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Globally, many coral reefs have fallen into negative carbonate budget states, where biological erosion exceeds carbonate production. The compounding effects of urbanization and climate change have caused reductions in coral cover and shifts in community composition that may limit the ability of reefs to maintain rates of vertical accretion in line...
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Carbonate budgets dynamically balance production and loss of calcium carbonate (CaCO3) on coral reefs. To sustain or expand the coral reef framework, CaCO3 production by calcifying organisms must be higher than erosion. However, global climate change has been negatively impacting carbonate production, with bleaching events causing widespread coral...
Abstract
The Musandam peninsula, situated on the Arabian peninsula in the Strait of Hormuz, comprise of coral reefs that appear to endure extremely harsh conditions when compared to corals in other parts of the world. Although resilience seems to exist among corals, scientists are increasingly concerned that any additional stress, imposed by globa...
There is a growing interest in the endolithic microbial biofilms inhabiting skeletons of living corals because of their contribution to coral reef bioerosion and the reputed benefits they provide to live coral hosts. Here, we sought to identify possible correlations between coral interspecific patterns in skeletal morphology and variability in the...
Synopsis
Bioeroding organisms play an important part in shaping structural complexity and carbonate budgets on coral reefs. Species interactions between various bioeroders are an important area of study, as these interactions can affect net rates of bioerosion within a community and mediate how bioeroders respond to environmental change. Here we te...
The development and maintenance of the physical structure of coral reefs depends on the balance between production (accretion) and removal (erosion) of CaCO3 produced mainly by reef-building corals. This calcareous material may be removed from the coral skeleton by means of physical, chemical, or biological agents, with the latter being the most in...
Citations
... Coral growth is restricted to < 10 m due to a combination of rapid light attenuation and sedimentation (Chow et al. 2019;Martin et al. 2021;Morgan et al. 2020). Hantu reef is relatively more sheltered and has higher sediment deposition rates than the more exposed Kusu reef ), but the hard-coral communities of both reefs are dominated by stress-tolerant and generalist species (Januchowski-Hartley et al. 2020). The Singapore Strait is subject to two monsoon seasons, and the prevailing ocean current reverses direction between the two ( Fig. 1; van Maren and Gerritsen 2012). ...
Coral reef productivity depends on nutrient supply, mediated partly by enzymatic breakdown of organic matter. Alkaline phosphatases hydrolyse phosphomonoesters and are one of the key enzymes involved in marine phosphorus cycling. They are expressed by many marine organisms including planktonic microbes and metazoans such as corals, often in response to phosphate limitation, and are potentially important for coral P nutrition and reef biogeochemical cycling. However, most alkaline phosphatase activity (APA) data are from open-ocean environments, and the rates and drivers of APA in coastal waters are not well understood. Here, we measured both bulk seawater APA and the extracellular APA of three coral species at reefs in Singapore, where the monsoonal ocean current reversal creates strong seasonal changes in dissolved nutrient availability. Seawater APA was always measurable, averaging 9 ± 10 nmol l⁻¹ h⁻¹, but was not correlated with dissolved phosphate or other biogeochemical parameters. Experimental phosphate addition did not reduce seawater APA but addition of labile organic carbon increased seawater APA, indicating that seawater APA was driven by heterotrophic activity rather than phosphate stress. Coral APA ranged from 12 to 163 µmol m⁻² h⁻¹ depending on species, which was equivalent to the seawater APA in several metres of the overlying water column. While most coral APA was associated with the coral holobiont rather than the coral mucus, the mucus release added 13–56 µmol m⁻² h⁻¹ of APA into the water column, suggesting that corals can potentially contribute significantly to seawater APA.
... Conversely, reef-building potential can be maintained under extreme environmental conditions. For example, high reef accretion rates occur on inshore reefs of the Great Barrier Reef despite high turbidity (Browne et al., 2013b), and active (though very low) net calcium carbonate production has been documented in Singapore despite the highly urbanized setting (Browne et al., 2015;Januchowski-Hartley et al., 2020) or on the very high latitude reefs at Iki Island, Japan (34°N) (Yamano et al., 2001). ...
... For example, both Singapore's inshore reefs and several inshore reef sites on the Great Barrier Reef are labelled as turbid reefs (Browne et al., 2015(Browne et al., , 2013bMorgan et al., 2016). Yet, based on our new conceptual framework, here we re-define (most) of Singapore's reefs as extreme and marginal due to both chronic environmental conditions influenced by land reclamation and reduced coral cover and reef function (Januchowski-Hartley et al., 2020). In contrast, Great Barrier Reef sites such as Paluma Shoals, which experience extreme fluctuations in turbidity due to wind-driven resuspension, have both high coral cover and species richness, and, therefore, would be considered to be an extreme but not marginal reef. ...
The worldwide decline of coral reefs has renewed interest in coral communities at the edge of environmental limits because they have the potential to serve as resilience hotspots and climate change refugia, and can provide insights into how coral reefs might function in future ocean conditions. These coral communities are often referred to as marginal or extreme but few definitions exist and usage of these terms has therefore been inconsistent. This creates significant challenges for categorising these often poorly studied communities and synthesising data across locations. Furthermore, this impedes our understanding of how coral communities can persist at the edge of their environmental limits and the lessons they provide for future coral reef survival. Here, we propose that marginal and extreme coral communities are related but distinct and provide a novel conceptual framework to redefine them. Specifically, we define coral reef extremeness solely based on environmental conditions (i.e., large deviations from optimal conditions in terms of mean and/or variance) and marginality solely based on ecological criteria (i.e., altered community composition and/or ecosystem functioning). This joint but independent assessment of environmental and ecological criteria is critical to avoid common pitfalls where coral communities existing outside the presumed optimal conditions for coral reef development are automatically considered inferior to coral reefs in more traditional settings. We further evaluate the differential potential of marginal and extreme coral communities to serve as natural laboratories, resilience hotspots and climate change refugia, and discuss strategies for their conservation and management as well as priorities for future research. Our new classification framework provides an important tool to improve our understanding of how corals can persist at the edge of their environmental limits and how we can leverage this knowledge to optimise strategies for coral reef conservation, restoration and management in a rapidly changing ocean.
... (2) What are the depth limits of coral species in relation to rising sea levels, and can their potential mortality due to light limitation be explained by their depth distributions? Given the key roles that reef corals play as habitats for other marine organisms and in maintaining biodiversity in Southeast Asia, this study is important in providing valuable insights on reef persistence under climate change impacts on top of ongoing intensification of coastal development in the region [26,48,54,[82][83][84][85]. Indeed, the combination of sea-level rise and continuing coastal development and urbanisation planned over the next few decades, which also include coastal protection against seawater inundation of land areas [86], could have additive and even interactive impacts that would further erode coral cover and abundance. ...
Sea-level rise (SLR) is expected to elevate the depth of seawater above shallow coral reefs, reducing light availability to the benthic environment, and impacting the survival and growth of corals especially on turbid reefs. However, the extent of impact at the deepest reef zones remains unknown. Coral growth could continue to keep pace above light thresholds as sea level rises, but mortality due to light limitation could vary between localities and local conditions. Here, we examine possible outcomes of corals inhabiting Singapore's turbid reefs in the years 2050 and 2100 by characterising their depth distributions and predicting potential mortality rates based on SLR projections. Our results reveal that in 2050, under both RCP4.5 and RCP8.5 sea level projections, up to 6.24% of colonies could face mortality if their growth is not considered. In 2100, up to 7.68% mortality under RCP4.5 and up to 10.7% mortality under RCP8.5 are predicted. When coral linear extension is considered, in 2050, under both RCP4.5 and RCP8.5 sea level projections, up to 1.03% of colonies could face mortality. In 2100, up to 0.87% mortality under RCP4.5 and up to 1.84% mortality under RCP8.5 are predicted. Species-specific losses could amount to 20% of colonies primarily at the deepest zones. The most vulnerable species exhibit a depth distribution with most colonies situated at the deeper parts of their depth ranges. Our findings suggest that sea-level rise may potentially result in the loss of coral cover for some species, but overall mortality could be low.
... Conversely, reef-building potential can be maintained under extreme environmental conditions. For example, high reef accretion rates occur on inshore reefs of the Great Barrier Reef despite high turbidity (Browne et al., 2013b), and active (though very low) net calcium carbonate production has been documented in Singapore despite the highly urbanized setting (Browne et al., 2015;Januchowski-Hartley et al., 2020) or on the very high latitude reefs at Iki Island, Japan (34°N) (Yamano et al., 2001). ...
... For example, both Singapore's inshore reefs and several inshore reef sites on the Great Barrier Reef are labelled as turbid reefs (Browne et al., 2015(Browne et al., , 2013bMorgan et al., 2016). Yet, based on our new conceptual framework, here we re-define (most) of Singapore's reefs as extreme and marginal due to both chronic environmental conditions influenced by land reclamation and reduced coral cover and reef function (Januchowski-Hartley et al., 2020). In contrast, Great Barrier Reef sites such as Paluma Shoals, which experience extreme fluctuations in turbidity due to wind-driven resuspension, have both high coral cover and species richness, and, therefore, would be considered to be an extreme but not marginal reef. ...
... These islands were selected as Pteraeolidia semperi is known to be found around them. In brief, the reefs of the four islands are dominated by dead corals and sediment with a similar coral cover of 20-30% [30][31][32], except for Pulau Jong with a lower coral cover of only 5% [31]. Subtidal sampling was conducted via SCUBA, and Life 2022, 12,1988 3 of 15 specimens were identified based on known anatomical descriptions [33]. ...
... These islands were selected as Pteraeolidia semperi is known to be found around them. In brief, the reefs of the four islands are dominated by dead corals and sediment with a similar coral cover of 20-30% [30][31][32], except for Pulau Jong with a lower coral cover of only 5% [31]. Subtidal sampling was conducted via SCUBA, and Life 2022, 12,1988 3 of 15 specimens were identified based on known anatomical descriptions [33]. ...
... These islands were selected as Pteraeolidia semperi is known to be found around them. In brief, the reefs of the four islands are dominated by dead corals and sediment with a similar coral cover of 20-30% [30][31][32], except for Pulau Jong with a lower coral cover of only 5% [31]. Subtidal sampling was conducted via SCUBA, and specimens were identified based on known anatomical descriptions [33]. ...
Despite the increasing recognition and importance surrounding bacterial and fungal interactions, and their critical contributions to ecosystem functioning and host fitness, studies examining their co-occurrence remain in their infancy. Similarly, studies have yet to characterise the bacterial and fungal communities associated with nudibranchs or their core microbial members. Doing this can advance our understanding of how the microbiome helps a host adapt and persist in its environment. In this study, we characterised the bacterial and fungal communities associated with 46 Pteraeolidia semperi nudibranch individuals collected from four offshore islands in Singapore. We found no distinct spatial structuring of microbial community, richness, or diversity across sampling locations. The bacterial genera Mycoplasma and Endozoicomonas were found across all samples and islands. The fungal genus Leucoagaricus was found with the highest occurrence, but was not found everywhere, and this is the first record of its reported presence in marine environments. The co-occurrence network suggests that bacterial and fungal interactions are limited, but we identified the bacterial family Colwelliaceae as a potential keystone taxon with its disproportionately high number of edges. Furthermore, Colwelliaceae clusters together with other bacterial families such as Pseudoalteromonadaceae and Alteromonadaceae, all of which have possible roles in the digestion of food.
... bleaching: Perry and Morgan 2017; Manzello et al. 2018;Lange and Perry 2019) and across environmental gradients (e.g. Roik et al. 2018;Januchowski-Hartley et al. 2020). However, the relatively recent uptake of budget assessment methodologies, and a general paucity of budget data from prior to about 2010 (Lange et al. 2020), means that our understanding of how budget states have changed, even over the recent period of rapidly accelerating ecological degradation, is sparse. ...
Recent interest in assessing coral reef functions has raised questions about how carbonate production rates have altered over the past few decades of ecological change. At the same time, there is growing interest in quantifying carbonate production on larger reef-scales. Resolving these issues is challenging because carbonate production estimates require three-dimensional survey data, which are typically collected in-situ over small spatial scales. In contrast, data that can be extracted from archive photograph or video imagery and high-resolution aerial imagery are generally planar. To address this disconnect, we collected data on the relationship between linear planar and 3D contour lengths of 62 common Indo-Pacific hard coral genera-morphotypes to establish appropriate conversion metrics (i.e. coral class rugosity values, hereafter termed R coral ). These conversion values allow planar colony dimensions to be converted to estimates of 3D colony contour length, which can be employed within existing census budget methodologies like ReefBudget to estimate coral carbonate production (G, in kg CaCO 3 m ⁻² yr ⁻¹ ). We tested this approach by comparing in-situ carbonate production data collected using the ReefBudget methodology against estimates derived from converted colony length data from video imagery. The data show a high level of consistency with an error of ~ 10%. We then demonstrate potential applications of the conversion metrics in two examples, the first using time-series (2006 to 2018) photo-quadrat imagery from Moorea, and the second using high-resolution drone imagery across different reef flat habitats from the Maldives. Whilst some degree of error must necessarily be accepted with such conversion techniques, the approach presented here offers exciting potential to calculate coral carbonate production: (1) from historical imagery to constrain past coral carbonate production rates; (2) from high quality aerial imagery for spatial up-scaling exercises; and (3) for use in rapid photograph or video-based assessments along reef systems where detailed surveys are not possible.
... Regional declines in coral cover and species diversity from coastal development, pollution, and overfishing (Heery et al., 2018) are increasingly being compounded by coral bleaching events (e.g., Guest et al., 2012;Licuanan et al., 2019). As a result, community composition on many reefs across the region have shifted towards stress-tolerant and generalist taxa (e.g., Cleary et al., 2016;Guest et al., 2016), influencing rates of reef calcification and bioconstruction (Perry et al., 2018;Januchowski-Hartley et al., 2020), and driving increased vertical reef compression . Yet, few studies have quantified how local taxonomic composition and associated growth forms affect reef structural complexity following a bleaching event (McCowan et al., 2012;Denis et al., 2017). ...
... Similarly, spatial variation in coral bleaching and mortality across Singapore's small reef system (~10 km 2 ; Huang et al., 2009) may be associated with well-documented differences in local environmental gradients (Browne et al., 2015;Bauman et al., 2017;Morgan et al., 2020). Coral bleaching, mortality and the loss of coral cover was generally higher on Singapore's nearshore reefs that persist under more turbid conditions (Browne et al., 2015) with lower light (Chen et al., 2005) and higher macroalgae cover (Bauman et al., 2017) relative to the offshore reefs (Guest et al., 2016;Januchowski-Hartley et al., 2020). Poor water quality (Wooldridge and Done, 2009), elevated nutrient levels (Burkepile et al., 2020) and high macroalgal abundance (Donovan et al., 2021) can act synergistically with thermal stress and magnify bleaching responses and mortality. ...
... Reductions in the abundance of dominant coral genera (Merulina, Pachyseris, Pectinia) and loss of foliose-laminar corals coincided with decreasing structural complexity across reefs in Singapore. Small-scale rugosity and reef topography both showed a high degree of spatial variation among reefs, that remained consistent up to one-year post survey (Januchowski-Hartley et al., 2020). However, overall reef rugosity in Singapore was low prior to bleaching (mean 1.7 ± 0.02) ...
Increasing incidence of severe coral bleaching events caused by climate change is contributing to extensive coral losses, shifts in species composition and widespread declines in the physical structure of coral reef ecosystems. With these ongoing changes to coral communities and the concomitant flattening of reef structural complexity, understanding the links between coral composition and structural complexity in maintaining ecosystem functions and processes is of critical importance. Here, we document the impacts of the 2016 global-scale coral bleaching event on seven coral reefs in Singapore; a heavily degraded, turbid reef system. Using a combination of field-based surveys, we examined changes in coral cover, composition and structural complexity before, during and after the 2016 bleaching event. We also quantified differential bleaching responses and mortality among coral taxa and growth forms using a bleaching response index. Elevated SSTs induced moderate to severe coral bleaching across reefs in Singapore in July 2016, but low overall coral mortality (~12% of colonies). However, we observed high bleaching prevalence and post-bleaching mortality of the three most abundant coral genera (Merulina, Pachyseris and Pectinia), all generalists, declined significantly across reefs between March and November 2016. Four months post-bleaching (November 2016), small-scale structural complexity declined across all Singaporean reefs and no moderately complex reefs remained. Importantly, reductions in structural complexity occurred across reefs with a large range of live coral cover (19–62%) and was linked to the loss of dominant coral genera with low-profile foliose-laminar growth forms which resulted in flatter, less structurally complex reefs. And while generalist coral taxa remain highly competitive within Singapore’s reef environment, they may not have the capacity to maintain structural complexity or ensure the persistence of other reef functions, even within communities with high coral cover. The widespread loss of structurally complexity on Singapore’s degraded coral reefs may further impair ecosystem functioning, potentially compromising the long-term stability of reef biodiversity and productivity.
... As sea level rises, lateral expansion in Singapore's coral reefs is restricted by their narrow depth range and limited suitable topography. In the intensely urbanised environments of Singapore, high levels of turbidity and suspended sediments (Browne, Tay, Low, Larson, & Todd, 2015;Dikou & van Woesik, 2006) have over decades restricted the bathymetric range of coral reefs to a narrow band of 0 m to 6 m (Chow, Chan, Jain, & Huang, 2019;Huang, Tun, Chou, & Todd, 2009;Januchowski-Hartley et al., 2020). When sea level rises as the current level of turbidity is sustained throughout the century, the deepest corals might be lost or experience changes in species composition with more light stress-and sediment-tolerant species (Chow et al., 2019;Guest et al., 2016a;Guest et al., 2016b). ...
... As Singapore's shorelines are highly modified (Lai et al., 2015), the lateral gain when coral reefs move landward is limited by high gradients in slope and other topographic barriers such as seawalls along the shorelines. Furthermore, the mean vertical accretion potential of Singapore's reefs has been estimated to be below the current and future rates of SLR (Januchowski-Hartley et al., 2020), so it is likely that vertical growth would also not be able to keep up with SLR (van Woesik et al., 2015;Perry et al., 2018). This assessment excludes impacts from other climate threats such as rising temperature, ocean acidification and local stressors such as development and coastal pollution. ...
Coastal cities and their natural environments are vulnerable to the impacts of climate change, especially sea-level rise (SLR). Hard coastal defences play a key role in protecting at-risk urban coastal populations from flooding and erosion, but coastal ecosystems also play important roles in the overall sustainability and resilience of cities and urban centres by contributing to coastal protection. Conserving coastal ecosystems and maximising their resilience will ensure that urban coastal communities can continue to benefit from ecosystem services and improve their adaptive capacity to cope with adverse impacts in the future. Using the hyper-urbanised coast of Singapore as a case study, we modelled the resilience of coastal wetlands to SLR and integrated resilience in conservation planning. We found that the responses of coastal habitats to rising sea level vary across the modelling periods. While there is a slight net gain in the extent of mangrove forests and tidal flats by the end of the century due to potential habitat conversion, the existing habitats will experience a loss in coverage. Highly modified coastlines associated with urbanisation impede the ability of existing wetlands to migrate landward, which is a key mechanism for coastal habitats to cope with rising sea levels. Systematic conservation planning can identify sites that are potentially resilient to SLR and incorporate factors that influence an ecosystem’s capability to respond to change. Crucially, the relatively slow rates of SLR and persistence of coastal wetlands during the earlier half of this century present an opportunity to introduce management interventions aimed at enhancing ecosystem resilience.
... 85 85 E), were selected as study sites ( Figure 1). Both reefs are characterized by a shore-adjacent reef flat leading seaward to the reef crest and down the reef slope to~8-10 m maximum depth because of high levels of suspended sediments that cause extreme light attenuation [55,59]. ...
... Although the presence of the flatworms was reported in both the investigated sites, a significantly higher prevalence of the association was observed on Kusu Island compared to Pulau Hantu. This difference could be explained by the greater diversity of coral genera, rugosity, and reef complexity on Kusu Island [59], which may have contributed to available niches for Waminoa sp., as well as by the different environmental and physical characteristics of the sites. Pulau Hantu is sheltered by adjacent and heavily developed islands in an area of intense industrialization and ship traffic, while Kusu Island experiences comparatively lower anthropogenic impacts and higher exposure to wave action [80,81]. ...
Waminoa spp. are acoel flatworms mainly found as ectosymbionts on scleractinian corals. Although Waminoa could potentially represent a threat to their hosts, not enough information is available yet regarding their ecology and effect on the coral. Here, the Waminoa sp.–coral association was analyzed in Singapore reefs to determine the prevalence, host range, and preference, as well as the flatworm abundance on the coral surface. Moreover, the impact of Waminoa sp. on the expression of putative immune- and stress-response genes (C-type lectin, C3, Hsp70 and Actin) was examined in the coral Lobophyllia radians. The association prevalence was high (10.4%), especially in sites with lower sedimentation and turbidity. Waminoa sp. showed a wide host range, being found on 17 coral genera, many of which are new association records. However, only few coral genera, mostly characterized by massive or laminar morphologies appeared to be preferred hosts. Waminoa sp. individuals displayed variable patterns of coral surface coverage and an unequal distribution among different host taxa, possibly related to the different coral growth forms. A down-regulation of the expression of all the analyzed genes was recorded in L. radians portions colonized by Waminoa individuals compared to those without. This indicated that Waminoa sp. could affect components of the immune system and the cellular homeostasis of the coral, also inhibiting its growth. Therefore, Waminoa sp. could represent a potential further threat for coral communities already subjected to multiple stressors.
... The loss of microhabitats can cause communities to become homogenized and dominated by habitat generalists (Stuart-Smith et al., 2021;Wilson et al., 2008). The loss of complexity is especially pronounced on urban reefs (Januchowski-Hartley et al., 2020), but its effects on trait communities and functioning remain poorly known. ...
Urbanized coral reefs experience anthropogenic disturbances caused by coastal development, pollution, and nutrient runoff, resulting in turbid, marginal conditions in which only certain species can persist. Mortality effects are exacerbated by increasingly regular thermal stress events, leading to shifts towards novel communities dominated by habitat generalists and species with low structural complexity.
There is limited data on the turnover processes that occur due to this convergence of anthropogenic stressors, and how novel urban ecosystems are structured both at the community and functional levels. As such, it is unclear how they will respond to future disturbance events.
Here, we examine the patterns of coral reef community change and determine whether ecosystem functions provided by specialist species are lost post-disturbance. We present a comparison of community and functional trait-based changes for scleractinian coral genera and reef fish species assemblages subject to coastal development, coastal modification, and mass bleaching between two time periods, 1975–1976 and 2018, in Nakagusuku Bay, Okinawa, Japan.
We observed an increase in fish habitat generalists, a dominance shift from branching to massive/sub-massive corals and increasing site-based coral genera richness between years. Fish and coral communities significantly reassembled, but functional trait-based multivariate space remained constant, indicating a turnover of species with similar traits. A compression of coral habitat occurred, with shallow (<5 m) and deep (>8 m) coral genera shifting towards the mid-depths (5–8 m).
We show that although reef species assemblages altered post disturbance, new communities retained similar ecosystem functions. This result could be linked to the stressors experienced by urban reefs, which reflect those that will occur at an increasing frequency globally in the near future. Yet, even after shifts to disturbed communities, these fully functioning reef systems may maintain high conservation value.
