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Combating Ocean Acidification: The Role of Marine Photosynthesis
Volume 14, Number 49: 7 December 2011

In an insightful paper published in Global Change Biology, Anthony et al. (2011) demonstrate that coral reefs are not totally at the mercy of the CO2 concentration of the air above them when it comes to their ability to live long and prosper. Indeed, they are to some extent masters of their own fate in the watery realm in which they dwell, as they seem to get by quite well with a little help from their symbiotic friends, as well as with the unintentional help that is provided by larger seagrasses and other macro-algae that often compete with them for living quarters on coastal seabeds.

The challenge that earth's corals and other calcifying marine organisms face is that of falling seawater pH and aragonite saturation statea), both of which declines are driven by the enhanced transfer of gaseous carbon dioxide from the air above the surface of the sea to the water beneath it, along with the negative impacts the subsequent chemical alteration of the seawater has on the organisms' abilities to calcify and build skeletons to protect and support their bodies. And with atmospheric CO2 concentrations rising higher and higher with each passing year, many a climate alarmist has already assigned numerous such organisms to the ash heap of history. But the Australian, French and U.S. researchers who address the subject in a pair of papers (Anthony et al., 2011; Kleypas et al., 2011) suggest that the calcifiers' obituaries may have been a bit premature.

Anthony et al. begin by describing how they used "a carbon flux model for photosynthesis, respiration, calcification and dissolution coupled with Lagrangian transport to examine how key groups of calcifiers (zooxanthellate corals) and primary producers (macroalgae) on coral reefs contribute to changes in the seawater carbonate system as a function of water residence time." This work revealed, in their words, that "the carbon fluxes of corals and macroalgae drive Ωa in opposing directions," such that "areas dominated by corals elevate pCO2 and reduce Ωa, thereby compounding ocean acidification effects in downstream habitats, whereas algal beds draw CO2 down and elevate Ωa, potentially offsetting ocean acidification impacts at the local scale." And they also report that simulations for two significantly elevated CO2 scenarios (600 and 900 ppm CO2) suggested that "a shift in reef community composition from coral to algal dominance in upstream areas under ocean acidification will potentially improve conditions for calcification in downstream areas."

Lastly, field validation of the simulations of Anthony et al. was provided by Kleypas et al., who examined the roles of three key members of benthic reef communities (corals, macroalgae and sand) in modifying the chemistry of open-ocean source water, finding that "the drawdown of total dissolved inorganic carbon due to photosynthesis and calcification of reef communities can exceed the drawdown of total alkalinity due to calcification of corals and calcifying algae, leading to a net increase in aragonite saturation state." In addition, they note that there were no seagrasses on the reef flat they studied; and they state that "research suggests that seagrasses may have an additional impact on reef seawater chemistry because they enhance the alkalinity flux from sediments (Burdige and Zimmerman, 2002), and they respond to CO2 fertilization (Palacios and Zimmerman, 2007)."

In light of these several observations, one might logically expect reef communities to gradually alter their spatial compositions in a CO2-acreting world to the point where seagrasses and other macroalgae take up residence in upstream regions, while corals and other calcifying organisms lay claim to downstream regions. Therefore, as Anthony et al. conclude, "although the carbon fluxes of benthic reef communities cannot significantly counter changes in carbon chemistry at the scale of oceans, they provide a significant mechanism of buffering ocean acidification impacts at the scale of habitat to reef."

Sherwood, Keith and Craig Idso

References
Anthony, K.R.N., Kleypas, J.A. and Gattuso, J.-P. 2011. Coral reefs modify their seawater carbon chemistry -- implications for impacts of ocean acidification. Global Change Biology 10.1111/j.1365-2486.2011.02510.x.

Burdige, D.J. and Zimmerman, R.C. 2002. Impact of sea grass density on carbonate dissolution in Bahamian sediments. Limnology and Oceanography 47: 1751-1763.

Kleypas, J.A., Anthony, K.R.N. and Gattuso, J.-P. 2011. Coral reefs modify their seawater carbon chemistry -- case study from a barrier reef (Moorea, French Polynesia). Global Change Biology 10.1111/j.1365-2486.2011.02530.x.

Palacios, S.L. and Zimmerman, R.C. 2007. Response of eelgrass Zostera marina to CO2 enrichment: possible impacts of climate change and potential for remediation of coastal habitats. Marine Ecology Progress Series 344: 1-13.