An area of the remote Southern Ocean that's long confused ocean color satellites by reflecting large amounts of turquoise-colored light appears to be full of silica-rich diatoms, according to a new study. Surprisingly, there is also evidence in these polar waters of coccolithophores, a type of marine microalgae with elaborate calcium carbonate shells that plays a critical role in the global carbon cycle.
The study helps answer a longstanding mystery for satellite oceanographers as to what microbes dominate in this part of the ocean that's proven largely inaccessible, illuminating how the plankton community shifts in response to changing seawater temperature and chemistry. That, in turn, has important implications for the cycling of carbon in the Southern Ocean and the remote sensing tools scientists use to study it.
"This work takes a broad brush to understand the biological and geochemical dynamics of this far-flung body of water in ways that haven't been previously possible," said the study's lead author Barney Balch, a senior research scientist emeritus at Bigelow Laboratory for Ocean Sciences.
The study was published this week in the journal Global Biogeochemical Cycles .
In the early 2000s, Balch and colleagues identified a swath of seawater encircling Antarctica, that became known as the Great Calcite Belt. This area is marked by unusually high levels of particulate inorganic carbon, like calcium carbonate and limestone, that reflects light back to satellites. Scientists eventually confirmed that this was due to the shiny calcium carbonate shells of vast blooms of coccolithophores.
At the same time, though, they identified an area well south of the calcite belt that also appeared unusually bright in satellite images, even though the water was considered too cold for coccolithophores. This mystery has been harder to explain with heavy cloud cover, icebergs, and rough seas making it challenging to monitor this far south with either ships or satellites. Until now.
Researchers sailed aboard the R/V Roger Revelle from Hawaii, down to 60 degrees latitude, taking a brief easterly detour to monitor where water from further south appears to be pinched up into several eddies. Along the transect, the team measured ocean color; calcification and photosynthesis rates; and concentrations of inorganic carbon and silica, two minerals that reflect light and are critical for helping sequester organic carbon in the deep ocean.
"Satellites only see the top several meters of the ocean, but we were able to drill down with multiple measurements at multiple depths," Balch said. "We've never had such a complete suite of integrated measurements through the water column in this part of the ocean."
The multi-tiered approach — combining biogeochemical measurements, optical data, and even visual counts of microbes with microscopy — enabled the scientists to observe how the plankton community shifts moving south: from dinoflagellates in the warmer, stratified waters of the subtropics, to coccolithophores in the calcite belt, and finally to diatoms in the silica-rich, colder waters south of the Polar Front.
This combination of complementary methods provides a "smoking gun," Balch said, that the high levels of reflectance scientists have observed in satellite images south of the calcite belt can be explained by frustules. These silica-based structures that diatoms build, resembling microscopic pillboxes, reflect light in much the same way as coccolithophore shells (it takes far more frustules to produce the same optical effect as coccolithophores, though — a testament to how dense their concentration is).
Surprisingly, though, the team also observed small concentrations of inorganic carbon, some amount of calcification happening — a first — and visual evidence of coccolithophores in the far southern waters. This, Balch said, suggests that coccolithophores can survive in colder waters than expected. Perhaps, he said, the eddies coming up from the south even serve as "seed populations," providing a small but consistent stream of coccolithophores into the Great Calcite Belt.
The presence of coccolithophores across a wider geographic range than expected could influence how carbon moves through the Southern Ocean, which is considered one of the planet's most critical sinks for atmospheric carbon. Meanwhile, the dominance of diatoms south of the Polar Front highlights the need to improve the algorithms scientists use to translate satellite data into meaningful predictions of ocean biology. That means potentially combining measurements of other satellite-derived variables to help distinguish between diatoms and coccolithophores in satellite images.
"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 said. "There's nothing like measuring something multiple ways to tell a more complete story."
In addition to Balch, the interdisciplinary team includes Bigelow Laboratory researchers Dave Drapeau, Bruce Bowler, and Sunny Pinkham, as well as scientists from Woods Hole Oceanographic Institute, Arizona State University, Texas A&M University, and the Bermuda Institute of Ocean Sciences.
