Marine Biogeochemistry - Ecological Systems - page 210

variability in both of these processes, it is not surprising that surface water
concentrations of iron are also highly variable.
Mesoscale Iron Fertilization Experiments
In the 1990s a series of large-scale ocean manipulations were undertaken to test the
hypothesis that iron limited phytoplankton growth in the tropical Pacific. Two
competing hypotheses were offered to explain the equilibrium concentrations of
high concentrations of nitrate and low phytoplankton biomass which were (1) limit-
ing levels of bioavailable iron and (2) rates of loss processes from grazing kept
phytoplankton standing stocks at low levels. To test these, in situ additions of iron
were planned for limited regions of the ocean. The passive tracer sulfur
hexafluoride (SF 6 ), which can be detected at very low levels, was added with the
iron so that the enriched water could be followed over time. The first iron enrich-
ment experiment produced contradictory results. The photosynthetic capacity of
phytoplankton showed a clear enhancement that was correlated with iron additions,
but nitrate and CO 2 concentrations were unaffected [ 18 , 19 ]. Further analysis
showed that upon initial iron enrichment, the iron dropped to extremely low levels
because colloid formation rapidly converted soluble iron to insoluble iron oxides,
and the fertilized water patch was subducted to depth, which removed the iron-
enriched water from the high irradiance euphotic zone required for nutrient assimi-
lation. To further test the two hypotheses, the experiment was repeated, and this
experiment clearly demonstrated the critical role of iron in limiting phytoplankton
growth in high-nutrient, low-chlorophyll waters. Iron was added repeatedly to the
patch of water at 3-day intervals for almost 2 weeks [ 20 ], and the response of the
surface water was clear, showing decreased nitrate (which dropped to zero),
decreased CO 2 , increased phytoplankton biomass and photosynthetic activity, and
a quantifiable decrease in iron concentrations. That is, the concentration and supply
of iron was nevertheless the essential feature in driving the carbon and nitrogen
cycles of the equatorial Pacific Ocean.
Subsequent similar iron enrichment experiments have been conducted in other
HNLC regions in the Southern Ocean and the North Pacific. The former is extremely
important to global biogeochemical cycles, as it is the site of deep and intermediate
water mass formation, and thus regulates the concentrations of inorganic nutrients in
much of the world's surface waters. As an example, models suggested that if all the
inorganic nutrients were utilized (by iron fertilization) in the Southern Ocean that
within 300 years the waters being upwelled in the eastern tropical Pacific would be
greatly reduced in nutrient levels, and thus decrease productivity of commercially
important higher trophic levels and marine mammals dependent on ecosystem
processes in that region [ 21 ]. In all iron enrichment experiments to date, substantial
and positive responses to additions of inorganic iron were observed, and while
the details among experiments differ (and the causes debated), it is now accepted
that iron plays a major role in the biogeochemistry of the ocean [ 22 ].