Probing rates of growth and mortality in natural coccolithophore populations

Coccolithophores are biogeochemically important components of marine phytoplankton communities through the intracellular production and subsequent export of calcium carbonate scales, termed coccoliths. The species Emiliania huxleyi is considered the most cosmopolitan and dominant coccolithophore in the modern ocean, having the ability to form extensive blooms. A hypothesis for the evolution and function of coccoliths, as well as for how E. huxleyi can form large-scale blooms, is that the presence of coccoliths provides protection against microzooplankton grazers. This thesis sets out to directly test this hypothesis using several lines of evidence. In this thesis, I show how gradients in nutrients and irradiance control coccolithophore biogeography on spatial and seasonal scales, provide evidence that microzooplankton grazers exert strong controls over coccolithophore populations. I also show that the possession of calcium carbonate coccoliths does not provide a protective function against microzooplankton ingestion of E. huxleyi, when grazing rates are compared with other similarly sized, but non-calcified, phytoplankton groups. I also observed no negative impact on community grazing rates by microzooplankton when E. huxleyi was dominant within the phytoplankton community. Although coccoliths do not appear to prevent ingestion of coccolithophores by microzooplankton, evidence is presented that rather the coccoliths may protect the organic cell from dissolution within the vacuole of microzooplankton grazers. Overall, this thesis provides a greater understanding of the role of microzooplankton in coccolithophore growth and mortality. Microzooplankton grazing could also reduce the export of calcite to depth, particularly if dissolution occurs within vacuoles, leading to enhanced CO2 recycling within the upper ocean.