Polynyas, Diatoms and CO2: How the Southern Ocean's Plankton Could Shift

Phytoplankton, the drifting, mostly single-celled algae and bacteria at the base of the marine food chain, are thought to produce at least 40 percent of the food made by photosynthesis on Earth each year. Because they take up carbon dioxide, they help keep the greenhouse gas in check: the more of them, the more carbon is pulled from the air. Work by a Stanford University geophysicist and colleagues, described in this NASA Earth Observatory feature, suggested that rising carbon dioxide could change which phytoplankton rule the Southern Ocean, with consequences for the climate itself.

Two kinds of plankton

In Antarctic and Arctic waters the main phytoplankton are Phaeocystis antarctica and diatoms.

• Diatoms, the dominant photosynthesizers of the world ocean, thrive where melting sea ice adds fresh water and leaves a shallow, strongly layered (stratified) surface.
• P. antarctica, single-celled algae that can grow in lower light, prefer deeper, well-mixed water.

They also differ chemically: P. antarctica takes up carbon dioxide more efficiently, with a ratio of carbon to phosphorus uptake nearly twice that of diatoms. If the community shifted from P. antarctica to diatoms, the scientists wrote in Science, the ocean's biological capacity to draw carbon dioxide from the air could fall dramatically.

Diatoms build a glass (silica) shell; these chain-forming diatoms are from the San Francisco Bay area. USGS.

Why the ocean's layers matter

Phytoplankton cannot swim; they live near the surface where light is plentiful and are at the mercy of the currents, so the physics of the ocean governs how fast they grow. Seawater forms layers of different density, temperature and salinity, and the surface becomes more stratified when it warms or when rain adds fresh water. Climate models indicated that over the next half century more precipitation over the Southern Ocean could increase that stratification, which would favour diatoms, since they prefer poorly mixed water.

Polynyas in the Ross Sea

The team built a model of the link between ocean physics and phytoplankton in the Ross Sea, long considered one of the Southern Ocean's most productive regions and home to its largest and most persistent polynyas, open water surrounded by ice, where wind blows the ice away or warm water from below melts it. Getting models to form polynyas in the right places is hard, and the biology depends on it. Because freezing seawater releases salt, the model used sea ice concentrations, from a long record of satellite microwave measurements held by the National Snow and Ice Data Center, to calculate the ocean's salt and heat balance. A surprise: much of the Southern Ocean's productivity happens in the ice, not in the water.

The models of productivity combine light, nutrients and chlorophyll, measured from space as subtle changes in ocean colour caused by plant pigments invisible to the eye. They resolved an old paradox. Earlier estimates of the Southern Ocean's yearly production were too low to support the Antarctic food web; the satellite-based model put it at four to five times higher than estimates from ship measurements.

Winds, polynyas and blooms

Comparing ocean pigments from the Coastal Zone Color Scanner with microwave sea ice data, the team linked the timing of polynya formation to the timing of the phytoplankton bloom in the southwestern Ross Sea. When the polynya opens early, stronger and more frequent winds mix plankton out of the surface and delay the bloom; if it opens after the winds ease, the bloom comes sooner.

Biological responses to changing ocean physics had been poorly understood, so including the shift between plankton types in large climate models was a first. The Southern Ocean, remote and little studied compared with the Pacific or Indian oceans, is among the richest regions on Earth for biology and chemistry, and knowing when and where its phytoplankton grow is crucial to understanding its part in the global carbon cycle.

Sources

• NASA Earth Observatory, "Polynyas, CO2, and Diatoms in the Southern Ocean"; rewritten in hubnx's own words. Photograph: USGS. Satellite images co-credited to a company, a journal figure and an uncredited image on the original page are not reproduced here.