Volume 13, Number 9: 3 March 2010
In the most comprehensive analysis ever conducted of experimental studies that have explored the effects of rising atmospheric CO2 concentrations on marine biota, Hendriks et al. (2010) assembled a database of 372 experimentally-evaluated responses of 44 different marine species to ocean acidification that was induced by equilibrating seawater with CO2-enriched air. This they did because, as they describe it, "warnings that ocean acidification is a major threat to marine biodiversity are largely based on the analysis of predicted changes in ocean chemical fields," which are derived from theoretical models that do not account for numerous biological phenomena and have only "limited experimental support."
Of the published reports they scrutinized, only 154 assessed the significance of responses relative to controls; and of those reports, 47 reported no significant response, so that "only a minority of studies," in their words, demonstrated "significant responses to acidification." And when the results of that minority group of studies were pooled, there was no significant mean effect. Nevertheless, the three researchers found that some types of organisms and certain functional processes did exhibit significant responses to seawater acidification. However, since their analyses to this point had included some acidification treatments that were extremely high, they repeated their analyses for only those acidification conditions that were induced by atmospheric CO2 concentrations of 2000 ppm or less, which latter limiting concentration had been predicted to occur around the year 2300 by Caldeira and Wickett (2003).
In this second analysis, Hendriks et al. once again found that the overall response, including all biological processes and functional groups, was not significantly different from that of the various control treatments, although calcification was reduced by 33 ± 4.5% and fertility by 11 ± 3.5% across groups, while survival and growth showed no significant overall responses. And when the upper limiting CO2 concentrations were in the range of 731-759 ppm, or just below the value predicted by the IPCC (2007) for the end of the 21st century (790 ppm) -- calcification rate reductions of only 25% were observed. What is more, the three researchers say that this decline "is likely to be an upper limit, considering that all experiments involve the abrupt exposure of organisms to elevated pCO2 values, while the gradual increase in pCO2 that is occurring in nature may allow adaptive and selective processes to operate," citing the work of Widdicombe et al. (2008) and noting that "these gradual changes take place on the scale of decades, permitting adaptation of organisms even including genetic selection."
Yet even this mitigating factor is not the end of the good news, for Hendriks et al. write that "most experiments assessed organisms in isolation, rather than [within] whole communities," and they say that the responses of other entities and processes within the community may well buffer the negative impacts of CO2-indced acidification on earth's corals. As an example, they note that "sea-grass photosynthetic rates may increase by 50% with increased CO2, which may deplete the CO2 pool, maintaining an elevated pH that may protect associated calcifying organisms from the impacts of ocean acidification."
In describing another phenomenon that benefits corals, the researchers write that "seasonal changes in pCO2 are in the range of 236-517 ppm in the waters of the northern East China Sea (Shim et al., 2007)," and that "metabolically-active coastal ecosystems experience broad diel changes in pH, such as the diel changes of >0.5 pH units reported for sea grass ecosystems (Invers et al., 1997)," which they say represent "a broader range than that expected to result from ocean acidification expected during the 21st century." And they remark that these fluctuations also "offer opportunities for adaptation to the organisms involved."
Hendriks et al. additionally state that the models upon which the ocean acidification threat is based "focus on bulk water chemistry and fall short of addressing conditions actually experienced by [marine] organisms," which are "separated from the bulk water phase by a diffusive boundary layer," adding that "photosynthetic activity" -- such as that of the zooxanthellae that are hosted by corals -- "depletes pCO2 and raises pH (Kuhl et al., 1995) so that the pH actually experienced by organisms may differ greatly from that in the bulk water phase (Sand-Jensen et al., 1985)."
Last of all, the insightful scientists note that "calcification is an active process where biota can regulate intracellular calcium concentrations," so that "marine organisms, like calcifying coccolithophores (Brownlee and Taylor, 2004), actively expel Ca2+ through the ATPase pump to maintain low intracellular calcium concentrations (Corstjens et al., 2001; Yates and Robbins, 1999)." And they say that "as one Ca2+ is pumped out of the cell in exchange for 2H+ pumped into the cell, the resulting pH and Ca2+ concentrations increase the CaCO3 saturation state near extracellular membranes and appear to enhance calcification (Pomar and Hallock, 2008)," so much so, in fact, that they indicate "there is evidence that calcification could even increase in acidified seawater, contradicting the traditional belief that calcification is a critical process impacted by ocean acidification (Findlay et al., 2009)."
In summation, Hendriks et al. write that the world's marine biota are "more resistant to ocean acidification than suggested by pessimistic predictions identifying ocean acidification as a major threat to marine biodiversity," noting that this phenomenon "may not be the widespread problem conjured into the 21st century" by the world's climate alarmists. We agree, having reached much the same conclusion back at the turn of the last millennium (Idso et al., 2000). Hence, we are happy to endorse Hendriks et al.'s conclusion that "biological processes can provide homeostasis against changes in pH in bulk waters of the range predicted during the 21st century."
Sherwood, Keith and Craig Idso
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