Biogeochemical-Argo float

Robotic floats provide new insight into ocean health

Image credit: Natalie Freeman © 2019 SOCCOM

Marine scientists have shown how a fleet of robotic floats could provide important insight into ocean primary productivity on a global scale.

Data collected by these floats 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, according to researchers from the Monterey Bay Aquarium Research Institute (MBARI).

Microscopic marine life plays a fundamental role in the ocean’s health. 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.

Any changes in phytoplankton productivity can have profound consequences, such as affecting the ocean’s ability to store carbon and altering ocean food webs. Faced with 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 Ken Johnson, a senior scientist at MBARI.

By converting carbon dioxide into organic matter, phytoplankton not only supports oceanic food webs but are also the first step in the ocean’s biological carbon pump.

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. Directly measuring productivity in the ocean requires collecting and analysing samples. Limitations in resources and human effort make direct observations at a global scale with seasonal to annual resolution challenging and cost-prohibitive. Instead, remote sensing by satellites or computer-generated circulation models offers the spatial and temporal resolution required.

“Satellites can make global maps of primary productivity, but the values are based on models and aren’t direct measurements,” Johnson cautioned.

“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.

According to the scientists, these robots are giving them a glimpse of marine primary productivity across the area, depth, and time, and are transforming our ability to estimate how much carbon the global ocean accumulates each year. For example, the Indian Ocean and the middle of the South Pacific Ocean are regions where scientists have very little information about primary productivity. This changed with the deployment of Biogeochemical-Argo (BGC-Argo) floats across the globe.

The BGC-Argo profiling floats measure temperature, salinity, oxygen, pH, chlorophyll and nutrients. When scientists first deploy a BGC-Argo float, it sinks to 1,000m deep and drifts at this depth. Then, its autonomous programming gets to work profiling the water column. The float descends to 2,000m, then ascends to the surface. Once at the surface, the float communicates with a satellite to send its data to scientists onshore. This cycle is then repeated every 10 days.

To confirm the accuracy of the primary productivity estimates computed from the BGC-Argo floats, Johnson and Bif compared their float data to ship-based sampling data in two regions – the Hawaii Ocean Time-series (HOT) Station and the Bermuda Atlantic Time-series Station (BATS). The data gained from the profiling floats near those regions gave similar results as monthly sampling from ships at these two sites over many years.

Johnson and Bif found phytoplankton produced about 53 petagrams of carbon per year. This measurement was close to the 52 petagrams (one petagram is roughly the equivalent of the weight of 200 million elephants) of carbon per year estimated by the most recent computer models. This study validated recent biogeochemical models and highlighted how robust these models have become, the researchers said.

High-resolution data from the BGC-Argo floats can help scientists better calibrate computer models to simulate productivity and ensure they represent real-world ocean conditions. These new data will allow scientists to better predict how marine primary productivity will respond to changes in the ocean by simulating different scenarios, such as warming temperatures; shifts in phytoplankton growth; ocean acidification, and changes in nutrients. As more floats are deployed, Johnson and Bif expect the results of their study can be updated, decreasing uncertainties.

“We can’t yet say if there is a change in ocean primary productivity because our time series is too short,” cautioned Bif, “but it establishes a current baseline from which we might detect future change. We hope our estimates will be incorporated into models, including those used for satellites, to improve their performance.”

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