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Caught in the Middle: Combined Impacts of Shark Removal and Coral Loss on the Fish Communities of Coral Reefs

Researchers used long-term monitored reefs off the coast of north-western Australia to understand how fishing changed shark communities in coral reef ecosystems. Two uninhabitated areas with atoll-like reefs with differing management regimes, closed marine protected area and open to shark fishing, were studied. Researchers found evidence that the loss of sharks due to fishing can have a cascading effect on the food chain with effects on mesopredators and primary consumers. Both habitat and shark fishing, individually and interactively, affected reef fish community composition and trophic structure across sites. Bottom-up processes such as bleaching and cyclones appear to affect herbivores, planktivores, and corallivores, but do not affect carnivores. Healthy reef shark populations should be a target of management of coral reefs, since the presence of sharks may promote the abundance of herbivores.

Author: Ruppert, J.L.W., M.J. Travers, L.L. Smith, M-J Fortin, and M.G. Meekan
Year: 2013
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PLoS ONE 8(9): e74648. doi:10.1371/journal.pone.0074648.

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Mass Coral Bleaching in 2010 in the Southern Caribbean

The authors, Alemu and Clement, monitored the bleaching response at 3 sites across Tobago during a bleaching event in 2010. The purpose was to find nodes of reef resilience in Tobago by identifying taxa resilient to bleaching. This will assist local coral reef managers in the decision making process by recognizing reefs that should be conserved. For 6 months, the authors monitored about 650 colonies (composed of 30 taxa) at three sites, and gained an understanding of which influences (freshwater lenses, water temperature, terrestrial run-off) may be most important at those sites. The study highlighted the importance of understanding local variables in the management of reefs.

Author: Alemu I, J. B. and Y. Clement
Year: 2014
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PLoS ONE 9(1): e83829. doi:10.1371/journal.pone.0083829

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Restrictions in Marine Protected Areas in the Coral Triangle – The coral reef crisis: The critical importance of <350 ppm CO2

The authors of this paper outline the grim state and future of coral reef ecosystems, and the consequential domino effects to follow for other systems associated with reefs. Mass bleaching and mortality are identified as the current crisis to corals, and based on the current rate of increase in global CO2 emissions (now exceeding 3% per year), most reefs world-wide are committed to an irreversible decline. Three issues of importance to the future of coral reefs are highlighted: (1) the role of multiple stressors and synergies; including sea level rise, storm impacts, fisheries impacts, water quality, and biotic responses, (2) the nature of resilience, and (3) the importance of domino effects. While the outlook for reefs in the fact of today’s rapid global warming is exceptionally serious, the authors provide remedial options for management interventions that will increase reef resilience, including: a) reduce the harvest of herbivorous fish to sustainable levels, b) protect sharks and other top predators, c) manage all aspects of water quality, and d) diminish direct anthropogenic impacts and stressors.

Author: Veron, J.E.N., O. Hoegh-Guldberg, T.M. Lenton, J.M. Lough, D.O. Obura, P. Pearce-Kelly, C.R.C. Sheppard, M. Spalding, M.G. Stafford-Smith, and A.D. Rogers
Year: 2009
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Marine Pollution Bulletin 58: 1428-1436. doi:10.1016/j.marpolbul.2009.09.009

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Do Some Corals Like It Hot?

The Adaptive Bleaching Hypothesis is a controversial theory that states that stress resistant coral-zooxanthellae associations can develop from frequent and severe environmental stress. This hypothesis is reviewed and future directions for research are suggested. Discussions include the following topics, spatial and temporal variation in Symbiodinium, costs to Symbiodinium D due to heat tolerance, and future directions. This paper is critical reading for any MPA practitioner dealing with coral reef persistence as coral-Symbiodinium interactions impact reef resilience.

Author: Sotka, E.E. and R.W. Thacker
Year: 2005
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Trends in Ecology and Evolution 20(2): 59-62. doi:10.1016/j.tree.2004.11.015

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Do Fluctuating Temperature Environments Elevate Coral Thermal Tolerance?

While differences in thermal tolerance among regions with distinct annual mean temperatures were first recognized as evidence of coral thermal adaptation or acclimatization, thermal variation on much smaller spatial and temporal scales also appears to affect coral thermal tolerance. In this study, the authors measured the timing of high thermal pulses in a pair of pools in Ofu, American Samoa, and tested whether a coral population exposed to more frequent and more extreme pulses was associated with higher thermal tolerance relative to a population exposed to more moderate variation in the bleaching sensitive coral Acropora hyacinthus. The results showed that corals from the thermally variable pool, all of which hosted heat-resistant symbionts, evidenced lower mortality and less severe declines in photochemical efficiency than corals from the thermally moderate pool, regardless of symbiont type. The results highlighted the importance of monitoring multiple, potentially interacting mechanisms when considering the potential for reef corals to resist rising temperatures.

Author: Oliver, T.A. and S.R. Palumbi
Year: 2011
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Coral Reefs 30(2): 429-440. doi:10.1007/s00338-011-0721-y

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Large-scale Stress Factors Affecting Coral Reefs: Open Ocean Sea Surface Temperature and Surface Seawater Aragonite Saturation Over the Next 400 Years

This study seeks to investigate three IPCC (Intergovernmental Panel on Climate Change) representative pathways (RCP) and their differing impacts of environmental stressors to coral reefs including rising sea surface temperatures and changes in seawater aragonite saturation, which relates to atmospheric carbon and ocean acidification.

A climate model, the UVic Earth System Climate Model, was used in this study to predict sea surface temperatures, open water aragonite saturation levels, and susceptibility of coral reefs to thermal stress over the next 400 years. Results of this work showed that by year 2030, 66–85% of the reef locations considered will become ‘thermally marginal’ and experience severe bleaching events at least once every 10 years.

Regardless of which RCP concentration was used in researcher’s simulations, virtually every reef considered in this study (97%) would experience severe thermal stress by 2050. Study simulations also showed that annual mean seawater aragonite thresholds will be exceeded within the first half of this century.

Author: Meissner, K. J., T. Lippmann, and A. Sen Gupta
Year: 2012
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Coral Reefs 31(2): 309-319. doi:10.1007/s00338-011-0866-8

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Preparing to Manage Coral Reefs for Ocean Acidification: Lessons from Coral Bleaching

The authors examine the lessons learned from the implementation of management strategies to address the impacts of coral reef bleaching and how these strategies may be modified to address the impacts of ocean acidification. Because stabilizing CO2 emissions is the most critical need to address ocean acidification, and this is out of the scope of reef managers’ jobs, it is important to manage for local stressors and protect resilient areas.

The authors have several recommendations: First, because evidence suggests local-scale processes and local stressors have more impact on ocean chemistry, it is important to manage for local stressors (such as land-based sources of pollution and over-fishing). Second, the priority areas to protect include the most resilient, least vulnerable sites. These sites may already be adapted to large variations in pH, may have surrounding seagrass beds, or be connected to “source reefs” to maximize larval influx. Finally the authors recommend further research into the impacts of lowered pH on reef species and on how local and regional processes can affect ocean chemistry.

Author: McLeod, E., K.R.N. Anthony, A. Andersson, R. Beeden, Y. Golbuu, J. Kleypas, K. Kroeker, D. Manzello, R.V. Salm, H. Schuttenberg, and J.E. Smith 
Year: 2013
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Frontiers in Ecology and the Environment 11:20-27. doi:10.1890/110240

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A Framework for Comparing Coral Bleaching Thresholds

The authors found that the “MMMmax” method for predicting bleaching has the highest predictive power at all spatial and temporal resolutions.Coral reef managers who subscribe to NOAA’s Coral Reef Watch can better rely on real-time bleaching alerts.

Author: Logan, C.A., J.P. Dunne, C.M. Eakin, and S.D. Donner 
Year: 2012
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Proceedings of the 12th International Coral Reef Symposium, Cairns, Australia, 9-13 July 2012, 10A Modelling reef futures

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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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Email for the full article: resilience@tnc.org

Science 318(5857): 1737-1742. doi: 10.1126/science.1152509

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