What was done
The authors, as they describe it, "compared variations in spatial and taxonomic patterns between two bleaching events at the scale of an island (Moorea Island, French Polynesia)." More specifically, they write that "nine stations involving three locations (Haapiti, Tiahura, and Viapahu) and three depths (6, 12 and 18 m) have been routinely surveyed for various scientific purposes since 2001," as described by Penin et al. (2007a, 2010). And more specifically still, they write that "bleaching intensity was measured at these nine stations in 2002 and 2007," being initiated "two weeks following the first signs of bleaching (Penin et al., 2007b)" in each of the two years. And they state that "this sampling strategy was repeated by the same observer in 2007." Also in both cases, they focused on six coral genera (Acropora, Montipora, Montastrea, Pavona, Pocillopora and Porites), which together accounted for fully 88% of the total coral assemblage.

What was learned
First of all, Penin et al. demonstrated that "thermal stress was similar between the two years studied." Then they reported that "the bleaching intensity was lower in 2007 (25-49%) than in 2002 (39-72%)," and that "the response to elevated temperature was delayed in the latter year." In addition, they report that "the outer slopes at Moorea Island are located in an oligotrophic oceanic environment and are not under terrestrial influence, which makes it unlikely that there was any effect of nutrients on the response of the corals, as has been proposed for other reef ecosystems (Wooldridge and Done, 2009)."

What it means
"Based on the above analysis," in the words of the three researchers, "it is likely that the observed decrease in stress response was the result of acclimatization of the coral/algal holobionts (Berkelmans et al., 2004; Maynard et al., 2008) or an influx of thermo-tolerant colonies between 2002 and 2007." And providing further support for their conclusions, they note that several "similar decreases in susceptibility to thermal stress have been documented between successive bleaching events, including between 1991 and 1994 at Moorea Island (Adjeroud et al., 2002), between 1998 and 2002 on the Great Barrier Reef in Australia (Maynard et al., 2008), and between 1982-83 and 1997-98 in Panama (Glynn et al., 2001), Costa Rica (Jimenez et al., 2001), and at the Galapagos Islands (Podesta and Glynn, 2001)."

References
Berkelmans, R., De'ath, G., Kininmonth, S. and Skirving, W.J. 2004. A comparison of the 1998 and 2002 coral bleaching events on the Great Barrier Reef: spatial correlation, patterns and predictions. Coral Reefs 23: 74-83.

Glynn, P.W., Mate, J.L., Baker, A.C. and Calderon, M.O. 2001. Coral bleaching and mortality in Panama and Ecuador during the 1997-1998 El Niņo-Southern Oscillation event: Spatial/temporal patterns and comparisons with the 1982-1983 event. Bulletin of Marine Science 69: 79-109.

Jimenez, C., Cortes, J., Leon, A. and Ruiz, E. 2001. Coral bleaching and mortality associated with the 1997-1998 El Niņo in an upwelling environment in the eastern Pacific (Gulf of Papagyo, Costa Rica). Bulletin of Marine Science 69: 151-169.

Maynard, J.A., Anthony, K.R.N., Marshall, P.A. and Masiri, I. 2008. Major bleaching events can lead to increased thermal tolerance in corals. Marine Biology 155: 173-182.

Penin, L., Adjeroud, M., Pratchet, M.S. and Hughes, T.P. 2007a. Spatial distribution of juvenile and adult corals around Moorea (French Polynesia): implications for population regulation. Bulletin of Marine Science 80: 379-389.

Penin, L., Adjeroud, M., Schrimm, M. and Lenihan, H. 2007b. High spatial variability in coral bleaching around Moorea (French Polynesia): patterns across locations and water depths. Comptes Rendus Biologies 330: 171-181.

Penin, L., Michonneau, F., Baird, A.H., Connolly, S.R., Pratchett, M.S., Kayal, M. and Adjeroud, M. 2010. Early post-settlement mortality and the structure of coral assemblages. Marine Ecology Progress Series 408: 55-64.

Podesta, G.P. and Glynn, P.W. 2001. The 1997-98 El Niņo event in Panama and Galapagos: an update of thermal stress indices relative to coral bleaching. Bulletin of Marine Science 69: 43-59.

Wooldridge, S.A. and Done, T.J. 2009. Improved water quality can ameliorate effects of climate change on corals. Ecological Applications 19: 1492-1499.