Monitoring coral bleaching using a colour reference card

Assessment of the extent of coral bleaching has become an important part of studies that aim to understand the condition of coral reefs. In this study a reference card that uses differences in coral colour was developed as an inexpensive, rapid and non-invasive method for the assessment of bleaching. The card uses a 6 point brightness/saturation scale within four colour hues to record changes in bleaching state. Changes on the scale of 2 units or more reflect a change in symbiont density and chlorophyll a content, and therefore the bleaching state of the coral. When used by non-specialist observers in the field (here on an intertidal reef flat), there was an inter-observer error of ± 1 colour score. This technique improves on existing subjective assessment of bleaching state by visual observation and offers the potential for rapid, wide-area assessment of changing coral condition.     

Annual coral bleaching and the long-term recovery capacity of coral

Mass bleaching events are predicted to occur annually later this century. Nevertheless, it remains unknown whether corals will be able to recover between annual bleaching events. Using a combined tank and field experiment, we simulated annual bleaching by exposing three Caribbean coral species (Porites divaricata, Porites astreoides and Orbicella faveolata) to elevated temperatures for 2.5 weeks in 2 consecutive years. The impact of annual bleaching stress on chlorophyll a, energy reserves, calcification, and tissue C and N isotopes was assessed immediately after the second bleaching and after both short- and long-term recovery on the reef (1.5 and 11 months, respectively). While P. divaricata and O. faveolata were able to recover from repeat bleaching within 1 year, P. astreoides experienced cumulative damage that prevented full recovery within this time frame, suggesting that repeat bleaching had diminished its recovery capacity. Specifically, P. astreoides was not able to recover protein and carbohydrate concentrations. As energy reserves promote bleaching resistance, failure to recover from annual bleaching within 1 year will likely result in the future demise of heat-sensitive coral species.     

Microscopic observations of recovery in the reef-building scleractinian coral,Montastrea annularis,after bleaching on a Cayman reef

Living tissues of a single massive colony of the reef-building scleractinian coral, M. annularis, were sampled at one month intervals following the observation that areas of marked discoloration (bleaching) had developed by the beginning of the summer of 1988. Histological analysis of replicate plugs of tissue from bleached and unbleached regions of that colony allowed comparison of time-matched samples as well as zone-matched samples over a period of six months. During that interval, the entire colony was restored to a uniform and normal degree of pigmentation. Histology of the bleached zones documents the gradual accumulation of zooxanthellae and the thickening of the gastrodermal epithelium of the coral. Once reconstituted, the previously bleached gastrodermis, repopulated with algal symbionts, is indistinguishable from unbleached tissue in the coral colony.     

Marine heatwaves reveal coral reef zones susceptible to bleaching in the Red Sea

As the Earth's temperature continues to rise, coral bleaching events become more frequent. Some of the most affected reef ecosystems are located in poorly monitored waters, and thus, the extent of the damage is unknown. We propose the use of marine heatwaves (MHWs) as a new approach for detecting coral reef zones susceptible to bleaching, using the Red Sea as a model system. Red Sea corals are exceptionally heat‐resistant, yet bleaching events have increased in frequency. By applying a strict definition of MHWs on >30 year satellite‐derived sea surface temperature observations (1985–2015), we provide an atlas of MHW hotspots over the Red Sea coral reef zones, which includes all MHWs that caused major coral bleaching. We found that: (a) if tuned to a specific set of conditions, MHWs identify all areas where coral bleaching has previously been reported; (b) those conditions extended farther and occurred more often than bleaching was reported; and (c) an emergent pattern of extreme warming events is evident in the northern Red Sea (since 1998), a region until now thought to be a thermal refuge for corals. We argue that bleaching in the Red Sea may be vastly underrepresented. Additionally, although northern Red Sea corals exhibit remarkably high thermal resistance, the rapidly rising incidence of MHWs of high intensity indicates this region may not remain a thermal refuge much longer. As our regionally tuned MHW algorithm was capable of isolating all extreme warming events that have led to documented coral bleaching in the Red Sea, we propose that this approach could be used to reveal bleaching‐prone regions in other data‐limited tropical regions. It may thus prove a highly valuable tool for policymakers to optimize the sustainable management of coastal economic zones.     

Abiotic and biotic controls on coral recovery 16 years after mass bleaching

Coral Reefs - As climate changes increase heat stress on tropical ecosystems, the long-term persistence of coral reefs requires rapid coral recovery following coral bleaching events. Using the...     

Assessing vegetation recovery from energy development using a dynamic reference approach

Ecologically relevant references are useful for evaluating ecosystem recovery, but references that are temporally static may be less useful when environmental conditions and disturbances are spatially and temporally heterogeneous. This challenge is particularly acute for ecosystems dominated by sagebrush (Artemisia spp.), where communities may require decades to recover from disturbance. We demonstrated application of a dynamic reference approach to studying sagebrush recovery using three decades of sagebrush cover estimates from remote sensing (1985–2018). We modelled recovery on former oil and gas well pads (n = 1200) across southwestern Wyoming, USA, relative to paired references identified by the Disturbance Automated Reference Toolset. We also used quantile regression to account for unmodelled heterogeneity in recovery, and projected recovery from similar disturbance across the landscape. Responses to weather and site‐level factors often differed among quantiles, and sagebrush recovery on former well pads increased more when paired reference sites had greater sagebrush cover. Little (<5%) of the landscape was projected to recover within 100 years for low to mid quantiles, and recovery often occurred at higher elevations with cool and moist annual conditions. Conversely, 48%–78% of the landscape recovered quickly (within 25 years) for high quantiles of sagebrush cover. Our study demonstrates advantages of using dynamic reference sites when studying vegetation recovery, as well as how additional inferences obtained from quantile regression can inform management.     

Forecasting decadal changes in sea surface temperatures and coral bleaching within a Caribbean coral reef

Elevated sea surface temperature (SST) caused by global warming is one of the major threats to coral reefs. While increased SST has been shown to negatively affect the health of coral reefs by increasing rates of coral bleaching, how changes to atmospheric heating impact SST distributions, modified by local flow environments, has been less understood. This study aimed to simulate future water flow patterns and water surface heating in response to increased air temperature within a coral reef system in Bocas del Toro, Panama, located within the Caribbean Sea. Water flow and SST were modeled using the Delft3D-FLOW© computer simulation package. Locally measured physical parameters, including bathymetry, astronomic tidal forcing, and coral habitat distribution were input into the model and water flow, and SST was simulated over a four-month period under present day, as well as projected warming scenarios in 2020s, 2050s, and 2080s. Changes in SST, and hence the thermal stress to corals, were quantified by degree heating weeks. Results showed that present-day reported bleaching sites were consistent with localized regions of continuous high SST. Regions with highest SST were located within shallow coastal sites adjacent to the mainland or within the interior of the bay, and characterized by low currents with high water retention times. Under projected increases in SSTs, shallow reef areas in low flow regions were found to be hot spots for future bleaching.     

Seawater temperature and sublethal coral bleaching in Jamaica

Permanent study sites were established at 6 m, 12 m and 18 m on the West Fore Reef at Discovery Bay, Jamaica. Colonies of Montastrea annularis, Porites astreoides, Porites porites and Agaricia spp. were assessed for presence and extent of bleached tissue at two month intervals between October 1986 and September 1987. In 98% of all corals exhibiting a bleaching response, less than 25% of the colony appeared pale. In the remaining 2%, more than 25% of the tissue appeared pale. M. annularis, P. astreoides and Agaricia spp. showed a significant positive correlation between the percent of colonies exhibiting a partial bleaching response and seawater temperature. There was no significant difference in the percentage of colonies bleached between the three depths. M. annularis and Agaricia spp. exhibited a significantly higher percentage of colonies bleached than P. astreoides and P. porites. For M. annularis 15% of coral colonies studied showed 1–2 cm² randomly seattered patches of pale tissue which remained constant throughout the study. The partial bleaching patterns observed in this study were never lethal and are considered, in part, to be a response to seasonal variations in seawater temperature. Study location: Discovery Bay Marine Laboratory, P.O. Box 35, Discovery Bay, Jamaica, W. Indies     

Assessing the cost of living with elephants (Loxodonta africana) in areas adjacent to Meru National Park,Kenya

Crop raiding by the elephants is a serious and recurring management problem around protected areas in Kenya such as Meru National Park. Crop-raiding menace is one of the most significant of human–elephant conflicts in Meru National Park. The distribution, impact, and conservation implications of the increased elephant crop raiding in areas adjacent to Meru National Park is attributed to the changes in land use systems within these areas. Crop raiding by African elephants (Loxodonta africana) was monitored in the area adjacent to Meru National Park between August 2010 and July 2011. From the study, 144 farms were raided and farmers lost crops amounting to USD 120,308.60. Crop raiding was higher during the month of August 2010 (KES 2,714,295 or USD 33,928). The study suggests capacity building for communities in order to safeguard their crops against elephant raiding. Other probable measures put forward to mitigate this include the development of alternative water sources and the need to implement electric fence around the remaining section of the Park boundary which is affected by elephant.     

Marine plants in a coral reef ecosystem

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

Synchronous biological feedbacks in parrotfishes associated with pantropical coral bleaching

Biological feedbacks generated through patterns of disturbance are vital for sustaining ecosystem states. Recent ocean warming and thermal anomalies have caused pantropical episodes of coral bleaching, which has led to widespread coral mortality and a range of subsequent effects on coral reef communities. Although the response of many reef‐associated fishes to major disturbance events on coral reefs is negative (e.g., reduced abundance and condition), parrotfishes show strong feedbacks after disturbance to living reef structure manifesting as increases in abundance. However, the mechanisms underlying this response are poorly understood. Using biochronological reconstructions of annual otolith (ear stone) growth from two ocean basins, we tested whether parrotfish growth was enhanced following bleaching‐related coral mortality, thus providing an organismal mechanism for demographic changes in populations. Both major feeding guilds of parrotfishes (scrapers and excavators) exhibited enhanced growth of individuals after bleaching that was decoupled from expected thermal performance, a pattern that was not evident in other reef fish taxa from the same environment. These results provide evidence for a more nuanced ecological feedback system—one where disturbance plays a key role in mediating parrotfish–benthos interactions. By influencing the biology of assemblages, disturbance can thereby stimulate change in parrotfish grazing intensity and ultimately reef geomorphology over time. This feedback cycle operated historically at within‐reef scales; however, our results demonstrate that the scale, magnitude, and severity of recent thermal events are entraining the biological responses of disparate communities to respond in synchrony. This may fundamentally alter feedbacks in the relationships between parrotfishes and reef systems.     

Facultative mimicry: cues for colour change and colour accuracy in a coral reef fish

Mimetic species evolve colours and body patterns to closely resemble poisonous species and thus avoid predation (Batesian mimicry), or resemble beneficial or harmless species in order to approach and attack prey (aggressive mimicry). Facultative mimicry, the ability to switch between mimic and non-mimic colours at will, is uncommon in the animal kingdom, but has been shown in a cephalopod, and recently in a marine fish, the bluestriped fangblenny Plagiotremus rhinorhynchos, an aggressive mimic of the juvenile cleaner fish Labroides dimidiatus. Here we demonstrate for the first time that fangblennies adopted mimic colours in the presence of juvenile cleaner fish; however, this only occurred in smaller individuals. Field data indicated that when juvenile cleaner fish were abundant, the proportion of mimic to non-mimic fangblennies was greater, suggesting that fangblennies adopt their mimic disguise depending on the availability of cleaner fish. Finally, measurements of spectral reflectance suggest that not only do mimic fangblennies accurately resemble the colour of their cleaner fish models but also mimic other species of fish that they associate with. This study provides insights into the cues that control this remarkable facultative mimicry system and qualitatively measures its accuracy.     

Greater functional diversity and redundancy of coral endolithic microbiomes align with lower coral bleaching susceptibility

The skeleton of reef-building coral harbors diverse microbial communities that could compensate for metabolic deficiencies caused by the loss of algal endosymbionts, i.e., coral bleaching. However, it is unknown to what extent endolith taxonomic diversity and functional potential might contribute to thermal resilience. Here we exposed Goniastrea edwardsi and Porites lutea, two common reef‐building corals from the central Red Sea to a 17-day long heat stress. Using hyperspectral imaging, marker gene/metagenomic sequencing, and NanoSIMS, we characterized their endolithic microbiomes together with ¹⁵N and ¹³C assimilation of two skeletal compartments: the endolithic band directly below the coral tissue and the deep skeleton. The bleaching-resistant G. edwardsi was associated with endolithic microbiomes of greater functional diversity and redundancy that exhibited lower N and C assimilation than endoliths in the bleaching-sensitive P. lutea. We propose that the lower endolithic primary productivity in G. edwardsi can be attributed to the dominance of chemolithotrophs. Lower primary production within the skeleton may prevent unbalanced nutrient fluxes to coral tissues under heat stress, thereby preserving nutrient-limiting conditions characteristic of a stable coral-algal symbiosis. Our findings link coral endolithic microbiome structure and function to bleaching susceptibility, providing new avenues for understanding and eventually mitigating reef loss.Subject terms: Microbial ecology, Microbial ecology     

Anthropogenic mortality on coral reefs in Caribbean Panama predates coral disease and bleaching

Caribbean reef corals have declined precipitously since the 1980s due to regional episodes of bleaching, disease and algal overgrowth, but the extent of earlier degradation due to localised historical disturbances such as land clearing and overfishing remains unresolved. We analysed coral and molluscan fossil assemblages from reefs near Bocas del Toro, Panama to construct a timeline of ecological change from the 19th century-present. We report large changes before 1960 in coastal lagoons coincident with extensive deforestation, and after 1960 on offshore reefs. Striking changes include the demise of previously dominant staghorn coral Acropora cervicornis and oyster Dendrostrea frons that lives attached to gorgonians and staghorn corals. Reductions in bivalve size and simplification of gastropod trophic structure further implicate increasing environmental stress on reefs. Our paleoecological data strongly support the hypothesis, from extensive qualitative data, that Caribbean reef degradation predates coral bleaching and disease outbreaks linked to anthropogenic climate change.     

Delayed coral recovery in a warming ocean

Climate change threatens coral reefs across the world. Intense bleaching has caused dramatic coral mortality in many tropical regions in recent decades, but less obvious chronic effects of temperature and other stressors can be equally threatening to the long‐term persistence of diverse coral‐dominated reef systems. Coral reefs persist if coral recovery rates equal or exceed average rates of mortality. While mortality from acute destructive events is often obvious and easy to measure, estimating recovery rates and investigating the factors that influence them requires long‐term commitment. Coastal development is increasing in many regions, and sea surface temperatures are also rising. The resulting chronic stresses have predictable, adverse effects on coral recovery, but the lack of consistent long‐term data sets has prevented measurement of how much coral recovery rates are actually changing. Using long‐term monitoring data from 47 reefs spread over 10 degrees of latitude on Australia's Great Barrier Reef (GBR), we used a modified Gompertz equation to estimate coral recovery rates following disturbance. We compared coral recovery rates in two periods: 7 years before and 7 years after an acute and widespread heat stress event on the GBR in 2002. From 2003 to 2009, there were few acute disturbances in the region, allowing us to attribute the observed shortfall in coral recovery rates to residual effects of acute heat stress plus other chronic stressors. Compared with the period before 2002, the recovery of fast‐growing Acroporidae and of “Other” slower growing hard corals slowed after 2002, doubling the time taken for modest levels of recovery. If this persists, recovery times will be increasing at a time when acute disturbances are predicted to become more frequent and intense. Our study supports the need for management actions to protect reefs from locally generated stresses, as well as urgent global action to mitigate climate change.     

Weather and hydrographic conditions associated with coral bleaching: Lee Stocking Island, Bahamas

Thermograph, current-meter, and coastal-weather data from Lee Stocking Island, Exuma Cays, Bahamas, are used to investigate hydrographic and meteorological conditions preceding and during a bleaching event in August 1990. Shelf water temperatures recorded at three locations rise to just over 30 °C. Weather data provide estimates of local heating and cooling by insolation, net outgoing long-wave radiation, and sensible and latent heat fluxes. Weather data do not indicate a period of unusually clear skies during the days and weeks preceding the bleaching event. Rather, calculations suggest that low wind speeds during late July and early August reduced evaporative cooling. A tidal channel near the bleaching site provided a source of hyperpycnal 31 °C water that had been heated in the shallow waters of Great Bahama Bank. Current-meter data suggest an along-shelf transport of water from the mouth of the tidal channel to the bleaching site. A comparison of wind-stress and water-temperature data suggests that a downwelling pattern contributed to heating at the reef by flooding the shelf with warm surface water. Results suggest that heating at the reef was a combination of local warming, enhanced by reduced evaporation, and advective warming resulting from both an along-shelf transport of bank water and a landward across-shelf transport of warm surface water.     

Amphibian abundance and diversity in Meru National Park, Kenya

The diversity and abundance of amphibians were investigated in Meru National Park, Kenya, using transect sampling, drift‐fence and pitfall trapping and opportunistic collecting. A total of 430 individuals under seven genera (Amietophrynus, Hemisus, Hyperolius, Phrynobatrachus, Phrynomantis, Ptychadena, Xenopus) comprising eleven species were sampled in three different habitats (apart from this, two additional species are known from Meru National Park): Acacia‐wooded grassland; Combretum‐wooded grassland; Acacia–Commiphora bushland. The sex ratio for almost all species was balanced (chi‐square, χ²; P > 0.5) and was not affected by habitat type (ANOVA: F = 8.3026, P = 0.6914). Shannon–Weaver Index (2.227) and Simpson's Index (8.244) were relatively high, and most of the eleven species sampled appeared to have a relatively even distribution (Shannon's Evenness Index, E = 0.927). However, Hemisus marmoratus and Phrynomantis bifasciatus were exclusively recorded in Acacia‐wooded grassland and in low abundances. There was a positive linear relationship of body weight against snout–vent length for two randomly selected anurans (Hyperolius glandicolor, Phrynobatrachus natalensis) among all three vegetation communities.     

Suppressed recovery of functionally important branching Acropora drives coral community composition changes following mass bleaching in Indonesia

Mass coral bleaching events may have disproportionate effects on branching corals, leading to coral community restructuring, reduced biodiversity, and decreased structural complexity. This affects overall reef health and resilience. Functionally important, fast-growing branching Acropora corals were a historically dominant and vital component of Indonesian reefs throughout the twentieth century, yet the genus is also one of the most vulnerable to external stressors. This study used long-term annual reef monitoring data from Indonesia’s Wakatobi Marine National Park (WMNP) to investigate the effects of a mass bleaching event in 2010 on Acropora and other branching corals, evaluate their post-disturbance recovery trajectories, and analyse shifts in coral community composition. Post-bleaching scleractinian coral cover decreased across study sites, with losses in branching corals especially evident. Long-term branching Acropora cover decreased significantly and failed to demonstrate the significant post-disturbance recovery of other branching corals (especially Porites). In areas characterised by relatively high branching Acropora cover (> 15% mean cover) prior to bleaching, long-term coral community composition changes have trended predominately towards branching and massive Porites and branching Montipora. The novelty and key contribution of this study is that results suggest suppressed recovery of Acropora in the WMNP. Contributing factors may include the Allee effect (inhibition of reproduction at low population densities), other forms of inhibited larval recruitment, direct and indirect spatial competition, and changes in the physical reef habitat. These findings have critical implications for this functionally important taxon, future reef conservation efforts, and overall reef health and resilience in the park.