euphotic zone [N+N] : P ratios: high in the Atlantic, low in the Pacific. The Sargasso
is more abundantly supplied with iron from Saharan dust carried westward by trade
winds. This is largely depleted by the rains extracted when those winds rise against
the Americas, leaving even the eastern Pacific less fertilized. No comparable dust is
exported from the Americas, and the eastern Pacific is largely dependent on distant,
Asian sources. This enables diazotrophs to utilize phosphate in the Atlantic until it
becomes the limiting nutrient, leaving extremely low residual phosphate relative to
fixed nitrogen. Intermittent measures of fixation rates may not be particularly high
because of the phosphate limitation. In the eastern NPSG, on the other hand, eolian
iron is supplied less frequently and in lesser amounts, such that phosphate is not
exhausted, while phytoplankton generally reduce fixed nitrogen to very low levels
(reducing [N+N] : P).
(^) Several indices of long-term nitrogen fixation effects in different areas have been
generated by examination of the discrepancy between the N : P ratio and the canonical
Redfield ratio (N : P ≈ 16 : 1, or a little less). Indices of the differences are:
(^) (Michaels et al. 1996) and
(^) (Deutsch et al. 2007).
(^) More sophisticated versions (Gruber and Sarmiento 1997 use an adjusted Redfield
ratio; Deutsch et al. 2007 do that and subtract the intercept offset from P) to account
for variations of deep ratios from Redfield. Rather oddly, the conclusions about the
global patterns of nitrogen fixation, Gruber and Sarmiento using N and Deutsch
using P*, are almost opposite. More strangely, the discrepancy is not discussed in the
later paper, and we leave the whole issue for readers to study in the primary literature.
Eventually, these or similar indices may provide a rough evaluation of N 2 fixation in
the global nitrogen budget. Both analyses do suggest that fixation by marine
diazotrophs is a substantial factor in the global nitrogen cycle, at least of the order of
1014 g N yr−1 – the estimate by Gruber and Sarmiento (1997). Continuing work on
evaluating fixation rates using ratios of the stable isotopes of nitrogen in nitrate (e.g.
Casciotti et al. 2008) may contribute to quantification of global N 2 fixation.
Higher Trophic Levels
(^) Grazing in the NPSG and similar systems keeps their phytoplankton stocks in almost
exact balance between cell division and mortality; effectively, grazers consume all
production (not all biomass, all increase in biomass) every day. Banse (1995) has
made an extended argument for this. He summarized (Fig. 11.31) upper-ocean
phytoplankton growth-rate data for oligotrophic mid-ocean areas like the NPSG.