A Mysterious Glow in The Ocean Turns Out to Be Billions of Tiny Creatures : ScienceAlert

A beaming patch of turquoise amidst the Southern Ocean’s blue-grey waters that’s baffled scientists ever since it was first spotted in satellite images in the early 2000s may have finally been deciphered by oceanographers.

Mapping concentrations of phytoplankton and biogeochemical compounds has revealed a curious combination of microorganisms that challenges assumptions about the way the frigid waters of the world’s southernmost oceans soak up carbon.

To the north of the turquoise patch flows a reflective ring of seawater referred to as the great calcite belt. Also discovered around two decades ago, it has since been found to contain billions of alien-looking, sunlight-eating ‘coccolithophores’, named for their reflective scales known as coccoliths.

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Using inorganic carbon to make their calcite scales, the belt’s coccolithophores play an essential role in the global carbon cycle, concentrating an estimated 30 million tonnes of the element each year.

High concentrations of the coccoliths increase ocean reflectance, which satellite oceanographers typically use to estimate ocean calcite concentrations. The mysterious patch of glinting blue-green to the south of the great calcite ring could also be explained by coccoliths, if not for the fact the waters ought to be too cold for the microorganims to flourish.

Often concealed by rough seas, heavy clouds and icebergs, the turquoise tides have until now been difficult to peer into from space. Oceanographer Barney Balch and his colleagues decided the only way to find out what was really going on was to venture out to sea. As they write in their paper, “there have been few sea-truth measurements in the region due to its remoteness.”

Aboard the research vessel Roger Revelle, Balch and his team traveled from Hawaii toward the South Pole, passing through the great calcite belt, which, it being summer in the Southern Hemisphere, was in full bloom.

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“Satellites only see the top several meters of the ocean, but we were able to drill down with multiple measurements at multiple depths,” Balch explains. “There’s nothing like measuring something multiple ways to tell a more complete story.”

Those measurements included ocean color, calcification rate, photosynthesis rate, and most importantly, inorganic carbon and silica concentrations; minerals that represent coccolithophores and their rivals, diatoms, which make their own microscopic shells from silica glass.

Both of these planktons – diatoms and coccolithophores – fill such similar niches that they are destined competitors, sequestering organic carbon in the deep ocean and producing energy that feeds massive marine food chains that extend worldwide.

The great calcite belt has always been thought of as coccolithophore territory; anywhere south of its polar front is the realm of the diatoms.

“High-reflectance waters further south of the great calcite belt have been regularly observed, but questioned, due to the fact that coccolithophores are not typically found in such cold waters,” the authors write.

“Instead, it has been suggested that the elevated reflectance could be due to other high-reflectance materials such as loose ice, glacial flour, Phaeocystis [algae] blooms, increased incidence of bubbles, or other suspended particulate material like high concentrations of suspended opal associated with diatoms.”

Water samples returned not only the first proof of calcification happening in these southern waters, but direct visual evidence of coccolithophores living where no-one expected they could.

“Moderate concentrations of plated coccolithophores and detached coccoliths were observed south of the great calcite belt all the way to 60°S,” the authors report.

But a few stray coccoliths cannot reflect enough light to explain the brightness in satellite imagery.

Diatoms, it seems, are so dense in these waters that their glassy, reflective structures were able to produce a similar optical effect as coccolithophores.

“Our results suggest that these highly reflective polar waters result from scattering by diatom frustules, not coccolithophores, and have been misidentified as particulate inorganic carbon in satellite measurements,” they write.

Balch and colleagues suggest the way satellites estimate particulate organic carbon will have to be reviewed in light of all this.

“We’re expanding our view of where coccolithophores live and finally beginning to understand the patterns we see in satellite images of this part of the ocean we rarely get to go to,” Balch says.

This research was published in Global Biogeochemical Cycles.