Article

Zone specific trends in coral cover, genera and growth-forms in the World-Heritage listed Ningaloo Reef

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Abstract

On coral reefs, changes in the cover and relative abundance of hard coral taxa often follow disturbance. Although the ecological responses of common coral taxa have been well documented, little is known about the ecological responses of uncommon coral taxa or of coral morphological groups across multiple adjacent reef zones. We used Multivariate Auto-Regressive State-Space modelling to assess the rate and direction of change of hard coral cover across a variety of coral genera, growth-forms, and susceptibility to bleaching and physical damage covering multiple reef zones at northern Ningaloo Reef in Western Australia. Trends were assessed between 2007 and 2016, during which multiple episodic disturbances occurred including cyclones and a heatwave. We provide evidence of zone specific trends, not only in total hard coral cover, but also in taxonomic and morphological groups of corals at Ningaloo Reef. Declines in total coral cover on the reef flat corresponded with declines in fast growing corals, particularly Acropora. In contrast, total coral cover on the reef slope and inshore (lagoon) did not undergo significant change, despite divergent trajectories of individual genera. Importantly, we also show that changes in the composition of coral assemblages can be detected using a morphological based approach when changes are not evident using a taxonomic approach. Therefore, we recommend that future assessments of coral reef trends incorporate not just standard metrics such as total coral cover, but also metrics that provide for detailed descriptions of trends in common and uncommon taxa and morphological groups across multiple reef zones.

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... Fish movement among reef zones is largely unrestricted over the full tidal range, with the exception of shallow reef flat areas, which are inaccessible to larger fish (>50 cm TL) during spring low tides (<0.3 m deep: 20-24 h per month). Live coral cover ranges from 15-25% on the reef slope, 25-90% on the reef flat and back reef and 0-5% in the lagoon and within the reef passages [26], with Acroporidae and Poritidae corals dominating reef slope and flat zones [27]. Ningaloo Reef is relatively isolated from large human populations, with the nearest major town, Exmouth, having a population of less than 2500. ...
... Adults are also known to seek shelter in deep interreef passages at night [33,34]. Reef crest zones at northern Mangrove Bay are currently characterised by moderate live coral cover (mean ± SE = 19% ± 5.8%: [27]) and close proximity to the only mangroves stands at northern Ningaloo. It is therefore possible that the highly restricted spatial extent of observations of B. muricatum observed in this study are due to the co-location of ideal habitats for feeding and recruitment, although we are unable to rule other factors such as temperature (see Table S4 for additional factors considered). ...
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The dynamic nature of coral communities can make it difficult to judge whether a reef system is resilient to the current disturbance regime. To address this question of resilience for near-shore coral communities of the Great Barrier Reef (Australia) a data set consisting of 350 annual observations of benthic community change was compiled from existing monitoring data. These data spanned the period 1985–2007 and were derived from coral reefs within 20km of the coast. During years without major disturbance events, cover increase of the Acroporidae was much faster than it was for other coral families; a median of 11% per annum compared to medians of less than 4% for other coral families. Conversely, Acroporidae were more severely affected by cyclones and bleaching events than most other families. A simulation model parameterised with these observations indicated that while recovery rates of hard corals were sufficient to compensate for impacts associated with cyclones and crown-of-thorns starfish, the advent of mass bleaching has lead to a significant change in the composition of the community and a rapid decline in hard coral cover. Furthermore, if bleaching events continue to occur with the same frequency and severity as in the recent past, the model predicts that the cover of Acroporidae will continue to decline. Although significant cover of live coral remains on near-shore reefs, and recovery is observed during inter-disturbance periods, it appears that this system will not be resilient to the recent disturbance regime over the long term. Conservation strategies for coral reefs should focus on both mitigating local factors that act synergistically to increase the susceptibility of Acroporidae to climate change while promoting initiatives that maximise the recovery potential from inevitable disturbances. KeywordsCoral community dynamics-Disturbance-Recovery-Simulation
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In the northeast Caribbean, doldrum-like conditions combined with elevated water temperatures in the summer/fall 2005 created the most severe coral bleaching event ever documented within this region. Video monitoring of 100 randomly chosen, permanent transects at five study sites in the US Virgin Islands revealed over 90% of the scleractinian coral cover showed signs of thermal stress by paling or becoming completely white. Lower water temperatures in October allowed some re-coloring of corals; however, a subsequent unprecedented regional outbreak of coral disease affected all sites. Five known diseases or syndromes were recorded; however, most lesions showed signs similar to white plague. Nineteen scleractinian species were affected by disease, with >90% of the disease-induced lesions occurring on the genus Montastraea. The disease outbreak peaked several months after the onset of bleaching at all sites but did not occur at the same time. The mean number of disease-induced lesions increased 51-fold and the mean area of disease-associated mortality increased 13-fold when compared with pre-bleaching disease levels. In the 12months following the onset of bleaching, coral cover declined at all sites (average loss: 51.5%, range: 42.4–61.8%) reducing the five-site average from 21.4% before bleaching to 10.3% with most mortality caused by white plague disease, not bleaching. Continued losses through October 2007 reduced the average coral cover of the five sites to 8.3% (average 2-year loss: 61.1%, range: 53.0–79.3%). Mean cover by M. annularis (complex) decreased 51%, Colpophyllia natans 78% and Agaricia agaricites 87%. Isolated disease outbreaks have been documented before in the Virgin Islands, but never as widespread or devastating as the one that occurred after the 2005 Caribbean coral-bleaching event. This study provides insight into the effects of continued seawater warming and subsequent coral bleaching events in the Caribbean and highlights the need to understand links between coral bleaching and disease.
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Formal monitoring of the Great Barrier Reef was initiated in 1986 in response to the clear scientific evidence (and growing public concern) over the loss of corals caused by two protracted outbreaks of crown-of thorns starfish, which began in 1962 and 1979. Using monitoring data from manta tows along and across the Great Barrier Reef, Sweatman et al. (Coral Reefs 30:521–531, 2011) show that coral cover after these outbreaks declined further from 28 to 22% between 1986 and 2004. Pointing to the current levels of protection of the Great Barrier Reef, they state that earlier estimates of losses of coral cover since the early 1960s have been exaggerated. However, the loss of close to one-quarter of the coral cover over the past two decades represents an average loss of 0.34% cover per year across the whole GBR after 1986, which is very similar to previously reported rates of annual loss measured over a longer timeframe. The heaviest recent losses have occurred on inshore and mid-shelf reefs, which Sweatman et al. (Coral Reefs 30:521–531, 2011) attribute to a natural cycle of disturbance and recovery. But there has been very limited recovery. While coral cover has increased for short periods on some individual reefs, it has declined sharply on many more to produce the observed system-wide trend of declining cover. Close to 40% of coral cover on inner reefs has been lost since 1986. Of particular significance is the new evidence that coral cover has remained unchanged or declined further from a low 1986 baseline in 28 out of 29 sub-regions of the Great Barrier Reef, indicating a gradual erosion of resilience that is impeding the capacity of this huge reef system to return towards its earlier condition. This result, and other clear evidence of widespread incremental degradation from overfishing, pollution, and climate change, calls for action rather than complacency or denial. KeywordsShifting base-lines–Climate change–Monitoring–Coral cover
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Staghorn corals (genus Acropora) are the most obvious and important corals on coral reefs throughout the world, providing much of the beauty and variety seen on the reefs. This invaluable reference tool is the first major review of Acropora in over 100 years. It assesses all the known species worldwide, describing each in detail and illustrating the range of variability of form with habitat and geographic location. The classification, evolution and worldwide distribution of all species are reviewed and illustrated with colour plates, full page black and white plates and distribution maps. Details of the general biology of staghorn corals are discussed and illustrated.
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Sea-level rise (SLR) is predicted to elevate water depths above coral reefs and to increase coastal wave exposure as ecological degradation limits vertical reef growth, but projections lack data on interactions between local rates of reef growth and sea level rise. Here we calculate the vertical growth potential of more than 200 tropical western Atlantic and Indian Ocean reefs, and compare these against recent and projected rates of SLR under different Representative Concentration Pathway (RCP) scenarios. Although many reefs retain accretion rates close to recent SLR trends, few will have the capacity to track SLR projections under RCP4.5 scenarios without sustained ecological recovery, and under RCP8.5 scenarios most reefs are predicted to experience mean water depth increases of more than 0.5 m by 2100. Coral cover strongly predicts reef capacity to track SLR, but threshold cover levels that will be necessary to prevent submergence are well above those observed on most reefs. Urgent action is thus needed to mitigate climate, sea-level and future ecological changes in order to limit the magnitude of future reef submergence.
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Tropical reef systems are transitioning to a new era in which the interval between recurrent bouts of coral bleaching is too short for a full recovery of mature assemblages. We analyzed bleaching records at 100 globally distributed reef locations from 1980 to 2016. The median return time between pairs of severe bleaching events has diminished steadily since 1980 and is now only 6 years. As global warming has progressed, tropical sea surface temperatures are warmer now during current La Niña conditions than they were during El Niño events three decades ago. Consequently, as we transition to the Anthropocene, coral bleaching is occurring more frequently in all El Niño–Southern Oscillation phases, increasing the likelihood of annual bleaching in the coming decades.
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In late April 2014 Ningaloo Reef was exposed to significant freshwater and sediment outflow following an extreme rainfall event (>200 mm in 48 h). It produced a plume of brown water of 9.63 km² that was present two days after the rainfall event. The extent of the plume decreased by 55.9% within ten days. Benthic surveys were conducted at eight sites at Ningaloo (three of them within the plume and five outside the plume) using 25 m line transects to assess the percentage cover of all major benthic organisms, including hard corals, soft corals, algae and turf algae. Results from a survey done in May 2014 (approximately one month after the flooding) were compared with one done in March 2014 (before the extreme rainfall event). Corals from the genus Acropora were the dominant species in the coral assemblages surveyed. Our results indicate that percentage cover of Acropora did not vary significantly between sites inside and outside of the plume, but percentage cover of non-Acropora coral varied significantly among plume and non-plume sites. Both patterns were present before the flood. The results show that the flood-induced plumes caused limited damage to the coral along this part of Ningaloo Reef.
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During 2015-2016, record temperatures triggered a pan-tropical episode of coral bleaching, the third global-scale event since mass bleaching was first documented in the 1980s. Here we examine how and why the severity of recurrent major bleaching events has varied at multiple scales, using aerial and underwater surveys of Australian reefs combined with satellite-derived sea surface temperatures. The distinctive geographic footprints of recurrent bleaching on the Great Barrier Reef in 1998, 2002 and 2016 were determined by the spatial pattern of sea temperatures in each year. Water quality and fishing pressure had minimal effect on the unprecedented bleaching in 2016, suggesting that local protection of reefs affords little or no resistance to extreme heat. Similarly, past exposure to bleaching in 1998 and 2002 did not lessen the severity of bleaching in 2016. Consequently, immediate global action to curb future warming is essential to secure a future for coral reefs.
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Coral reefs are biologically diverse and ecologically complex ecosystems constructed by stony corals. Despite decades of research, basic coral population biology and community ecology questions remain. Quantifying trait variation among species can help resolve these questions, but progress has been hampered by a paucity of trait data for the many, often rare, species and by a reliance on nonquantitative approaches. Therefore, we propose filling data gaps by prioritizing traits that are easy to measure, estimating key traits for species with missing data, and identifying 'supertraits' that capture a large amount of variation for a range of biological and ecological processes. Such an approach can accelerate our understanding of coral ecology and our ability to protect critically threatened global ecosystems.
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We investigated the prevalence and potential drivers of coral disease across Ningaloo Reef on the Western Australian coast. Coral disease assessment surveys were undertaken at 2 spatial scales, the first over a small area of reef (Bill's Bay, 2.5 x 5.0 km), where human use is high and where several anoxic events have caused significant coral mortality, and the second over a broader area (spanning similar to 200 km of the Ningaloo coast). Throughout Ningaloo, 2.3% of coral colonies showed signs of disease, although disease prevalence varied at both broad and local scales, ranging from 1.1 to 7% along the coast, and from 0.1 to 3.1% locally-all within the range of values recorded in other Indo-Pacific regions. Seven diseases were identified, the most common being 'skeletal eroding band' (which affected similar to 1% of colonies). At a broad spatial scale, prevalence of skeletal eroding band was positively related to the number of coral colonies exhibiting Drupella spp. feeding scars, whilst black-band disease (BBD) was positively associated with density of coral colonies. At the local scale, severity of anoxic events and occurrence of Drupella spp. feeding scars were positively related to prevalence of BBD, whilst other cyanobacterial bands were associated only with Drupella spp. scars. We saw no strong indication that human activities, measured as density of people or vessels in the water, were related to disease prevalence. Positive relationships amongst disease, anoxic events and Drupella spp. feeding suggest that natural stressors are potential drivers of disease at Ningaloo.
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We examined the seasonal and spatial variability in the temperatures of nearshore reef waters over 19 months across Coral Bay at Ningaloo Reef, Western Australia. Local deviations in the mean daily temperature of nearshore reef waters from offshore values (dT) were a linear function of the combined effect of net atmospheric heating (Qnet) and offshore wave height and period. Whereas intra-annual variation in local heat exchange was driven mainly by seasonal changes in shortwave radiation, intra-annual variation in local cooling was driven mostly by changes in relative humidity (r^{2} = 0.60) and wind speed (r^{2} = 0.31) that exhibited no apparent seasonality. We demonstrate good agreement between nearshore reef temperatures modeled from offshore sea surface temperatures, offshore wave forcing, and local atmospheric heat fluxes with observed temperatures using a simple linear model (r^{2} = 0.31–0.69, root-mean-square error = 0.4–0.9degC). Using these modeled nearshore reef temperature records, we show that thermal stresses across the reef reached between 16uC weeks and 22 degC weeks in the summer of 2011 when a mass coral bleaching event was reported, and between 12 degC weeks and 13 degC weeks in the following summer of 2012 when no mass bleaching was reported. After compensating for differences between observed and modeled thermal stresses, we found that maximum thermal stresses across the reef likely reached as high as 18–34 degC weeks in the summer of 2011. The approach used here could thus improve our ability to predict spatial variation in thermal stress and bleaching across other wave-driven nearshore reef systems.
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The most extensive and severe bleaching of coral ever recorded occurred in 1997 and 1998 as a result of anomalously high sea-surface temperatures. In the Ryukyu Islands, extensive bleaching occurred from late July to early October in 1998. A time-series transect study on a reef flat revealed the different responses of coral populations among species to this event. Coral cover was monitored just before, during, and 6 times after the bleaching occurred, along 5 fixed transects, 710 to 800 m long, across the reef flat of Shiraho Reef, Ishigaki Island, in the Ryukyu Islands of Japan. The dominant corals were Heliopora coerulea, massive Porites, and branching Porites, Montipora, Acropora, and Pavona (these latter 4 genera could not always be distinguished to species level, and are thus grouped by genus and ecoform). H. coerulea was the least susceptible to bleaching and maintained almost constant coverage before and after the bleaching. Massive Porites were susceptible to bleaching, but regained their algae after the bleaching and sustained their coverage. On the other hand, the branching Porites, Montipora, and Acropora were susceptible to bleaching and mortality was high. Coverage by branching Montipora was initially reduced by 66 %, but by 2 yr after the bleaching it had recovered to pre-bleaching coverage. However, only large patches of more than 70 in along a transect, with coverage of 10 to 40 %, recovered. Small patches of less than 30 m along a transect, with coverage of less than 10 %, died and did not recover, The response of corals to bleaching differs according to the strategy the coral uses against bleaching; moderately frequent and severe bleaching might permit the coexistence of both types of corals on the same reef flat, as in the case of Shiraho Reef.
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Coral bleaching is an increasingly prominent threat to coral reef ecosystems, not only to corals, but also to the many organisms that rely on coral for food and shelter. Coral-feeding fishes are negatively affected by coral loss caused by extensive bleaching, but it is unknown how feeding behaviour of most corallivorous fishes changes in response to coral bleaching. In this study, coral bleaching was experimentally induced in situ to examine the feeding response of two obligate corallivorous fish, Labrichthys unilineatus (Labridae) and Chaetodon baronessa (Chaetodontidae). Feeding rates were monitored before, during, and immediately after experimental bleaching of prey corals. L. unilineatus significantly increased its feeding on impacted corals during bleaching, but showed a steady decline in feeding once corals were fully bleached. Feeding response of L. unilineatus appears to parallel the expected stress-induced mucous production by bleaching colonies. In contrast, C. baronessa preferentially fed from healthy colonies over bleached colonies, although bleached colonies were consumed for five days following manipulation. Feeding by corallivorous fishes can play an important role in determining coral condition and mortality of corals following stress induced bleaching.
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Coral reef recovery from major disturbance is hypothesized to depend on the arrival of propagules from nearby undisturbed reefs. Therefore, reefs isolated by distance or current patterns are thought to be highly vulnerable to catastrophic disturbance. We found that on an isolated reef system in north Western Australia, coral cover increased from 9% to 44% within 12 years of a coral bleaching event, despite a 94% reduction in larval supply for 6 years after the bleaching. The initial increase in coral cover was the result of high rates of growth and survival of remnant colonies, followed by a rapid increase in juvenile recruitment as colonies matured. We show that isolated reefs can recover from major disturbance, and that the benefits of their isolation from chronic anthropogenic pressures can outweigh the costs of limited connectivity.
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Habitat degradation and fishing are major drivers of temporal and spatial changes in fish communities. The independent effects of these drivers are well documented, but the relative importance and interaction between fishing and habitat shifts is poorly understood, particularly in complex systems such as coral reefs. To assess the combined and relative effects of fishing and habitat we examined the composition of fish communities on patch reefs across a gradient of high to low structural complexity in fished and unfished areas of the Ningaloo Marine Park, Western Australia. Biomass and species richness of fish were positively correlated with structural complexity of reefs and negatively related to macroalgal cover. Total abundance of fish was also positively related to structural complexity, however this relationship was stronger on fished reefs than those where fishing is prohibited. The interaction between habitat condition and fishing pressure is primarily due to the high abundance of small bodied planktivorous fish on fished reefs. However, the influence of management zones on the abundance and biomass of predators and target species is small, implying spatial differences in fishing pressure are low and unlikely to be driving this interaction. Our results emphasise the importance of habitat in structuring reef fish communities on coral reefs especially when gradients in fishing pressure are low. The influence of fishing effort on this relationship may however become more important as fishing pressure increases.
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In March 1989, most of the corals near Coral Bay, off the north-western coastline of Australia, spawned several nights earlier than usual. Flood, rather than ebb, tides at the time of spawning combined with light north-west winds and low swell conditions to restrict the dispersal of coral propagules and, as a result, large amounts of coral spawn were trapped in the bay, forming extensive slicks. Fish and other animals began to die almost immediately, and over the next few days, over 1 million fish, representing at least 80 species, were washed ashore. A survey of the benthic communities revealed extensive mortality of corals and other reef animals over an area of about 3 km2. Live coral cover in this area decreased from 42.9% to 9.4% and several large coral colonies up to 10 m in diameter were killed. The observed mortality was presumably the result of hypoxia (oxygen depletion) created initially by the respiratory demand of the coral spawn and maintained by the biological oxygen demand of the decomposing spawn slicks and dead animals. Anecdotal reports of corals and other reef animals dying in the vicinity of coral spawn slicks on other reefs in Western Australia suggest that this phenomenon may be a relatively common event on shallow coral reefs where coral mass spawning occurs. These records and observations document, for the first time, a new source of natural disturbance that has a significant influence on the community structure of some coral reefs.