Microscopic marine life plays a fundamental role in the health of the ocean and, ultimately, the planet. Just like plants on land, tiny phytoplankton use photosynthesis to consume carbon dioxide and convert it into organic matter and oxygen. This biological transformation is known as marine primary productivity.
A fleet of robotic floats could revolutionize our understanding of primary productivity in the ocean on a global scale, according to scientists from the Monterey Bay Aquarium Research Institute (MBARI).
Data collected by the robots will allow scientists to more accurately estimate how carbon flows from the atmosphere to the ocean and shed new light on the global carbon cycle, Changes in phytoplankton productivity can have profound consequences, like affecting the ocean’s ability to store carbon and altering ocean food webs. In the face of a changing climate, understanding the ocean’s role in taking carbon out of the atmosphere and storing it for long periods of time is imperative.
“Based on imperfect computer models, we’ve predicted primary production by marine phytoplankton will decrease in a warmer ocean, but we didn’t have a way to make global-scale measurements to verify models. Now we do,” said MBARI Senior Scientist Ken Johnson.
By converting carbon dioxide into organic matter, phytoplankton not only support oceanic food webs, they are the first step in the ocean’s biological carbon pump.
Phytoplankton consume carbon dioxide from the atmosphere and use it to build their bodies. Marine organisms eat those phytoplankton, die, and then sink to the deep seafloor. This organic carbon is gradually respired by bacteria into carbon dioxide. Since a lot of this happens at great depths, carbon is kept away from the atmosphere for long periods of time. This process sequesters carbon in deep-sea water masses and sediments and is a crucial component in modelling Earth’s climate now and in the future.
Marine primary productivity ebbs and flows in response to changes in our climate system. “We might expect global primary productivity to change with a warming climate,” explained Johnson. “It might go up in some places, down in others, but we don’t have a good grip on how those will balance.” Monitoring primary productivity is crucial to understanding our changing climate, but observing the response on a global scale has been a significant problem.
“Scientists estimate about half of Earth’s primary productivity happens in the ocean, but the sparsity of measurements couldn’t give us a reliable global estimate for the ocean yet,” added Mariana Bif, a biogeochemical oceanographer and a former postdoctoral fellow at MBARI. Now, scientists have a new alternative for studying ocean productivity—thousands of autonomous robots drifting throughout the ocean.
“This work represents a significant milestone in ocean data acquisition,” emphasized Bif. “It demonstrates how much data we can collect from the ocean without actually going there.”
The BGC-Argo profiling floats measure temperature, salinity, oxygen, pH, chlorophyll, and nutrients. When scientists first deploy a BGC-Argo float, it sinks to 1,000 meters (3,300 feet) deep and drifts at this depth. Then, its autonomous programming gets to work profiling the water column. The float descends to 2,000 meters (6,600 feet), then ascends to the surface. Once at the surface, the float communicates with a satellite to send its data to scientists on shore. This cycle is then repeated every 10 days.
For the past decade, an increasing fleet of BGC-Argo floats has been taking measurements across the global ocean. The floats capture thousands of profiles every year. This trove of data provided Johnson and Bif with scattered measurements of oxygen over time.
Knowing the pattern of oxygen production allowed Johnson and Bif to compute net primary productivity at the global scale.
During photosynthesis, phytoplankton consume carbon dioxide and release oxygen at a certain ratio. By measuring how much oxygen phytoplankton release over time, researchers can estimate how much carbon phytoplankton produce and how much carbon dioxide they consume. “Oxygen goes up in the day due to photosynthesis, down at night due to respiration—if you can get the daily cycle of oxygen, you have a measurement of primary productivity,” explained Johnson. Although this is a well-known pattern, this work represents the first time that it has been quantitatively measured by instruments at the global scale rather than estimated through modeling and other tools.
Johnson, K.S. and M.B. Bif (2021). Constraint on net primary productivity of the global ocean by Argo oxygen measurements. Nature Geoscience. doi.org/10.1038/s41561-021-00807-z