A new study using a NASA satellite instrument orbiting Earth has found that small, environmental changes in polar food webs significantly influence the boom-and-bust, or peak and decline, cycles of phytoplankton. These findings will supply important data for ecosystem management, commercial fisheries and our understanding of the interactions between Earth’s climate and key ocean ecosystems.
“It’s really important for us to understand what controls these boom-and-bust cycles, and how they might change in the future so we can better evaluate the implications on all other parts of the food web,” said Michael Behrenfeld, a marine plankton expert at Oregon State University in Corvallis.
Phytoplankton also influence Earth’s carbon cycle. Through photosynthesis, they absorb a great deal of the carbon dioxide dissolved in the upper ocean and produce oxygen, which is vital for life on Earth. This reduces the amount of carbon dioxide in the atmosphere.
Behrenfeld, along with scientists from NASA’s Langley Research Center in Hampton, Virginia, and several other institutions collaborated on the study. The findings were published Monday in Nature Geoscience.
Coastal economies and wildlife depend on what happens to tiny green plants, or phytoplankton, at the base of the ocean food chain. Commercial fisheries, marine mammals and birds all depend on phytoplankton blooms. The new study shows that accelerations in growth rate cause blooms by allowing phytoplankton to outgrow the animals that prey on them. When this happens, the phytoplankton populations rapidly increase.
However, as soon as that acceleration in growth stops, the predatory animals catch up by eating the ocean plants and the bloom ends. This new understanding goes against traditional theories that blooms only occur when phytoplankton growth rates exceed a specific threshold of fast growth and that they end when these growth rates fall below that threshold again.
Behrenfeld compares the new idea to two rubber balls connected by a rubber band.
“A green ball represents the phytoplankton. A red one represents all the things that eat or kill the phytoplankton,” he said. “Take the green ball and whack it with a paddle. As long as that green ball accelerates, the rubber band will stretch and the red ball won’t catch the green ball. As soon as the green ball stops accelerating, the tension in the rubber band will pull that red ball up to it and the red ball will catch the green ball.”
NASA’s Cloud-Aerosol LIdar with Orthogonal Polarization (CALIOP), an instrument aboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite launched in 2006, uses a laser to take measurements. Scientists used the instrument to continuously monitor plankton in polar regions from 2006 to 2015.
“CALIOP was a game-changer in our thinking about ocean remote sensing from space,” said Chris Hostetler, a research scientist at Langley. “We were able to study the workings of the high-latitude ocean ecosystem during times of year when we were previously completely blind.”
Ocean ecosystems typically are monitored with satellite sensors that simply measure sunlight reflected back to space from the ocean. These instruments have a problem seeing the ocean plankton in polar regions because of limited sunlight and persistent clouds that obscure their view of the ocean surface. The lidar shines its own light – a laser – and can illuminate and measure the plankton day or night, in between clouds, and even through some clouds.
The study also reveals that year-to-year variations in this constant push and pull between predator and prey have been the primary driver of change in Arctic plankton stocks over the past decade. In the Southern Ocean around Antarctica, though, changes in the ice cover were more important to phytoplankton population fluctuations than were differences in growth rates and predation.
“The take home message is that if we want to understand the biological food web and 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 current CALIOP lidar was engineered to take atmospheric measurements, not optimized for ocean measurements. Nonetheless, the CALIOP ocean measurements are scientifically valuable, as demonstrated by the results of this study.
New lidar technology is being tested that would allow scientists to better measure how phytoplankton are distributed through the sunlit layer of the ocean. This new capability will improve knowledge of phytoplankton concentrations and photosynthesis and will reveal more about the causes of phytoplankton blooms. This knowledge is critical for understanding cycling of ocean carbon, and for determining and managing the health of global ocean ecosystems.
The CALIPSO satellite mission is a collaboration between NASA and France’s space agency, the Centre National d’Etudes Spatiales. The University of Maine in Orono, the University of California, Santa Barbara, and Princeton University also participated in the study.
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