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"Doom and Boom" on Australia's Great Barrier Reef
Volume 12, Number 33: 19 August 2009

Historically, "most degraded reefs," in the words of Diaz-Pulido et al. (2009), "have either failed to recover from events such as coral bleaching and other human induced disturbances, or [they] have taken several years to decades to return to pre-disturbance conditions." Thanks to the ten scientists' astute observations, however, we now know that some very dramatic exceptions can - and do - occur.

In early 2006, mass bleaching of corals on inshore reefs of the Keppel Islands in the southern Great Barrier Reef (GBR) caused high coral mortality, with severe bleaching affecting 77-95% of coral colonies (Weeks et al., 2008; Jones et al., 2008). This event, according to Diaz-Pulido et al., was followed by "an extraordinary bloom of the brown seaweed Lobophora variegata," which they say was "unprecedented in magnitude on the GBR" and "exacerbated coral mortality by overgrowing stressed coral tissue."

To most everyone's way of thinking, this tremendous one-two punch of extreme misfortune should have spelled The End for the "glory days" of the Keppel Islands' corals; but the spectacular sequel to the devastating events proved even better than the original tragedy.

Following the natural seasonal decline in L. variegata, which in some places had increased its cover by 200-300% by August 2006, "the cover of branching Acropora corals at most sites showed an extremely rapid recovery," according to the Australian researchers, "reaching pre-bleaching levels by December 2006-April 2007 ca 12-14 months after the onset of bleaching."

"Unexpectedly," as they describe it, "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." Most interesting, in this regard, was "the 'phoenix effect' in which apparently dead coral branches regenerate live tissue (Krupp et al., 1993; Jokiel et al., 1993; Riegl and Piller, 2001)." This "remnant surviving coral tissue," as Diaz-Pulido et al. continue, "rapidly expanded upwards along the dead coral branches and actively overgrew L. variegata, as well as a range of other algal types, including filamentous algal turfs, fleshy seaweeds and crustose coralline algae," resulting in "a 'seaweed sandwich' with algae engulfed between new and old layers of [coral] skeleton."

Noting that reefs of the Keppel Islands "have shown rapid recovery of coral dominance, despite repeated coral bleaching events (1998, 2002, and 2006), severe flood plumes (e.g. 1991, 2008), and dense algal overgrowth," Diaz-Pulido et al. conclude that these and other reefs that are "able to rapidly recover abundant corals may serve as key refugia, or sources of larvae for reef recovery at broader scales," and that the unique phenomena they documented in their research "may well be critical to the overall resilience and persistence of coral reef ecosystems globally."

Yes, the world is full of surprises; and one should never count the good guys out. The planet's oh-so-sensitive corals appear to have a few tricks up their sleeves that we are only now beginning to appreciate, not the least of which is rapid recovery from apparent death.

Sherwood, Keith and Craig Idso

Diaz-Pulido, G., McCook, L.J., Dove, S., Berkelmans, R., Roff, G., Kline, D.I., Weeks, S., Evans, R.D., Williamson, D.H. and Hoegh-Guldberg, O. 2009. Doom and boom on a resilient reef: Climate change, algal overgrowth and coral recovery. PLos ONE 4: e5239.

Jokiel, P.I., Hunter, C.I., Taguchi, S. and Watarai, I. 1993. Ecological impact of a freshwater "reef kill" in Kaneohe Bay, Oahu, Hawaii. Coral Reefs 12: 177-184.

Jones, A.M., Berkelmans, R., van Oppen, M.J.H., Mieog, J.C. and Sinclair, W. 2008. A community shift in the symbionts of a scleractinian coral following a natural bleaching event: field evidence of acclimatization. Proceedings of the Royal Society of London, Series B 275: 1359-1365.

Krupp, D.A., Jokiel, P.I. and Chartrand, T.S. 1993. Asexual reproduction by the solitary scleractinian coral Fungia scutaria on dead parent corolla in Kaneohe Bay, Oahu, Hawaiian Islands. Proceedings of the Seventh International Coral Reef Symposium 1: 527-534.

Riegl, B. and Piller, W.E. 2001. "Cryptic" tissues inside Acropora frameworks (Indonesia): a mechanism to enhance tissue survival in hard times while also increasing framework density. Coral Reefs 20: 67-68.

Weeks, S.J., Anthony, K.R.N., Bakun, A., Feldman, G.C. and Hoegh-Guldberg, O. 2008. Improved predictions of coral bleaching using seasonal baselines and higher spatial resolution. Limnology and Oceanography 53: 1369-1375.