by Kimberly Coombs
In 1998, a mass coral bleaching event resulted from increased water temperatures due to climate change and impacted corals world wide. This event caused much of the coral cover to be greatly reduced as many corals have a narrow set of temperature ranges that they can survive, and most live near their upper thermal maximum; therefore, slight increases in temperatures can have negative affects on coral survivorship. Not much is known about the ability of corals to recover after coral bleaching events or the likelihood of the environment switching to an algae dominated environment.
Graham et al. (2015) conducted a study in order to identify reef recovery, the amount of coral cover being greater than macroalgal cover post-disturbance, or a regime shift, the amount of macroalgal cover being greater than coral cover post-disturbance, at the Seychelles reefs. This study observed 21 reef sites from 1994 to 2011 in which about 90% of the coral cover was lost in 1998. They found that 12 of the 21 reef sites were able to recover post-disturbance, yet it took about 10 years to see any major improvements in the amount of coral cover. On the other hand, the other 9 reef sites switched to a macroalgae dominated environment. Before the mass bleaching event, the macroalgae and coral cover percent were the same between the 12 reefs and 9 reefs, suggesting that the regime shift resulted from coral bleaching.
In order to evaluate and predict whether a reef recovery or a regime shift would result following a coral bleaching event, Graham et al. (2015) identified five specific factors that are good indicators of whether or not a reef will recover or undergo a regime shift: water depth, density of juvenile corals, nutrient conditions of the reef, reef’s initial structural complexity, and the herbivorous fish biomass. Their results showed that reef recovery has a greater chance of occurring when the density of juvenile corals is more than 6.2 m2, the reef is at least moderately structurally complex and at least 6.6 m deep, herbivorous fish biomass is low, and carbon:nitrogen ratios are high. Because it can be difficult to gather data on juvenile coral densities, nutrient levels, and herbivorous fishes biomass, the authors focused their attention on water depth and coral reef structural complexity, and were able to correctly predict whether a reef would recover or would undergo a regime shift 98% of the time, using only these predictors. Graham et al. (2015) also gathered data from six other countries that overall, these two predictors correctly recognize the trajectory a reef will endure after a bleaching episode.
Graham, N.A., Jennings, S., MacNeil, M.A., Mouillot, D., Wilson, S.K., 2015. Predicting climate-driven regime shifts versus rebound potential in coral reefs. Nature doi:10.1038/nature14140.