Posts Tagged 'corals'

Flow-driven micro-scale pH variability affects the physiology of corals and coralline algae under ocean acidification

Natural variability in pH in the diffusive boundary layer (DBL), the discrete layer of seawater between bulk seawater and the outer surface of organisms, could be an important factor determining the response of corals and coralline algae to ocean acidification (OA). Here, two corals with different morphologies and one coralline alga were maintained under two different regimes of flow velocities, pH, and light intensities in a 12 flumes experimental system for a period of 27 weeks. We used a combination of geochemical proxies, physiological and micro-probe measurements to assess how these treatments affected the conditions in the DBL and the response of organisms to OA. Overall, low flow velocity did not ameliorate the negative effect of low pH and therefore did not provide a refugia from OA. Flow velocity had species-specific effects with positive effects on calcification for two species. pH in the calcifying fluid (pHcf) was reduced by low flow in both corals at low light only. pHcf was significantly impacted by pH in the DBL for the two species capable of significantly modifying pH in the DBL. The dissolved inorganic carbon in the calcifying fluid (DICcf) was highest under low pH for the corals and low flow for the coralline, while the saturation state in the calcifying fluid and its proxy (FWHM) were generally not affected by the treatments. This study therefore demonstrates that the effects of OA will manifest most severely in a combination of lower light and lower flow habitats for sub-tropical coralline algae. These effects will also be greatest in lower flow habitats for some corals. Together with existing literature, these findings reinforce that the effects of OA are highly context dependent, and will differ greatly between habitats, and depending on species composition.

Continue reading ‘Flow-driven micro-scale pH variability affects the physiology of corals and coralline algae under ocean acidification’

Multi-decadal change in reef-scale production and calcification associated with recent disturbances on a Lizard Island reef flat

Climate change is threatening the persistence of coral reef ecosystems resulting in both chronic and acute impacts which include higher frequency and severity of cyclones, warming sea surface temperatures, and ocean acidification. This study measured net ecosystem primary production (NEP) and net ecosystem calcification (NEC) on a reef flat after the most severe El Nino-driven mass bleaching event on Australia’s Great Barrier Reef (GBR) in 2016 and again in 2018 after another consecutive bleaching event in 2017. Our results indicate temporal changes in reef metabolism likely as result of both the continuing press disturbance of ocean acidification and severe pulse disturbances (cyclones and bleaching events). In 2016, NEP was within the range of values reported in past studies, however, it declined in 2018. NEC over a 12-h period was lower in 2016 than 2018; but when compared with past studies there was a severe decline in daytime net calcification from 2008–2009, to 2016 followed by an increase in 2018 (but still NEC remained lower than values reported in 2008–2009). Conversely, nighttime net calcification was similar to that reported in 2009 indicating nighttime dissolution did not increase over the past decade. Overall coral cover remained stable following recent disturbances, however, algal turf was the dominant benthic component on the reef flat, while calcifiers (corals and calcified algae) were minor components (<20% of total benthic cover). This study documented temporal changes in community function following major pulse disturbances (bleaching events and cyclones) within the context of ongoing OA at the same location over the last decade. Repeated pulse disturbances could jeopardize the persistence of the reef flat as a net calcifying entity, with the potential for cascading effects on other ecosystem services.

Continue reading ‘Multi-decadal change in reef-scale production and calcification associated with recent disturbances on a Lizard Island reef flat’

Mangrove lagoons of the Great Barrier Reef support coral populations persisting under extreme environmental conditions

Global degradation of coral reefs has increased the urgency of identifying stress-tolerant coral populations, to enhance understanding of the biology driving stress tolerance, as well as identifying stocks of stress-hardened populations to aid reef rehabilitation. Surprisingly, scientists are continually discovering that naturally extreme environments house established coral populations adapted to grow within extreme abiotic conditions comparable to seawater conditions predicted over the coming century. Such environments include inshore mangrove lagoons that carry previously unrecognised ecosystem service value for corals, spanning from refuge to stress preconditioning. However, the existence of such hot-spots of resilience on the Great Barrier Reef (GBR) remains entirely unknown. Here we describe, for the first time, 2 extreme GBR mangrove lagoons (Woody Isles and Howick Island), exposing taxonomically diverse coral communities (34 species, 7 growth morphologies) to regular extreme low pH (<7.6), low oxygen (7°C) conditions. Coral cover was typically low (0.5 m diameter), with net photosynthesis and calcification rates of 2 dominant coral species (Acropora millepora, Porites lutea) reduced (20-30%), and respiration enhanced (11-35%), in the mangrove lagoon relative to adjacent reefs. Further analysis revealed that physiological plasticity (photosynthetic ‘strategy’) and flexibility of Symbiodiniaceae taxa associations appear crucial in supporting coral capacity to thrive from reef to lagoon. Prevalence of corals within these extreme conditions on the GBR (and elsewhere) increasingly challenge our understanding of coral resilience to stressors, and highlight the need to study unfavourable coral environments to better resolve mechanisms of stress tolerance.

Continue reading ‘Mangrove lagoons of the Great Barrier Reef support coral populations persisting under extreme environmental conditions’

Deconvolving the long-term impacts of ocean acidification and warming on coral biomineralisation

Highlights

• Evaluation of temperature and pH effects in coral carbonate chemistry over 1939-2013.

• Coral calcifying fluid pH influenced by both, seawater pH and temperature.

• Temperature principal influence on calcifying fluid pH on seasonal scales.

• Long-term changes in calcifying fluid pH mainly influenced by seawater pH.

• Decline in carbonate ion and calcification consistent with ocean acidification.

Abstract

Identifying the long-term effects of ocean acidification (OA) and global warming on coral calcification has proven elusive yet has major implications for the continuing viability of coral reefs in the face of climate change. Here we address this question using seasonally and annually resolved boron proxies (11B/10B and B/Ca) of calcifying fluid (cf) pHcf and carbonate ion concentrations ([CO]cf) preserved in a long-lived Porites coral from the Great Barrier Reef (GBR). From 1939 to 2013 we find that the coral pHcf closely followed the decline in seawater pH of ∼0.1 units, but at a reduced rate of ∼60%, indicative of biological buffering. Of the decline in pHcf ∼82% is attributed to OA and ∼17% to the ∼0.5 °C long-term warming observed over this period. This long-term warming induced change in pHcf is consistent with the much larger seasonally modulated changes in pHcf where ∼4 to 6 °C seasonal changes in temperatures are accompanied by relatively large antithetic ∼0.1 changes in pHcf. Furthermore, we find that although the supply of dissolved inorganic carbon (DIC) of the coral cf has remained at constant elevated levels of ∼1.5 × seawater, there has been a significant long-term decline (4 to 11%) in the [CO]cf, due primarily to the OA-induced change in pHcf. This decline in [CO]cf, a critical parameter controlling calcification, is thus likely responsible for the ∼15% decline in coral calcification observed since 1939 and across the GBR generally.

Continue reading ‘Deconvolving the long-term impacts of ocean acidification and warming on coral biomineralisation’

Positive genetic associations among fitness traits support evolvability of a reef‐building coral under multiple stressors

Climate change threatens organisms in a variety of interactive ways that requires simultaneous adaptation of multiple traits. Predicting evolutionary responses requires an understanding of the potential for interactions among stressors and the genetic variance and covariance among fitness‐related traits that may reinforce or constrain an adaptive response. Here we investigate the capacity of Acropora millepora, a reef‐building coral, to adapt to multiple environmental stressors: rising sea surface temperature, ocean acidification, and increased prevalence of infectious diseases. We measured growth rates (weight gain), coral color (a proxy for Symbiodiniaceae density), and survival, in addition to nine physiological indicators of coral and algal health in 40 coral genets exposed to each of these three stressors singly and combined. Individual stressors resulted in predicted responses (e.g., corals developed lesions after bacterial challenge and bleached under thermal stress). However, corals did not suffer substantially more when all three stressors were combined. Nor were trade‐offs observed between tolerances to different stressors; instead, individuals performing well under one stressor also tended to perform well under every other stressor. An analysis of genetic correlations between traits revealed positive covariances, suggesting that selection to multiple stressors will reinforce rather than constrain the simultaneous evolution of traits related to holobiont health (e.g., weight gain and algal density). These findings support the potential for rapid coral adaptation under climate change and emphasize the importance of accounting for corals’ adaptive capacity when predicting the future of coral reefs.

Continue reading ‘Positive genetic associations among fitness traits support evolvability of a reef‐building coral under multiple stressors’

Heterotrophy of oceanic particulate organic matter elevates net ecosystem calcification

Coral reef calcification is expected to decline due to climate change stressors such as ocean acidification and warming. Projections of future coral reef health are based on our understanding of the environmental drivers that affect calcification and dissolution. One such driver that may impact coral reef health is heterotrophy of oceanic‐sourced particulate organic matter, but its link to calcification has not been directly investigated in the field. In this study, we estimated net ecosystem calcification (NEC) and oceanic particulate organic carbon (POCoc) uptake across the Kāneʻohe Bay barrier reef in Hawai‘i. We show that higher rates of POCoc uptake correspond to greater NEC rates, even under low aragonite saturation states (Ωar). Hence, reductions in offshore productivity may negatively impact coral reefs by decreasing the food supply required to sustain calcification. Alternatively, coral reefs that receive ample inputs of POCoc may maintain higher calcification rates, despite a global decline in Ωar.

Continue reading ‘Heterotrophy of oceanic particulate organic matter elevates net ecosystem calcification’

Ocean acidification effects on in situ coral reef metabolism

The Anthropocene climate has largely been defined by a rapid increase in atmospheric CO2, causing global climate change (warming) and ocean acidification (OA, a reduction in oceanic pH). OA is of particular concern for coral reefs, as the associated reduction in carbonate ion availability impairs biogenic calcification and promotes dissolution of carbonate substrata. While these trends ultimately affect ecosystem calcification, scaling experimental analyses of the response of organisms to OA to consider the response of ecosystems to OA has proved difficult. The benchmark of ecosystem-level experiments to study the effects of OA is provided through Free Ocean CO2 Enrichment (FOCE), which we use in the present analyses for a 21-d experiment on the back reef of Mo’orea, French Polynesia. Two natural coral reef communities were incubated in situ, with one exposed to ambient pCO2 (393 µatm), and one to high pCO2 (949 µatm). Our results show a decrease in 24-h net community calcification (NCC) under high pCO2, and a reduction in nighttime NCC that attenuated and eventually reversed over 21-d. This effect was not observed in daytime NCC, and it occurred without any effect of high pCO2 on net community production (NCP). These results contribute to previous studies on ecosystem-level responses of coral reefs to the OA conditions projected for the end of the century, and they highlight potential attenuation of high pCO2 effects on nighttime net community calcification.

Continue reading ‘Ocean acidification effects on in situ coral reef metabolism’


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Ocean acidification in the IPCC AR5 WG II

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