The response of the ambient nondiatom, predominantly picophytoplankton assemblage to the addition of iron was rapid and dramatic in both IronEx experiments. Within a half day after the completion of iron addition, nondiatom biomass had increased detectably (Figures 3c and 3d). Tight coupling between onset of favorable conditions and rapid increases in photochemical efficiency, net growth rates, and biomass are characteristic of the ambient nondiatom assemblage in the equatorial Pacific [Kolber et al., 1994; Landry et al., 1996, 2000; Foley et al., 1997; Landry and Kirchman, 2002]. In IronEx-2 the initial nondiatom chlorophyll concentration was 0.13 mg Chl m−3, about half the initial concentration in IronEx-1; the gap in biomass was closed in the first day after iron addition and further increases in chlorophyll and net μchl were remarkably similar in the two experiments. In both, nondiatom chlorophyll increased at a modest exponential rate to concentrations of about 0.4 to 0.6 mg Chl m−3, roughly a tripling of the initial nondiatom chlorophyll concentration. The mean net μchl was 0.24 d−1 in IronEx-1 and 0.21 d−1 in IronEx-2 for the first 4 days (Figure 4). After the rapid initial increase there was no further increase because, as Landry et al.  have shown, the protistan grazers of the microbial food web also increased in abundance and grazing rates in both experiments. The balance between net growth and grazing loss prevented a large accumulation of nondiatom biomass [Landry, 1977; Landry et al., 1997; Landry, 2002]. During the favorable growth transient, the autotrophs and their protistan grazers shift to higher, but still balanced, biomass and rate levels as described in the following equations, after Lindley et al. . In the balanced microbial food web:
where B is autotrophic picophytoplankton biomass, μ is its specific growth rate, and m is the specific mortality loss rate due to the sum of all loss processes. Since the majority of loss in the microbial food web is due to grazing, we refer to the sum of the losses as grazing loss. The balance between autotrophic growth rate and grazing (loss) rate requires that m is density dependent,
At steady state, the (loss) grazing constant a can be defined. Since μ = m,
Under the influence of the favorable transient, μ increases to μnew and biomass increases proportionally to Bnew,
For the duration of the favorable transient, then, this relationship predicts a higher steady state biomass, increased steady state growth rate of small autotrophs, plus increased grazing loss rate (mnew) [Lindley et al., 1995; Landry and Kirchman, 2002]. Protistan grazers in pelagic food webs are almost always capable of preventing the formation of high biomass blooms of picophytoplankton; we know of only two reports of picophytoplankton blooms >1.5 mg chl m−3 in the open ocean [Morel, 1997; Bidigare et al., 1997].
 In IronEx-1 and IronEx-2, nondiatom assemblages reached the shifted-up biomass levels (Bnew) quickly. In IronEx-1 the iron-enriched parcel of water subducted beneath a layer of water with ambient (low) iron concentrations between Day 4 and Day 5; therefore, the surface HPLC pigment values after Day 4 are not representative of the iron-stimulated community, making it impossible to determine Bnew for IronEx-1 with any confidence. Figures 3c and 3d suggest that the nondiatom assemblages in the two experiments were following similar trajectories. In IronEx-2 the mean Bnew value (0.46 mg Chl m−3) was reached between Day 1.4 and Day 2.4, and was maintained for at least 8 days with an oscillation of values between Days 2.4 and 6.4 (Figure 3e) that suggested protistan grazers and autotrophs were settling into the new Bnew equilibrium value through a series of damped oscillations.
 Chlorophyll-specific net growth rate calculations for cyanophyte and prochlorophyte chlorophyll indicated that the nondiatom response was representative of the prokaryotic picophytoplankton response (Figure 4). For the first 4 days of IronEx-1 the mean nondiatom net μchl = 0.24 d−1, the cyanophyte and prochlorophyte net μchl = 0.21 d−1; in IronEx-2 the net μchl values were similarly close, net μchl = 0.21 d−1 for nondiatoms and 0.19 d−1 for cyanophytes and prochlorophytes. These results confirm that the iron response of the two major prokaryotic groups is similar to the bulk nondiatom assemblage iron response; that is, they increased modestly in both biomass and chlorophyll-specific net growth rate as also reported by Mann and Chisholm .
 Analysis of photochemical efficiency with the Fast Repetition Rate Fluorometer (FRRF) has provided surprising results from the two IronEx experiments (Figure 5). FRRF observations in IronEx-1 and IronEx-2 show that ambient Fv/Fm values in the equatorial Pacific are very low, around Fv/Fm = 0.3, indicating that ambient picophytoplankton were iron limited when the IronEx experiments were carried out (Figure 5). After iron addition in both experiments, Fv/Fm increased to high values in the first 24 hours and up to maximal values after 48 hours. The IronEx-1 and IronEx-2 response curves of photochemical efficiency versus time were similar. In both experiments the phytoplankton assemblage in the first 24 and 48 hours was composed almost entirely of small phytoplankton. When the massive diatom accumulation did develop in IronEx-2, the Fv/Fm curve remained similar to the response curve of IronEx-1 where no diatoms were present.