Chapter 5
Nutrient Limitation: The
Example of Iron
P
hysical forcing is not a sufficient explanation of the seasonal and spatial pattern
of production in the ocean and, as noted in the previous chapter, a necessary
auxiliary assumption is that the surface mixed layer should be recharged with
inorganic nutrients. It will be useful, then, at this point to discuss the role of both
macro- and micro-nutrients in enabling and terminating phytoplankton growth. Because
iron limitation has been so widely discussed in recent years, this chapter will be focused
primarily on that element.
Although nitrate and other molecules have long been known also to reach the sea
surface in rainwater and dry aerosols, this flux has seemed unlikely to modify significantly
the assumptions of either Margalef or Sverdrup, both of whom assumed that flux across
a nutricline was dominant. Further, most biological oceanographers thought only about
the macronutrients nitrogen, phosphorus, and silica and ignored the early experiments
of Menzel and Ryther (1961), who clearly demonstrated that iron was the initial limiting
element in the Sargasso Sea off Bermuda and that iron limitation was due to differential
supply rates from below of iron and the macronutrients: winter mixing was found to
control the seasonal production cycle here. These experiments were not done with the
ultra-clean rigor of the subsequent studies performed by the Moss Bay team, but that is
no reason for their subsequent neglect.
Martin’s contrary suggestion, made in 1991, was extraordinarily influential: he proposed that in “open ocean upwelling regions far from Fe-rich continental margins, the
only way phytoplankton can obtain Fe is via long-range, wind-blown transport of Fe-rich
atmospheric dust derived from arid regions.” Iron-fertilization experiments at sea in the
tropical Pacific (IronEx I and II), the Southern Ocean (SOIREE), and in the NE Pacific
(SEEDS and SERIES) soon appeared to confirm his insight and were widely interpreted
as demonstrating that areas with residual nitrate are causally related to low rates of
deposition of terrestrial dust (Martin and Fitzwater, 1988). It came to be widely accepted
that such High Nutrient–Low Chlorophyll, or HNLC, regimes are characteristic of the
subarctic Pacific, much of the eastern tropical Pacific, and parts of the Southern Ocean.
But, as I hope to demonstrate, little evidence was presented to support the concept of
growth limitation in these regions by insufficiency of subaerial input.
Martin’s intervention was so uncritically accepted as to become mythic in the sense
of William Dickinson, quoted on the fly-leaf of this book. In the years that followed,
many authors turned to the assumption that in at least some parts of the ocean, the
regional pattern of productivity and chlorophyll accumulation was forced by the regional
pattern of subaerial deposition of Fe in terrestrial dust. Menzel and Ryther were soon
forgotten, and the Bermuda area was omitted from the paradigmatic “HNLC” regions.
71
Nutrient Limitation: The
Example of Iron
P
hysical forcing is not a sufficient explanation of the seasonal and spatial pattern
of production in the ocean and, as noted in the previous chapter, a necessary
auxiliary assumption is that the surface mixed layer should be recharged with
inorganic nutrients. It will be useful, then, at this point to discuss the role of both
macro- and micro-nutrients in enabling and terminating phytoplankton growth. Because
iron limitation has been so widely discussed in recent years, this chapter will be focused
primarily on that element.
Although nitrate and other molecules have long been known also to reach the sea
surface in rainwater and dry aerosols, this flux has seemed unlikely to modify significantly
the assumptions of either Margalef or Sverdrup, both of whom assumed that flux across
a nutricline was dominant. Further, most biological oceanographers thought only about
the macronutrients nitrogen, phosphorus, and silica and ignored the early experiments
of Menzel and Ryther (1961), who clearly demonstrated that iron was the initial limiting
element in the Sargasso Sea off Bermuda and that iron limitation was due to differential
supply rates from below of iron and the macronutrients: winter mixing was found to
control the seasonal production cycle here. These experiments were not done with the
ultra-clean rigor of the subsequent studies performed by the Moss Bay team, but that is
no reason for their subsequent neglect.
Martin’s contrary suggestion, made in 1991, was extraordinarily influential: he proposed that in “open ocean upwelling regions far from Fe-rich continental margins, the
only way phytoplankton can obtain Fe is via long-range, wind-blown transport of Fe-rich
atmospheric dust derived from arid regions.” Iron-fertilization experiments at sea in the
tropical Pacific (IronEx I and II), the Southern Ocean (SOIREE), and in the NE Pacific
(SEEDS and SERIES) soon appeared to confirm his insight and were widely interpreted
as demonstrating that areas with residual nitrate are causally related to low rates of
deposition of terrestrial dust (Martin and Fitzwater, 1988). It came to be widely accepted
that such High Nutrient–Low Chlorophyll, or HNLC, regimes are characteristic of the
subarctic Pacific, much of the eastern tropical Pacific, and parts of the Southern Ocean.
But, as I hope to demonstrate, little evidence was presented to support the concept of
growth limitation in these regions by insufficiency of subaerial input.
Martin’s intervention was so uncritically accepted as to become mythic in the sense
of William Dickinson, quoted on the fly-leaf of this book. In the years that followed,
many authors turned to the assumption that in at least some parts of the ocean, the
regional pattern of productivity and chlorophyll accumulation was forced by the regional
pattern of subaerial deposition of Fe in terrestrial dust. Menzel and Ryther were soon
forgotten, and the Bermuda area was omitted from the paradigmatic “HNLC” regions.
71