The greatest animal migration on Earth is likely something you never heard of and few have witnessed: legions of tiny marine creatures rising to the ocean surface every night to feed on tiny plants, then sinking back into the deep, dark water at dawn.
Called the diel vertical migration, it was first recorded nearly 200 years ago by hauling ship nets through the water column. Today, marine scientists still sample the movement using shipboard nets. They also shoot acoustic signals into the water to track the sound "backscattering" off the zooplankton as they migrate up and down. Some collect data from aircraft, deploying a lidar system that uses a kind of laser radar to create the backscatter.
But such methods take time and resources and are limited.
Then, a couple years ago, oceanographers working with NASA Langley Research Center in Virginia discovered that NASA's CALIPSO orbiting satellite and its lidar instrument -- built to study clouds, not oceans -- has been collecting data on this migration on a global scale by happenstance since 2006.
Scientists were astonished -- and delighted.
"The lidar has given us our first measurements of animals -- in fact, animal behavior -- from space," said Michael Behrenfeld, a biological oceanographer at Oregon State University. "What (it) has allowed us to do for the first time is actually to study this migration on a global scale every 16 days for 10 years. And that's a very powerful place to be."
Behrenfeld is lead author of a paper on this research that recently published in the science journal Nature.
Behrenfeld has collaborated for years with Yongxiang Hu, an atmospheric scientist at Langley, on ways to use NASA's current assets in space to collect ocean data. Measurements taken by CALIPSO's lidar instrument, called CALIOP, enabled him to study plankton biomass in the surface oceans, for instance, and changes in polar ecosystems.
Then one night on a research cruise in the North Atlantic, Behrenfeld noticed a wedge of phytoplankton eaten away by migrating animals in a very short period of time.
"I looked at it and said, 'This is something I need to pay attention to,' " Behrenfeld recalled.