In 1997, the first global survey of coral reefs was carried out by teams of volunteer scientists using an innovative standard method to measure coral reef health (Hodgson, 1999). Reef Check was the name given to the one-time survey timed to coincide with the first International Year of the Reef. The Reef Check training and surveys proved so popular that scientists in many countries asked if they could form teams and participate in 1998. By chance, the first global coral bleaching event occurred during 1998-99, wiping out corals in dozens of countries. So, with Reef Check teams already in place and new teams joining, they were able to document the damage (Wilkinson and Hodgson, 1999), and many more scientists and coral reef managers began to appreciate the value of the annual surveys. As a result, the Reef Check organization was born and has monitored reefs worldwide for 25 years now.
Sadly, coral reefs have been damaged by an increasing list of impacts since 1997, including increasingly frequent bleaching events and cyclones/hurricanes, overfishing, new diseases, and pollution. In most cases, scientists trying to determine the cause of reef declines in an area such as Florida or the Great Barrier Reef, are faced with interpreting several of these impacts at the same time. How is it possible to pinpoint which impacts are the most important? A standardized method of studying reefs globally helps answer these questions. This is the advantage of the Reef Check tropical database; it contains over 20 years of data using one standard method and participants, globally, are trained and certified in this one method.
Why is a standard method important? Scientists have studied the results of surveys using different types of measurements and found that they can differ a great deal. For example, counting crown-of-thorns starfish from the surface in water 10 m (~30 ft) deep will produce much lower numbers than by searching for them at depth. The Reef Check dataset allows researchers to avoid incorrect conclusions due to differences in how the data were collected by different teams or because protocols have changed over time. The two-decade long dataset allows researchers to track trends over this unusually long time period during which so many changes have occurred on reefs. So, it is no surprise that since 1999, dozens of peer-reviewed scientific publications have used Reef Check’s tropical dataset to answer questions at a local reef scale, at regional and even a global scale.
The most recent paper published on May 27th in the top scientific journal “Science” is getting a lot of attention because it provides a clear answer to an important question. Lead author, Mary K. Donovan, wanted to know what variables might be affecting survival of corals following bleaching. An expert in statistics, she and co-authors used a technique called “Bayesian hierarchical modeling” to determine which of 19 variables were associated with coral survival or death one year after bleaching. The variables included purely physical ones such as water depth, water temperature and wave exposure as well as biological factors such as the numbers of parrot fish, sea urchins and macro-algae. Not surprisingly, the strongest association with corals dying was a measure of hot water exposure called Degree Heating Weeks that includes a temperature and duration component. This had been shown by other studies, but the global nature as well as the long time series of the data led to some other interesting and important discoveries.
The key discovery is that as the amount of macro-algae increases on a reef above about 5%, coral mortality following bleaching increases. By macro-algae we mean any species of fleshy algae that has grown taller than 3 cm (about 1 inch). When the macroalgae occupies more than 25% of the reef surface, the mortality of corals increases much more. This finding is important because it is possible to control the amount of algae on a reef by careful fisheries management of herbivores and even direct removal of algae. But as overfishing has become an increasing problem, more reefs have become dominated by algae due to a lack of herbivory, especially in the Caribbean where a typical reef now has about 20-30% of its surface covered by macro-algae. Prior to 1980, there is little quantitative data on macroalgal cover on reefs, but those reefs surveyed had less than about 5% cover (Jackson et al., 2014). Increased mortality of corals following bleaching was also associated with more long-spined black sea urchins. Sea urchins are helpful herbivores at low densities but at high densities they can damage corals by scraping so much that they can destabilize coral heads.
The results of this study will be immediately useful to coral reef managers who have been saddened as they have watched their favorite reefs bleach and die. The primary solution for stopping coral bleaching is reversing climate change by reducing greenhouse gas emissions at the global level. But this study highlights some other ways in which local managers and communities can act. Any intervention that helps corals survive bleaching, such as increasing herbivorous fish populations by reducing fishing, will be extremely useful in allowing corals to survive the onslaught of hot water.
We look forward to seeing more excellent publications from the expert team of scientists that Mary brought together.
Donovan, MK , DE Burkepile, C Kratochwill, T Shlesinger, S Sully, TA.Oliver, G Hodgson, J Freiwald, RV Woesik. 2021. Local conditions magnify coral loss following marine heatwaves. Science. Vol. 372, Issue 6545, pp. 977-980.
Hodgson, G. 1999. A global assessment of human effects on coral reefs. Marine Pollution Bulletin. 38 (5) 345-355.
Jackson, JBC, MK Donovan, KL Cramer, VV Lam (editors). 2014. Status and Trends of Caribbean Coral Reefs: 1970-2012. Global Coral Reef Monitoring Network. IUCN, Switzerland, 306 p.
Wilkinson, C. and G. Hodgson. 1999. Coral reefs and the 1997-1998 mass bleaching and mortality. Nature and Resources. 35(2):17-25.