Reef State and Resilience in a Climatically Changing Environment

by Kimberly Coombs

Climate change has been impacting coral reefs all over the world, and many models have been created to predict how coral reefs are going to respond to global climate change, in particular, global warming. It has been reported that the effects of greenhouse gas emissions have reduced coral reefs resilience, causing them to be more susceptible to stressors in their environment. As a result, coral reef state, the percent of coral cover, has begun to be greatly lessened, with a noticeable shift from coral dominated environments to macroalgae environments.

Bozec and Mumby (2015) conducted a study to look at the Caribbean coral reefs’ resilience to increases in sea surface temperatures. There are acute and chronic stressors that arise from sea surface temperature increases. Acute stresses may cause unexpected mortality amongst coral reefs. Chronic stresses may influence the carbonate skeleton rate of extension, which impacts the mortality rates of size-dependent corals, rates of recovery among populations, and ecological interactions of coral reefs. Bozec and Mumby looked specifically at whether or not acute and chronic stresses from sea surface temperature rises would interact antagonistically, synergistically, or additively.

In order to evaluate the impact of rising sea surface temperatures, Bozec and Mumby developed a model of coral populations that could simulate the impacts of chronic and acute stresses. The model allows for chronic and acute stresses to be assessed together and separately. The model is also individual-based and has taken a mid-depth Caribbean coral reef as its model. The outputs of the model include the percent cover of the algal and coral species (reef state) from 2010 to 2060. Resilience of the coral reef was quantified every ten years from 2010 to 2060 as the probability that the coral reef was on a recovery trajectory. The thermal stresses encompassed in the model followed the RCP8.5 greenhouse gas emissions trajectory, which is simply the current greenhouse gas emission trends today and assumes there will be no mitigation strategies put in place.

Bozec and Mumby found that under no thermal stresses from increased sea surface temperatures, coral cover is able to proliferate over time. However, acute stress causes the reef state to show a significant reduction in coral cover by 2025 and chronic stress causes the reef state to show a significant reduction in coral cover by 2035. In terms of reef resilience, acute stress reduces coral reef’s resilience and by 2060, coral reefs have less than a 50% chance of being able to recover. Chronic stress has less of an impact, as coral reefs have at least a 70% chance of recovering by 2060. When chronic stress and acute stress both impact the coral reefs, their affect is additive on reef state, and their interactions show synergism on coral reef resilience. Together, they cause a far greater affect on coral reefs with recovery becoming almost impossible by the year 2040.

The authors attribute these results to the reactions of corals to global warming. The reef state of corals is extremely sensitive to bleaching as bleaching results in loss of medium-sized to large corals that tend to have the greatest influence in coral cover. Bozec and Mumby believe that the two stressors are additive because of the way they act on the corals. Chronic stress acts directly on growth rate, reducing it with repeated bleaching events, and acute stress acts directly on mortality of corals; therefore, the interaction between these two stressors is indirect. For resilience, the acute and chronic stress interaction is synergistic due to both stressors working together to drive the coral reef towards the point of not being able to recover. Resilience and reef state are thus important indicators of a coral reef’s health.

Bozec, Y., Mumbery, P.J., 2015. Synergistic impacts of global warming on the resilience of coral reefs. Philosophical Transactions of the Royal Society B, 370: 20130267.

http://dx.doi.org/10.1098/rstb.2013.0267

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