Not All Coral Bleaching Is Bad

In an Opinion article published in Global Change Biology, Suggett and Smith (2011) write that "observations of non and sublethal bleaching (and subsequent recovery) are arguably not as readily reported as those of lethal bleaching since (1) the convenient tools used to quantify bleaching yield major ambiguity as to the severity of bleaching; and (2) lethal bleaching events inevitably receive higher profile (media) attention and so are more readily reported." And because of these facts, they go on to express concern that "under-representation of non and sublethal bleaching signs may over-classify the severity of bleaching, under-estimate the potential resilience of reefs against environmental change, and thus ultimately limit (if not depreciate) the validity and effectiveness of reef management policies and practices."

The two UK researchers note, for example, that nonlethal coral discoloration, and by inference coral bleaching, "is and always has been a natural phenomenon that is driven by inherent system variability in environmental condition, such as temperature, light, sediment loading and salinity (Stimson, 1997; Brown et al., 1999; Fagoonee et al., 1999; Fitt et al., 2000), as well as biological interactions, including disease and microbial and corallivore activity (Rosenberg et al., 2007; Rotjan and Lewis, 2008)." Indeed, they report that it is commonplace for corals to "exhibit sublethal bleaching during extreme environmental conditions whereby mortality does not occur," and to "subsequently recover once ambient environmental conditions return." Most importantly of all, they write that "it is the frequency and extent of these non and sublethal processes that yield key evidence as to how coral species and reef systems will likely withstand environmental and thus climate change."

In concluding, Suggett and Smith say "we must move beyond the most common perception that discolored corals are always less healthy," because "nonlethal (and arguably sublethal) bleaching events are part of a natural rhythm that enables corals to successfully respond to environmental variability or indeed, in the case of corallivore and microbial activity, contributes to sustain trophic interactions and hence reef biodiversity."

Additional References
Brown, B.E., Dunne, R.P., Ambarsaril, Le Tissier, M.D.A. and Satapoomin, U. 1999. Seasonal fluctuations in environmental factors and variations in symbiotic algae and chlorophyll pigments in four indo-pacific corals. Marine Ecology Progress Series 191: 53-69.

Fagoonee, I., Wilson, H.B., Hassell, M.P. and Turner, J.R. 1999. The dynamics of zooxanthellae populations: a long-term study in the field. Science 283: 843-845.

Fitt, W.K., McFarland, F.K., Warner, M.E. and Chilcoat, G.C. 2000. Seasonal patterns of tissue biomass and densities of symbiotic dinoflagellates in reef corals and relation to coral bleaching. Limnology and Oceanography 45: 677-685.

Rosenberg, E., Koren, O., Reshef, L., Efrony, R. and Zilber-Rosenberg, I. 2007. The role of microorganisms in coral health, disease and evolution. Nature Reviews Microbiology 5: 355-362.

Rotjan, R.D. and Lewis, S.M. 2008. Impact of coral predators on tropical reefs. Marine Ecology Progress Series 367: 73-91.

Stimson, J. 1997. The annual cycle of density of zooxanthellae in the tissues of field and laboratory-held Pocillopora damicornis. Journal of Experimental Marine Biology and Ecology 214: 35-48.