Marine Biogeochemistry - Ecological Systems - page 209

the Southern Ocean, the equatorial Pacific, and the north Pacific, have substantial
standing stocks of nitrate and phosphate, as well as adequate irradiance, but exhibit
very low standing stocks of phytoplankton (high-nutrient, low-chlorophyll regions,
or HNLCs). During glacial-interglacial periods, atmospheric concentrations of CO 2
showed substantial variations and were strongly negatively correlated with iron
deposition [ 14 ]. Thus iron limitation could explain CO 2 variations over geological
time as well. Given that iron is the fourth most abundant element on earth, how can
such low concentrations exist in the ocean, and how did oceanographers unequivo-
cally demonstrate the ecological importance of iron?
Iron is derived from terrestrial and hydrothermal sources, but upon entry into
oxygenated, saline waters, it rapidly forms iron oxides. The precipitates are
largely insoluble under aerobic conditions, and attach to particles or remain in
the water as colloids. The colloids can be solubilized by irradiance, contributing to
a pool of dissolved inorganic iron, which consists of two forms, Fe +2 and Fe +3 .
Both of these ions can be removed by plankton for their growth, although Fe +2 is
generally oxidized to Fe +3 and kept at low levels. The mean ocean concentration
of dissolved inorganic iron in the upper 200 m of the ocean is 0.07 nmol kg 1 [ 12 ].
Both forms can also be chelated by organic molecules, and thus become part of the
dissolved ferro-organic pool. In general, there are two classes of organic ligands
that bind with iron, a strong-binding ligand and a weak-binding ligand. The latter
exchanges iron easily with biota, and thus makes iron bioavailable. There is also
a class of special ligands called siderophores, which are low molecular weight
organics that are produced and excreted primarily by prokaryotic organisms
(bacteria, cyanobacteria) and that bind dissolved inorganic iron [ 15 ]. The ferro-
ligand complex can be assimilated by bacteria, phytoplankton, and cyanobacteria,
and the iron incorporated into a variety of cellular processes. Transformations
among all of these pools are both biologically and irradiance mediated; entirely
different transformations and equilibria are established in anoxic waters and
sediments.
Iron in ocean surface waters derives from either atmospheric or deep ocean
sources. Atmospheric deposition varies by latitude (proximity to terrestrial sources)
and temporally (dependent on source region wind variability). Aerosols can be
measured by satellite-borne sensors, which have shown that some oceanic systems
receive substantial periodic depositions of iron from industrial sources (the North
Atlantic) and from dust derived from terrestrial deserts in China (the western
Pacific) and the Sahara in Africa (the coast of North Africa). Dissolution of aerosols
in ocean water (fractional solubility) depends on the type of mineral in the aerosol,
and can range from 1-90% [ 16 , 17 ]. Small aerosol particles can rapidly aggregate

with biological particles and exit the surface layer by sinking. Residence times for
particulate iron can be as short as 6 days [ 17 ]. Conversely, other regions are rarely
impacted by atmospheric deposition events (e.g., the Southern Ocean, the equato-
rial Pacific) by virtue of large-scale wind patterns that isolate them from terrestrial
sources. These regions have their iron inputs driven by oceanographic processes
such as deep vertical mixing and upwelling. Given the spatial and temporal

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