platform. Probably that’s correct. Results of the entire global suite of iron-addition
studies are summarized by Boyd et al. (2007). Chlorophyll increases by two- to 25-
fold. Greater phytoplankton responses are observed in shallow mixed layers than in
deep. More rapid responses are seen in warmer waters. Dominant phytoplankton
species change to medium sizes and more diatoms. Bacteria increase by two- to 15-
fold. The duration of experiments is usually too short to see a mesozooplankton
response, but in two experiments (IronEx II and SEEDS I) where copepods were
abundant, they played a role in controlling the blooms (Boyd et al. 2007). To date, no
response of fish to iron additions has been observed. Note that in regard to all of these
scenarios that they depend upon more than the phytoplankton and their responses to
light, nutrients, and mixing. Protists and animals grazing upon the phytoplankton also
play a large part, a point to be repeated shortly.
(^) Biological processes in the eastern equatorial Pacific studied intensively during the
EqPAC Program during 1992 (Murray et al. 1995, 1997) showed that the HNLC
character of this region results in part from low availability of iron interacting with
grazing pressure from micro- and mesozooplankton. Upwelled water has a greater
NO 3 − to Si(OH) 4 ratio than optimal for diatoms, and Dugdale et al. (2007) suggested
that silicate limitation may play a role in restricting levels of primary production.
However, it has not prevented diatom blooms after in situ iron additions. The
Equatorial Biodiversity (EB) Program (Nelson & Landry 2011) conducted
experimental studies in 2004–2005 to clarify the relative roles of grazing, iron-
limitation, and silicate-limitation in controlling primary production and the
phytoplankton community structure. The phytoplankton community was dominated
by small cells that were grazed at the same rate as their growth (Landry et al. 2011). A
majority (70%) of the grazing pressure was from microzooplankton. These
heterotrophic protists served as a major food source for the mesozooplankton.
Prochlorococcus was the only phytoplankton species that showed an in situ increase
in abundance with increasing iron concentrations. In 5–7-day shipboard microcosm
experiments, additions of iron led to an increase in rare large diatoms and to depletion
of NO 3 − and Si(OH) 4 . Additions of silicate resulted in increased biogenic silica
production, but did not result in the depletion of major nutrients (Brzezinski et al.
2011).
(^) Mesozooplankton standing stocks in the eastern tropical Pacific appear to have
increased by two-fold between 1992 and 2004–2005; however, different nets likely to
capture different subsets of zooplankton in respect to size and activity were used for
the two sampling programs (Décima et al. 2011). Rates of primary production and the
assimilation index were similar during the two sampling periods (Table 11.5), so it is
not clear how a doubling of mesozooplankton stocks could have been supported.
(^) Despite its HNLC character, relatively high productivity in the eastern equatorial