clean additions of PO 4 3− (0.2 μM), FeCl 3 (2 nM), P + Fe, N as NH 4 + (1 μM), N + Fe,
N + P and N + P + Fe to samples of near-surface water from five stations across the
Sargasso Sea. After 24 hours of incubation they measured ^14 C uptake, and after 48
hours they measured chlorophyll levels. Results were consistent among stations (Fig.
11.26): no effect from just P, just Fe or P + >Fe. Ammonium supplement, however,
gave strong increases, and adding both NH 4 + and P was even more effective. Iron
produced no increment in any combination. Clearly, the immediately limiting nutrient
for the resident BATS phytoplankton community, which was typical on the
observation dates, that is, dominated by Prochlorococcus with modest numbers of
Synechococcus and picoeukaryotes, was fixed nitrogen. However, given ammonium,
the cells benefited further from added phosphate. That result from Moore et al. is
almost pure Liebig: relief of the most limiting nutrient opens the potential for growth
stimulation by the next-scarcest relative to requirements. In the Sargasso, the effect of
combined N and P additions is likely due to the low phosphate concentration relative
to fixed nitrogen (high N : P) (Fig. 11.27). This experiment is different from
determinations of what might limit the rare, often large-cell component of the
phytoplankton. Incubations over many days might allow the “grow-out” of cells with
completely different limiting factors.
Fig. 11.26 Response of the bulk phytoplankton community abundance and carbon
fixation to additions of different nutrients at different locations across the subtropical
Atlantic. Upper row: carbon fixation measured from 24 to 48 h after the addition of
the indicated nutrients. Lower row: chlorophyll concentration measured at 48 hours.
Vertical lines in bar tops are the standard error among replicates. Bars with the
different letters were statistically distinguishable (i.e. p < 0.05).
(After Moore et al. 2008.)