How does rising atmospheric CO2 affect marine organisms?

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The Resiliency of the Coral Communities on the Outer Slope of Moorea, French Polynesia
Adjeroud, M., Augustin, D., Galzin, R. and Salvat, B. 2002. Natural disturbances and interannual variability of coral reef communities on the outer slope of Tiahura (Moorea, French Polynesia): 1991 to 1997. Marine Ecology Progress Series 237: 121-131.

What was done
The authors assessed the interannual variability of corals and algae on the outer slope of the Tiahura sector of Moorea Island, French Polynesia, between 1991 and 1997, focusing on the impacts of "bleaching events caused by thermal stress ? when sea surface temperatures rose above 29.2C."

What was learned
With respect to corals, Adjeroud et al. report that "a severe decline in coral cover on the outer slope of Tiahura followed the bleaching event which started in March 1991," but they say that "the bleaching event in March 1994 did not have an important impact on coral cover," even though "the proportion of bleached colonies ? and the order of susceptibility of coral genera were similar in 1991 and 1994 (Gleason, 1993; Hoegh-Guldberg and Salvat, 1995)." In fact, they report that between 1991 and 1992, total coral cover dropped from 51.0% to 24.2%, but that "coral cover did not decrease between 1994 and 1995."

With respect to algae, the four scientists report that "the percent cover of macroalgae did not show any significant temporal variation during the 7-year period."

What it means
With respect to algae, Adjeroud et al. note that "several studies have documented an increase in turf and macroalgal covers following man-induced or natural disturbances," which observations have been of much concern to people who worry that such things, as well as the ongoing rise in the air's CO2 content, will favor macroalgal growth at the expense of corals. Fortunately, Langdon et al. (2003) have recently reported results they describe as "good news for undisturbed coral reefs in as much as rising atmospheric CO2 will not give macroalgal growth a boost and hasten the transformation of reef community structure from coral to algal dominance." And now we have the added good news from Adjeroud et al. that "the decrease in the percent cover of corals, which opens up new substrate, does not always result in an increase in macroalgal cover."

With respect to corals themselves, Adjeroud et al. say "it is generally believed that mortality rates following bleaching events are related to the intensity and duration of sea temperature anomalies (see Hoegh-Guldberg, 1999)," but they note that their results "do not fit this general rule." Indeed, they say their results "suggest that there is no direct relationship between the proportion of bleached colonies and the mortality at the end of the event, as bleached colonies have the capacity to recover (Williams and Bunkley-Williams, 1990; Gleason, 1993)." They also write that "another possible explanation of the low mortality following the bleaching event in 1994 is that most of the colonies in place in 1994 were those that survived the 1991 event or were young recruits derived from those colonies," noting that "one may assume that these coral colonies and/or their endosymbiotic zooxanthellae were phenotypically (acclimation) and possibly genotypically (adaptation) resistant to bleaching events (Rowan et al., 1997; Hoegh-Guldberg, 1999)," as per the concept discussed in the Symbiont Shuffling section of our website, which describes an effective means for coral colonies to cope with rising temperatures. Hence, they further state that "this result demonstrates the importance of understanding the ecological history of reefs (i.e., the chronology of disturbances) in interpreting the specific impacts of a particular disturbance."

Gleason, M.G. 1993. Effects of disturbance on coral communities: bleaching in Moorea, French Polynesia. Coral Reefs 12: 193-201.

Hoegh-Guldberg, O. 1999. Climate change, coral bleaching and the future of the world's coral reefs. Marine and Freshwater Research 50: 839-866.

Hoegh-Guldberg, O. and Salvat, B. 1995. Periodic mass-bleaching and elevated sea temperatures: bleaching of outer reef slope communities in Moorea, French Polynesia. Marine Ecology Progress Series 121: 181-190.

Langdon, C., Broecker, W.S., Hammond, D.E., Glenn, E., Fitzsimmons, K., Nelson, S.G., Peng, T.-S., Hajdas, I. and Bonani, G. 2003. Effect of elevated CO2 on the community metabolism of an experimental coral reef. Global Biogeochemical Cycles 17: 10.1029/2002GB001941.

Rowan, R., Knowlton, N., Baker, A. and Jara, J. 1997. Landscape ecology of algal symbionts creates variation in episodes of coral bleaching. Nature 388: 265-269.

Williams, E.H. and Bunkley-Williams, L. 1990. The worldwide coral reef bleaching cycle and related sources of coral mortality. Atoll Research Bulletin 335: 1-71.

Reviewed 24 December 2003