chemical alarm cue. When visual and chemical cues were presented together they yielded a stronger antipredator response than when gobies were exposed solely to conspecific alarm cues. This suggests additivity of risk assessment information at the levels of threat used, however, the goby’s response is also likely to depend on the environmental and social context of the predator–prey encounter. This study highlights the importance of chemical cues in the assessment of predation risk for a coral reef fish.

Symbiont acquisition strategy drives host–symbiont associations in the southern Great Barrier Reef

Abstract: Coral larvae acquire populations of the symbiotic dinoflagellate Symbiodinium from the external environment (horizontal acquisition) or inherit their symbionts from the parent colony (maternal or vertical acquisition). The effect of the symbiont acquisition strategy on Symbiodinium-host associations has not been fully resolved. Previous studies have provided mixed results, probably due to factors such as low sample replication of Symbiodinium from a single coral host, biogeographic differences in Symbiodinium diversity, and the presence of some apparently host-specific symbiont lineages in coral with either symbiont acquisition strategies. This study set out to assess the effect of the symbiont acquisition strategy by sampling Symbiodinium from 10 coral species (five with a horizontal and five with a vertical symbiont acquisition strategy) across two adjacent reefs in the southern Great Barrier Reef. Symbiodinium diversity was assessed using single-stranded conformational polymorphism of partial nuclear large subunit rDNA and denaturing gradient gel electrophoresis of the internal transcribed spacer 2 region. The Symbiodinium population in hosts with a vertical symbiont acquisition strategy partitioned according to coral species, while hosts with a horizontal symbiont acquisition strategy shared a common symbiont type across the two reef environments. Comparative analysis of existing data from the southern Great Barrier Reef found that the majority of corals with a vertical symbiont acquisition strategy associated with distinct species- or genus-specific Symbiodinium lineages, but some could also associate with symbiont types that were more commonly found in hosts with a horizontal symbiont acquisition strategy.

Distribution and photobiology of Symbiodinium types in different light environments for three colour morphs of the coral Madracis pharensis: is there more to it than total irradiance?

Abstract: The role of symbiont variation in the photobiology of reef corals was addressed by investigating the links among symbiont genetic diversity, function and ecological distribution in a single host species, Madracis pharensis. Symbiont distribution was studied for two depths (10 and 25 m), two different light habitats (exposed and shaded) and three host colour morphs (brown, purple and green). Two Symbiodinium genotypes were present, as defined by nuclear internal transcribed spacer 2 ribosomal DNA (ITS2-rDNA) variation. Symbiont distribution was depth- and colour morph-dependent. Type B15 occurred predominantly on the deeper reef and in green and purple colonies, while type B7 was present in shallow environments and brown colonies. Different light microhabitats at fixed depths had no effect on symbiont presence. This ecological distribution suggests that symbiont presence is potentially driven by light spectral niches. A reciprocal depth transplantation experiment indicated steady symbiont populations under environment change. Functional parameters such as pigment composition, chlorophyll a fluorescence and cell densities were measured for 25 m and included in multivariate analyses. Most functional variation was explained by two photobiological assemblages that relate to either symbiont identity or light microhabitat, suggesting adaptation and acclimation, respectively. Type B15 occurs with lower cell densities and larger sizes, higher cellular pigment concentrations and higher peridinin to chlorophyll a ratio than type B7. Type B7 relates to a larger xanthophyll-pool size. These unambiguous differences between symbionts can explain their distributional patterns, with type B15 being potentially more adapted to darker or deeper environments than B7. Symbiont cell size may play a central role in the adaptation of coral holobionts to the deeper reef. The existence of functional differences between B-types shows that the clade classification does not necessarily correspond to functional identity. This study supports the use of ITS2 as an ecological and functionally meaningful marker in Symbiodinium.

Effects of temperature, light and heterotrophy on the growth rate and budding of the temperate coral Cladocora caespitosa

Abstract: Recent investigations have shown the temperate scleractinian coral Cladocora caespitosa to be a new potential climate archive for the Mediterranean Sea. Whilst earlier studies have demonstrated a seasonal variation in growth rates, they were unable to distinguish which environmental parameter (light, temperature, or food) was influencing growth. In this study, the effect of these three factors on the coral physiology and calcification rate was characterized to aid the correct interpretation of skeletal trace element variations. Two temperatures (13 and 23°C), irradiances (50 and 120 μmol m−2 s−1), and feeding regimes (unfed and fed with nauplii of Artemia salina) were tested under controlled laboratory conditions on the growth, zooxanthellae density, chlorophyll (chl) content, and asexual reproduction (budding) of C. caespitosa during a 7-week factorial experiment. Unlike irradiance, which had no effect, high temperature and food supply increased the growth rates of C. caespitosa. The effect of feeding was however higher for corals maintained at low temperature, suggesting that heterotrophy is especially important during the cold season, and that temperature is the predominant factor affecting the coral’s growth. At low temperature, fed samples had higher zooxanthellae density and chl content, possibly for maximizing photosynthetic efficiency. Sexual reproduction investment of C. caespitosa was higher during favourable conditions characterised by high temperatures and zooplankton availability.

Fungi in healthy and diseased sea fans ( Gorgonia ventalina): is Aspergillus sydowii always the pathogen?

Abstract: Caribbean corals, including sea fans (Gorgonia spp.), are being affected by severe and apparently new diseases. In the case of sea fans, the pathogen is reported to be the fungus Aspergillus sydowii, and the disease is named aspergillosis. In order to understand coral diseases and pathogens, knowledge of the microbes associated with healthy corals is also necessary. In this study the fungal community of healthy Gorgonia ventalina colonies was contrasted with that of diseased colonies. In addition, the fungal community of healthy and diseased tissue within colonies with aspergillosis was contrasted. Fungi were isolated from healthy and diseased fans from 15 reefs around Puerto Rico, and identified by sequencing the nuclear ribosomal ITS region and by morphology. Thirty fungal species belonging to 15 genera were isolated from 203 G. ventalina colonies. Penicillum and Aspergillus were the most common genera isolated from both healthy and diseased fans. However, the fungal community of healthy fans was distinct and more diverse than that of diseased ones. Within diseased fans, fungal communities from diseased tissues were distinct and more diverse than from healthy tissue. The reduction of fungi in diseased colonies may occur prior to infection due to environmental changes affecting the host, or after infection due to increase in dominance of the pathogen, or because of host responses to infection. Data also indicate that the fungal community of an entire sea fan colony is affected even when only a small portion of the colony suffers from aspergillosis. An unexpected result was that A. sydowii was found in healthy sea fans but never in diseased ones. This result suggests that A. sydowii is not the pathogen causing aspergillosis in the studied colonies, and suggests several fungi common to healthy and diseased colonies as opportunistic pathogens. Given that it is not clear that Aspergillus is the sole pathogen, calling this disease aspergillosis is an oversimplification at best.

Habitat associations of juvenile versus adult butterflyfishes

Abstract: Many coral reef fishes exhibit distinct ontogenetic shifts in habitat use while some species settle directly in adult habitats, but there is not any general explanation to account for these differences in settlement strategies among coral reef fishes. This study compared distribution patterns and habitat associations of juvenile (young of the year) butterflyfishes to those of adult conspecifics. Three species, Chaetodon auriga, Chaetodon melannotus, and Chaetodon vagabundus, all of which have limited reliance on coral for food, exhibited marked differences in habitat association of juvenile versus adult individuals. Juveniles of these species were consistently found in shallow-water habitats, whereas adult conspecifics were widely distributed throughout a range of habitats. Juveniles of seven other species (Chaetodon aureofasciatus, Chaetodon baronessa, Chaetodon citrinellus, Chaetodon lunulatus, Chaetodon plebeius, Chaetodon rainfordi, and Chaetodon trifascialis), all of which feed predominantly on live corals, settled directly into habitat occupied by adult conspecifics. Butterflyfishes with strong reliance on corals appear to be constrained to settle in habitats that provide access to essential prey resources, precluding their use of distinct juvenile habitats. More generalist butterflyfishes, however, appear to utilize distinct juvenile habitats and exhibit marked differences in the distribution of juveniles versus adults.

Acquisition of symbiotic dinoflagellates ( Symbiodinium) by juveniles of the coral Acropora longicyathus

Abstract: Scleractinian corals may acquire Symbiodinium from their parents (vertically) or from the environment (horizontally). In the present study, adult colonies of the coral Acropora longicyathus from One Tree Island (OTI) on the southern Great Barrier Reef (Australia) acquired two distinct varieties of symbiotic dinoflagellates (Symbiodinium) from the environment. Adult colonies had either Symbiodinium from clade C (86.7%) or clade A (5.3%), or a mixture of both clades A and C (8.0% of all colonies). In contrast, all 10-day-old juveniles were associated with Symbiodinium from clade A, while 83-day-old colonies contained clades A, C and D even though they were growing at the same location. Symbiodinium from clade A were dominant in both 10- and 83-day-old juveniles (99 and 97% of all recruits, respectively), while clade D was also found in 31% of 83-day-old juveniles. Experimental manipulation also revealed that parental association (with clade A or C), or the location within the OTI reef, did not influence which clade of symbiont was acquired by juvenile corals. The differences between the genetic identity of populations of Symbiodinium resident in juveniles and adult A. longicyathus suggest that ontogenetic changes in the symbiosis may occur during the development of scleractinian corals. Whether or not these changes are due to host selective processes or differences in the physical environment associated with juvenile versus adult colonies remains to be determined.

Changes in coral reef communities among the Florida Keys, 1996-2003

Abstract: Hard coral (Scleractinia and Milleporina) cover data were examined from 37 sites surveyed annually from 1996 to 2003 in the Florida reef tract, USA. Analyses of species numbers and total cover showed that site-to-site differences were generally very much greater than differences among times within sites. There were no significant differences among different geographical areas within the reef tract (Upper, Middle and Lower Keys). Large-scale changes documented included a reduction in species numbers and total cover on both deep and shallow offshore reefs between 1997 and 1999 followed by no recovery in cover, and only scant evidence of any recovery in species numbers by 2003. These changes coincided with bleaching events in 1997 and 1998, and the passage of Hurricane Georges through the Lower Keys in 1998. The lack of recovery among offshore reefs suggests that they were no longer resilient. Multivariate analyses revealed that some sites showed relatively little temporal variation in community composition, essentially random in direction, while others showed relatively large year-on-year changes. There was little evidence of any major region-wide changes affecting assemblage composition, or of any events that had impacted all of the sampling sites in any single year. Instead, different sites exhibited differing patterns of temporal variation, with certain sites displaying greater variation than others. Changes in community composition at some sites are interpreted in the light of knowledge of events at those sites and the relative sensitivities of species to various stressors, such as changes in cover of Acropora palmata and Millepora complanata at Sand Key following the bleaching events and hurricane in 1998, and declines in Montastraea annularis at Smith Shoal following a harmful algal bloom in 2002. For most sites, however, it is impossible to determine the causes of observed variation.

Characterization and management of the commercial sector of the Pohnpei coral reef fishery, Micronesia

Abstract: Commercial coral reef fisheries in Pohnpei (Micronesia) extract approximately 1,521 kg of reef fish daily (*500 MT year -1 ) from 152 km 2 of surrounding reef. More than 153 species were represented during sur- veys, with 25 species very common or common within combined-gear catch. Acanthurids contributed the greatest to catch volume, with bluespine unicornfish, Naso uni- cornis, and orangespine unicornfish, Naso lituratus, among the most frequently observed herbivores. Nighttime spearfishing was the dominant fishing method and inner lagoon areas were primarily targeted. A seasonal sales ban (March-April), intended to reduce pressure on reproduc- tively active serranids, significantly increased the capture volume of other families. Catch was significantly greater during periods of low lunar illumination, suggesting higher fishing success or greater effort, or both. The marketed catch was dominated by juveniles and small adults, based on fishes of known size at sexual maturity. Artificially depressed market prices appear to be catalyzing (potential or realized) overfishing by increasing the volume of fish needed to offset rising fuel prices. These results support the need for comprehensive fisheries management that pro- duces sustainable fishing and marketing practices and promotes shared management and enforced responsibilities between communities and the state. To be effective, management should prohibit nighttime spearfishing.