Avoiding Coral Reef Functional Collapse Requires Local and Global Action

This paper uses modeling on Caribbean reefs to emphasize the need for both local action and a low-carbon economy to prevent further degradation of coral reefs. The authors find that no-take marine reserves (leading to protection of herbivorous fish) must be combined with low carbon emissions (keeping the global mean temperature below 2 degrees Celsius) to prevent erosion of coral skeletons. Additionally, they found that coral reefs must be initially relatively healthy (they use a measure of 20% coral cover) to start, even with lowered carbon emissions and no-take reserves. This paper provides a clear message that global action to reduce carbon emissions must go hand-in-hand with local efforts, such as no-take reserves and watershed protection.

Author: Kennedy, E.V., C.T. Perry, P.R. Halloran, R. Iglesias-Prieto, C.H.L. Schonberg, M. Wisshak, A.U. Form, J.P. Carricart-Ganivet, M. Fine, C.M. Eakin, and P.J. Mumby
Year: 2013
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Current Biology  23(10): 912–918

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Projected Changes to Growth and Mortality of Hawaiian Corals over the Next 100 Years

This study investigates the use of modeling techniques to quantitatively examine rates of coral cover change due to these effects. Broad-scale probabilities of change in shallow-water reef-building coral cover in the Hawaiian Archipelago for years 2000–2099 were calculated using a single middle of the range of future greenhouse gas emissions scenario.

Model results suggest that under a regime of warming temperatures over the 21st century, mean growth rates of surviving corals have a high likelihood of increasing significantly towards the northernmost end of the Hawaiian Archipelago (e.g. Kure, Midway, Pearl and Hermes Atolls); increasing to a lesser degree towards the center of the chain (e.g. Maro Reef, French Frigate Shoals) and remain roughly stable to the South (the main Hawaiian Islands and Johnston). However, the contribution of increasing growth rates to increasing coral cover will most likely be more than offset by mortality associated with increasing incidence of episodic heat stress events (coral bleaching), especially in the northern end of the archipelago, where projected probabilities of episodic mortality are much higher.

If Hawaiian corals are not able to increase their tolerance to future levels of heat stress, model output suggests it is extremely unlikely that viable coral populations will exist in the shallow waters of the Hawaiian Archipelago in 2100. Despite large uncertainties, the analysis quantitatively illustrates that a large decline in coral cover is highly likely in the 21st Century, but that there are significant spatial and temporal variances in outcomes, even under a single climate change scenario.

Author: Hoeke, R.K., P.L. Jokiel, R.W. Buddemeier, and R.E. Brainard
Year: 2011
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PLoS ONE 6(3): e18038. doi: 10.1371/journal.pone.0018

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Coral Reefs Under Rapid Climate Change and Ocean Acidification

Levels of atmospheric CO2 continues to rise and threaten coral reefs globally. This is because atmospheric CO2 reacts with water in the ocean to produce carbonic acid which in turn forms bicarbonate ions that react with carbonate ions to produce more bicarbonate ions (reducing availability of carbonate in the ocean). Declines in available carbonate can reduce the calcification of coral reefs and marine organisms. The authors describe the consequences of increased atmospheric CO2 and subsequent warming, as predicted. Even under the best case scenario, ocean acidification will likely cause contractions of carbonate coral reefs if CO2 levels exceed 500 ppm. Although these global threats require changes at a global scale, local factors such as poor water quality, coastal pollution, and overexploitation of certain organisms, should be reduced to lesson the overall stressors to coral reef communities. The authors also suggest that healthy grazing populations should help to improve a coral reefs ability to bounce back from future disturbances; thus, healthy herbivore populations should be managed for explicitly.

Author: Hoegh-Guldberg, O.,  P.J. Mumby, A.J. Hooten, R.S. Steneck, P. Greenfield, E. Gomez, C.D. Harvell, P.F. Sale, A.J. Edwards, K. Caldeira, N. Knowlton, C.M. Eakin, R. Iglesias-Prieto, N. Muthiga, R.H. Bradbury, A. Dubi, and M.E. Hatziolos
Year: 2007
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Science 318(5857): 1737-1742. doi: 10.1126/science.1152509

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High CO2 Enhances the Competitive Strength of Seaweeds Over Corals

This study presents the case that increasing concentrations of atmospheric CO2 may be an additional process driving a shift from corals to seaweeds on reefs. The authors tested the combined effects of ocean acidification and algal–coral competition on coral survivorship and growth through simultaneous manipulation of competitors and CO2 levels in an experiment carried out on Heron Island Research Station (HIRS), southern GBR.

In the experiments, the authors demonstrate that ocean acidification enhances the ability of a common algae (Lobophora) to kill, and potentially out-compete, a coral (Acropora). Only corals in contact with live seaweeds showed significant mortality, and mortality was exacerbated by elevated pCO2. This indicates that coral mortality can be attributed to the presence of, and interaction with, seaweeds. Accordingly, increased coral mortality with increasing pCO2 is therefore likely to be a consequence of CO2, enhancing the competitive strength of the seaweeds.

These results suggest that coral (Acropora) reefs may become increasingly susceptible to seaweed proliferation under ocean acidification, and processes regulating algal abundance (e.g. herbivory) will play an increasingly important role in maintaining coral abundance.

Author: Diaz-Pulido, G., M. Gouezo, B. Tilbrook, S. Dove, and K.R.N. Anthony
Year: 2011
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Ecology Letters 14: 156-162. doi:10.1111/j.1461-0248.2010.01565.x

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Ocean Acidification and Warming will Lower Coral Reef Resilience

In this study, the authors quantitatively analyzed how different combinations of CO2 and fishing pressure on herbivores will affect the ecological resilience of a benthic reef community consisting of three common groups: branching corals, fleshy macroalgae and turfs (free space for coral and algal colonization). Resilience was defined by the community’s capacity to maintain and recover to coral-dominated states. They developed a dynamic community model that was run for changes in sea surface temperature and water chemistry predicted by the rise in atmospheric CO2 projected from the IPCC’s fossil-fuel intensive scenario during this century. The findings demonstrate, using on the dynamics of a species pair of corals (Acropora) and fleshy macroalgae (Lobophora), that the effects of ocean acidification and warming on coral growth and mortality will have important impacts on coral reef resilience under increasing CO2. Specifically, by reducing coral growth (due to acidification) and survivorship (due to warming), increasing CO2 will lower the threshold value at which local and regional processes like herbivore overfishing and nutrification drive the study community from predominantly coral-dominated to predominantly algal-dominated states. Therefore, warming, acidification, overfishing and nutrification all drive the dynamics of the system in the same direction, suggesting that reduced coral resilience in a high-CO2 world is likely to be a consequence of both global threats and local-scale disturbances

Author: Anthony, K.R.N., J.A. Maynard, G. Diaz-Pulido, P.J. Mumby, P.A. Marshall, L. Cao, and O. Hoegh-Guldberg
Year: 2011
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Global Change Biology 17(5): 1798-1808. doi:10.1111/j.1365-2486.2010.02364.x

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