Scientists have uncovered the ocean conditions that support a massive summertime bloom of algae that spans 16 percent of the global ocean. Known as the Great Calcite Belt, this dense group of a microscopic phytoplankton, coccolithophores, can be seen in satellite images as turquoise swirls in the dark blue water of the Southern Ocean.
“Satellites see the light reflected by the coccolithophores,” said Barney Balch, a senior research scientist at Bigelow Laboratory for Ocean Sciences. “Under a high-powered microscope, you can see this is because they surround themselves with intricate, white plates made of calcium carbonate. These chalk plates act like millions of tiny suspended mirrors, reflecting sunlight back out of the ocean toward NASA’s Earth-observing satellites.”
Balch was part of an international team of researchers from Bigelow Laboratory, University of Southampton (UK), the National Oceanography Centre in Southampton, and the Bermuda Institute of Ocean Science that studied the conditions that make the bloom possible and the ecology of the phytoplankton species that compose it. They recently published their results in the journal Biogeosciences.
“The links between ocean chemistry and populations of coccolithophores and diatoms are not straightforward,” said lead author Helen Smith, a researcher from the University of Southampton. “To fully understand the interplay between these two important phytoplankton groups and the ocean environment, we had to maintain a holistic approach to data collection and analysis.”
The team found that sea surface temperature, nutrient levels, and carbon dioxide concentration were the most important factors in determining where species of coccolithophores and diatoms, another type of microscopic phytoplankton, grow. As expected, dissolved iron was a key factor in controlling the plankton populations.
“All phytoplankton need iron to grow, and it is generally in short supply in the Southern Ocean,” said Ben Twining, a senior research scientist and interim president at Bigelow Laboratory. “Coccolithophores – but not necessarily diatoms – were more abundant at locations with elevated iron.”
Indeed, diatoms also need silica to build their glass exoskeletons. Great Calcite Belt water doesn’t have enough silicic acid to support large diatoms, which are typically found in the most productive parts of the world’s oceans. This creates an opportunity for species of tiny phytoplankton to thrive, including coccolithophores and extremely small diatom species. As a result, small phytoplankton dominate the region.