Phytoplankton are responsible for approximately 40% of the planet's total annual photosynthetic (i.e., `primary') production. The magnitude of their contribution to global primary production is often unappreciated because they represent such a small fraction of the total photosynthetic biomass on earth. Their high output per unit biomass is possible because of their high surface to volume ratio, because they are free-floating and thus have no supporting structures to maintain, and because they are embedded in a very dynamic ecosystem where time scales of environmental forcing are on the order of days.
Light and nutrients are the primary factors regulating phytoplankton growth. Light attenuates dramatically with depth, penetrating roughly 200m in the open ocean, thus illuminating only a tiny fraction of the total ocean volume. Solar energy heats the surface waters, generating a temperature gradient (the thermocline) which, along with the salinity gradient (the halocline) generated by surface rainfall, creates a density gradient (the pycnocline) that effectively isolates the surface wind-mixed layer from the waters below. Inorganic nutrients such as nitrogen and phosphorus, central to the cells' biochemistry, are constantly stripped from the surface waters by the growing phytoplankton. Most of the phytoplankton are eaten on the spot and the nutrients comprising their biomass are regenerated at the surface and are available for another round of production. Some fraction of the primary production, however, finds its way to the deep sea, either through the settling of dead cells and fecal matter or through advection in the course of global circulation patterns. Most of the organic carbon which finds its way to the deep sea is ultimately assimilated by bacteria, which regenerate it (and N and P) into inorganic forms (Fig. 1). The collective action of this so-called ``biological pump'' over time scales of centuries is to create a sharp concentration gradient of nutrients with depth (Fig. 2a). The gradient is larger in the Pacific than the Atlantic because deep ocean currents flow from the Atlantic to the Pacific, collecting as they travel the ``rain'' of nutrient-rich particles from the productive surface waters above.
This biochemical pump---comprised of phytoplankton cells,
their animal predators, and the bacteria that assimilate their
waste---plays a central role in the global carbon cycle. Because
the elemental composition of phytoplankton is C:N:P = 106:16:1
(commonly referred to as the ``Redfield Ratio''---see Redfield,
1934, 1958), about 100 units of carbon are delivered to the deep
sea for every unit of phosphorus. As such, the biological pump
delivers carbon from the atmosphere to the deep sea, where it is
concentrated and sequestered for centuries (Fig. 2b). Superimposed
on the biochemical pump is a ``solubility pump'' which also serves
to concentrate dissolved inorganic carbon (CO
plus
bicarbonate and carbonate ions) in the deep ocean. The driving
force of this pump is increased solubility of CO
in the cold
surface waters at high latitudes where the water sinks to join the
deep ocean currents. Although estimates of the relative importance
of these two pumps have a wide range, the most recent estimate
(Sarmiento and Bender, 1994) suggests that 75% of the difference in
dissolved inorganic carbon concentration between the surface and
deep oceans is due to the biological pump. If it were eliminated
tomorrow, and the dynamics of the oceanic carbon cycle were driven
solely by physical and chemical processes, the amount of carbon
released from the deep ocean as it equilibrated with the atmosphere
would more than double the CO
concentration in the
atmosphere.
Although there are many cyclic processes in the surface waters
which are coupled to the biological pump, the net flux of
biologically-produced materials is one way---from the surface to
the deep ocean. So how do the nutrients find their way back to the
surface waters to refuel the pump? The answer is critical because
the rate of resupply of nutrients from the deep water to the
surface regulates the rate of phytoplankton production globally on
the time scales relevant to anthropogenic change. There are
essentially three mechanisms by which the nutrients are returned to
the surface: Upwelling of the nutrient-rich deep water (e.g., along
the western coasts of continents); diffusion across the thermocline
(e.g., the central ocean gyres); and seasonal, wind-driven deep
mixing which erodes the thermocline and entrains the deep water to
the surface (e.g., the N. Atlantic). As the phytoplankton pump is
refueled by N and P to generate more organic carbon through
photosynthesis, CO
from the deep water is being supplied as
well. Put quite simply, inorganic N and P are delivered to the
surface waters in the biologically dictated Redfield Ratio (along
with CO
in excess), are converted to organic compounds (i.e.,
biomass) in the Redfield proportions of C:N:P, are recycled in the
surface waters many times in those proportions, and are ultimately
re-pumped to the deep sea---in those proportions. Thus, on a
global scale, any changes in upwelling and the supply of nutrients
to the surface waters will not influence the biological pump's role
in the annual atmospheric carbon budget, so long as the supply and
assimilation of N, P, and C are tightly coupled.
In some areas of the world's oceans the supply and assimilation of N, P and C are not tightly coupled, even on an annual average, because N and P do not appear to be limiting phytoplankton growth. These are the so called ``high-nutrient, low-chlorophyll'' (HNLC) waters of the subarctic Pacific, the Southern ocean around Antarctica, and the equatorial Pacific (see Cullen, 1991). Here the biological pump is inefficient: the phytoplankton standing stocks are not large enough to assimilate the N and P in the surface waters fast enough to deplete them at any time throughout the year. These regions, particularly the Southern ocean, are of major interest to paleooceanographers because they could have played a role in past climate change (Sarmiento and Togweiler, 1984). The cold surface waters of the Southern ocean steadily sink to the abyss as part of the global ocean circulation and, in today's ocean, carry with them a large reservoir of unused nutrients---a potential carbon sink of some magnitude. The actual significance of this potential sink in the global scheme of things will be discussed below. The more immediate question is: What factor(s) are responsible for the inefficiency of the biological pump in these regions? Why can't the phytoplankton use up the nutrients, as they do so effectively in the other regions of the world's oceans?
Many hypotheses have been proposed, which primarily involve light limitation or control by zooplankton grazing (see Cullen, 1991). Each has merit and describes important factors controlling phytoplankton in these regions. The most provocative, and now compelling, hypothesis in recent years, however, is the suggestion that the supply of iron to these regions is regulating primary productivity.