According to the simplest version of the so called Iron Hypothesis, plankton blooms move atmospheric carbon down to the deep sea and increased carbon dioxide levels in the atmosphere can therefore be counteracted by promoting plankton blooms. The Iron Hypothesis derives its names from the suggestion that global warming can be thwarted by fertilizing plankton with iron in regions that are iron-poor but rich in other nutrients like nitrogen, silicon, and phosphorus, such as the Southern Ocean.

New research from the U.S. Department of Energy’s Lawrence Berkeley National Laboratory is now dealing a powerful blow to this hypothesis by showing that most of the carbon used for plankton blooms never reaches to deep sea.

Using data collected around the clock for over a year by deep-diving Carbon Explorer floats, Oceanographers Jim Bishop* and Todd Wood** have revealed that a lot of the carbon tied up by plankton blooms never sink very far.

“Just adding iron to the ocean hasn’t been demonstrated as a good plan for storing atmospheric carbon,” says Bishop. “What counts is the carbon that reaches the deep sea, and a lot of the carbon tied up in plankton blooms appear not to sink very fast or very far.”

The reasons behind this behaviour are complex, but the seasonal feeding behaviour of planktonic animal life is believed to play major part.

Photoplankton

The Carbon Explorer floats used in the study were launched in January 2002, as a part of the Southern Ocean Iron Experiment (SOFeX)***, and experiment meant to test the Iron Hypothesis in the waters between New Zealand and Antarctica during the Antarctic summer.

SOFeX fertilized and measured two regions of ocean, one in an HNLC (high-nutrient, low-chlorophyl) region at latitude 55 degrees south and another at 66 degrees south. Carbon Explorers were launched at both these sites while a third Carbon Explorer was launched well outside the iron-fertilized region at 55 degrees south as a control.

Bishop and Wood were originally assigned to the project to monitor the iron-fertilization experiment for 60 days, but the Carbon Explorers continued to transmit data throughout the Antarctic fall and winter and on into the following spring.

“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 explains. “Assumptions about the biological pump – the way ocean life circulates carbon – are mostly based on averaging measurements that have been made from ships, at intervals widely separated in time. Cost, not to mention the environment, would have made continuous ship-based observations impossible in this case. Luckily one Carbon Explorer float costs only about as much as a single day of ship time.”

The scientific hypothesis that iron can be used to stimulate phytoplankton growth in regions low in iron but rich in other nutrients is still intact and experiments show that algal blooms do in fact occur if you add iron to such waters. The study by Bishop and Wood only shows that the carbon bound by the plankton do not end up far down in the depths of the sea.

Jim Bishop’s team. (From left) Christopher Guay,
Phoebe Lam, Jim Bishop, Todd Wood, and David Kaszuba.

During the early stages of the South Sea experiment, the Iron Hypothesis seemed to hold up to scrutiny as the Berkeley researchers could detect not only a vigorous plankton bloom in the fertilized region at 55°N, but also how carbon particles sank beneath the bloom carrying 10-20 percent of the fixed carbon away from the surface layer and down to a dept of at least 100 meters. These results were published in the 2004April issue of Science.

But since the Carbon Explorers continued to submit information even when the 3-month study was officially over, Bishop and Wood could continue their monitoring of South Sea carbon levels throughout fall and winter and well into the following spring; a continued monitoring that would prove invaluable.

The two Carbon Explorers released at 55 degrees south continued to report for over 14 months and almost reached South America before they turned silent. After this, the explorer launched at 66 degrees south continued to transmit for another four months, despite having spent much of the Arctic winter recording at a dept of 800 meters where the pressure is immense. This explorer also had several encounters with the underside of chunky sea ice as it tried to surface to report during the Arctic winter.

All this new data surprisingly showed that there seemed to be much less particulate matter reaching the depth where the biomass was highest, i.e. in plankton blooms. Reports from the 66°S explorer showed how particulate carbon levels decreased sharply as the perpetually dark Arctic winter commenced and ice began to cover previously open waters. As the sun returned in spring and melted the ice the levels made a modest increase, but no sinking (sedimentation) of large amounts of carbon to the deep ocean was observed.

Another even more surprising report came from the control float, dubbed 55 C, which reported higher sedimentation of carbon 800 meters under a region with no plankton bloom than what the other 55°S (dubbed 55A) reported from the fertilized, blooming region.

Researchers are currently pondering several ideas as to explain these unforeseen results but have not reached any conclusion. A higher biomass seems to be linked to a lower export of carbon, but one knows why. One of the most promising hypotheses takes into account how phytoplankton needs sufficient amounts of light to survive and grow. Latitude 55°S is located far enough from the Arctic for light to reach the ocean year round, even though the amount is severely reduced during the winter months. But the notorious winter storms occurring in these waters can cause mixing between near-surface water and underlying water layers all the way down to a dept of 400-500 meters. Phytoplankton are dragged down to depths where it is too dark for them to grow and where hungry zooplankton waits for them.

“Mixing is the dumbwaiter that brings food down,” says Bishop. “The question is whether the dumbwaiter is empty or full.”

If mixing is consistently below the critical light level, phytoplankton can not grow, i.e. the dumbwaiter stays empty and the zooplankton gets no food. As the winter storms stop with the advent of spring, the phytoplankton can quickly rebound, aided by increased levels of sunlight. But since a lot of zooplankton starved to death during the winter, the zooplankton population is not large enough to keep steps with the phytoplankton bloom and intercept carbon loaded material as it sinks between 100 and 800 meters.

In the part of the South Sea where Carbon Explorer 55C spent the winter collecting data, storms where not continuous and the mixing was therefore halted now and then. More zooplankton survived, zooplankton which fed on the phytoplankton in spring, keeping their numbers down and increasing carbon sedimentation.

Bishop says these observations point to an important lesson: “Iron is not the only factor that

determines phytoplankton growth in HNLC regions. Light, mixing, and hungry zooplankton are fundamentally as important as iron.”

You can find more information about Bishop and Wood’s study in the journal Global Biogeochemical Cycles. Preprints of the issue are already available to subscribers at http://www.agu.org/journals/gb/papersinpress.shtml.

* Jim Bishop is a member of Berkeley Lab’s Earth Sciences Division and a professor of Earth and planetary sciences at the University of California at Berkeley.

** Todd Wood is a staff researcher with the U.S. Department of Energy’s Laurence Berkley National Laboratory.

*** The Southern Ocean Iron Experiment (SOFeX) is a collaboration led by scientists from Moss Landing Marine Laboratory and the Monterey Bay Aquarium Research Institute.

This year, fishermen in the southern ocean of Australia report seeing more tuna than in 20 years. They report not only bigger catches, but also that the average tuna is about 20% lager than previous years.

Australian Tuna Association chief executive Brian Jeffries says: “There’s a lot more fish out there than there has been in the past 10 years and the fish are bigger in that period.”

This leads some to believe that the tuna population is recovering and that it has been doing so since 2006 when it was discovered that Japanese boats were catching 40,000 tonnes of southern blue fin tuna illegally each year and had been doing so for at least 20 years. The discovery lead to a crack down on Japanese illegal fishing and the illegal fishing in the region has, although still preset, dramatically dropped since 2006.

Is the tuna population in the Southern Ocean starting to recover, or is this just a sign of an imminent collapse – similar to the good years that preceded the cod collapse in the Atlantic? Only the future can tell.

Vast amounts of creatures looking like jelly balls have begun to appear off the eastern coast of Australia, and researchers now suspect that these animals may help slow down global warming by moving carbon dioxide from the atmosphere to the ocean floor.

The proper English name for this “jelly ball” being is salp. A salp is a barrel-shaped free-floating tunicate that moves around in the ocean by contracting and relaxing its gelatinous body. Just like the other tunicates, the salp is a filter feeder that loves to eat phytoplankton and this is why it has caught the attention of scientists researching global warming.

Phytoplankton are famous for their ability to absorb carbon dioxide from the top level of the sea, and a salp feasting on phytoplankton will excrete that carbon dioxide in the form of faeces. The faeces will drop to the ocean floor; thus lowering the amount of carbon dioxide present in the upper part of the ocean. Since the carbon dioxide found in this level of the sea chiefly hails from the atmosphere, phytoplankton and salps are a great aid when it comes to removing carbon dioxide from the air. Salps will also bring carbon down to the ocean floor when they die, which happens fairly often since the life cycle of this organism is no more than a few weeks.

Salp species can be found in marine environments all over the world, but they are most abundant and concentrated in the Southern Ocean near Antarctica where it is possible to encounter enormous swarms of salp. Over the last 100 years, krill populations in the Southern Ocean have declined and salp populations seem to be replacing them in this cold ecosystem. According to researcher Mark Baird of the Australian Commonwealth Scientific and Research Organization (CSIRO), the amount of salps in the waters off Australia are also on the increase, at least according to a survey carried out last month by CSIRO and the University of New South Wales.

While salp may help slow down global warming, their increase may also cause problems. Salp has a fairly low nutrient content and salps replacing nutrient rich krill in the Southern Sea may therefore prove detrimental for oceanic animals higher up in the food chain.