A niche comparison of <i>Emiliania huxleyi</i> and <i>Gephyrocapsa oceanica</i> and potential effects of climate change

Abstract

Abstract. Coccolithophore responses to changes in carbonate chemistry speciation such as CO 2 and H + are highly modulated by light intensity and temperature. Here we fit an analytical equation, accounting for simultaneous changes in carbonate chemistry speciation, light and temperature, to published and original data for Emiliania huxleyi, and compare the projections with those for Gephyrocapsa oceanica. Based on our analysis, the two most abundant coccolithophores in today's oceans appear to be adapted for a similar fundamental light niche but slightly different ones for temperature and CO 2 , with E. huxleyi having 5 a tolerance to lower temperatures and higher CO 2 levels than G. oceanica. Based on growth rates, a dominance of E. huxleyi over G. oceanica is projected below temperatures of 22• C at current atmospheric CO 2 levels. This is similar to a global surface sediment compilation of E. huxleyi and G. oceanica coccolith abundances suggesting temperature dependent dominance shifts. For a future RCP 8.5 climate change scenario (1000 µatm f CO 2 and + 4.8