Growth and Mortality of Coccolithophores during spring in the Celtic Sea

Coccolithophores are an important group of single celled protists which dominate pelagic calcite production, however little is currently known about the mortality rates within this group, or their importance in shelf seas regarding productivity and nutrient recycling. Measurements of coccolithophore calcification and cellular calcite quotas, as well as dilution experiments for microzooplankton grazing rates, were made during a spring cruise (April, 2015) in the Celtic Sea (NW European Shelf) and within an April bloom of Emiliania huxleyi.

Calcite production and coccolithophore cell numbers showed a general positive trend throughout the progression of the spring bloom, ranging from 15 - 34µmol C m^-3d^-1 and 6 - 94 cells ml^-1. Cell normalised calcification rates declined from 3 – 0.6 pmol C cell^-1d^-1 due to a shift from a mixed community to an E. huxleyi dominated one. Within the E. huxleyi bloom we recorded high daily calcite production (6049 µmol C m^-3d^-1) and cell normalised calcification of 3 pmol C cell^-1d^-1. This is significantly higher than E. huxleyi dominated sites in the Iceland Basin and more similar to a bloom on the Patagonian Shelf. Within the E. huxleyi bloom, mortality rates were 0.23 d^-1 compared with growth rates of 0.29 d^-1, meaning ~80% of daily calcification was removed by grazers.

In this study, coccolithophore mortality rates are presented from the central Celtic Sea throughout spring, and compared with an April E. huxleyi bloom in terms of species composition, trends in calcite production and composition of the phytoplankton community. These observations will potentially elucidate the role grazing plays in the fate of calcium carbonate, bloom formation and community composition.