What controls phytoplankton production in nutrient-rich areas of the open sea? Page: 8 of 23
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during past and present interglacial periods, which suggests that this "natural" fertilization with iron
might have resulted in increased phytoplankton production and consequent reduction in atmospheric
CO2
+ Addition of small amounts of iron to plankton communities enclosed in bottles stimulates the net
rate of particulate chlorophyll, carbon, and nitrogen production and in addition, changes the phyto-
plankton community composition. We note, however, that the magnitude of biomass increase might
not be achieved in the presence of the large grazers, which are defacto excluded from such bottle
experiments.
+ Iron additions have been shown to cause a shift in the nitrogen utilization patterns of the phyto-
plankton from ammonium to nitrate, presumably because iron is required for nitrate reduction.
+ Marine algal species exhibit wide differences in their growth requirements for iron, which closely
match the differences in iron levels in the waters from which the species were isolated. This suggests
that iron availability is an important agent of natural selection in the oceans.
Although the collected evidence is compelling, it has not yet been demonstrated that iron enrich-
ment stimulates the specific growth rate (as opposed to final yield) of phytoplankton species in bottle
experiments. Moreover, we have no way of predicting, at present, whether iron enrichment in the
presence of the entire food web would result in increased net community production (i.e. carbon that
would ultimately be sequestered in the deep ocean). The first of these questions can be addressed
through bottle experiments, because the answer is independent of grazing pressure. The second,
however, could only be addressed through an unenclosed enrichment experiment in the ocean.
Recommendations for Future Research
Because iron is required in trace amounts by phytoplankton (C:Fe ratios in cultures range from
30,000 to 500,000) it is theoretically possible to carry out moderate-scale enrichment experiments
with this element in areas of the oceans where it is hypothesized to limit plankton production. The
power of this type of experimental manipulation of natural systems has been amply demonstrated by
limnologists in their studies of the conditions that control productivity and food web dynamics in
lakes.
Nutrient-rich seas that have very low in situ iron concentrations and very low rates of atmospheric
iron input provide the perfect natural setting for such an experiment. The challenge, though, is not
simply to demonstrate that iron limitation of phytoplankton production in these regions could be
artificially alleviated, but to determine the implications of such increased productivity for carbon
sequestration in the deep ocean. As described above, without a full study of the effect on planktonic
food web dynamics, there is no assurance that an increase in productivity would result in a grater
storage o' arbon in the ocean.
It is important, therefore, that we examine this hypothesis in depth, and consider designing a
modestly scaled iron-enrichment experiment in a high-nutrient region of the open sea. The scale of4
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Weiler, C. S. What controls phytoplankton production in nutrient-rich areas of the open sea?, article, June 25, 1991; United States. (https://digital.library.unt.edu/ark:/67531/metadc1273023/m1/8/: accessed July 17, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.