Review: Diaz-Pulido G, McCook LJ, Dove S, Berkelmans R, Roff G, Kline DI, Weeks S, Evans RD, Williamson DH, Hoegh-Guldberg O (2009). Doom and boom on a resilient reef: Climate change, algal overgrowth and coral recovery. PLoS ONE, 4(4):e5239 1-9.

Feature Paper: Diaz-Pulido G, McCook LJ, Dove S, Berkelmans R, Roff G, Kline DI, Weeks S, Evans RD, Williamson DH, Hoegh-Guldberg O (2009). Doom and boom on a resilient reef: Climate change, algal overgrowth and coral recovery. PLoS ONE, 4(4):e5239 1-9.

Author Abstract: Background: Coral reefs around the world are experiencing large-scale degradation, largely due to global climate change, overfishing, diseases and eutrophication. Climate change models suggest increasing frequency and severity of warming-induced coral bleaching events, with consequent increases in coral mortality and algal overgrowth. Critically, the recovery of damaged reefs will depend on the reversibility of seaweed blooms, generally considered to depend on grazing of the seaweed, and replenishment of corals by larvae that successfully recruit to damaged reefs. These processes usually take years to decades to bring a reef back to coral dominance.

Methodology / Principal Findings: In 2006, mass bleaching of corals on inshore reefs of the Great Barrier Reef caused high coral mortality. Here we show that this coral mortality was followed by an unprecedented bloom of a single species of unpalatable seaweed (Lobophora variegata), colonizing dead coral skeletons, but that corals on these reefs recovered dramatically, in less than a year. Unexpectedly, this rapid reversal did not involve reestablishment of corals by recruitment of coral larvae, as often assumed, but depended on several ecological mechanisms previously underestimated.

Conclusions / Significance: These mechanisms of ecological recovery included rapid regeneration rates of remnant coral tissue, very high competitive ability of the corals allowing them to out-compete the seaweed, a natural seasonal decline in the particular species of dominant seaweed, and an effective marine protected area system. Our study provides a key example of the doom and boom of a highly resilient reef, and new insights into the variability and mechanisms of reef resilience under rapid climate change.


Note to Readers: Follow links above for author email, full article text, or the publishing scientific journal. Author notes in my review are in quotes.

Review: This paper is important because it deals with a subject that is becoming all too common on coral reefs worldwide: high temperatures cause coral bleaching (the expulsion and loss of symbiotic microalgae living within reef-building corals' tissue and without which, corals die); The symbiotic algae (zooxanthellae) don't return in time; The bleached corals die; Algae overgrow coral skeletons; Coralline algae cover goes down; Less recruitment area is available for coral larvae (which can't settle on filamentous algae); If coupled with overfishing of herbivores, the algae grows out of control (especially when also coupled with increased nutrients, as from agricultural and sewarage runoff); A phase shift occurs and the "coral" reef that once was is no longer and won't be again until environmental conditions return to normal.

This paper deals with the situation when just such a scenario occurs (corals bleaching and dying and their skeletons being overgrown by a seaweed; in this case, herbivores were present but the seaweed overgrowth was of a noxious seaweed that prevented herbivore grazing) but later, the corals still recover quicker than expected.

An important point to keep in mind is that this study occurred on the Great Barrier Reef, which is one of the best managed reef ecosystems in the world. Unlike the US Government, which has something like 40+ government agencies with a say in the fate of coral reefs under US jurisdictional waters, Australia is unique in having a single government agency (The Great Barrier Marine Park Authority) overseeing all aspects of recreation, fishing, use, and take on Australia's major reef.

As Raymundo and her team showed in our third review, healthy coral communities are better able to recover from stress. The same was seen in this study, where fragments of coral tissue that still remained were able to regrow and chemically kill microalgae in their path as they regrew on the skeletons where most of their tissue died back from. When coupled with a natural seasonal decline in the particular algae in question and healthy waters without overfishing, the coral reef was able to completely recover in less than three years.

As El Niño Southern Oscillation (ENSO) events increase in frequency due to anthropogenic climate change, coral reefs are bleaching more frequently. With major fisheries around the world near collapse stressors on reefs continue mostly unabated. Yet this paper shows that if there is proper management of reefs, their ability to recover from catastrophic events is relatively great. However, if catastrophic events keep occurring more frequently than the ability of corals to recover fully, ecosystem collapse may occur. And as seen in the Caribbean, once that happens, it can take decades for reefs to recover, if at all.

The take-home message is that coral reefs still have some surprises up their sleeves, with previously undocumented ways to recover still being found. However, for these protective mechanisms (likely acquired over millions of years) to "kick in" reefs have to be resilient in the first place, which they can only do when they are not under undue stress. Coral reefs seem relatively able to recover from brief, catastrophic events (such as a single hurricane or ENSO event) but are less able to cope when continuously bombarded with stressors, which can compound decline by leading to more disease and algal overgrowth.

Now that we've looked at enough examples of how coral reefs can decline, we'll look at the extent of coral decline throughout the Indo-Pacific region of highest coral diversity.