A three-dimensional niche comparison of <i>Emiliania huxleyi</i> and <i>Gephyrocapsa oceanica</i>: reconciling observations with projections

Abstract

Abstract. Coccolithophore responses to changes in carbonate chemistry speciation such as CO2 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 common bloom-forming species in present-day coccolithophore communities appear to be adapted for a similar fundamental light niche but slightly different ones for temperature and CO2, with E. huxleyi having a tolerance to lower temperatures and higher CO2 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 CO2 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 Representative Concentration Pathway (RCP) 8.5 climate change scenario (1000 µatm fCO2), we project a CO2 driven niche contraction for G. oceanica to regions of even higher temperatures. However, the greater sensitivity of G. oceanica to increasing CO2 is partially mitigated by increasing temperatures. Finally, we compare satellite-derived particulate inorganic carbon estimates in the surface ocean with a recently proposed metric for potential coccolithophore success on the community level, i.e. the temperature-, light- and carbonate-chemistry-dependent CaCO3 production potential (CCPP). Based on E. huxleyi alone, as there was interestingly a better correlation than when in combination with G. oceanica, and excluding the Antarctic province from the analysis, we found a good correlation between CCPP and satellite-derived particulate inorganic carbon (PIC) with an R2 of 0.73, p < 0.01 and a slope of 1.03 for austral winter/boreal summer and an R2 of 0.85, p < 0.01 and a slope of 0.32 for austral summer/boreal winter.

Particulate inorganic to organic carbon production as a predictor for coccolithophorid sensitivity to ongoing ocean acidification

“…Interestingly, the finding that calcification rates are typically more sensitive to OA than photosynthetic carbon fixation and growth rates in a single species (i.e., Sett et al ; Bach et al ; Zhang et al ; Gafar and Schulz ) does also apply to our multispecies analysis. This is reflected in process‐specific slopes of sensitivity to PIC : POC ratios (e.g., Fig.…”

“…The influence of average cell size, cell volume, and surface area to volume ratio (calculated from Table ) on OA sensitivity was also examined. Fit coefficients for G. oceanica , E. huxleyi , and S. apsteinii were taken from Gafar et al (), Gafar and Schulz (), and N. A. Gafar et al (). Fit coefficients for C. leptoporus and C. pelagicus strains RCC1198 and RCC1200 are presented in Supporting Information Tables S1–S3, respectively.…”

“…The influence of average cell size, cell volume, and surface area to volume ratio (calculated from Table 1) on OA sensitivity was also examined. Fit coefficients for G. oceanica, E. huxleyi, and S. apsteinii were taken from Gafar et al (2018b), Gafar and Schulz (2018), and N. A. Gafar et al (2019).…”

“…As a result, OA influences the relative production of PIC and POC in coccolithophores. Sensitivities to OA also vary between species with some such as G. oceanica being much more sensitive than others like E. huxleyi and S. apsteinii (Gafar et al 2018b;Gafar and Schulz 2018;N. A. Gafar et al 2019).…”

“…A. Gafar et al 2019). It is not yet clear what drives these differences; however, they are likely to result in shifts in coccolithophore community composition under climate change conditions (Rost and Riebesell 2004;Gafar and Schulz 2018). Through effects on both community composition and physiological rates, OA has the potential to impact PIC and POC production on a global scale.…”