The Anthropogenic Forcing of Coccolithophore Growth

Global increases in atmospheric CO₂ concentration and temperature are causing changes in ocean chemistry, temperature and circulation, altering light and nutrient regimes. Such changes are expected to impact the coccolithophore community with organisms responding through phenotypic and physiological plasticity, genetic change and/or dispersal to more hospitable habitats. Understanding how the coccolithophore community responds to climate variability is vital for predicting ecosystem functioning and the fate of the global carbon cycle under a changing climate. Despite these urgent concerns, the coccolithophore response to recent global climate change remains poorly understood. Fossil coccospheres, the external calcite structure produced by the excretion of interlocking plates by coccolithophores, can provide a rare window into cell size in the past and an archive of the response of coccolithophores to environmental change. In turn, coccolithophore cell size has the potential to impose fundamental constraints on ecological and biogeochemical processes. In this thesis, I develop novel techniques combining imaging flow cytometry and cross-polarised light (ISX+PL) to rapidly and reliably visually isolate and quantify the morphological characteristics of coccospheres from marine sediment to enable cell size reconstructions. This method overcomes the constraints of labour-intensive manual microscopy allowing rapid identification and analysis of coccospheres within sediment. By employing ISX+PL, coccolithophore community cell size and community size structure of a subpolar North Atlantic community is reconstructed from ~1750 to ~2014 C.E. to investigate how modern coccolithophores may be responding to recent climate change. Results suggest average community cell size and community size structure was insensitive to increases in CO₂ concentration and SST since the mid-1900s. Stability in the species assemblage may be attributed to the low magnitude of environmental change and broad tolerances of the dominant species of the subpolar North Atlantic. The results imply environmental changes in the subpolar North Atlantic are not yet considerable enough to impact fundamental traits that affect the biogeochemical and ecological functioning of the coccolithophore ecosystem. In regions where climate change is resulting in higher local environmental variability and in communities that have a broader cell size composition with a subordinate community of larger cells, changes in cell size may be observed and should be a focus for future investigations.