Space-based lasers track Earth's largest animal migration in unprecedented global study
A joint study by NASA and the French space agency, Centre National d'Etudes Spatiales (CNES), has used space-based lasers to produce the first global study of the great migration of tiny marine animals that takes place twice every day. Using the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite that was launched in 2006, the unprecedented 10-year study may provide new insights into the Earth's climate.
When we think of animal migrations we usually think of herds of wildebeests roaming across the African veldt or flocks of geese seeking warmer climes as winter approaches, but these only occur a couple of times of the year and are small potatoes to what happens in the vast oceans.
Twice a day, when the sun rises and then when it sets, there is a tremendous movement of animal life in the sea. Called the Diel Vertical Migration (DVM), it is the largest known migration in terms of both numbers and biomass. Huge schools of many different species, including krill, baby squid, larval crabs, and tiny fish take part in the DVM, but we don't notice it as a rule because these animals are tiny – mostly microscopic.
Put simply, at night, these animals, called collectively zooplankton, rise to near the surface to feed on the tiny plants, called phytoplankton. When the day returns, these zooplankton dive back into the depths to hide in the dark waters from predators.
This gigantic movement has great importance in a wide number of areas, so NASA and CNES used the CALIPSO Light Detection and Ranging (LIDAR) laser from 2008 to 2017 to penetrate the top 20 m (66 ft) of the ocean to detect and study these tiny animals on a global scale.
"What the lidar from space allowed us to do is sample these migrating animals on a global scale every 16 days for 10 years," says Mike Behrenfeld, the lead for the study and a senior research scientist and professor at Oregon State University in Corvallis, Oregon. "We've never had anywhere near that kind of global coverage to allow us to look at the behavior, distribution, and abundance of these animals."
There are numerous reasons to study the DVM. For decades, the major navies of the world have wanted to learn more about this layer of migrating animals because it is very noisy and reflects sonar signals – making it an ideal place for submarines to hide. In addition, zooplanktons are one of the major sources of food for bigger fish, and a larger the DVM signal indicates areas that can sustain larger fish stocks.
The the DVM is also important in terms of climatology. As phytoplankton indulges in a bit of photosynthesis, they capture a large amount of carbon dioxide. When the zooplankton eats them, they carry this carbon down into the deep ocean, where it can be trapped as the animal defecates or dies. The result could be a significant factor in understanding the Earth's carbon cycle and how this affects climate models.
"What these modelers haven't had is a global dataset to calibrate these models with, to tell them where these migrators are most important, where they're most abundant, and how they change over time," says Behrenfeld. "The new satellite data give us an opportunity to combine satellite observations with the models and do a better job quantifying the impact of this enormous animal migration on Earth’s carbon cycle."
The research was published in Nature.