Morphology of Emiliania huxleyi coccoliths on the northwestern European shelf – is there an influence of carbonate chemistry?

Abstract. Within the context of the UK Ocean Acidification project, Emiliania huxleyi (type A) coccolith morphology was examined from samples collected during cruise D366. In particular, a morphometric study of coccolith size and degree of calcification was made on scanning electron microscope images of samples from shipboard CO₂ perturbation experiments and from a set of environmental samples with significant variation in calcite saturation state (Ω_calcite). One bioassay in particular (E4 from the southern North Sea) yielded unambiguous results – in this bioassay exponential growth from a low initial cell density occurred with no nutrient enrichment and coccosphere numbers increased tenfold during the experiment. The samples with elevated CO₂ saw significantly reduced coccolithophore growth. However, coccolithophore morphology was not significantly affected by the changing CO₂ conditions even under the highest levels of perturbation (1000 μatm CO₂). Environmental samples similarly showed no correlation of coccolithophore morphology with calcite saturation state. Some variation in coccolith size and degree of calcification does occur but this seems to be predominantly due to genotypic differentiation between populations on the shelf and in the open ocean.