<p><strong>Abstract.</strong> The Great Calcite Belt (GCB) of the Southern Ocean is a region of elevated summertime upper ocean calcite concentration derived from coccolithophores, despite the region being known for its diatom predominance. The overlap of two major phytoplankton groups, coccolithophores and diatoms, in the dynamic frontal systems characteristic of this region provides an ideal setting to study environmental influences on the distribution of different species within these

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... ic groups. Samples for phytoplankton enumeration were collected from the upper mixed layer (30<span class="thinspace"></span>m) during two cruises, the first to the South Atlantic sector (January–February 2011; 60°<span class="thinspace"></span>W–15°<span class="thinspace"></span>E and 36–60°<span class="thinspace"></span>S) and the second in the South Indian sector (February–March 2012; 40–120°<span class="thinspace"></span>E and 36–60°<span class="thinspace"></span>S). The species composition of coccolithophores and diatoms was examined using scanning electron microscopy at 27 stations across the Subtropical, Polar, and Subantarctic fronts. The influence of environmental parameters, such as sea surface temperature (SST), salinity, carbonate chemistry (pH, partial pressure of CO₂ (<i>p</i>CO₂), alkalinity, dissolved inorganic carbon), macronutrients (nitrate<span class="thinspace"></span>+<span class="thinspace"></span>nitrite, phosphate, silicic acid, ammonia), and mixed layer average irradiance, on species composition across the GCB was assessed statistically. Nanophytoplankton (cells 2–20<span class="thinspace"></span>µm) were the numerically abundant size group of biomineralizing phytoplankton across the GCB, with the coccolithophore <i>Emiliania huxleyi</i> and diatoms <i>Fragilariopsis nana</i>, <i>F. pseudonana</i>, and <i>Pseudo-nitzschia</i> spp. as the most numerically dominant and widely distributed. A combination of SST, macronutrient concentrations, and <i>p</i>CO₂ provided the best statistical descriptors of the biogeographic variability in biomineralizing species composition between stations. <i>Emiliania huxleyi</i> occurred in silicic acid-depleted waters between the Subantarctic Front and the Polar Front, a favorable environment for this species after spring diatom blooms remove silicic acid. Multivariate statistics identified a combination of carbonate chemistry and macronutrients, covarying with temperature, as the dominant drivers of biomineralizing nanoplankton in the GCB sector of the Southern Ocean.</p>