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The Significance of Size and Age in the Bleaching of Coral Colonies
Reference
Bena, C. and van Woesik, R.  2004.  The impact of two bleaching events on the survival of small coral colonies (Okinawa, Japan).  Bulletin of Marine Science 75: 115-125.

What was done
The authors evaluate what is known about the bleaching of large and small coral colonies along the western coast of Okinawa, Japan, during the summers of 1998 and 2001, based on data acquired in Oct-Dec 1998, Jun-Sep 1999, Jan-Mar 2000, Jul-Aug 2000 and Nov-Dec 2001, as well as pertinent related findings of others scientists.

What was learned
Bena and van Woesik report some interesting observations.  First, they found that "the percentage cover of corals declined in the summer of 1998 when exposed to water temperatures 2.8C above average, but the total number of juvenile coral colonies only declined significantly at shallow depths," which finding, in their words, "agrees with Loya et al. (2001) who showed a high survival of small Acropora colonies on reefs farther north of our study sites, but an almost complete elimination of adult Acropora spp. in 1998 during elevated SST."  In addition, they note that "after the 1998 bleaching event there were no adult Acropora colonies surviving at either site, yet the number of small corals on the shallow reefs increased gradually over time," replenished by larvae that they conclude "were most likely transported from distant reefs."

The two scientists also observed that "on the nearshore fringing reefs where [their] study was conducted, small Acropora and pocilloporid communities were less affected by the 2001 thermal anomaly than by the 1998 event, although both anomalies were of similar intensity and duration."  They say these observations suggest that the lesser impact of the 2001 event "may have resulted from a shift in species composition through selective mortality and differential settlement of more thermally tolerant species." This hypothesis is also in harmony with the findings of Loya et al. (2001), who observed the local extinction of adult Pocillopora damicornis in 1998 while noting that a large proportion of P. verrucosa colonies survived at nearby sites, which findings fit well with the fact that P. verrucosa, in the words of Bena and van Woesik, "generally supports high concentrations of photoprotective pigments (Salih et al., 2000)."

Another intriguing idea that may explain some of these observations has been proposed by Nakamura and van Woesik (2001), who argue that small coral colonies should survive thermal and light stress more readily than large coral colonies based on mass transfer theory, which suggests that rates of passive diffusion are more rapid for small colonies than for large colonies.  Still another reason why large coral colonies suffer more than small colonies during environmental conditions conducive to bleaching is the fact that small Acropora recruits, according to Bena and van Woesik, "contain high concentrations of fluorescent proteins (Papina et al., 2002), which have photoprotective properties (Salih et al., 2000)," and they note that "a high concentration of photoprotective pigments in early life, when planulae are near the surface and as newly settled recruits, may facilitate survival during this phase as well as during stress events involving both high irradiance and thermal anomalies (van Woesik, 2000)."

What it means
These several intriguing observations demonstrate just how easy it would be for corals to survive and reestablish themselves, even after massive bleaching events, such as those that may have been associated with the multi-century warm periods of Roman and Medieval times and that might now occur during the Modern Warm Period.  The fact that all four of the prior interglacials were warmer than the current interglacial by more than 2C (Petit et al., 1999) also suggests that something of this nature has indeed enabled earth's corals to survive much warmer temperatures than anything we have seen of late or will likely ever see this time around the glacial-interglacial cycle.  Consequently, it would appear that climate-alarmist claims of impending coral extinctions due to global warming are rather far removed from reality.

References
Loya, Y., Sakai, K., Yamazato, K., Nakano, Y., Sambali, H. and van Woesik, R.  2001.  Coral bleaching: the winners and the losers.  Ecology Letters 4: 122-131.

Nakamura, T. and van Woesik, R.  2001.  Differential survival of corals during the 1998 bleaching event is partially explained by water-flow rates and passive diffusion.  Marine Ecology Progress Series 212: 301-304.

Papina, M., Sakihama, Y., Bena, C., van Woesik, R. and Yamasaki, H.  2002.  Separation of highly fluorescent proteins by SDS-PAGE in Acroporidae corals.  Comp. Biochem. Phys. 131: 767-774.

Petit, J.R., Jouzel, J., Raynaud, D., Barkov, N.I., Barnola, J.-M., Basile, I., Bender, M., Chappellaz, J., Davis, M., Delaygue, G., Delmotte, M., Kotlyakov, V.M., Legrand, M., Lipenkov, V.Y., Lorius, C., Pepin, L., Ritz, C., Saltzman, E., and Stievenard, M.  1999.  Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica.  Nature 399: 429-436.

Salih, A., Larkum, A., Cox, G., Kuhl, M. And Hoegh-Guldberg, O.  2000.  Fluorescent pigments in corals are photoprotective.  Nature 408: 850-853.

van Woesik, R.  2000.  Modelling processes that generate and maintain coral community diversity.  Biodiversity and Conservation 9: 1219-1233.

Reviewed 2 February 2005