The Great Calcite Belt can be defined as an elevated particulate inorganic carbon (PIC) feature occurring alongside seasonally elevated chlorophyll a in austral spring and summer in the Southern Ocean. It plays an important role in climate fluctuations, accounting for over 60% of the Southern Ocean area (30–60° S). The region between 30° and 50° S has the highest uptake of anthropogenic carbon dioxide (CO) alongside the North Atlantic and North Pacific oceans. Knowledge of the impact of interacting environmental influences on phytoplankton distribution in the Southern Ocean is limited. For example, more understanding is needed of how light and iron availability or temperature and pH interact to control phytoplankton biogeography. Hence, if model parameterizations are to improve to provide accurate predictions of biogeochemical change, a multivariate understanding of the full suite of environmental drivers is required.
The Southern Ocean has often been considered as a microplankton-dominated (20–200 µm) system with phytoplankton blooms dominated by large diatoms and Phaeocystis sp.  However, since the identification of the Great Calcite Belt (GCB) as a consistent feature and the recognition of picoplankton (< 2 µm) and nanoplankton (2–20 µm) importance in high-nutrient, low-chlorophyll (HNLC) waters, the dynamics of small (bio)mineralizing plankton and their export need to be acknowledged. The two dominant biomineralizing phytoplankton groups in the GCB are coccolithophores and diatoms. Coccolithophores are generally found north of the polar front, though Emiliania huxleyi has been observed as far south as 58° S in the Scotia Sea, at 61° S across Drake Passage, and at 65°S south of Australia.
Diatoms are present throughout the GCB, with the polar front marking a strong divide between different size fractions. North of the polar front, small diatom species, such as Pseudo-nitzschia spp. and Thalassiosira spp., tend to dominate numerically, whereas large diatoms with higher silicic acid requirements (e.g., Fragilariopsis kerguelensis) are generally more abundant south of the polar front. High abundances of nanoplankton (coccolithophores, small diatoms, chrysophytes) have also been observed on the Patagonian Shelf and in the Scotia Sea. Currently, few studies incorporate small biomineralizing phytoplankton to species level. Rather, the focus has often been on the larger and noncalcifying species in the Southern Ocean due to sample preservation issues (i.e., acidified Lugol’s solution dissolves calcite, and light microscopy restricts accurate identification to cells > 10 µm. In the context of climate change and future ecosystem function, the distribution of biomineralizing phytoplankton is important to define when considering phytoplankton interactions with carbonate chemistry, and ocean biogeochemistry.
The Great Calcite Belt spans the major Southern Ocean circumpolar fronts: the Subantarctic front, the polar front, the Southern Antarctic Circumpolar Current front, and occasionally the southern boundary of the Antarctic Circumpolar Current. The subtropical front (at approximately 10 °C) acts as the northern boundary of the GCB and is associated with a sharp increase in PIC southwards. These fronts divide distinct environmental and biogeochemical zones, making the GCB an ideal study area to examine controls on phytoplankton communities in the open ocean. A high PIC concentration observed in the GCB (1 µmol PIC L) compared to the global average (0.2 µmol PIC L) and significant quantities of detached E. huxleyi coccoliths (in concentrations > 20,000 coccoliths mL) both characterize the GCB. The GCB is clearly observed in satellite imagery spanning from the Patagonian Shelf across the Atlantic, Indian, and Pacific oceans and completing Antarctic circumnavigation via the Drake Passage.