preference to oxidized forms of fixed nitrogen (Fig. 3.14). So, their concentrations are
held at low levels, rarely exceeding 0.5 μM, except in estuaries.
Fig. 3.14 Ratio of nitrate uptake to total fixed nitrogen uptake (the f-ratio) versus
available ammonium concentration in Chesapeake Bay
(^) (from McCarthy et al. 1975).
Inset: nitrate uptake rate vs. available ammonium estimated with ^15 NO 3 − in the
oceanic subarctic Pacific.
(^) (After Wheeler & Kokkinakis 1990.)
In both habitats, ammonium availability suppresses nitrate utilization by
phytoplankton.
Fixed nitrogen, like all nutrients, is eroded from the euphotic zone by the downward
movement of photosynthetically generated organic matter. This occurs through the
sinking of phytoplankton; vertical mixing of particles and dissolved organic matter;
sinking of particulate wastes of grazers; and downward swimming of zooplankton and
fish. Subsequent mineralization of organic matter introduces ammonium at depth,
where nitrifiers can function. The second oxidation step, mediated by Nitrobacter, is
more light-inhibited than the first, which is mediated in the ocean by archaea, so there
is generally a thin layer with measurable concentration of NO 2 −. Below that, most
fixed nitrogen is oxidized to NO 3 −. In most regions, the most important return supply
of fixed nitrogen to the upper layer is the upward return of nitrate by vertical mixing,
which is often seasonally accentuated, and by upwelling, which is geographically
localized. Once in the euphotic zone, nitrate can be returned to the biological cycle