Coccolithophores grown in a tank equilibrated to an “atmosphere” with CO2 levels at 800 ppm (double modern day levels) also show degraded shells. Credit: Reibesell et al., 2000.

Decreased carbonate levels have been linked to slower shell growth and weaker shells among many marine species.  Although further studies are required in order to establish significant evidence of this trend occurring naturally, an early study of calcification rates among small Southern Ocean pteropods suggests a linear reduction in mean shell weight from 1998-2006, possibly due to the changing chemistry of seawater (Roberts et al., 2008).  Laboratory studies suggest that ocean acidification leads to a reduction in calcification rates among most marine calcifiers (Fabry et al., 2008).

Much of the recent research on the impact of ocean acidification has examined its effect on reef-building corals.  Calcification rates among these organisms are expected to decrease by approximately 30% once atmospheric CO2 levels reach 560 ppm, or double the preindustrial level (McLeod et al., 2008).  This increase may be reached in this century.  Reduced calcification has already been observed in corals located in the Great Barrier Reef (Cooper et al., 2008).

By extrapolating from data collected near seafloor vents that emit CO2 at ambient temperature and from regions with naturally varying pH gradients, such as coastal upwelling systems, scientists have developed hypotheses regarding the ability of ecosystems and individual organisms to adapt to high-CO2 environments, such as those expected to result from the continuation of current carbon emission trends (Orr et al., 2009).

One such study, conducted near a natural submarine CO2 vent in the Mediterranean Sea, concluded that increasing acidification might result in a shift away from carbonate-dominated ecosystems (Hall-Spencer et al., 2008).  Calcifying organisms, such as corals, sea urchins and coralline algae, which are prevalent in the larger community, were found to be unusually rare or altogether absent near the vent, where they were replaced by seagrasses and other algal species.  The pH of this community (7.8-7.9) is within a tenth of a point from the pH level expected to prevail throughout the ocean when atmospheric CO2 levels reach 560 ppm – that is, pH 7.91.

Research suggests that some plankton groups may do better than others in seawater with lower pH. Scientists have run experiments on four species of coccolithophore by measuring the weight of their shells–or coccoliths–in response to seawater with lower pH levels (Grigarov, 2008; Iglesias-Rodriguez et al., 2008).

In a 2008 paper in Science, M. Debora Iglesias-Rodriguez found that calcification and net primary production increased for the coccolithophore species, Emiliania huxleyi, raised in a lab under high CO2 conditions, contradicting earlier studies. She also presented evidence from deep-sea sediment cores that over the last 220 years, average coccolith mass has increased 40%. There is also evidence from sediment cores that another species of coccolithophore, Calcidiscus leptoporus, adapted to low carbon dioxide levels during the last ice age and is adapting now to high levels today (Doney et al., 2009). As scientists have pointed out, however, only four of the 250 to 500 living species of coccolithophore have been studied.

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