| Summary: | Increasing anthropogenic atmospheric CO₂ is altering the chemistry of surface seawater worldwide, resulting in ocean acidification. Experiments have begun to demonstrate the detrimental consequences that a CO₂-mediated decline in ocean pH can have on the growth and survival of calcifying organisms. However, significant knowledge gaps exist both in our understanding of the mechanisms driving the observed reductions in shell growth rates of organisms exposed to increases in CO₂, and in our ability to predict how these individual-level effects could scale-up to the population or community-level. Using laboratory exposure experiments in which levels of dissolved CO₂ were carefully manipulated, we tested the effects of climatically relevant increases in CO₂ levels on a) both shell deposition rate and shell dissolution rate in the intertidal snail, Nucella lamellosa, and b) the growth and feeding behaviour of juvenile red sea urchins, Strongylocentrotus franciscanus. Based on the results of the former study, we found that shell weight gain per day in live snails decreased linearly with increasing CO₂ level while shell weight loss per day in empty shells more than doubled over this same range. These results suggest that for some species, elevated CO₂ levels may have a much greater effect on shell dissolution than shell deposition. In the latter study, although we found no effect of a doubling of current CO₂ concentration on the individual feeding rates or absorption efficiency of juvenile urchins, there was a significant reduction in relative growth rates at the higher CO₂ concentration after 4 months of exposure. Applying the urchin growth data to a simple demographic matrix model, and incorporating empirical relationships between urchin test diameter, biomass and kelp consumption rates to the model outputs, we estimated that if current CO₂ levels were to double by the end of the century, it would take significantly longer for urchins to reach reproductive and harvestable sizes, and their per capita kelp grazing rates would be significantly reduced. These simple model applications illustrate how CO₂-mediated reductions in individual growth rates could indirectly impact important population-level attributes such as time to first reproduction, and could have community- or ecosystem-level effects by moderating the importance of top-down biological control.
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