A study from the US Department of Energy’s Lawrence Berkeley National Laboratory has revealed that most of the carbon from plankton blooms does not reach the deep ocean.
This discovery could deal a blow to the proponents of the Iron Hypothesis who believe that global warming can be slowed or even reversed by fertilizing plankton with iron to induce rapid growth or bloom.

Oceanographers Jim Bishop and Todd Wood of the Lawrence Berkeley National Laboratory have measured the fate of carbon particles originating in plankton blooms in the Southern Ocean, using data that deep-diving Carbon Explorer floats collected around the clock for over a year.

“Just adding iron to the ocean hasn’t been demonstrated as a good plan for storing atmospheric carbon,” says Bishop, a member of Berkeley Lab’s Earth Sciences Division and a professor of Earth and planetary sciences at the University of California at Berkeley. “What counts is the carbon that reaches the deep sea, and a lot of the carbon tied up in plankton blooms appears not to sink very fast or very far.”

The Carbon Explorers involved in the study were launched in January, 2002, as part of the Southern Ocean Iron Experiment (SOFeX), a collaboration led by scientists from Moss Landing Marine Laboratory and the Monterey Bay Aquarium Research Institute.

SOFeX was meant to test the Iron Hypothesis in waters between New Zealand and Antarctica during the Antarctic summer. The Berkeley Lab Carbon Explorers were originally intended to monitor the iron-fertilization experiment for 60 days, but they continued to report by satellite throughout the Antarctic fall and winter and on into the following year.

“We would never have made these surprising observations if the autonomous Carbon Explorer floats hadn’t been recording data 24 hours a day, seven days a week, at depths down to 800 meters or more, for over a year after the experiment’s original iron signature had disappeared,” Bishop said.

Previous studies of iron-fertilization and its effects were based on averaging measurements that were made from ships at intervals widely separated in time. Cost made continuous ship-based observations impossible.

The Carbon Explorer floats, which cost only about as much as a single day of ship time, proved to be an affordable way to continuously observe the “biological pump,” how ocean life circulates carbon, and test the Iron Hypothesis.

The Iron Hypothesis came about in the 1980s when oceanographer, John Martin, of the Moss Landing Marine Laboratories, proposed that iron be added to regions of the ocean which are rich in nutrients but poor in iron (high-nutrient, low-chlorophyll or HNLC) to stimulate the growth of phytoplankton. This growth could also change the climate.
Oceans are the largest active carbon dioxide (CO2) sinks on earth. The biological pump transports carbon to the ocean floor. Algae absorb carbon and, when algae dies, sinks to the bottom of the ocean floor, taking carbon with it.

In testing the Iron Hypothesis, SOFeX’s investigators acknowledged that matters were not quite that simple, and that the crucial question was not whether plankton blooms could be induced but whether the carbon they captured was removed to the deep sea.

The SOFeX research vessels fertilized and measured two regions of ocean, one in an HNLC region at latitude 55 degrees south and another at 66 degrees south. Carbon Explorers were launched at both these sites; a third Carbon Explorer was launched well outside the iron-fertilized region at 55°S as a control.

One question was whether the relatively silicate-poor waters of the more northerly 55° region would allow plankton, known as diatoms, to form silicon skeletons. If large diatoms could not grow in this HNLC region, the SOFeX researchers theorized, enhanced carbon sinking would not occur. Partly for this reason, most of the effort by the ships was at 66°S, where silicon was not considered a limiting factor.

To the researchers’ surprise, the iron-augmented region at 55°N did form a vigorous plankton bloom. Dubbed the North Patch, Carbon Explorers tracked this bloom throughout the Antarctic summer, measuring carbon particles, including waste from grazing zooplankton and other aggregates, sinking beneath the bloom and carrying 10% to 20% of the fixed carbon out of the surface layer – at least to below 100 meters.

The two North Patch floats operated for over 14 months, diving, recording, and surfacing to report data in the world’s stormiest waters, traveling almost to South America before falling silent. The Carbon Explorer launched at 66° south lasted 18 months, spending much of its first winter recording at 800 meters depth and surfacing at weekly intervals to report, although occasionally prevented by bumping into the underside of the sea ice. The Carbon Explorer data accumulated, awaiting analysis.

The observed results showed that while plankton bloom did occur, strong sedimentation or the sinking of large amounts of carbon into the deep ocean was not observed. Particulate carbon levels were severely reduced with the onset of perpetual darkness and sea ice formation, and then modestly increased again with the return of light and the melting of the ice.

Data from the two floats deployed farther north at 55°S produced a startling picture. As noted above, the Explorer launched inside the original SOFeX iron-amended region, dubbed 55A, had recorded a long-lasting bloom immediately following iron fertilization, with sedimentation to 100 meters. Remarkably, 55A found an equally strong bloom the next spring in the same region – long after the SOFeX iron was gone – with equally strong sedimentation down to 100 meters.

And while it was unsurprising that the original control float, dubbed 55C, did not see a bloom, a bigger surprise came at depth: a rain of particulate organic carbon at 800 meters down. Carbon Explorer 55C had measured sedimentation beneath a region with no plankton bloom much greater than the 55A float measured under the bloom itself.

To explain this counterintuitive “High Biomass, Low Export” result the researchers considered a number of ideas, the most convincing being one that took into account the fact that lighting conditions encourage or limit the growth of algae and other zooplankton.

Latitude 55°S is far enough north for light to reach into the water year round (although in winter it is much reduced). But mixing between near-surface and deeper waters can carry phytoplankton too deep to grow – out of the light, beneath the critical depth where growth is sufficient to meet the energy demands of the whole plant and animal community.

To survive the winter, zooplanktons have to stay deep, where it is too dark for the phytoplankton they live on to survive. Storms and deep mixing keep the zooplanktons alive. When mixing is consistently below the critical level, the phytoplankton are carried out of the light and cannot grow, but when mixing stops in the spring, the phytoplankton rebound.

Sinking carbon particles may be abundant at 100 meters down, but the starving animals intercept all the material they can reach as it falls between 100 and 800 meters, explaining why Carbon Explorer 55A observed a low carbon export to the deep ocean beneath the lush bloom.

Continual deep mixing can starve zooplankton, but if the mixing is regularly interrupted, more phytoplankton grows during the winter to supply the zooplankton lurking at depth. In the region where Carbon Explorer 55C spent the winter, storms were intermittent and mixing below 400 meters was interrupted on a daily basis. When phytoplankton growth began in the spring, the healthy zooplankton were there to “mow the lawn,” as it were – probably accounting for the modest phytoplankton growth near the surface, with increased carbon sedimentation from the zooplankton.

The Iron Hypothesis isn’t wrong, but it’s much more subtle than usually stated. Achieving optimum carbon sedimentation from plankton growth may require the right “recipe” of iron and other trace nutrients to grow the right kind of phytoplankton.
This research was funded by the Biological and Environmental Research Program of the US Department of Energy’s Office of Science, and by the National Oceanographic Partnership Program administered by the Office of Naval Research.


-  Katrice R. Jalbuena

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Source:

¹ http://newscenter.lbl.gov/press-releases/2009/05/06/ocean-carbon-iron/