NASA's space-based sensor maps plankton behaviour on Earth

A NASA space-based sensor that can 'see' through fog, clouds and darkness has given scientists the first continuous look at the boom-bust cycles of polar phytoplanktons - microscopic marine plants that are the foundation of the ocean's food web. The decade-long set of images shows that phytoplankton cycles are more tied to the push-pull relationship between them and their predators than was initially thought.

Phytoplankton are the foundation of the ocean's food web. Commercial fisheries, marine mammals and birds all depend on the blooms, said Michael Behrenfeld from the Oregon State University's College of Agricultural Sciences. "It's really important for us to understand what controls these boom-bust cycles and how they might change in the future, because the dynamics of plankton communities have implications for all the other organisms throughout the web," Behrenfeld said.

Phytoplankton also influence Earth's carbon cycle. Through photosynthesis, they absorb a great deal of the carbon dioxide near the ocean's surface. That, in turn, allows carbon dioxide from the atmosphere to go into the ocean. The satellite-mounted LIDAR instrument, dubbed Cloud-Aerosol Lidar with Orthogonal Polarisation (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. CALIOP'S measurements show that, as the phytoplankton growth accelerates, the blooms are able to outpace the organisms that prey on them. As soon as that acceleration stops, however, the predatory organisms catch up and the bloom ends.

The finding goes against the commonly held belief that blooms begin when phytoplankton growth rates reach a threshold rate and then stop when growth rates crash, he said. Instead, blooms start when growth rates are extremely slow, and then stop when phytoplankton growth is at its maximum but the acceleration of the bloom has hit its peak. It is only then that the predatory organisms catch up and the bloom terminates.

The study also shows that in Arctic waters the year-to-year changes in this constant push and pull between predator and prey has been the primary driver of change over the past 10 years. The situation is different in the southern ocean around Antarctica, where changes in the ice cover held more sway. 

"The take-home message is that if we want to understand the production of the polar systems as a whole, we have to focus both on changes in ice cover and changes in the ecosystems that regulate this delicate balance between predators and prey," said Behrenfeld. The study was published in the journal Nature Geoscience.