Coccolithophores in high latitude and Polar regions:Relationships between community composition, calcificationand environmental factors

Coccolithophores are a unique group of calcifying phytoplankton that dominate pelagic biogenic calcification and facilitate carbon export. Changes in coccolithophore calcite production through changes in their abundance, species distribution or cellular calcification could affect the oceanic carbon cycle. Ocean acidification, global warming and future changes in nutrient and light conditions might affect coccolithophore populations. This study investigated the relationships between coccolithophore distribution and calcification and environmental factors, between the North Sea and the Arctic Ocean and in the Southern Ocean. Large gradients in carbonate chemistry and other variables provided insights into coccolithophore response to concurrent changes in the future ocean. Freshwater inputs and biological processes were driving the carbonate chemistry changes in the surface waters of the North Sea, the Norwegian Sea and the Svalbard Arctic region. Even though biological processes seemed to play a major role in shaping the saturation state (calcite) and pH of these regions, the carbonate chemistry of the freshwater sources (Baltic Sea, sea-ice melt, riverine input/ terrestrial runoff) was also important and had accentuated the effects of biological activity. A multivariate approach showed that changes in pH and mixed layer irradiance explained most of the variation in coccolithophore distribution and community composition between the North Sea and Svalbard. Differences between the Svalbard population (dominated by the family Papposphaeraceae) and those from other regions were mostly explained by pH, whereas mixed layer irradiance explained most of the variation between the North Sea, Norwegian Sea and Arctic water assemblages. Estimates of cell specific calcification rates showed that species composition can considerably affect community calcification. At Drake Passage, the coccolithophore community was dominated by Emiliania huxleyi B/C. Diversity and abundance were highest in the Subantarctic and Polar Frontal Zones, respectively, where temperature and mixed layer irradiance were high. Community and cell specific calcification, as well as coccolith production rates, showed an overall decreasing trend towards Antarctica and were correlated with the strong latitudinal gradients in temperature and calcite and anti-correlated with nutrient concentrations. Additionally, coccolith production rates and cell specific calcification were also correlated with mixed layer irradiance. Coccolithophore calcification rates at Svalbard and at Drake Passage were low compared to other oceanic regions. At Svalbard, the low calcification rates were the result of very low abundances of species that have both a low (e.g. Papposphaeraceae) and high (e.g. Coccolithus pelagicus) calcite content. At Drake Passage low calcification rates were the result of low to moderate abundances of E. huxleyi B/C, which has a low calcite content. The results of this study suggest that changes in future pelagic calcite production may result from physiological changes acting on single species and/or from shifts in the species composition of coccolithophore assemblages, as well as poleward biome migrations induced by ocean acidification, sea surface warming and stratification.