Coccolith-derived turbidity and hydrological conditions in May in the Bay of Biscay

Coccolithophore blooms occur regularly from April to June in the Bay of Biscay where they have been observed for many years from ocean-colour imagery thanks to the ability of their calcite plates to scatter light. They are easily depicted on interpolated images of non-algal Suspended Particulate Matter (SPM) derived from satellite reflectance data in May, at the time of the PELGAS (PELagique GAScogne) annual surveys. During the spring from 2012-2015, the PELGAS surveys provided in-situ data on the turbidity throughout the water column and on the hydrological environment prevailing during the blooms in the vicinity of the continental shelf break. The satellite-derived SPM in the area of coccolithophore blooms was closely related to measured turbidity in the euphotic zone. The deep maxima of coccolithophore-derived biomass were well observed in the turbidity signal throughout the water column. Despite the recurrence of SPM patterns due to coccolithophores in the well-mixed waters over the shelf break in spring, the variability of the satellite-derived SPM and the turbidity profiles was particularly high during the four years studied. The year 2013 showed persistent blooms from mid-April to the end of May over large areas inside and outside the shelf waters, whereas no bloom was observed in 2014, neither on SPM images nor through turbidity profiles. The presence or absence of coccolithophore blooms during the years 2012-2015 seem to be related to the degree of stratification of the water column, with more intense blooms occurring during mixed conditions. These results should improve classification of the PELGAS surveys from those occurring in a coccolith-rich environment, synonymous with mixed and cold waters, to those occurring in a coccolith-poor environment where surface waters are warmer and more stratified.