Coccolithophore surface distributions in the North Atlantic and their modulation of the air-sea flux of CO 2 from 10 years of satellite Earth observation data
Coccolithophores are the primary oceanic phytoplankton responsible for the production of calcium carbonate (CaCO 3). 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 (CO 2) through increasing the seawater partial pressure of CO 2 ( pCO 2). 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 km 2, which results in a net CaCO 3 carbon (CaCO 3-C) production of 0.14–1.71 Tg CaCO 3-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 pCO 2 and, thus, decrease the localised air-sea flux of atmospheric CO 2. In regions where the blooms are prevalent, the average reduction in the monthly air-sea CO 2 flux can reach 55%. The maximum reduction of the monthly air-sea CO 2 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 pCO 2 should be considered if we are to fully understand the variability of the North Atlantic air-to-sea flux of CO 2. We estimate that these blooms can reduce the annual N. Atlantic net sink atmospheric CO 2 by between 3–28%.