Until recently, the two species nitrate and nitrite have
typically been combined in isotopic analysis, such
that the presence of low d
15 N nitrite may have
masked some of the
15 N enrichment of nitrate in the
oxygen-deficient zone.
Ammonium The d
15 N of ammonium reflects the
production of ammonium by the degradation of
organic N and its consumption by nitrification,
ammonium assimilation, and perhaps anammox
(Figure 3). Analytical constraints have limited
isotopic studies of ammonium to environments with
ammonium concentrations greater than 1 mM,
excluding studies in the open ocean. In estuarine
systems, where ammonium can be abundant, its
d
15 N is often high (commonly higher than þ 10%,
with one observation of þ 70%) and it increases as
the ammonium concentration decreases along
transects from riverine to marine waters, due
to discrimination associated with ammonium
consumption by nitrification and/or ammonium
assimilation.
In the open ocean interior, below the depth of algal
assimilation, essentially all ammonium generated
from particles is oxidized to nitrite and then nitrate
before it can be transported into or out of a given
region. Thus, nitrification should be of limited importance for the isotope dynamics of both particulate
and dissolved N once the former has sunk out of the
upper ocean. In the open ocean surface mixed layer,
it is generally assumed that ammonium generated by
remineralization is quickly and entirely assimilated
by plankton, in which case the isotope effect associated with its consumption would not play an important role in N isotope dynamics of the open
ocean. However, in at least some regions of the upper
ocean, ammonium oxidation and assimilation are
likely to co-occur. If the isotope effect of ammonium
oxidation is greater than that of ammonium assimilation, low-d
15 N N will preferentially be routed to
the nitrate pool by oxidation and high-d
15
N N will
be routed back to the PN pool by assimilation. If the
isotope effect of oxidation is less than that of assimilation, the opposite will occur. Thus, with better
constaints on the isotope effects of ammoniumconsuming processes, the isotopes of upper ocean N
pools promise to provide an integrative constraint on
the relative rate of nitrification in the upper ocean,
especially when paired with the O isotopes of nitrate
(see above).
Dissolved organic nitrogen DON concentrations
are significant in the open ocean, typically Z4 mM
in surface waters, decreasing to B2 mM in deep water.
Fluxes associated with the DON pool are among
the least constrained terms in the modern marine N
budget and may be important. Studies to date of
bulk DON have been in the subtropical ocean,
where DON is by far the dominant N pool in
the surface ocean. In the surface mixed layer at
the Bermuda Atlantic Time-series site in the Sargasso
Sea, the concentration and d
15
N of TON are B4 mM
and B4% (TON being total organic N, or DON plus
the small pool of PN) (Figure 6). This d
15
N is similar
to or slightly higher than the shallow subsurface
nitrate that is entrained into the euphotic zone during
wintertime vertical mixing (Figure 6(b)). Minimal
gradients in the concentration and d
15
N of DON
in this region of the upper ocean hinder reconstruction of fluxes of DON or the d
15
N of those
fluxes. There is a weak increase in the concentration
of TON into the surface layer and an accompanying
decrease in its d
15
N (Figures 6(a) and 6(b)).
Thus, there may be an input of low-d
15
N N into
the surface DON pool, which is remineralized
at depth, but this requires further validation. Progress
on DON d
15
N dynamics would be aided by a method
to remove nitrate from samples without compromising the DON pool, which would make subsurface waters and high-nitrate surface waters
more accessible to study. Ongoing work on separable fractions of the DON pool (e.g., the highmolecular-weight fraction and its components) is also
promising.
Dissolved gases Dissolved N 2 in equilibrium with
atmospheric N 2 at the surface has a d
15 N of 0.6%.
The isotopic composition of dissolved N 2 does not
vary greatly in ocean profiles, except in zones of
denitrification. Production of low-d
15 N N 2 in
denitrification zones results in measured N 2 d
15 N as
low as 0.2% (Figure 8(a)). Since N 2 is the main
product of denitrification, its d
15 N provides a test of
the nitrate-based estimates of the isotope effect for
this process.
Dissolved N 2 O is produced by nitrification and is
both produced and consumed by denitrification. The
marine flux of N 2 O is perhaps one-third of the global
flux of this greenhouse gas to the atmosphere;
therefore, an understanding of the mechanisms of
N 2 O production and their regulation in the ocean is
an important goal. Culture studies indicate that
bacterial production of N 2 O by nitrification and
denitrification produces gas depleted in
15
N and
18
O
relative to the source material. Consumption of N 2 O
by denitrification leaves the residual gas enriched in
15 N and
18
O, with d
15 N of N 2 O as high as 40%
measured in the Arabian Sea. In oxygenated waters
of the open ocean, nitrification likely dominates N 2 O
production and its isotopic profile. A depth profile in
NITROGEN ISOTOPES IN THE OCEAN 559
typically been combined in isotopic analysis, such
that the presence of low d
15 N nitrite may have
masked some of the
15 N enrichment of nitrate in the
oxygen-deficient zone.
Ammonium The d
15 N of ammonium reflects the
production of ammonium by the degradation of
organic N and its consumption by nitrification,
ammonium assimilation, and perhaps anammox
(Figure 3). Analytical constraints have limited
isotopic studies of ammonium to environments with
ammonium concentrations greater than 1 mM,
excluding studies in the open ocean. In estuarine
systems, where ammonium can be abundant, its
d
15 N is often high (commonly higher than þ 10%,
with one observation of þ 70%) and it increases as
the ammonium concentration decreases along
transects from riverine to marine waters, due
to discrimination associated with ammonium
consumption by nitrification and/or ammonium
assimilation.
In the open ocean interior, below the depth of algal
assimilation, essentially all ammonium generated
from particles is oxidized to nitrite and then nitrate
before it can be transported into or out of a given
region. Thus, nitrification should be of limited importance for the isotope dynamics of both particulate
and dissolved N once the former has sunk out of the
upper ocean. In the open ocean surface mixed layer,
it is generally assumed that ammonium generated by
remineralization is quickly and entirely assimilated
by plankton, in which case the isotope effect associated with its consumption would not play an important role in N isotope dynamics of the open
ocean. However, in at least some regions of the upper
ocean, ammonium oxidation and assimilation are
likely to co-occur. If the isotope effect of ammonium
oxidation is greater than that of ammonium assimilation, low-d
15 N N will preferentially be routed to
the nitrate pool by oxidation and high-d
15
N N will
be routed back to the PN pool by assimilation. If the
isotope effect of oxidation is less than that of assimilation, the opposite will occur. Thus, with better
constaints on the isotope effects of ammoniumconsuming processes, the isotopes of upper ocean N
pools promise to provide an integrative constraint on
the relative rate of nitrification in the upper ocean,
especially when paired with the O isotopes of nitrate
(see above).
Dissolved organic nitrogen DON concentrations
are significant in the open ocean, typically Z4 mM
in surface waters, decreasing to B2 mM in deep water.
Fluxes associated with the DON pool are among
the least constrained terms in the modern marine N
budget and may be important. Studies to date of
bulk DON have been in the subtropical ocean,
where DON is by far the dominant N pool in
the surface ocean. In the surface mixed layer at
the Bermuda Atlantic Time-series site in the Sargasso
Sea, the concentration and d
15
N of TON are B4 mM
and B4% (TON being total organic N, or DON plus
the small pool of PN) (Figure 6). This d
15
N is similar
to or slightly higher than the shallow subsurface
nitrate that is entrained into the euphotic zone during
wintertime vertical mixing (Figure 6(b)). Minimal
gradients in the concentration and d
15
N of DON
in this region of the upper ocean hinder reconstruction of fluxes of DON or the d
15
N of those
fluxes. There is a weak increase in the concentration
of TON into the surface layer and an accompanying
decrease in its d
15
N (Figures 6(a) and 6(b)).
Thus, there may be an input of low-d
15
N N into
the surface DON pool, which is remineralized
at depth, but this requires further validation. Progress
on DON d
15
N dynamics would be aided by a method
to remove nitrate from samples without compromising the DON pool, which would make subsurface waters and high-nitrate surface waters
more accessible to study. Ongoing work on separable fractions of the DON pool (e.g., the highmolecular-weight fraction and its components) is also
promising.
Dissolved gases Dissolved N 2 in equilibrium with
atmospheric N 2 at the surface has a d
15 N of 0.6%.
The isotopic composition of dissolved N 2 does not
vary greatly in ocean profiles, except in zones of
denitrification. Production of low-d
15 N N 2 in
denitrification zones results in measured N 2 d
15 N as
low as 0.2% (Figure 8(a)). Since N 2 is the main
product of denitrification, its d
15 N provides a test of
the nitrate-based estimates of the isotope effect for
this process.
Dissolved N 2 O is produced by nitrification and is
both produced and consumed by denitrification. The
marine flux of N 2 O is perhaps one-third of the global
flux of this greenhouse gas to the atmosphere;
therefore, an understanding of the mechanisms of
N 2 O production and their regulation in the ocean is
an important goal. Culture studies indicate that
bacterial production of N 2 O by nitrification and
denitrification produces gas depleted in
15
N and
18
O
relative to the source material. Consumption of N 2 O
by denitrification leaves the residual gas enriched in
15 N and
18
O, with d
15 N of N 2 O as high as 40%
measured in the Arabian Sea. In oxygenated waters
of the open ocean, nitrification likely dominates N 2 O
production and its isotopic profile. A depth profile in
NITROGEN ISOTOPES IN THE OCEAN 559