nutrients in the upper euphotic zone and different concentrations available at depth for
upward mixing. The main candidates as limiting nutrients are fixed nitrogen (nitrate at
depth), phosphate, and iron.
(^) Except after the strongest winter mixing, the upper water columns in both the NPSG
and Sargasso Sea are sufficiently depleted in N and P that special techniques are
required to determine their concentration. A chemoluminescent method for NO 3 − +
NO 2 − can determine levels down to ∼2 nM (nanomolar, 10−9 mol liter−1). A technique
called MAGIC can determine “soluble reactive phosphate” (SRP), including an
uncertain part of organic-bound phosphate, down to 3 nM. Some of the organic
phosphate is available to at least those phytoplankton-bearing phosphatases on their
cell surfaces (e.g. Trichodesmium, White et al. 2010), so SRP may be close to the
right measure of sufficiently labile phosphate to support cell growth. At HOT (Karl et
al. 2001) the bulk euphotic zone values of NO 3 − + NO 2 − ( = [N+N]) range from the
detection limit to ∼8 nM and of SRP from 20 to 100 nM. The ratio of means, [N+N] :
SRP is ∼3 : 50. Because ammonium is held extremely low, these are certainly the
available forms of the major nutrients, and they are far from the usual Redfield values
for phytoplankton composition, N : P ≈ 16. On the other hand, the dissolved organic N
and P (Karl et al. 2001) have a mean molar ratio DON : DOP = 5 μM N to 0.23 μM P,
or ∼22. Inorganic N : P = 13.5 (∼40 μM NO 3 − : 3 μM PO 4 3−) slowly diffusing up
from below 800 m. The amounts of DON and DOP available for recycling are
uncertain; however, it seems likely, based on the low levels and ratios of inorganic
species, that N is consistently more limiting than P.
(^) The upper euphotic zone [N+N] and [SRP] are seasonally variable at BATS, rising
from extremely low concentrations most of the year to 0.2–1 μM and 20–100 nM,
respectively, during deep winter mixing (N : P ≈ 40). Summer to early winter values
are much lower: 2–10 nM and 1–20 nM (N : P ≈ 0.3–7) (Cavender-Bares et al. 2001).
Thus, nitrogen would appear to be more available relative to phosphate (and to
relative phytoplankton requirements) than in the NPSG, although DON and DOP are,
again, more abundant and partially available. Explanations mostly depend upon N 2 -
fixing phytoplankton producing exported organic matter with no requirement for fixed
nitrogen, stripping the P from the upper layers.
(^) The obvious tests to determine which nutrients set the proximal limits on
phytoplankton growth rates are modest additions of nutrients singly and in
combinations that potentially interact, as for example of N and P together, or of Fe
and P. Nitrogenase catalyzing N 2 -fixation requires a good deal of iron, 15 iron atoms
for each active enzyme complex. It also requires molybdenum, but that apparently is
never limiting, despite having the typical surface depletion exhibited by
phytoplankton nutrients. Moore et al. (2008) in March and April made trace-metal-