Feast or Famine:
Investigating Iron in the Sargasso Sea
Dr. Peter Sedwick, Associate Research Scientist
The 1990s have been dubbed the "iron age" of oceanography because of the realization that one chemical element — iron — may control the
growth of phytoplankton over much of the ocean's surface. Phytoplankton are single-celled microscopic plants that form the foundation of the marine ecosystem, and they are important players in controlling
the amount of carbon dioxide in the atmosphere, which, in turn, may help to control the earth's climate. To oceanographers, the realization that iron plays such a significant role in the growth of
phytoplankton was both important and surprising. It is important because phytoplankton account for a large portion of the planet's primary production, or the use of light energy to convert carbon dioxide
into plant material. The surprise is that iron is in such short supply in the ocean, considering it is the fourth most abundant element in the earth's crust. We now know that iron levels can be very low in
the upper ocean: often less than 10 parts per trillion. Iron has a very low solubility in seawater, so it rapidly "settles out" of the surface ocean. In the open ocean, away from land, iron levels remain
low, except where iron is continuously supplied in airborne soil dust. Levels are so low, in fact, that phytoplankton growth is severely limited by iron deficiency in vast areas of the open ocean surrounding
Antarctica, in the eastern Equatorial Pacific, and in the far North Pacific. So why have oceanographers only recently discovered the importance of iron in controlling phytoplankton growth? The simple
reason is that it is very difficult to measure the low concentrations of iron in open-ocean waters when using rusty steel ships to collect samples. Only with the application of specialized "clean" techniques
in the 1980s and early 1990s were marine scientists able to accurately measure iron in seawater and conduct biological experiments to investigate "iron limitation" using samples uncontaminated by iron from
their research vessels. Initially controversial, the idea of iron limitation has since been proven in a number of ocean fertilization experiments, in which massive blooms of phytoplankton were produced by
adding several tons of iron to the surface ocean. Despite being one of the most heavily studied areas in the open ocean, there have been very few measurements of iron in the Sargasso Sea near Bermuda.
This is largely because surface waters of the Sargasso Sea, and the North Atlantic in general, have been widely regarded as "iron replete" — that is, to contain adequate levels of iron to support
phytoplankton growth. Every year, winds carry several hundred million tons of soil dust across the Atlantic Ocean from arid regions in Saharan and sub-Saharan Africa, which are the largest sources of dust on
earth. Much of this iron-bearing dust enters the North Atlantic Ocean, adding iron to surface waters. Thus it has been assumed that surface waters in the Sargasso Sea contain plenty of iron. But recent
research suggests that this assumption may not hold up. In 2002, oceanographers Jingfeng Wu and Ed Boyle, of the
Massachusetts Institute of Technology, reported quite low levels of iron in waters northeast of Bermuda that were sampled during the
spring. They argued that the large amount of iron added to these waters during summer, when most of the African dust is transported, might be removed during the winter and spring due to deep mixing and
biological activity in the upper ocean. Perhaps the Sargasso Sea is not so iron rich after all. As an oceanographer who has previously worked in "iron-limited" Antarctic waters, this piqued my interest, and
I set out to take a closer look at iron in the Sargasso Sea. For this work I collaborated with Thomas Church of the University of Delaware, who has studied the transport of soil dust to Bermuda over the last two
decades. With funding from the U.S. National Science Foundation, our joint research project aimed to answer the following questions: How do iron levels in the Sargasso Sea vary with the seasons? Are these levels ever low enough
to limit phytoplankton growth? What are the effects and timing of the desert dust inputs from North Africa?We have tried to answer these questions by measuring iron in both the upper ocean and in the
lower atmosphere during several cruises aboard BBSR's research vessel, Weatherbird II. Starting in
late 2002, our first task was to "get clean" — this alone amounted to many months of preparation, as sampling and measuring iron at low levels requires the use of highly specialized "trace-metal clean"
laboratories and equipment in order to avoid contamination of samples.With our preparations complete, we embarked on our first cruise in late summer 2003, joined by other iron aficionados Andrew Bowie,
from the University of Tasmania
, and Simon Ussher, from the University of Plymouth. Satellite sea-level analyses provided by Dennis McGillicuddy
(Woods Hole Oceanographic Institution) and assistance from BBSR's Bermuda Atlantic Time-series Study
team allowed us to follow a single patch of surface water to the southeast of Bermuda over a two-week period. This cruise coincided with a massive African dust event; with regard to observing atmospheric iron input to the Sargasso Sea, we had hit the jackpot! On a subsequent cruise in spring 2004, we collected samples during the low-dust, pre-summer period, and another cruise in early summer 2004 coincided with the first significant inputs of African dust to the Bermuda region.
Our first analyses of iron in samples from the summer 2003 and spring 2004 cruises have produced some exciting and unexpected results. The summer 2003 samples have revealed, as expected, a dramatic
increase in dissolved iron levels in the surface waters impacted by the African dust plume, with levels as high as 100 parts per trillion. A great surprise, however, was that iron levels were as low as one
part per trillion at a depth of 100 to 150 meters below the sea surface. During summer in the Sargasso Sea, most phytoplankton are found within these dim, sub-surface waters because they contain higher
levels of the major nutrients nitrogen and phosphorous, which are depleted from surface waters in summer. It was a real shock to discover that phytoplankton growth might be limited by iron deficiency at this
depth, given the surfeit of iron in overlying surface waters. Our samples also tell us that dissolved iron levels are indeed low during spring, when soil dust inputs are low. This result supports the idea
that there is a huge seasonal variation in iron content at the Sargasso Sea's surface. This may have important implications for phytoplankton growth in this region during winter and spring. In addition, the
likely seasonal increase in surface iron due to summer dust inputs provides an ideal natural laboratory to study the processes of atmospheric iron deposition and the dissolution of dust-iron in seawater,
which are still very poorly understood. We plan to continue our field studies in this region, addressing questions that pertain not only to the Sargasso Sea, but to the deposition of dust-iron to the
global ocean, the role of this iron in controlling primary production by phytoplankton, and the effects of changes in dust deposition that are likely to result from human activities.
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