Coccolithophores are a biogeochemically important group of phytoplankton, responsible for around half of oceanic carbonate production through the formation of calcite coccoliths. Globally distributed, Emiliania huxleyi is generally perceived to be the key calcite producer, yet it has a relatively low cellular calcite content (~ 0.4 – 0.7 pmol C cell-1) compared to heavily calcified species such as Coccolithus pelagicus (~ 15 – 21 pmol C cell-1). This study set out to test the central hypothesis that C. pelagicus has a significant biogeochemical role, dominating calcite production within mixed communities in the North Atlantic. Cultures of Coccolithus species (C. pelagicus, C. braarudii) and E. huxleyi were grown in parallel to examine relative growth rates, and relative calcite production was modelled. While E. huxleyi grew faster than C. pelagicus, it was estimated that at the average relative growth rate observed, C. pelagicus calcified at a rate equivalent to 34 cells of E. huxleyi. This was compared to field samples of abundances from the North Atlantic, where C. pelagicus was found to be the major calcite producer in 69 % of the samples. At two sites in the North Atlantic (Iceland Basin, Norwegian Sea), repeat samples were collected during the early stages of the spring bloom to examine phytoplankton community dynamics and the role of coccolithophores. The two sites had contrasting communities with diatoms dominant in the Iceland Basin, but absent in the Norwegian Basin. The coccolithophore community was generally similar, with E. huxleyi dominating abundance and C. pelagicus dominating coccolithophore calcite. In situ growth rates found that C. pelagicus grew faster than E. huxleyi. In the Arctic and North Atlantic, species-specific calcite production was determined from measurements of calcite production and community composition. The results of this study indicated that C. pelagicus dominated calcite production despite forming only a small fraction (< 2 %) of the community. In the synthesis, the seasonal contribution of C. pelagicus to calcite production was determined, using measured growth rates to estimate calcite production in spring. Coccolithus pelagicus was found to be the major source of calcite in the North Atlantic throughout the early to late spring and summer. When C. pelagicus was absent, other heavily calcified species, such as Coronosphaera mediterranea and Helicosphaera carteri, were important sources of calcite.