Seasonal and interannual variability of coccolithophore blooms in the North East-Atlantic Ocean from a 18-year time-series of satellite water-leaving radiance

Abstract. Evaluate the impact of anthropogenic CO₂ uptake and acidification on the most abundant calcareous phytoplankton, coccolithophores, requires a better knowledge of the temporal and spatial evolution of their blooms. Here we determine, from satellite radiance, the seasonal and interannual variability of coccolithophore blooms for 18 years (1998 to 2015) across the North-East Atlantic region covering the Bay of Biscay and the Celtic Sea. The identification of coccolithophores is carried out using a modified version of the fuzzy method developed by Moore (2009) applied to a time series of SeaWiFS (1998–2003) and MODIS (2003–2015) spectral radiance. After identification of the coccolith pixels, the abundance of coccoliths is assessed from a data base of non-algal Suspended Particulate Matter (SPM), a product initially developed for estimating the content of mineral particles, mainly due to resuspension effects, in coastal waters. The results, in terms of identification and quantification, are consistent with in situ observations in the area and with those of algorithms addressing coccolithophore blooms at global scale (CALCITE and PHYSAT). Although a regular pattern in the phenology of the blooms is observed, starting south in April in Biscay and moving northwards until July near Ireland, there is a high seasonal and interannual variability in the extent of the blooms. Year 2014 shows very low concentrations of coccoliths from space (twice less than average) and anomalies point out the maximum level for 2001. The cause of the seasonal and interannual variability of the coccolithophores blooms in this Atlantic region remains an open question.