Posts Tagged 'corals'

Large-scale interventions may delay decline of the Great Barrier Reef

On the iconic Great Barrier Reef (GBR), the cumulative impacts of tropical cyclones, marine heatwaves and regular outbreaks of coral-eating crown-of-thorns starfish (CoTS) have severely depleted coral cover. Climate change will further exacerbate this situation over the coming decades unless effective interventions are implemented. Evaluating the efficacy of alternative interventions in a complex system experiencing major cumulative impacts can only be achieved through a systems modelling approach. We have evaluated combinations of interventions using a coral reef meta-community model. The model consisted of a dynamic network of 3753 reefs supporting communities of corals and CoTS connected through ocean larval dispersal, and exposed to changing regimes of tropical cyclones, flood plumes, marine heatwaves and ocean acidification. Interventions included reducing flood plume impacts, expanding control of CoTS populations, stabilizing coral rubble, managing solar radiation and introducing heat-tolerant coral strains. Without intervention, all climate scenarios resulted in precipitous declines in GBR coral cover over the next 50 years. The most effective strategies in delaying decline were combinations that protected coral from both predation (CoTS control) and thermal stress (solar radiation management) deployed at large scale. Successful implementation could expand opportunities for climate action, natural adaptation and socioeconomic adjustment by at least one to two decades.

Continue reading ‘Large-scale interventions may delay decline of the Great Barrier Reef’

The Bouraké semi-enclosed lagoon (New Caledonia). A natural laboratory to study the life-long adaptation of a coral reef ecosystem to climate change-like conditions

According to current experimental evidence, coral reefs could disappear within the century if CO2 emissions remain unabated. However, recent discoveries of diverse and high cover reefs that already thrive under extreme conditions seem to contradict these projections. Volcanic CO2 vents, semi-enclosed lagoons and mangrove estuaries are unique study sites where one or more ecologically relevant parameters for life in the oceans are close or even worse than currently projected for the year 2100. These natural analogues of future conditions hold new hope for the future of coral reefs and provide unique natural laboratories to explore how reef species could keep pace with climate change. To achieve this, it is essential to characterize their environment as a whole, and accurately consider all possible environmental factors that may differ from what is expected in the future and that may possibly alter the ecosystem response.

In this study, we focus on the semi-enclosed lagoon of Bouraké (New Caledonia, SW Pacific Ocean) where a healthy reef ecosystem thrives in warm, acidified and deoxygenated water. We used a multi-scale approach to characterize the main physical-chemical parameters and mapped the benthic community composition (i.e., corals, sponges, and macroalgae). The data revealed that most physical and chemical parameters are regulated by the tide, strongly fluctuate 3 to 4 times a day, and are entirely predictable. The seawater pH and dissolved oxygen decrease during falling tide and reach extreme low values at low tide (7.2 pHT and 1.9 mg O2 L−1 at Bouraké, vs 7.9 pHT and 5.5 mg O2 L−1 at reference reefs). Dissolved oxygen, temperature, and pH fluctuates according to the tide of up to 4.91 mg O2 L−1, 6.50 °C, and 0.69 pHT units on a single day. Furthermore, the concentration of most of the chemical parameters was one- to 5-times higher at the Bouraké lagoon, particularly for organic and inorganic carbon and nitrogen, but also for some nutrients, notably silicates. Surprisingly, despite extreme environmental conditions and altered seawater chemical composition, our results reveal a diverse and high cover community of macroalgae, sponges and corals accounting for 28, 11 and 66 species, respectively. Both environmental variability and nutrient imbalance might contribute to their survival under such extreme environmental conditions. We describe the natural dynamics of the Bouraké ecosystem and its relevance as a natural laboratory to investigate the benthic organism’s adaptive responses to multiple stressors like future climate change conditions.

Continue reading ‘The Bouraké semi-enclosed lagoon (New Caledonia). A natural laboratory to study the life-long adaptation of a coral reef ecosystem to climate change-like conditions’

Factors controlling coral skeletal U/Ca ratios with implications for their use as a proxy for past ocean conditions

Seawater temperature, salinity and carbonate chemistry have been shown to influence the uranium/calcium (U/Ca) ratios of scleractinian coral skeletons. This apparent sensitivity of U/Ca to multiple environmental parameters calls into question whether there is one environmental variable that most strongly controls coral U/Ca, and whether U/Ca can be straightforwardly applied as a paleoenvironmental proxy due to the tendency of environmental variables to covary in space and time. In this study, uranium concentration data from an existing compilation of tropical scleractinian coral U-series measurements is paired with environmental data from the World Ocean Atlas (WOA) and the Global Ocean Data Analysis Project (GLODAP) to examine the sensitivity of coral skeletal U/Ca to multiple seawater properties including temperature, salinity, pH, and saturation state. First, univariate linear regressions and multiple linear regressions were used to compare relationships between uranium and environmental parameters in the dataset with relationships observed in previous studies. Next, principal component analysis and regularized regression were used to identify the most likely predictors of coral U/Ca in order to create a multiple linear regression model. Results indicate that pH,  Ω, alkalinity, and temperature are all significant predictors of uranium concentrations in coral. The magnitude and strength of relationships between U/Ca and environmental variables also differ across different genera. Seawater properties with strong correlations and small ranges make interpretation of these results difficult. However, results of these analyses indicate that U/Ca is dependent on multiple environmental parameters and that previously developed univariate regressions may be insufficient to characterize the full range of variables that influence coral [238U].

Continue reading ‘Factors controlling coral skeletal U/Ca ratios with implications for their use as a proxy for past ocean conditions’

Effects of ocean acidification on carbon and nitrogen fixation in the hermatypic coral Galaxea fascicularis

The supply of metabolites from symbionts to scleractinian corals is crucial to coral health. Members of the Symbiodiniaceae can enhance coral calcification by providing photosynthetically fixed carbon (PFC) and energy, whereas dinitrogen (N2)-fixing bacteria can provide additional nutrients such as diazotrophically-derived nitrogen (DDN) that sustain coral productivity especially when alternative external nitrogen sources are scarce. How these mutualistic associations benefit corals in the future acidifying ocean is not well understood. In this study, we investigated the possible effects of ocean acidification (OA; pHs 7.7 and 7.4 vs. 8.1) on calcification in the hermatypic coral Galaxea fascicularis with respect to PFC and DDN assimilation. Our measurements based on isotopic tracing showed no significant differences in the assimilation of PFC among different pH treatments, but the assimilation of DDN decreased significantly after 28 days of stress at pH 7.4. The decreased DDN assimilation suggests a nitrogenous nutrient deficiency in the coral holotiont, potentially leading to reduced coral calcification and resilience to bleaching and other stressful events. This contrasting impact of OA on carbon and N flux demonstrates the flexibility of G. fascicularis in coping with OA, apparently by sustaining a largely undamaged photosystem at the expense of N2 fixation machinery, which competes with coral calcification for energy from photosynthesis. These findings shed new light on the critically important but more vulnerable N cycling in hospite, and on the trade-off between coral hosts and symbionts in response to future climate change.

Continue reading ‘Effects of ocean acidification on carbon and nitrogen fixation in the hermatypic coral Galaxea fascicularis’

Effects of ocean acidification on bleaching, survival, and calcification of Porites porites and P. astreoides in Cartagena, Colombia

Estimations of the ocean acidification-OA effects on marine environments indicate that coral reefs’ structure will collapse. This study aimed to determine the effects of OA, and its associated carbon chemistry in the sea water, on corals near the Colombian Caribbean city of Cartagena, taking as model organisms of the species Porites astreoides and P. porites. For each species, the effect of OA on bleaching, survival, and calcification was determined using artificial systems with pH of 7.879 ± 0.004 and 7.789 ± 0.007. The results showed that under the first pH, the bleaching of P. astreoides increased by 24.92% and its survival decreased by 80.56%, while at lowest pH, bleaching increased in 32.78% and survival decreased by 87.5%. In the case of P. porites, at first pH bleaching increased by 29.42% and survival decreased by 30.56% and at the lowest, bleaching increased in 37.32% and survival decreased by 13.39%. In both species, calcification was reduced in more than 90% at 7.879 ± 0.004 and their skeleton began to dissolve at 7.789 ± 0.007. This study represents the first effort to determine OA effects on Colombian Caribbean’s marine biota.

Continue reading ‘Effects of ocean acidification on bleaching, survival, and calcification of Porites porites and P. astreoides in Cartagena, Colombia’

Change of coral carbon isotopic response to anthropogenic Suess effect since around 2000s

Highlights

  • The declining trend in the coral δ13C time series slowed or reversed after 2000.
  • The change of the declining rate in coral δ13C is not due to seawater chemistry.
  • The response of coral δ13C to Suess effect has changed since around 2000s.
  • The change results from coral acclimatization to external environmental stressors.

Abstract

The stable carbon isotope composition (δ13C) in coral skeletons can be used to reconstruct the evolution of the dissolved inorganic carbon (DIC) in surface seawater, and its long-term declining trend during the past 200 years (∼1800-2000) reflects the effect of anthropogenic Suess effect on carbonate chemistry in surface oceans. The global atmospheric CO2 concentration still has been increasing since 2000, and the Suess effect is intensifying. Considering the coral’s ability of resilience and acclimatization to external environmental stressors, the response of coral δ13C to Suess effect may change and needs to be re-evaluated. In this study, ten long coral δ13C time series synthesized from different oceans were used to re-evaluate the response of coral carbonate chemistry to Suess effect under the changing environments. These δ13C time series showed a long-term declining trend since 1960s, but the declining rates slowed in eight time series since around 2000s. Considering that the declining rates of the DIC-δ13C in surface seawater from the Hawaii Ocean Time-series Station and Bermuda Atlantic Time-series Station has not changed since 2000 compared with those during 1960-1999, the change in the coral δ13C trends at eight of ten locations may indicate that the response of coral δ13C to the anthropogenic Suess effect has changed since around 2000s. This change may have resulted from coral acclimatization to external environmental stressors. To adapt to acidifying oceans, coral may have the ability to regulate the source of DIC in extracellular calcifying fluid and/or the utilization way of DIC, therefore the response of coral δ13C to anthropogenic Suess effect will change accordingly.

Continue reading ‘Change of coral carbon isotopic response to anthropogenic Suess effect since around 2000s’

Climate change impacts on corals in the UK overseas territories of BIOT and the Pitcairn Islands

BIOT

The British Indian Ocean Territory (BIOT) consists of five atolls of low-lying islands, including the largest atoll in the world, Great Chagos Bank, and a number of submerged atolls and banks. Diego Garcia is the only inhabited island. The BIOT Marine Protected Area (MPA) was designatedin 2010. It covers the entire maritime zone and coastal waters, an approximate area of 640,000 km2. The marine environment is rich and diverse, attracting sea birds, sharks, cetaceans and sea turtles and with extensive seagrass and coral reef habitats. It includes the endangered Chagos brain coral (Ctenella chagius), an endemic massive coral unique to BIOT. BIOT reefs have suffered extensive bleaching and mortality, and they remain vulnerable to current and future climate change and other pressures, including:

Bleaching
The heavy mortality has been caused by recurrent marine heatwaves since the 1970s. Reefs have not yet recovered from the most severe bleaching in 2016 and 2017, with increasingly severe events expected. Deeper fore-reefs may act as refuges, but those colonies are likely to be more sensitive to temperature change. Limiting other pressures will not guarantee resilience to future bleaching.

Ocean acidification
There has been a low impact of ocean acidification on coral reefs so far, but when combined with future bleaching therisk of decalcification and erosion will increase. Under high emissions scenarios, BIOT is projected to become less suitable for corals by the end of the century.

Continue reading ‘Climate change impacts on corals in the UK overseas territories of BIOT and the Pitcairn Islands’

Will community calcification reflect reef accretion on future, degraded coral reefs?

Coral bleaching events continue to drive the degradation of coral reefs worldwide, causing a shift in the benthic community from coral to algae dominated ecosystems. Critically, this shift may decrease the capacity of degraded coral reef communities to maintain net positive accretion during warming-driven stress events (e.g., reef-wide coral bleaching). Here we measured rates of net ecosystem calcification (NEC) and net ecosystem production (NEP) on a degraded coral reef lagoon community (coral cover < 10 % and algae cover > 20 %) during a reef-wide bleaching event in February of 2020 at Heron Island on the Great Barrier Reef. We found that during this bleaching event, rates of community NEP and NEC across replicate transects remained positive and did not change in response to bleaching. Repeated benthic surveys over a period of 20 d indicated an increase in the percent area of bleached coral tissue, corroborated by relatively low Symbiodiniaceae densities (~0.6 × 106 cm−2) and dark-adapted photosynthetic yields in photosystem II of corals (~0.5) sampled along each transect over this period. Given that a clear decline in coral health was not reflected in the overall community NEC estimates, it is possible that elevated temperatures in the water column that compromise coral health enhanced the thermodynamic favourability for calcification in other, ahermatypic benthic calcifiers. These data suggest that positive NEC on degraded reefs may not equate to the net positive accretion of reef structure in a future, warmer ocean. Critically, our study highlights that if coral cover continues to decline as predicted, NEC may no longer be an appropriate proxy for reef growth as the proportion of the community NEC signal owed to ahermatypic calcification increases and coral dominance on the reef decreases.

Continue reading ‘Will community calcification reflect reef accretion on future, degraded coral reefs?’

Ecosystem composition and environmental factors as drivers of pH on barrier reefs

Tropical coral reefs are both biologically diverse and economically important ecosystems, yet are under threat globally, facing a multitude of stressors including global warming, ocean acidification, nutrient loading, over-fishing and sedimentation. Reef building corals precipitate an aragonite skeleton (CaCO3), which forms the base of the coral reef ecosystem, but it is this skeleton, which makes them sensitive to changes in ocean pH. To precipitate their skeletons, corals raise their internal pH, as seawater pH decreases this increases the energy demands needed to facilitate calcification. Furthermore, reductions in coral calcification has significant implications for reef health, potentially altering community structure with reef-wide consequences. Global ocean pH is decreasing due to rising atmospheric concentrations of CO2, however, dynamic ecosystems, alongside carbon and freshwater input from land, may result in coastal ocean pH being lower than is predicted by open ocean models. While it is predicted than ocean pH will decrease by 0.3 units by 2100 if emissions are not curbed, coral reefs, particularly those near major river outflow, may already be experiencing pH values similar to that of future scenarios.

Our aim was to determine the factors which influence pH in coastal reef systems and thus potentially mitigate or exacerbate atmospheric CO2 mediated ocean acidification. This was achieved by contrasting reefs in distinct environmental settings and collecting data over a sufficient temporal resolution to permit the identification of pertinent drivers. To accomplish this we deployed fixed point observatories in the distinct reefs of Belize (fore and back reef sites), Fiji and Dominica. These custom-built platforms were equipped with a spectrophotometric pH sensor and a conductivity, temperature and dissolved oxygen (CT-DO) sensor from which data was logged at 30-120 minute intervals.

A strong diel cycle in pH, O2 and temperature was observed at all reef sites in response to the changing balance of respiration and photosynthesis. However, the range of these changes varied between the different sites – Belize fore reef (pH 7.849­ – 8.000), Belize back reef (pH 7.897 – 8.039), Fiji (pH 7.951 – 8.0950) and Dominica (pH 7.843 – 8.144). Meteorological conditions, such as wind direction, affected the amplitude of diurnal pH variability and its relationship with other parameters, likely by influencing mixing and the spatial distribution of seawater and freshwater endmembers. The relationship between pH and O2 varied between sites reflecting differences in ecosystem processes (e.g. calcification and primary production) and ecosystem composition (e.g. hard coral and algae cover, proximity to seagrass). Our data confirms that different reef sites are subject to varying degrees of ocean acidification and that controls on pH vary between environments. Furthermore, it highlights the need for widespread high-resolution monitoring to identify, and where possible enact protective measures, in vulnerable reef regions. As coral reefs continue to experience ocean acidification our data also serves to document baseline conditions against which future changes can be assessed.

Continue reading ‘Ecosystem composition and environmental factors as drivers of pH on barrier reefs’

Unexpected role of communities colonizing dead coral substrate in the calcification of coral reefs

Global and local anthropogenic stressors such as climate change, acidification, overfishing, and pollution are expected to shift the benthic community composition of coral reefs from dominance by calcifying organisms to dominance by non‐calcifying algae. These changes could reduce the ability of coral reef ecosystems to maintain positive net calcium carbonate accretion. However, relationships between community composition and calcification rates remain unclear. We performed field experiments to quantify the metabolic rates of the two most dominant coral reef substrate types, live coral and dead coral substrate colonized by a mixed algal assemblage, using a novel underwater respirometer. Our results revealed that calcification rates in the daytime were similar for the live coral and dead coral substrate communities. However, in the dark, while live corals continued to calcify at slower rates, the dead coral substrate communities exhibited carbonate dissolution. Daytime net photosynthesis of the dead coral substrate communities was up to five times as much as for live corals, which we hypothesize may have created favorable conditions for the precipitation of carbonate minerals. We conclude that: (1) calcification from dead coral substrate communities can contribute to coral reef community calcification during the day, and (2) dead coral substrate communities can also contribute to carbonate mineral dissolution at night, decreasing ecosystem calcification over a diel cycle. This provides evidence that reefs could shift from slow, long‐term accretion of calcium carbonate to a state where large daily cycling of calcium carbonate occurs, but with little or no long‐term accumulation of the carbonate minerals needed to sustain the reef against erosional forces.

Continue reading ‘Unexpected role of communities colonizing dead coral substrate in the calcification of coral reefs’

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Ocean acidification in the IPCC AR5 WG II

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