Posts Tagged 'corals'

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Exploring coral reef benefits: a systematic SEEA-driven review

Published 19 August 2024 Science Closed
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Highlights

  • Understand the value and assessment of coral reef ecosystem services using the SEEA framework
  • Identify features of the SEEA in coral reef ecosystem services accounting studies.
  • Explore the characteristics of extent, condition, physical and monetary values associated with implementing ecosystem services accounting in coral reef ecosystems.

Abstract

Coral reefs are among the most diverse and valuable ecosystems on the planet, providing numerous benefits to human societies, including fisheries, coastal protection, and biodiversity conservation. In order to effectively manage and conserve coral reefs, it is essential to understand the value of the ecosystem services they provide. The System of Environmental-Economic Accounting (SEEA) framework offers a comprehensive approach for accounting for ecosystem services, which can be useful for assessing the value of natural environments. While the validity of SEEA for many marine ecosystems is increasingly acknowledged, there remains a scarcity of studies that have investigated SEEA in the context of coral reef ecosystems. To bridge this gap, this study offers extensive examination and investigates the evolution of coral reef ecosystem service research under the SEEA framework in over nearly three decades, providing a rich dataset for understanding trends and gaps. The research findings reveal interdisciplinary methodological integration in coral reef ecosystem research, incorporating remote sensing, environmental science, ecology, environmental economics, ecological economics, computer science, and citizen science. Across different time periods, within the shared focus of coral reef health and sustainability, there has been a transition from concerns about the impacts of human activities to a concentration on climate change, supported by empirical evidence and case studies. These research results contribute to our better understanding of the value of coral reef ecosystems.

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Elevated heterotrophic capacity as a strategy for Mediterranean corals to cope with low pH at CO2 vents

Published 2 August 2024 Science Closed
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The global increase in anthropogenic CO2 is leading to ocean warming and acidification, which is threatening corals. In Ischia, Italy, two species of Mediterranean scleractinian corals–the symbiotic Cladocora caespitosa and the asymbiotic Astroides calycularis–were collected from ambient pH sites (average pHT = 8.05) and adjacent CO2 vent sites (average pHT = 7.8) to evaluate their response to ocean acidification. Coral colonies from both sites were reared in a laboratory setting for six months at present day pH (pHT ~ 8.08) or low pH (pHT ~7.72). Previous work showed that these corals were tolerant of low pH and maintained positive calcification rates throughout the experiment. We hypothesized that these corals cope with low pH by increasing their heterotrophic capacity (i.e., feeding and/or proportion of heterotrophically derived compounds incorporated in their tissues), irrespective of site of origin, which was quantified indirectly by measuring δ13C, δ15N, and sterols. To further characterize coral health, we quantified energy reserves by measuring biomass, total lipids, and lipid classes. Additional analysis for C. caespitosa included carbohydrates (an energy reserve) and chlorophyll a (an indicator of photosynthetic capacity). Isotopic evidence shows that ambient-sourced Mediterranean corals, of both species, decreased heterotrophy in response to six months of low pH. Despite maintaining energy reserves, lower net photosynthesis (C. caespitosa) and a trend of declining calcification (A. calycularis) suggest a long-term cost to low heterotrophy under ocean acidification conditions. Conversely, vent-sourced corals maintained moderate (C. caespitosa) or high (A. calycularis) heterotrophic capacity and increased photosynthesis rates (C. caespitosa) in response to six months at low pH, allowing them to sustain themselves physiologically. Provided there is sufficient zooplankton and/or organic matter to meet their heterotrophic needs, vent-sourced corals are more likely to persist this century and potentially be a source for new corals in the Mediterranean.

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Symbiosis under stress: unraveling the interplay of ocean acidification and rising temperatures on Acropora Samoensis (staghorn coral)

Published 18 July 2024 Science Closed
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This experiment explores the response of marine organisms to the combined effects of ocean acidification and rising temperatures, with a focus on coral reef ecosystems. As global climate change threatens profound declines in coral reefs, understanding the multifaceted impacts of these stressors becomes crucial. The study observes cellular changes in A. samoensis tissues in four different groups, including control, Temp, pH, and Temp and pH. Results indicate significant breakdown of membrane compartmentalization and cell junctions, with notable degradation and calcium carbonate crystallization in pH-stressed samples. Additionally, chlorophyll extraction data support coral bleaching due to the expulsion of zooxanthellae. These findings underscore the severe impact of pH and temperature variations on coral health, with pH conditions exhibiting a stronger effect. The study concludes by proposing a follow-up experiment involving the addition of carbon-fixing plants to mitigate environmental stressors and enhance coral resilience, aiming to contribute to the conservation of coral reef ecosystems in the face of climate change challenges.

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Bottom’s up – focusing on habitat shifts as mediators of anthropogenic impacts on marine ecosystems

Published 16 July 2024 Science Closed
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Marine ecosystems face unprecedented challenges in the Anthropocene, an age characterized by escalating environmental stressors such as ocean acidification, warming and the intrusion of human infrastructure into coastal seascapes. As we hasten to understand the ecological consequences of these mounting pressures, much attention has been devoted to characterizing the traits of individual taxa that are likely to dictate their response to future conditions. However, we are increasingly recognizing the pivotal role that habitat may play in shaping the response of communities to such broad-scale changes. In this thesis, I present empirical evidence of the capacity of habitat-level responses to stress to propagate upwards through the broader ecosystem, inducing substantial and meaningful changes in supported fish assemblages. In my first project, I trace the indirect effects of ocean acidification from the habitat level through to the structure of an assemblage of small-bodied reef fish. I use the natural laboratory provided by a volcanic seep in Papua New Guinea to approximate future acidification conditions under current climate change projections. Here, coral communities chronically exposed to elevated CO2 exhibit a shift in competitive interactions that favours fast-growing, morphologically simple taxa, with the implication that other coral reefs globally may undergo an equivalent structural simplification in coming decades in response to ocean acidification. I show that several common, ecologically important reef fishes display strong and relatively inflexible associations with branching corals, with some even preferencing structure over living tissue when selecting habitat. I then demonstrate that acidified and structurally simplified reefs show a drastically reduced capacity to support healthy populations of these fishes. This chapter contributes two important findings: first, that simplification of coral morphology in response to ocean acidification can induce substantial negative changes in supported reef fish assemblages, even if the total cover of live coral remains unchanged; and secondly, that reef fish may be more vulnerable to these indirect, habitat level changes than to the simple direct effects wrought by acidification. Shifting focus to temperate ecosystems, my second project examines how warming, coastal urbanisation and marine protection interact to influence the distributions and assemblage structures of rangeshifting tropical fishes as they venture poleward in response to ocean warming. Using breakwalls as a ubiquitous and readily accessible test case, I reveal that the structural complexity and shelter from wave action offered by coastal infrastructure can render these environments hotspots for tropical fish recruitment. Importantly, this chapter both identifies coastal infrastructure as potentially significant contributors to the process of tropicalisation, highlighting the need for further research attention and monitoring, but also recognises that marine protected areas can offer an effective means of mitigating the effects of coastal urbanisation. Together, the two projects presented in this thesis demonstrate the power of both the direct and indirect effects of habitat changes. In light of the ongoing and accelerating accumulation of anthropogenically induced stressors, my research underscores the necessity of accounting for habitat-level responses when projecting future fish assemblages, and frames habitat protection as a vital element of safeguarding healthy ecosystems.

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Out of shape: ocean acidification simplifies coral reef architecture and reshuffles fish assemblages

Published 3 July 2024 Science Closed
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  1. Climate change stressors are progressively simplifying biogenic habitats in the terrestrial and marine realms, and consequently altering the structure of associated species communities.
  2. Here, we used a volcanic CO2 seep in Papua New Guinea to test in situ if altered reef architecture due to ocean acidification reshuffles associated fish assemblages.
  3. We observed replacement of branching corals by massive corals at the seep, with simplified coral architectural complexity driving abundance declines between 60% and 86% for an assemblage of damselfishes associated with branching corals. An experimental test of habitat preference for a focal species indicated that acidification does not directly affect habitat selection behaviour, with changes in habitat structural complexity consequently appearing to be the stronger driver of assemblage reshuffling. Habitat health affected anti-predator behaviour, with P. moluccensis becoming less bold on dead branching corals relative to live branching corals, irrespective of ocean acidification.
  4. We conclude that coral reef fish assemblages are likely to be more sensitive to changes in habitat structure induced by increasing pCO2 than any direct effects on behaviour, indicating that changes in coral architecture and live cover may act as important mediators of reef fish community structures in a future ocean.
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Revisiting 20 years of coral–algal interactions: global patterns and knowledge gaps

Published 1 July 2024 Science Closed
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Coral–algal interactions are pivotal in reef ecosystems globally as they can scale up ecosystem levels and lead to dominance shifts. In this study, we conducted a systematic review of global coral–algal interactions, identifying the most studied locations, species, and types of interactions. We then assessed how these interactions may be impacted by consumers and climate change. Over the past 20 years (2001–2020), coral and algae interactions were mostly explored in the Pacific, and the Caribbean and US East Coast, where branching and massive corals were the focus, while other coral growth forms received less attention, and effects on algae were often overlooked. Adult corals were generally reported to be damaged when directly interacting with algae through physical abrasion or allelopathy. Conversely, algae interactions were found to have a positive impact on juvenile corals by facilitating larval recruitment and settlement. As expected, coral–algal interactions and the type of coral–algal relationships vary globally, most likely due to differences in abiotic conditions, community composition and the number of studies performed in a region. Despite the large emphasis on the role of consumers in controlling coral–algal interactions, few studies directly explored the effects of herbivory on coral–algal interactions. Given the growing evidence that ocean warming and acidification can reduce the competitive ability of corals, understanding the dynamic relationships between coral, algae, and consumers under future climate change conditions is crucial in predicting future coral recruitment potential and reef composition patterns. Here, we highlight the main findings from coral–algal interaction studies performed in the last 20 year and point to future directions, such as: 1) diversifying location, coral species, growth forms and life phases; 2) considering effects on both sides of interaction, not neglecting effects on algae; and 3) taking a closer look into the role of consumers and microbiomes. Advancing our understanding of coral–algal interactions, as well as how these interactions shift under changing conditions, is critical in predicting how coral reef ecosystems may operate in the future.

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Ocean acidification in the tropical Indian Ocean over the past 37 years: insights from 𝛿11B and B/Ca records in a Maldives coral

Published 1 July 2024 Science Closed
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Highlights

  • This is the first coral’s carbonate chemistry record in the tropical Indian Ocean.
  • Porites calcification pH is sensitive to ocean acidification, but the transfer function to seawater pH needs re-evaluation.
  • A different internal energy/dissolved inorganic carbon (DIC) supply might explain the lower [DIC] CF observed in Maldives corals.
  • Data from a wider geographic range is required to constrain the impacts of ocean acidification on corals.

Abstract

Boron isotopes (𝛿11B) in coral skeletons of Porites have been widely applied to reconstruct past seawater pH (pHSW) on decadal to centennial timescales. However, due to biological regulation within corals, an additional transfer function is required to estimate ambient seawater chemistry during the skeleton growth under the calcification site fluid pH. Temperature may also interfere with coral calcification fluid pH (pHCF) due to changes in kinetics of coral aragonite precipitation, or buffering capacity in coral calcification fluid. To decipher how coral Porites adjusts pHCF in response to pHSW from complex environmental controls, long-term records from sites with least fluctuations in environmental conditions other than pHSW are essential. Here we present a 37-year record of coral 𝛿11B and B/Ca ratios derived from a coral core collected from southern Maldives, the tropical Indian Ocean. Our results show no clear seasonality in the coral 𝛿11B and B/Ca ratios between monsoons, but a long-term decline in coral pHCF is evident across the entire record. When applying different existing transfer functions, we also observe discrepancies among the calculated pHCF values, model results and short-term instrumental data. Calculated calcification fluid dissolved inorganic carbon concentration ([DIC]CF) values are relatively low compared to literature, suggesting that coral calcification fluid carbonate chemistry may be under different levels of control, even within the same coral taxa. Thus, coral records from a wider geographic range are required to better quantify coral response to ocean acidification, and our results can serve as a baseline for future comparisons.

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Intraspecific variation in response to elevated pCO2 and temperature in the branching reef coral Acropora digitifera from different habitats

Published 27 June 2024 Science Closed
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Ocean acidification (OA) and ocean warming (OW) affect the calcification of corals, and intraspecific variations in response to these stressors in the population level need to be clarified for better future predictions. Using Acropora digitifera as our subject, we examined the intraspecific variability in calcification and maximum quantum yield (Fv/Fm) of photosystem II of symbiotic zooxanthella in responses to OA, OW, and OA + OW. Samples were taken from two different sites: Sesoko Station (warmer) and Sesoko South (cooler) in Okinawa, Japan. Calcification rates varied between the two sites, and noticeable differences were observed only among coral colonies from the Sesoko South site, specifically under control and OA treatments. Furthermore, Fv/Fm showed no variation between the sites due to those stresses. Hence, the calcification rates among A. digitifera colonies varied by habitat, and we found within-site variation only in the lower temperature location, Sesoko South. We observed diminished variation in response among colonies in the warmer site. The adapting to diverse environmental conditions and responding to changes such as seawater pCO2 and temperature, may lead to differences in sensitivity between the two populations to OA, OW, and OA + OW. These intraspecific variation could arise from factors like acclimatizations, the influence of specific genotypes, or phenotypic plasticity of the colonies.

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Sulphate reduction and carbonate precipitation in a high-energy algal rim framework

Published 24 June 2024 Science Closed
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Algal ridges are protective features for coral reefs that form through the accretion and encrustation of reef rubble and debris by crustose coralline algae (CCA) and processes of diagenetic cementation. Carbonate precipitation and dissolution dynamics on and within algal ridge frameworks are poorly understood. We studied the surface and subsurface geochemistry of the algal ridge framework at One Tree Island, Australia. Measurable quantities of hydrogen sulphide were detected in most porewater samples collected from bores, indicating a largely anoxic ridge framework. Total alkalinity (TA) and pH measurements indicate that the precipitation of carbonate minerals within the interior of the ridge framework occurs under largely anoxic conditions and is likely to be driven by TA changes associated with sulphate-reducing bacteria. Modelling of porewater hydrogen sulphide concentrations in combination with TA and dissolved inorganic carbon (DIC) indicates anoxic respiration processes produce alkalinity within the algal ridge framework. However, significantly more TA is removed via the precipitation of mineral carbonate, resulting in porewater TA concentrations falling below the open seawater values. The precipitation of mineral carbonate also lowers interstitial water pH, such that pH changes are not solely from organic carbon diagenesis. The simultaneous precipitation and dissolution of carbonate minerals within the algal ridge framework are key to forming and cementing algal ridges, which are important physical protective features for coral reefs.

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Rubble persistence under ocean acidification threatened by accelerated bioerosion and lower-density coral skeletons

Published 20 June 2024 Science Closed
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As the balance between erosional and constructive processes on coral reefs tilts in favor of framework loss under human-induced local and global change, many reef habitats worldwide degrade and flatten. The resultant generation of coral rubble and the beds they form can have lasting effects on reef communities and structural complexity, threatening the continuity of reef ecological functions and the services they provide. To comprehensively capture changing framework processes and predict their evolution in the context of climate change, heavily colonized rubble fragments were exposed to ocean acidification (OA) conditions for 55 days. Controlled diurnal pH oscillations were incorporated in the treatments to account for the known impact of diel carbonate chemistry fluctuations on calcification and dissolution response to OA. Scenarios included contemporary pH (8.05 ± 0.025 diel fluctuation), elevated OA (7.90 ± 0.025), and high OA (7.70 ± 0.025). We used a multifaceted approach, combining chemical flux analyses, mass alteration measurements, and computed tomography scanning images to measure total and chemical bioerosion, as well as chemically driven secondary calcification. Rates of net carbonate loss measured in the contemporary conditions (1.36 kg m−2 year−1) were high compared to literature and increased in OA scenarios (elevated: 1.84 kg m−2 year−1 and high: 1.59 kg m−2 year−1). The acceleration of these rates was driven by enhanced chemical dissolution and reduced secondary calcification. Further analysis revealed that the extent of these changes was contingent on the density of the coral skeleton, in which the micro- and macroborer communities reside. Findings indicated that increased mechanical bioerosion rates occurred in rubble with lower skeletal density, which is of note considering that corals form lower-density skeletons under OA. These direct and indirect effects of OA on chemical and mechanical framework-altering processes will influence the permanence of this crucial habitat, carrying implications for biodiversity and reef ecosystem function.

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Macroalgal presence decreases coral calcification rates more than ocean acidification

Published 12 June 2024 Science Closed
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Global coral reef degradation has precipitated phase shifts toward macroalgal-dominated communities. Despite the negative repercussions for reefscapes, higher abundances of primary producers have the potential to positively impact the physicochemical environment and mitigate negative impacts of ocean acidification (OA). In this study, we investigated the influence of macroalgal (cf. Sargassum vulgare) density on coral (Acropora millepora and A. hemprichii) calcification rates under current and future OA conditions. Corals were resistant to OA up to ~ 1100 µatm, with no changes in calcification rates. However, the presence of (low and high density) algae reduced calcification rates by ~ 41.8%, suggesting either a chemical defense response due to algal metabolites or potential physical impacts from shading or abrasion. Documented beneficial buffering effects of macroalgae in OA may also elicit negative impacts on coral calcification, suggesting further work is needed to elucidate how species interactions influence responses to projected climate change.

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Effects of water flow and ocean acidification on oxygen and pH gradients in coral boundary layer

Published 10 June 2024 Science Closed
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Reef-building corals live in highly hydrodynamic environments, where water flow largely controls the complex chemical microenvironments surrounding them—the concentration boundary layer (CBL). The CBL may be key to alleviate ocean acidification (OA) effects on coral colonies by partially isolating them. However, OA effects on coral CBL remain poorly understood, particularly under different flow velocities. Here, we investigated these effects on the reef-building corals Acropora cytherea, Pocillopora verrucosa, and Porites cylindrica. We preconditioned corals to a control (pH 8.0) and OA (pH 7.8) treatment for four months and tested how low flow (2 cm s−1) and moderate flow (6 cm s−1) affected O2 and H+ CBL traits (thickness, surface concentrations, and flux) inside a unidirectional-flow chamber. We found that CBL traits differed between species and flow velocities. Under OA, traits remained generally stable across flows, except surface pH. In all species, the H+ CBL was thin and led to lower surface pH. Still, low flow thickened H+ CBLs and increased light elevation of surface pH. In general, our findings reveal a weak to null OA modulation of the CBL. Moreover, the OA-buffering capacity by the H+ CBL may be limited in coral species, though low flow could enhance CBL sheltering.

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Skeleton-forming responses of reef-building corals under ocean acidification

Published 5 June 2024 Science Closed
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Ocean acidification is increasing in frequency and is considered one of the most important causes of severe damage to global coral reefs. Therefore, there is an urgent need to study the impact of acid stress on the growth patterns of major reef-building corals. Here, we studied the skeleton forming strategies of four widely distributed coral species in a simulated acidified habitat with a pH of 7.6–7.8. We reconstructed and visualized the skeleton building process, quantified elemental calcium loss, and determined gene expression changes. The results suggest that different reef-building corals have diverse growing strategies in acidified seawater. A unique ‘cavity-like’ forming process starts from the inside of the skeletons of Acropora muricata, which sacrifices skeleton density to protect its polyp-canal system. The forming patterns in Pocillopora damicornis, Montipora capricornis, and M. foliosa were characterized by ‘osteoporosis’, exhibiting disordered skeletal structures, insufficient synthesis of adhesion proteins, and low bone mass, correspondingly. In addition, we found that skeletal areas near coral polyps suffered less and had later acidified damage than other skeletal areas in the colony. These results help to understand the skeleton-forming strategies of several major coral species under acid stress, thereby laying a foundation for coral reef protection and restoration under increasing ocean acidification.

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Century‐Long records of sedimentary input on a caribbean reef from coral Ba/Ca ratios

Published 14 May 2024 Science Closed
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Abstract

Coral reef ecosystems are delicately balanced and are thus prone to disruption by stressors such as storms, disease, climate variability and natural disasters. Most tropical coral populations worldwide are now in rapid decline owing to additional anthropogenic pressures, such as global warming, ocean acidification and a variety of local stressors. One such problem is the addition of excess sediment and nutrients flux to reefs from increased soil erosion from land use changes. Here we present century-long Ba/Ca records from two Siderastrea siderea colonies as a proxy for local riverine discharge and sediment flux to the southern Mesoamerican Barrier Reef System (MBRS). The coral colonies have linear extension trends, which can be seen as a first-order indicator for coral health and response. The coral colony that exhibits a decline in linear extension rate from the forereef of the MBRS, mainly receives riverine input from Honduras, whilst the coral from the backreef, which does not exhibit a decline in extension rate, primarily receives riverine input from more sparsely populated regions of Belize. Coral Ba/Ca increased (>70%) through time in the forereef colony, while the backreef colony showed little long-term increase in Ba/Ca over the last century. Our results suggest that increasing sediment supply may have played a role in the decline of forereef skeletal extension in the southernmost MBRS region, likely stemming from increasing land-use changes in Honduras.

Key Points

  • Coral skeletal Ba/Ca measured by LA-ICP-MS is a proxy for river discharge and sediment flux to the reefs in this study
  • Skeletal Ba/Ca has increased in forereef corals that mainly receive riverine flux from Honduras
  • Results suggest that changing land-use around the Mesoamerican Barrier Reef System could be influencing long-term coral growth trends
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Decline of a distinct coral reef holobiont community under ocean acidification

Published 22 April 2024 Science Closed
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Background

Microbes play vital roles across coral reefs both in the environment and inside and upon macrobes (holobionts), where they support critical functions such as nutrition and immune system modulation. These roles highlight the potential ecosystem-level importance of microbes, yet most knowledge of microbial functions on reefs is derived from a small set of holobionts such as corals and sponges. Declining seawater pH — an important global coral reef stressor — can cause ecosystem-level change on coral reefs, providing an opportunity to study the role of microbes at this scale. We use an in situ experimental approach to test the hypothesis that under such ocean acidification (OA), known shifts among macrobe trophic and functional groups may drive a general ecosystem-level response extending across macrobes and microbes, leading to reduced distinctness between the benthic holobiont community microbiome and the environmental microbiome.

Results

We test this hypothesis using genetic and chemical data from benthic coral reef community holobionts sampled across a pH gradient from CO2 seeps in Papua New Guinea. We find support for our hypothesis; under OA, the microbiome and metabolome of the benthic holobiont community become less compositionally distinct from the sediment microbiome and metabolome, suggesting that benthic macrobe communities are colonised by environmental microbes to a higher degree under OA conditions. We also find a simplification and homogenisation of the benthic photosynthetic community, and an increased abundance of fleshy macroalgae, consistent with previously observed reef microbialisation.

Conclusions

We demonstrate a novel structural shift in coral reefs involving macrobes and microbes: that the microbiome of the benthic holobiont community becomes less distinct from the sediment microbiome under OA. Our findings suggest that microbialisation and the disruption of macrobe trophic networks are interwoven general responses to environmental stress, pointing towards a universal, undesirable, and measurable form of ecosystem change.

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Seasonally varying biogeochemical regime around the coral habitats off central west coast of India

Published 9 April 2024 Science Closed
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Highlights

  • Waters around the Grande Island (harboring patchy coral growth) along the central west coast of India experience low- O2, pH, and temperature during seasonal upwelling, while episodes of low- O2 and pH are observed during plankton blooms in the summer season.
  • Changes in benthic communities and a steady decline in coral cover associated with a sharp increase in the macro-algae and rubble were observed at the Grande Island.
  • Angria Bank, the offshore coral site, does not show extreme seasonal change in the environmental conditions, rather it shows open ocean biogeochemical characyteristics.

Abstract

The Western Indian Continental Shelf (WICS) experiences upwelling during the Southwest Monsoon (SWM), leading to deoxygenation and acidification of subsurface waters. The region has patchy growth of corals, e.g. in the Grande Island and Angria Bank. Measurements made during the late SWM of 2022 reveal that the shelf waters around the Grande Island were subject to varying environmental conditions, viz. lower temperature (21.3–26.1°C), oxygen (0–4.9 mL L−1) and pHT (7.506–7.927). Complete anoxia was associated with sulphide build-up to a maximum of 5.9 μmol L−1 at 17 m depth. An additional episodic condition (high temperature, low oxygen and pH) also occurred associated presumably with a plankton bloom in April 2017. Hence, unlike the offshore coral site Angria Bank, waters around the Grande Island experiences extreme changes in physico-chemical conditions (e.g. Ωarg ∼1.2–1.8 during October 2022) seasonally as reported here. The biogeochemical conditions are however not as intense (Ωarg = 0.6) as observed along the eastern boundary upwelling system of the Pacific Ocean.

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Systematic review of the uncertainty of coral reef futures under climate change

Published 3 April 2024 Science Closed
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Climate change impact syntheses, such as those by the Intergovernmental Panel on Climate Change, consistently assert that limiting global warming to 1.5 °C is unlikely to safeguard most of the world’s coral reefs. This prognosis is primarily based on a small subset of available models that apply similar ‘excess heat’ threshold methodologies. Our systematic review of 79 articles projecting coral reef responses to climate change revealed five main methods. ‘Excess heat’ models constituted one third (32%) of all studies but attracted a disproportionate share (68%) of citations in the field. Most methods relied on deterministic cause-and-effect rules rather than probabilistic relationships, impeding the field’s ability to estimate uncertainty. To synthesize the available projections, we aimed to identify models with comparable outputs. However, divergent choices in model outputs and scenarios limited the analysis to a fraction of available studies. We found substantial discrepancies in the projected impacts, indicating that the subset of articles serving as a basis for climate change syntheses may project more severe consequences than other studies and methodologies. Drawing on insights from other fields, we propose methods to incorporate uncertainty into deterministic modeling approaches and propose a multi-model ensemble approach to generating probabilistic projections for coral reef futures.

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Gene expression of pocillopora damicornis coral larvae in response to acidification and ocean warming

Published 27 March 2024 Science Closed
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Objectives

The endosymbiosis with Symbiodiniaceae is key to the ecological success of reef-building corals. However, climate change is threatening to destabilize this symbiosis on a global scale. Most studies looking into the response of corals to heat stress and ocean acidification focus on coral colonies. As such, our knowledge of symbiotic interactions and stress response in other stages of the coral lifecycle remains limited. Establishing transcriptomic resources for coral larvae under stress can thus provide a foundation for understanding the genomic basis of symbiosis, and its susceptibility to climate change. Here, we present a gene expression dataset generated from larvae of the coral Pocillopora damicornis in response to exposure to acidification and elevated temperature conditions below the bleaching threshold of the symbiosis.

Data description

This dataset is comprised of 16 samples (30 larvae per sample) collected from four treatments (Control, High pCO2, High Temperature, and Combined pCO2 and Temperature treatments). Freshly collected larvae were exposed to treatment conditions for five days, providing valuable insights into gene expression in this vulnerable stage of the lifecycle. In combination with previously published datasets, this transcriptomic resource will facilitate the in-depth investigation of the effects of ocean acidification and elevated temperature on coral larvae and its implication for symbiosis.

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Short periods of decreased water flow may modulate long-term ocean acidification in reef-building corals

Published 14 March 2024 Science Closed
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Ocean acidification (OA) poses a major threat to reef-building corals. Although water flow variability is common in coral reefs and modulates coral physiology, the interactive effects of flow and OA on corals remain poorly understood. Therefore, we performed a three-month OA experiment investigating the effect of changes in flow on coral physiology. We exposed the reef-building corals Acropora cytherea, Pocillopora verrucosa, and Porites cylindrica to control (pH 8.0) and OA (pH 7.8) conditions at moderate flow (6 cm s-1) and monitored OA effects on growth. Throughout the experiment, we intermittently exposed all corals to low flow (2 cm s-1) for 1.5 h and measured their photosynthesis:photosynthesis (P:R) ratio under low and moderate flow. On average, corals under OA calcified 18 % less and grew 23 % less in surface area than those at ambient pH. We observed species-specific interactive effects of OA and flow on coral physiology. P:R ratios decreased after 12 weeks of OA in A. cytherea (22 %) and P. cylindrica (28 %) under moderate flow, but were unaffected by OA under low flow. P:R ratios were stable in P. verrucosa. These results suggest that short periods of decreased water flow may modulate OA effects on some coral species, indicating that flow variability is a factor to consider when assessing long-term effects of climate change.

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Investigation of the Kuroshio-coastal current interaction and marine heatwave trends in the coral habitats of Northeastern Taiwan

Published 6 March 2024 Science Closed
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Highlights

  • The Kuroshio branch exhibits a northwestward intrusion trend into the ECS shelf.
  • The coral habitats experienced over 400 days of marine heatwaves from 2020 to 2022.
  • The coral habitats reached the bleaching alert level 2 for the first time in 2020.
  • The PDO negative phase and La Niña occurrence have increased MHW days.
  • The lack of periodic cooling is harmful to this area’s coral habitats.

Abstract

The continually rising concentration of the surface aqueous partial pressure of carbon dioxide has led to sustained ocean acidification and increased sea surface temperature (SST) in the coral habitats of northeastern Taiwan. Since 2016, this region has been experiencing intense marine heatwave (MHW) events, with the accumulated thermal stress reaching its peak between 2020 and 2022. Apart from the attributing factor of the increasing atmospheric carbon dioxide concentration, the Kuroshio (KC) path along the eastern coast of Taiwan has exhibited a westward tendency towards the coast of Taiwan from October to April. The westward and northward components of the KC’s branch into the East China Sea (ECS) shelf have rapidly increased. The interplay between the KC and the northeastern Taiwan coastal countercurrent (NETCC) near the coral habitats has formed a counterclockwise circulation, which continues to show a westward trend. This has resulted in the influx of warmer waters into northeastern Taiwan. On another note, the pronounced negative phase of the Pacific decadal oscillation (PDO) and La Niña conditions from 2020 to 2022 have further contributed to the increased SST, with the average MHW event accumulating to 172 days annually. The coral bleaching index, degree heating week (DHW), indicates that 2020 was historically the first year for this region to experience a DHW exceeding 8°C-weeks, reaching an Alert Level 2 for bleaching, and 2022 saw even more severe conditions with an average of 12 days at this level. With a reduced number of typhoon incursions in recent years in northeastern Taiwan, and the absence of periodic cold waters to mitigate the heat, the future marine environment of the coral habitats in this region is of significant concern.

Continue reading ‘Investigation of the Kuroshio-coastal current interaction and marine heatwave trends in the coral habitats of Northeastern Taiwan’

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