Phytoplankton community dynamics during late spring coccolithophore blooms at the continental margin of the Celtic Sea (North East Atlantic, 2006–2008)

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We determined the spatial and temporal dynamics of major phytoplankton groups in relation to biogeochemical and physical variables during the late spring coccolithophore blooms (May–June) along and across the continental margin in the Celtic Sea (2006–2008). Photosynthetic biomass (chl a) of the dominant plankton groups was determined by CHEMTAX analysis of chromatographic (HPLC) pigment signatures.Phytoplankton standing stock biomass varied substantially between and during the campaigns (areal chl a [mgchl am−2] in June 2006: 63.8±26.5, May 2007: 27.9±8.4, and May 2008: 41.3±21.8), reflecting the different prevailing conditions of weather, irradiance, and sea surface temperature between the campaigns. Coccolithophores, represented mainly by Emiliania huxleyi, and diatoms were the dominant phytoplankton groups, with a maximal contribution of, respectively, 72% and 89% of the total chl a. Prasinophytes, dinoflagellates, and chrysophytes often co-occurred during coccolithophorid blooms, while diatoms dominated the phytoplankton biomass independently of the biomass of other groups. The location of the stations on the shelf or on the slope side of the continental margin did not influence the biomass and the composition of the phytoplankton community despite significantly stronger water column stratification and lower nutrient concentrations on the shelf. The alternation between diatom and coccolithophorid blooms of similar biomass, following the mostly diatom-dominated main spring bloom, was partly driven by changes in nutrient stoichiometry (N:P and dSi:N). High concentrations of transparent exopolymer particles (TEP) were associated with stratified, coccolithophore-rich water masses, which probably originated from the slope of the continental margin and warmed during advection onto the shelf. Although we did not determine the proportion of export production attributed to phytoplankton groups, the abundance of coccolithophores, together with TEP and coccoliths may affect the carbon export efficiency through increased sinking rates of particles formed by aggregation of TEP and coccoliths.