Robotic floats provide new look at ocean health and global carbon cycle


Date:
August 16, 2021
Source:
Monterey Bay Aquarium Research Institute
Summary:
Researchers demonstrated how a fleet of robotic floats can
revolutionize our understanding of ocean primary productivity.



FULL STORY ========================================================================== 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.


==========================================================================
In a new study in Nature Geoscience today, MBARI Senior Scientist Ken
Johnson and former MBARI postdoctoral fellow Mariana Bif demonstrated
how a fleet of robotic floats could revolutionize our understanding of
primary productivity in the ocean 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. 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 modeling 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.

Directly measuring productivity in the ocean requires collecting
and analyzing 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 offer the spatial and temporal resolution required.

"Satellites can be used to make global maps of primary productivity,
but the values are based on models and aren't direct measurements,"
cautioned Johnson.

"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.

These robots are giving scientists a glimpse at marine primary
productivity across area, depth, and time. They are dramatically
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. But this changed with the
deployment of Biogeochemical-Argo (BGC-Argo) floats across the globe.

"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.

But profiling floats only sample once every 10 days, and Johnson and
Bif needed multiple measurements in one day to get a daily cycle. A
novel approach to analyzing the float data allowed Johnson and Bif to
calculate ocean primary productivity. With each profiling float coming up
at a different time of day, combining data from 300 floats and samples
from various times of day allowed Johnson and Bif to recreate the daily
cycle of oxygen going up and down and then calculate primary productivity.

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 acquired 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 that phytoplankton produced about 53 petagrams of
carbon per year. This measurement was close to the 52 petagrams of carbon
per year estimated by the most recent computer models. (One petagram is 1,000,000,000,000 kilograms, or one gigaton, and roughly the equivalent
of the weight of 200 million elephants.) This study validated recent biogeochemical models and highlighted how robust these models have become.

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 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
that our estimates will be incorporated into models, including those
used for satellites, to improve their performance." But already, the
wealth of data from these floats has proved invaluable in bettering our understanding of marine primary productivity and how Earth's climate is
linked to the ocean.

The BGC-Argo floats have been instrumental to the Southern Ocean Carbon
and Climate Observations and Modeling project (SOCCOM), an NSF-sponsored program focused on unlocking the mysteries of the Southern Ocean and determining its influence on climate. And last year marked the debut of
the Global Ocean Biogeochemistry Array (GO-BGC Array) project, which will
allow scientists to pursue fundamental questions about ocean ecosystems, observe ecosystem health and productivity, and monitor the elemental
cycles of carbon, oxygen, and nitrogen in the ocean through all seasons
of the year.

The information gathered by these collaborative global initiatives
provides data essential to improving computer models of ocean fisheries
and climate and monitoring and forecasting the effects of ocean warming
and ocean acidification on marine life.

This work was supported by the Global Ocean Biogeochemical Array project
(NSF OCE-1946578), the Southern Ocean Carbon and Climate Observations and Modeling project (NSF PLR-1425989 and OPP-1936222), and the David and
Lucile Packard Foundation. Profiling floats in the Equatorial Pacific
were supported by NOAA under grant NA16OAR4310161 to the University
of Washington.

========================================================================== Story Source: Materials provided by
Monterey_Bay_Aquarium_Research_Institute. Note: Content may be edited
for style and length.


========================================================================== Journal Reference:
1. Johnson, K.S. and M.B. Bif. Constraint on net primary productivity
of the
global ocean by Argo oxygen measurements. Nature Geoscience,
2021 DOI: 10.1038/s41561-021-00807-z ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/08/210816112044.htm

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