Coccolitophore calcification is independent of carbonate chemistry in the tropical ocean

Abstract

Short-term experiments indicate that seawater acidification can cause a decrease in the rate of calcification by coccolithopores, but the relationship between carbonate chemistry and coccolithophore calcification rate in natural assemblages is still unclear. During the Malaspina 2010 circumnavigation, we measured primary production, calcificatiion, coccolithophore abundance, particulate inorganic carbon (PIC) concentration, and the parameters of the carbonate system, along basin-scale transects in the tropical Atlantic, Indian and Pacific oceans. Euphotic layer-integrated calcification and mean cell-specific calcification to primary production (CP/PP) ratio was relatively invariant among ocean basins, with an overall mean value of 0.05+-0.04. Extrapoling this value to the entire ocean would result in a global pelagic calcification rate of 2.4 PtC /year. The mean PIC concentration in surface waters was 1.8 +-1.6 mgC/m3 and its turnover time averaged 20 d. We combined our data of calcification, primary production, and carbonate chemistry from Malaspina 2010 with those obtained during two previous cruises in the northern Arabian Sea. Both the CP/PP ratio and cell-specific calcification were largely constant across a wide range of calcite saturation rate (1.5-6.5), [HCO3-]/[H+] (0.08-0.24; mol:umol), and pH (7.6-8.1), which indicates that calcification by natural coccolithophore assemblages was independent of carbonate chemistry. Our results suggest that coccolithophore calcification, at least in tropical regions, may not be decreasing in the currently acidifying ocean.