Responses of urban reef corals during the 2016 mass bleaching event

Decadal stability in coral cover could mask hidden changes on reefs in the East Asian Seas

Coral reefs in the Central Indo-Pacific region comprise some of the most diverse and yet threatened marine habitats. While reef monitoring has grown throughout the region in recent years, studies of coral reef benthic cover remain limited in spatial and temporal scales. Here, we analysed 24,365 reef surveys performed over 37 years at 1972 sites throughout East Asia by the Global Coral Reef Monitoring Network using Bayesian approaches. Our results show that overall coral cover at surveyed reefs has not declined as suggested in previous studies and compared to reef regions like the Caribbean. Concurrently, macroalgal cover has not increased, with no indications of phase shifts from coral to macroalgal dominance on reefs. Yet, models incorporating socio-economic and environmental variables reveal negative associations of coral cover with coastal urbanisation and sea surface temperature. The diversity of reef assemblages may have mitigated cover declines thus far, but climate change could threaten reef resilience. We recommend prioritisation of regionally coordinated, locally collaborative long-term studies for better contextualisation of monitoring data and analyses, which are essential for achieving reef conservation goals.

Coral persistence despite marginal conditions in the Port of Miami

Coral cover has declined worldwide due to anthropogenic stressors that manifest on both global and local scales. Coral communities that exist in extreme conditions can provide information on how these stressors influence ecosystem structure, with implications for their persistence under future conditions. The Port of Miami is located within an urbanized environment, with active coastal development, as well as commercial shipping and recreational boating activity. Monitoring of sites throughout the Port since 2018 has revealed periodic extremes in temperature, seawater pH, and salinity, far in excess of what have been measured in most coral reef environments. Despite conditions that would kill many reef species, we have documented diverse coral communities growing on artificial substrates at these sites-reflecting remarkable tolerance to environmental stressors. Furthermore, many of the more prevalent species within these communities are now conspicuously absent or in low abundance on nearby reefs, owing to their susceptibility and exposure to stony coral tissue loss disease. Natural reef frameworks, however, are largely absent at the urban sites and while diverse fish communities are documented, it is unlikely that these communities provide the same goods and services as natural reef habitats. Regardless, the existence of these communities indicates unlikely persistence and highlights the potential for coexistence of threatened species in anthropogenic environments, provided that suitable stewardship strategies are in place.

Species richness and the dynamics of coral cover in Bangka Belitung Islands, Indonesia

Pressures on the world's tropical coral reefs that threaten their existence have been reported worldwide due to many stressors. Loss of coral cover and declines in coral richness are two of the most common changes often reported in coral reefs. However, a precise estimate of species richness and the coral cover dynamics for most Indonesian regions, particularly in the Bangka Belitung Islands, have been poorly documented. Annual monitoring data from 2015 to 2018 at 11 fixed sites in the Bangka Belitung Islands using the photo quadrat transect method identified 342 coral species from 63 genera. Of these, 231 species (>65%) were rare or uncommon, occurring in <40% of all sites. The species richness of hard corals was categorized as moderate compared to other studies in Indonesia, averaging 53 species across sites and years, and there was an increasing number of sites with high species richness. The percent cover of live and dead hard corals was greater than other benthic and substrate categories in all sites; revealing a live-dead hard corals pattern with dead coral cover averaged 12% higher than live hard coral across the years, but they did not show a significant difference (P > 0.05). There was a slightly increasing trend in hard coral cover in ten out of 11 sites in 2018, indicating the reefs are in a recovery process. The results support the need to identify recovering or stable areas despite apparent anthropogenic and natural variations recently. This vital information is essential for early detection and preparation for management strategies in the current context of climate change and for ensuring future coral reef survival.

Reef on the edge: resilience failure of marginal patch coral reefs in Eastern Arabian Sea under recurrent coral bleaching, coral diseases, and local stressors

Marked by strong El Niño-Southern Oscillation (ENSO) effects during 2014-2016, global coral reefs underwent mass bleaching. Here, we conducted a comprehensive (2014-2019) study, coinciding with the 2014-16 ENSO, to investigate the response and resilience potential of marginal coral communities to the combined impact of recurrent thermal anomalies and multiple anthropogenic stressors before, during, and after the mass bleaching episodes. Our result unveiled that thermal-stress-driven back-to-back annual coral bleaching episodes caused coral mortality and significantly decimated coral cover, primarily in 2015 and 2016. Subsequent benthic regime shifts toward macroalgal and algal turf colonization, followed by an increase in coral disease prevalence and recruitment failure was observed after the recurrent bleaching episodes. Algal cover increased from 21% in 2014 to 52.90% in 2019, and a subsequent increase in coral disease occurrence was observed from 16% in 2015 to 29% in 2019. The cascading negative effect of multiple stressors magnified coral loss and decreased the coral cover significantly from 45% in 2014 to 20% in 2019. The corals in the intensive recreational diving activity sites showed higher disease prevalence, concurring with high mechanical coral damage. The present study demonstrates that consecutive thermal bleaching episodes combined with local stressors can cause declines in coral cover and promote an undesirable regime shift to algal dominance in marginal coral reef habitats within a short duration. These results are of particular interest given that marginal reefs were traditionally perceived as resilient reef habitats due to their higher survival threshold to environmental changes. The present study indicates that mitigation of local stressors by effective management strategies, in conjunction with globally coordinated efforts to ameliorate climate change, can protect these unique coral reefs.

Climate Change Induced Thermal Stress Caused Recurrent Coral Bleaching over Gulf of Kachchh and Malvan Marine Sanctuary, West Coast of India

Coral reefs are one of the most sensitive, productive, and invaluable biological resources on the earth. However, coral reefs are facing unprecedented stress due to ongoing climate changes and intensified anthropogenic disturbances globally. Elevated Sea Surface Temperature (SST) has emerged as the most imminent threat to the thermos-sensitive reef-building corals. The 2010–2014-2016 El Niño Southern Oscillation (ENSO) caused prolonged marine heat waves (MHWs) that led to the most widespread coral bleaching and mortality in the tropical Indi-Pacific regions. Coral bleaching prediction is vital for the management of the reef biodiversity, ecosystem functioning, and services. Recent decades, satellite remote sensing has emerged as a convenient tool for large-scale coral reef monitoring programs. As thermal stress is a critical physical attribute for coral bleaching hence, the present study examines the effectiveness of the elevated SSTs as a proxy to predict coral bleaching in shallow water marginal reefs. Advanced Very High-Resolution Radiometer (AVHRR) satellite data from the NOAA Coral Reef Watch’s (CRW) platform has been used for this study. Coral bleaching indices like Bleaching Threshold (BT), Positive SST Anomaly (PA), and Degree Heating Weeks (DHW) are computed to analyze the thermal stress on the coral reefs. The computed thermal stress from satellite-derived SST data over regions concurrence with the mass coral bleaching (MCB) events. This study concludes that in the last decades (2010 to 2019) the coral cover around these regions has dramatically declined due to higher SST, which indicates that the thermal stress induced recurrent bleaching events attributed to the coral loss.

Characterization of anthropogenic marine macro-debris affecting coral habitat in the highly urbanized seascape of Mumbai megacity

Marine debris has become a major form of pollution and a serious ecosystem health concern. The present study evaluates the accumulation, origin, and fate of debris in intertidal coral habitats of Mumbai-one of the world's highly populated coastal cities on the west coast of India. Predominantly, seven hermatypic coral species belonging to seven genera and five families were identified and mainly represented by Pseudosidastrea, Porites, and Bernardpora. In terms of number, the mean density of marine debris was 1.60 ± 0.13 SE items/m², which is higher than the global average. The mean density of plastic debris was 1.46 ± 0.14 SE items/m². Approximately 9% of total coral colonies were in physical contact with debris, and 22% of these colonies showed visible signs of partial bleaching. Single use plastic bags and wrappers were dominant plastic debris. The study area was characterized as 'very poor cleanliness' according to the Beach Quality Indexes, which include the Clean Coast Index, General Index, and Hazardous Items Index. The numerical model indicates the influence of river discharge and probable areas of plastic accumulation with high tidal currents in this region, maneuvering the spatial advection of litter in the nearshore areas. Combined analysis of ground-truthing and model simulation implies that the possible contributing sources of litter were representatives of land-based and sea-originated. The overall results point to increasing anthropogenic stressors threatening coastal coral communities, including marine debris pollution. It is advocated to adopt an integrated coastal zone management approach supported by coordinated policy frameworks could guide the mitigation of the debris footprint in coastal environments.

Coral community composition and carbonate production in an urbanized seascape

Coastal urbanization causes environmental modifications that directly and indirectly influence the distribution and functioning of coral reefs. However, the capacity of urban infrastructure to support corals and vertically accrete is less understood. Here, we investigated if coral communities on reefs and seawalls in Singapore are distinct, and examined the environmental variables influencing coral carbonate production. Surveys at 22 sites yielded 134 coral species, with richness significantly higher on reefs. Coral cover and Shannon index did not differ between habitat types. Community composition was distinct between habitat types, with seawalls supporting a higher proportion of massive and thick-plating species. 'Distance from mainland' was the single most important variable influencing normalized carbonate production rates (a function of species-specific linear extension rate and skeletal bulk density and site coral cover), which were higher further from the mainland where human activity and development pressures were greater. Our results indicate that environmental filtering strongly shapes coral communities and may influence ecosystem functioning in Singapore's urbanized reef system. The findings will guide the management of reefs on increasingly urbanized coastlines.

Drivers of coral bleaching in a Marine Protected Area of the Southern Gulf of Mexico during the 2015 event

Here we report the bleached coral cover and its drivers observed at Alacranes Reef in 2015. Our results show that 2015 was an unprecedented heat stress event. However, we observed low coral bleaching, with the most substantial impact on sites with a 10-20% of coral cover with bleaching. Depth was the most relevant variable related to coral bleaching and the bleaching severity index, with deeper reefs being most affected. Further, our results show that reefs with higher structural complexity based on species composition were among the most affected. We identified that accumulated heat stress and thermal variation in the last 28 days were relevant drivers of coral bleaching. This work highlights the importance of multidimensional frameworks in assessing the spatial variation of coral bleaching, demonstrating the importance of structural habitat variables such as depth in high heat stress events and at a reef scale.

Low coral bleaching prevalence at the Bolinao-Anda Reef Complex, northwestern Philippines during the 2016 thermal stress event

Here, we examined the coral bleaching responses during the 2016 thermal stress event and post-bleaching changes in coral communities in the heavily disturbed reefs of the Bolinao-Anda Reef Complex (BARC), northwestern Philippines. Less than 25% of colonies bleached, with 77% attributed to five genera (Dipsastrea, Porites, Fungia, Seriatopora, and Montipora). Coral bleaching prevalence was associated with site location, coral composition, and coral abundance, suggesting that small-scale variation (<20 km) in coral communities (taxa and density) influences spatial variation in coral bleaching prevalence. There was no noticeable change in coral composition and cover two years after the bleaching event as exposure to chronic disturbance likely selected for the dominance of stress tolerant coral taxa and communities. Results show that the 2016 thermal stress event caused coral bleaching but with low prevalence at the BARC, which suggests that disturbed reefs may provide spatial refuge to coral communities from thermal stress.

Population dynamics and growth rates of free-living mushroom corals (Scleractinia: Fungiidae) in the sediment-stressed reefs of Singapore

The reefs of Singapore provide an excellent opportunity to study the population dynamics and growth rates of free-living mushroom corals (Fungiidae) under sediment-stressed conditions. Transect surveys at four study sites revealed a total of 11 free-living mushroom coral species-the same 11 species as those found by local studies since the 1980s. The abundance of the four most common species ranged from 1.0 to 68.3 Ind. per 100m², while their population size-structure showed a common pattern of a higher proportion of small-sized corals than large-sized ones (i.e. positively skewed size-structure), although very few individuals of the smallest-size classes were recorded for any of the four species. A more positively skewed size-structure for each of the four most common species was observed at the reef slope (5-6m depth) than at the reef crest (2-3m depth), possibly due to a slower growth rate caused by light reduction with depth. All the mushroom corals studied exhibited a decline in growth rate with increasing size and weight, indicating determinate growth. Growth rate of each of the four most common species was similar among the study sites, despite variation in environmental conditions. Our results demonstrate species richness stability over the past three decades, suggesting that these free-living mushroom coral assemblages comprise species that are well-adapted to the chronic high sedimentation characteristic of Singapore's reefs. However, if the paucity of individuals of the smallest-size classes reflects poor recruitment and/or early mortality, there may be some cause for concern. Our robust baseline data can contribute to a long-term monitoring strategy for determination of changes in mushroom coral population dynamics.

Homogenization of Endosymbiont Communities Hosted by Equatorial Corals during the 2016 Mass Bleaching Event

Thermal stress drives the bleaching of reef corals, during which the endosymbiotic relationship between Symbiodiniaceae microalgae and the host breaks down. The endosymbiont communities are known to shift in response to environmental disturbances, but how they respond within and between colonies during and following bleaching events remains unclear. In 2016, a major global-scale bleaching event hit countless tropical reefs. Here, we investigate the relative abundances of Cladocopium LaJeunesse & H.J.Jeong, 2018 and Durusdinium LaJeunesse, 2018 within and among Pachyseris speciosa colonies in equatorial Singapore that are known to host both these Symbiodiniaceae clades. Bleached and unbleached tissues from bleaching colonies, as well as healthy colonies, during and following the bleaching event were sampled and analyzed for comparison. The nuclear ribosomal internal transcribed spacer (ITS) regions were separately amplified and quantified using a SYBR Green-based quantitative polymerase chain reaction (qPCR) method and Illumina high-throughput sequencing. We found Cladocopium to be highly abundant relative to Durusdinium. The relative abundance of Durusdinium, known to be thermally tolerant, was highest in post-bleaching healthy colonies, while bleached and unbleached tissues from bleaching colonies as well as tissue from healthy colonies during the event had depressed proportions of Durusdinium. Given the importance of Durusdinium for thermal tolerance and stress response, it is surprising that bleached tissue showed limited change over healthy tissue during the bleaching event. Moreover, colonies were invariably dominated by Cladocopium during bleaching, but a minority of colonies were Durusdinium-dominant during non-bleaching times. The detailed characterization of Symbiodiniaceae in specific colonies during stress and recovery will provide insights into this crucial symbiosis, with implications for their responses during major bleaching events.

Gene Expression and Photophysiological Changes in Pocillopora acuta Coral Holobiont Following Heat Stress and Recovery

The ability of corals to withstand changes in their surroundings is a critical survival mechanism for coping with environmental stress. While many studies have examined responses of the coral holobiont to stressful conditions, its capacity to reverse responses and recover when the stressor is removed is not well-understood. In this study, we investigated among-colony responses of Pocillopora acuta from two sites with differing distance to the mainland (Kusu (closer to the mainland) and Raffles Lighthouse (further from the mainland)) to heat stress through differential expression analysis of target genes and quantification of photophysiological metrics. We then examined how these attributes were regulated after the stressor was removed to assess the recovery potential of P. acuta. The fragments that were subjected to heat stress (2 °C above ambient levels) generally exhibited significant reduction in their endosymbiont densities, but the extent of recovery following stress removal varied depending on natal site and colony. There were minimal changes in chl a concentration and maximum quantum yield (Fv/Fm, the proportion of variable fluorescence (Fv) to maximum fluorescence (Fm)) in heat-stressed corals, suggesting that the algal endosymbionts’ Photosystem II was not severely compromised. Significant changes in gene expression levels of selected genes of interest (GOI) were observed following heat exposure and stress removal among sites and colonies, including Actin, calcium/calmodulin-dependent protein kinase type IV (Camk4), kinesin-like protein (KIF9), and small heat shock protein 16.1 (Hsp16.1). The most responsive GOIs were Actin, a major component of the cytoskeleton, and the adaptive immune-related Camk4 which both showed significant reduction following heat exposure and subsequent upregulation during the recovery phase. Our findings clearly demonstrate specific responses of P. acuta in both photophysiological attributes and gene expression levels, suggesting differential capacity of P. acuta corals to tolerate heat stress depending on the colony, so that certain colonies may be more resilient than others.