Great Calcite Belt

Collaborative Research: The Great Southern Coccolithophore Belt

A consistent feature in all the global particulate inorganic carbon (PIC) imagery has been the presence of a great belt of elevated PIC concentrations near the sub-Antarctic front and polar front, all the way around the Southern Ocean. This large band of elevated PIC water (hereafter referred to as the “Great Belt”) has been observed all 7 years, with minor variation since the launch of MODISAqua. The great belt appears south of the ~30oS parallel and extends southwards to ~60oS with an area of ~88 x 106 km2.

The distribution and growth of coccolithophorids in the great belt may be influenced by the availability of micronutrient metals. The high latitudes of the southern hemisphere are generally characterized by low inputs of Aeolian dust resulting in potentially growth limiting Fe concentrations in many sectors of the Southern Ocean. Indeed, Fe limitation of phytoplankton growth in this High-Nutrient Low-Chlorophyll (HNLC) region has been demonstrated by numerous researchers conducting bottle incubations52, mesoscale fertilization experiments, and modeling studies. Most of these studies have observed diatoms to dominate the biological response to Fe enrichment, although Phaeocystis sp. can also respond to added Fe.

Work with cultured strains suggests that coccolithophores are well adapted to growth under low ambient iron conditions, enabling them to grow under conditions that limit coastal and even oceanic diatoms. Coccolithophores also appear to have an advantage with regards to Zn requirements and metabolism. Zinc is utilized by diatoms for carbonic anhydrase, however coccolithophores require very little, if any, CA since they can obtain CO2 formed as part of the calcification process. Additionally, coccolithophores can replace Zn with Co more effectively than can diatoms, further lowering their requirement for Zn relative to diatoms but imparting to coccolithophores an elevated Co requirement. Like Fe, Zn concentrations in oceanic surface waters are drawn down by biological uptake and export, with total dissolved Zn concentrations in sub-Antarctic and polar waters of the Pacific sector less than 0.5 nM. Dissolved Zn concentrations in the Atlantic Sector of the Southern Ocean are somewhat higher. However, deckboard incubation experiments indicate that Zn availability does not limit phytoplankton growth in the waters south of Australia and New Zealand. It thus appears that metal availability may provide a competitive advantage to coccolithophores growing in the calcite belt. However there may still be controlling interactions between trace metals and calcifying algae. Two recent studies in the sub-polar waters of the N. hemisphere have documented E. huxleyi to respond to Fe additions. Crawford69 also observed an increase in coccolithophore abundance in response to Zn alone. Additionally, macronutrient availability may also play a role in controlling the distribution of coccolithophores which have been shown to stop calcifying at high macronutrient concentrations 58, potentially explaining their absence south of the polar front. Diatoms are likely Si limited north of this boundary. Coccolithophore enrichment in the Great Belt to the east of the Patagonian Shelf suggests that Fe (or potentially other micronutrients supplied by the shelf or continental dust, such as Co) may be playing a role.