Mesocosm CO2 perturbaration studies: from organism to community level

The uptake of anthropogenic CO2 by the ocean and the corresponding increase in surface ocean CO2 concentrations have already caused a measurable decrease in seawater pH (Bindoff and Willebrand, 2007). Surface ocean acidification through this process will amplify as long as fossil fuel CO2 continues to enter the atmosphere and will transform the ocean to a new chemical state for tens of thousands of years (Caldeira and Wickett, 2003). While the magnitude of these changes can be estimated with reasonable certainty for any given CO2 emissions scenario, our understanding of their biological consequences is in its infancy. Effects of seawater acidification at the organismal level have been demonstrated in single species experiments and small-scale incubations of mixed assemblages. Among these, studies on plankton organisms with CO2 perturbation ranges relevant to non-deliberate ocean acidification have primarily focussed on coccolithophores (Riebesell et al., 2000; Zondervan et al., 2001, 2002; Sciandra et al., 2003; Leonardos and Geider, 2005; Langer et al., 2006; Feng et al., 2008), diatoms (Burkhardt et al., 2001; Rost et al., 2003), dinoflagellates (Rost et al., 2006), the diazotrophic cyanobacterium Trichodesmium (Barcelos e Ramos et al., 2007; Hutchins et al., 2007), foraminifera (Bijma et al., 2002), copepods (Kurihara et al., 2004), and oyster larvae (Kurihara et al., 2007). These studies have shown both adverse effects, including those on calcium carbonate production in calcifying organisms, and stimulating effects, such as on carbon and nitrogen fixation rates of some of the photoautotrophic organisms. In incu- bation experiments using mixed phytoplankton assemblages a shift in species composition from Phaeocystis to diatom dominance was observed with increasing pCO2 (Tortell et al., 2002).