Coral Reef Oases

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Coral reefs everywhere are being threatened by climate change and human activity, but some are proving to be more resilient than others. These rare reef communities, termed “oases” by scientists, tend to have increased coral cover compared to neighboring communities. The paper, led by Stanford lecturer, Dr. Robin Elahi, highlights the importance of light attenuation, human activity, and sea surface temperature in determining the presence of these resilient reefs. Experts theorize that these oases could serve to repopulate nearby depleted reef communities. Conservation and restoration efforts that focus on these special reefs may prove to be an effective strategy in the ongoing battle to foster reef persistence in a changing ocean.

A Collaborative Study

In collaboration with an international team of ten scientists assembled through an award from the USGS Powell Center for Analysis and Synthesis, Drs. Ilsa B. Kuffner and Lauren T. Toth of the USGS examined the correlation between environmental predictors—factors that could impact coral health—and the presence of oasis communities. The group considered ten different environmental predictors including light attenuation (light absorption in the water), human population density, and sea-surface temperature variability using data obtained from the United States National Oceanic and Atmospheric Administration’s (NOAA) Coral Reef Conservation Program’s (CRCP) National Coral Reef Monitoring Program at 3766 randomly selected sample sites across the reefs of the United States and its territories.

The team studied the predictors across four different geographic extents that ranged from across oceans to individual islands. Researchers used an approach to detect if a predictor’s influence on oasis presence changed based on the spatial extent—meaning, for instance, over 1000s of kilometers as opposed to 10s of kilometers— because it is possible that the same predictor could positively associate with oasis occurrence at one extent but have the reverse effect at another.

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What is a Coral Reef "Oasis"?

The initial goal for the working group was to establish the definition of an “oasis” for a coral reef system—a framework that was first published in 2018. The current work considers the coral reef oasis concept at different spatial extents and asks whether specific environmental conditions are associated with their occurrence. Though coral reef oases may appear and function similarly in multiple locations, they operate in different situations in the Atlantic and Indo-Pacific Oceans where coral cover is very different. Coral cover is the proportion of the seafloor that is occupied by living coral. Coral reefs in the Indo-Pacific, on average, have much higher coral cover than their Atlantic counterparts. Even within these two ocean basins, regions exhibited considerable variability in the prevalence of oases.

Our new work highlights the need for careful consideration of the area targeted for conservation and management, because the categorization of oases in the first place, and their environmental correlates in the second place, depend on the defined area

--Dr. Robin Elahi of Hopkins Marine Station, Stanford University, and lead author on the study

The Right Balance: Light, Temperature, and Human Activity

With thresholds in place to identify coral reef oases, researchers were able to determine which environmental predictors were associated with the presence of oases. High light attenuation—a result of lower water clarity—was the only factor found to be positively associated with the occurrence of coral reef oases, regardless of spatial scale. Though clear water is often associated with thriving coral reefs, the present results suggest that less clear water can function like sunglasses to filter bright sunlight, which is particularly problematic for corals exposed to “hot water.”

This is somewhat counterintuitive, as it is well accepted that corals need sunlight for their algal symbionts to photosynthesize. Other work has shown that less intense light can help corals survive bleaching events caused by ocean heatwaves, and productive—therefore less clear—waters can also mean more food in the water column for corals to eat.

--Dr. Ilsa Kuffner, USGS Research Marine Biologist and co-author on the study

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While increased human activity is linked to a decline in reef health globally, these researchers found that its relation to oasis occurrence is dependent on spatial extent. Larger human population sizes were negatively related to oasis presence in all areas, but the effect was most prominent on the largest (cross-basin) and smallest (subregional) scales. It is unclear why oasis occurrence at some scales was more affected than at others, but the group explains that the effects of people can be more variable than other environmental predictors due to factors such as seasonal tourist populations and the fact that human population size may not accurately reflect the local stressors that are detrimental to coral health. 

Sea-surface temperature variability was positively associated with oasis occurrence at a small spatial scale. This supports evidence that corals are able to positively respond to regular changes in surrounding water temperature. With rising global ocean temperatures, corals that develop tolerance to these effects have shown to be less prone to bleaching. Scientists at the USGS hope the framework outlined in this study can ultimately be used on a local level to identify coral communities that are resilient to anthropogenic stressors.

Our work supports a balanced approach to reef conservation and management: keeping up the good work at local and regional scales is important while mankind continues to grapple with the global scale of the climate crisis,

--Dr. Ilsa Kuffner

Coral reef oases provide new hope for at-risk coral communities and those tasked with managing them. With the help of the models developed in this study, conservation groups worldwide can locate and prioritize oases in their efforts. A shift to preserve, restore, and connect these oases may maximize the resources of conservation groups, as healthy oases have shown to help support neighboring reef communities.

Though oases shouldn’t solely be relied upon to save entire reef communities, they could buy some much needed time while work is done to mitigate the effects of climate change

--Dr. Peter Edmunds, Professor at California State University at Northridge and co-author of the paper

This paper, "Scale dependence of coral reef oases and their environmental correlates" was recently published in the journal Ecological Applications. The team of authors included Dr. Robin Elahi of Stanford University, Dr. Peter J. Edmunds of California State University Northridge, Dr. Ruth D. Gates of University of Hawa’i at Mānoa (deceased), Dr. Brian B. Barnes of the University of South Florida, Dr. Iliana Chollett of Sea Cottage in Ireland, Dr. Travis A. Courtney of the Scripps Institution of Oceanography, Dr. James R. Guest of Newcastle University, Dr. Elizabeth A. Lenz University of Hawa’i at Mānoa/Hawai’i Sea Grant, Dr. T. Shay Viehman of the NOAA National Ocean Science, and Dr. Ivor D. Williams of the NOAA Pacific Islands Fisheries Center.

Additionally, from all the authors,

This contribution recognizes the life-long commitment to solution-oriented coral reef science of our friend and colleague Dr. Ruth Gates, who sadly died before this study was completed. Ruth was instrumental in starting and leading this work, and she would be very happy with this outcome.

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