Coccolithophore surface distributions in the North Atlantic and their modulation of the air-sea flux of CO₂ from 10 years of satellite Earth observation data

Abstract. Coccolithophores are the primary oceanic phytoplankton responsible for the production of calcium carbonate (CaCO₃). These climatically important plankton play a key role in the oceanic carbon cycle as a major contributor of carbon to the open ocean carbonate pump (~50%) and their calcification can affect the atmosphere-to-ocean (air-sea) uptake of carbon dioxide (CO₂) through increasing the seawater partial pressure of CO₂ (pCO₂). Here we document variations in the areal extent of surface blooms of the globally important coccolithophore, Emiliania huxleyi, in the North Atlantic over a 10-year period (1998–2007), using Earth observation data from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS). We calculate the annual mean sea surface areal coverage of E. huxleyi in the North Atlantic to be 474 000 ± 104 000 km², which results in a net CaCO₃ carbon (CaCO₃-C) production of 0.14–1.71 Tg CaCO₃-C per year. However, this surface coverage (and, thus, net production) can fluctuate inter-annually by −54/+8% about the mean value and is strongly correlated with the El Niño/Southern Oscillation (ENSO) climate oscillation index (r=0.75, p<0.02). Our analysis evaluates the spatial extent over which the E. huxleyi blooms in the North Atlantic can increase the pCO₂ and, thus, decrease the localised air-sea flux of atmospheric CO₂. In regions where the blooms are prevalent, the average reduction in the monthly air-sea CO₂ flux can reach 55%. The maximum reduction of the monthly air-sea CO₂ flux in the time series is 155%. This work suggests that the high variability, frequency and distribution of these calcifying plankton and their impact on pCO₂ should be considered if we are to fully understand the variability of the North Atlantic air-to-sea flux of CO₂. We estimate that these blooms can reduce the annual N. Atlantic net sink atmospheric CO₂ by between 3–28%.