Space-Borne Lidar Enables Continuous View of Plankton Cycles in Earth’s Polar Regions

A space-based sensor that can penetrate fog, clouds and darkness has given scientists a decade-long set of images that provide a continuous view of polar phytoplankton biomass cycles. Traditional passive sensors, when used in regions that experience periods of constant night and prevailing low solar elevations, are unable to observe marine activity for many consecutive months each year. Through the use of space-borne lidar, scientists overcame the limitations of conventional passive satellite ocean color retrievals.

An artist's rendering shows a laser beam from a satellite-mounted lidar instrument probing the plankton-rich waters of Earth's northern and southern oceans. Courtesy of Tim Marvel, NASA.

The satellite-mounted lidar instrument, dubbed Cloud-Aerosol Lidar with Orthogonal Polarization, or CALIOP, uses a laser beam to map the ocean’s surface and immediate subsurface. CALIOP monitored plankton in the Arctic and Antarctic ocean waters from 2006 to 2015. Use of CALIOP led to the discovery that polar phytoplankton dynamics are characterized by “boom-bust” cycles resulting in slight imbalances in plankton predator-prey equiliabria.

“It’s really important for us to understand what controls these boom-bust cycles and how they might change in the future,” said professor Michael Behrenfeld of Oregon State University. “The dynamics of plankton communities have implications for all the other organisms throughout the [ocean's food] web.”

CALIOP, however, was engineered to take measurements of the atmosphere and does not have the resolution necessary to capture detailed information below the ocean’s surface. A higher-resolution instrument, now being developed at NASA but not yet deployed on a satellite, could collect subsurface samples at finely spaced depths as the laser pulse penetrates through the water column, allowing scientists to see the vertical structure of plankton blooms. That would reveal more about how plankton are being influenced by the ocean's currents and its other physical properties, Behrenfeld said.

The instrument could also determine what fraction of the signal is from the scattering of light versus the absorption of light.

"We can use the scattering information to quantify the concentration of the plankton, and we can use the absorption to say something about the plankton's physiology — in other words, the health of the cells," said Behrenfeld.

The CALIOP sensor is mounted on the CALIPSO satellite (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation), jointly owned by NASA and France's space agency.

The research was published in Nature Geoscience (doi: 10.1038/ngeo2861).