A Giant Effort for Tiny Marine Plankton

BIOS research specialist Becky Garley worked on board the research vessel Roger Revelle during a cruise out of Hawaii this winter to the central Southern Ocean between New Zealand and South America, “about as far from land as you can get,” she said. She worked 12 hour shifts each day collecting water samples at a variety of depths to study coccolithophores, microscopic shell-building plankton that live in huge numbers throughout the ocean. Photo by Giuliana Viglione.

The quest to understand a very small, yet critically important, part of the marine food web proved especially challenging this spring during the ongoing global health pandemic. For participating scientists and university students, the process started in December 2020 with a 14-day quarantine period in Hawaii, where they were prohibited from leaving their hotel rooms, even for a walk. Then there were five COVID-19 tests for each (all negative). Next came days of sea travel past the Tahitian Islands and the Equator, with all 39 people on board the research vessel Roger Revelle wearing masks and trying to stay socially distant for the first two weeks of the trip (as much as possible on a 277-foot ship).  

Finally, the ship reached the central Southern Ocean between New Zealand and South America, “about as far from land as you can get,” said BIOS research specialist Becky Garley. Then they could start their study of coccolithophores, microscopic shell-building plankton that live in huge numbers throughout the ocean.

Coccolithophores, which play a large role in the marine food web, are of particular interest to those studying global climate change. As ocean acidity increases, their outer shells, called coccoliths, may become even more important as a carbon sink, or reservoir, for carbon-containing chemical compounds.

Under 17,500-times magnification, an image of the coccolithophore Syracosphaera mediterranea collected in the waters off Bermuda clearly shows its calcite coccoliths. These delicate structures make the organisms more susceptible to ocean acidification, as a lower pH can dissolve existing coccoliths and make it difficult for the coccolithophores to build new ones. (The “hat” visible on this coccolithophore likely landed on it from another species). Photo credit: Amos Winter, Indiana State University.

“We wanted to understand how these plankton change the chemistry of the water at the surface before it is subducted, or transported down, into the deep ocean,” said participating oceanographer Guiliana Viglione. They also wanted to learn how different nutrients in the marine environment, like nitrate and iron, affect the plankton’s growth. “Combining all this information with the bigger-picture will help us better understand how the water in this region absorbs carbon dioxide from the atmosphere and affects conditions elsewhere in the ocean,” she said.

The coccolithophore study was one of several of the cruise’s research objectives. Another was to collect data along a “transect,” or straight line, across part of the ocean. This route follows the course of a previously completed transect; oceanographers try to collect the same key data, such as temperature, salinity, and nutrient amounts, along pre-defined lines at routine intervals. This helps them understand how the ocean changes over time. It also provides a larger-scale view of their target area, which provides important context for understanding other collected information.

A Quest for Coccolithophores
Coccolithophores, algae covered in miniscule plates of the mineral calcite, bloom in the Southern Ocean region near the sea surface in austral summertime (December through February). It is estimated that coccolithophores produce more than 1.5 million tons of calcite each year, making them the ocean's leading producer of calcite and a potentially large influence on the global carbon cycle.

Unfortunately, the calcite coccoliths also make the coccolithophores susceptible to ocean acidification. Changing ocean pH may dissolve the coccoliths or make it more difficult for the coccolithophores to build their plates.

The start of this research, prompted by BIOS chemical oceanographer Nicholas Bates and colleagues at a variety of marine science institutes, was a 2008 project that looked at coccolithophores around the Falkland Islands in the South Atlantic. In subsequent years, Bates’ work with colleagues at BIOS and other organizations grew to include the study of a phenomenon in the Southern Ocean known as the Great Calcite Belt, which can appear in satellite images as an enormous band of milky water surrounding Antarctica.

Since the project began, scientists have been able to draw some conclusions. In a study published in 2017, “we found really interesting scientific issues with the coccolithophores in the Great Calcite Belt, including the bloom producing carbon dioxide,” Bates said. He was surprised to find that, in places, this actually reversed the direction of air-sea carbon dioxide exchange, turning parts of the Southern Ocean into sources of carbon dioxide flowing into the atmosphere, rather than the large carbon dioxide sink that is usually observed across the Southern Ocean.

 

Garley (left) and the expedition’s chief scientist, biological oceanographer Barney Balch of the Bigelow Laboratory for Ocean Sciences in Maine, gathered in the ship’s computer lab while the crew ran a CTD cast on deck. The monitors showed the cast profile, and allowed them to see and select depths for remotely closing bottles on the CTD rosette. The collected water samples at different depths provide a complete cast profile. Photo by Giuliana Viglione.

Twelve-Hour Shifts
Garley– who works with Bates and has participated on cruises to the Southern Ocean in 2011, 2012, and again this spring–worked a 12 hour shift every 2 a.m. to 2 p.m. while on the ship. (Her BIOS colleague, research specialist Matt Enright, worked the other 12 hours.) To sample continuously for carbonate chemistry in seawater at 12 different depths, ranging from surface waters to depths up to 19,000 feet (6,000 meters), they relied on an instrument called a CTD, used to collect measurements of conductivity (a measure of salinity), temperature, and depth.

They also analyzed surface seawater samples using the ship’s underway surface sampling equipment, which collects water every 20 to 30 minutes to measure acidity, carbon dioxide, and alkalinity. The ship’s system also collected a huge amount of data related to currents, bathymetric (or depth) measurements, meteorological data, and surface water properties, including temperature, salinity, fluorescence, and oxygen, which will be processed and studied in the months ahead.

When the samples arrive back at BIOS this spring after shipment from Hawaii, they will be re-processed for quality control checks, entered into data sheets, then combined with other lab groups’ biogeochemical data as a means to understand Southern Ocean ecosystem dynamics.

This research cruise was part of a collaborative National Science Foundation award to BIOS scientist Nicholas Bates and colleagues, including lead scientist Barney Balch (Bigelow Laboratory of Ocean Sciences), Dennis McGillicuddy (Woods Hole Oceanographic Institution), Peter Morton (Florida State University), and Matthew Long (National Center for Atmospheric Research/University Corporation for Atmospheric Research).