The biogeography of Southern Ocean phytoplankton controls the local biogeochemistry and the export of macronutrients to lower latitudes and depth. Of particular relevance is the competitive interaction between coccolithophores and diatoms, with the former being prevalent along the Great Calcite Belt (40–60°S), while diatoms tend to dominate the regions south of 60°S, as illustrated in the diagram on the right.
The ocean is changing at an unprecedented rate as a consequence of increasing anthropogenic CO emissions and related climate change. Changes in density stratification and nutrient supply, as well as ocean acidification, lead to changes in phytoplankton community composition and consequently ecosystem structure and function. Some of these changes are already observable today and may have cascading effects on global biogeochemical cycles and oceanic carbon uptake. Changes in Southern Ocean (SO) biogeography are especially critical due to the importance of the Southern Ocean in fuelling primary production at lower latitudes through the lateral export of nutrients and in taking up anthropogenic CO. For the carbon cycle, the ratio of calcifying and noncalcifying phytoplankton is crucial due to the counteracting effects of calcification and photosynthesis on seawater pCO, which ultimately controls CO exchange with the atmosphere, and the differing ballasting effect of calcite and silicic acid shells for organic carbon export.
Calcifying coccolithophores and silicifying diatoms are globally ubiquitous phytoplankton functional groups. Diatoms are a major contributor to global phytoplankton biomass and annual net primary production. In comparison, coccolithophores contribute less to biomass and to global NPP.
However, coccolithophores are the major phytoplanktonic calcifier. thereby significantly impacting the global carbon cycle. Diatoms dominate the phytoplankton community in the Southern Ocean, but coccolithophores have received increasing attention in recent years. Satellite imagery of particulate inorganic carbon (PIC, a proxy for coccolithophore abundance) revealed the "Great Calcite Belt", an annually reoccurring circumpolar band of elevated PIC concentrations between 40 and 60°S. In situ observations confirmed coccolithophore abundances of up to 2.4×10 cells mL in the Atlantic sector (blooms on the Patagonian Shelf), up to 3.8×10 cells mL in the Indian sector, and up to 5.4×10 cells mL in the Pacific sector of the Southern Ocean with Emiliania huxleyi being the dominant species. However, the contribution of coccolithophores to total Southern Ocean phytoplankton biomass and NPP has not yet been assessed. Locally, elevated coccolithophore abundance in the GCB has been found to turn surface waters into a source of CO for the atmosphere, emphasising the necessity to understand the controls on their abundance in the Southern Ocean in the context of the carbon cycle and climate change. While coccolithophores have been observed to have moved polewards in recent decades, their response to the combined effects of future warming and ocean acidification is still subject to debate. As their response will also crucially depend on future phytoplankton community composition and predator–prey interactions, it is essential to assess the controls on their abundance in today's climate.