in ODZs and a quantitative relationship between
N loss and the flux of OM, the results presented
here also have the potential to explain much
of the variability in the relative contributions of
anammox and denitrification rates observed in
the ODZs. Although the average OM C/N in the
ocean is 6.6, large variations are observed, based
on the phytoplankton community composition or
the state of remineralization of the OM (14, 28).
For instance, newly formed OM rich in amino
acids can stimulate higher relative anammox rates
due to the preferential remineralization of N-rich
compounds (16). Older recalcitrant OM, poor in
organic N, however, will result in a smaller proportion of N loss catalyzed by anammox bacteria. Yet, integrated over both space and time,
the balance between anammox and denitrification must be constrained by the flux and C/N
ratio of the OM in and out of the anoxic zone.
It is also worth noting that the occurrence of
nitrite accumulation in the ODZs does not significantly alter the balance between anammox and
denitrification, because of the requirement that a
large percentage of nitrate reduction proceeds no
further than nitrite before a noticeable enrichment in fraction anammox results (fig. S3).
Furthermore, these constraints have applicability to estimating future fixed N loss. The model
developed here, predicting water column denitrification rates using POM fluxes and C/N ratios,
can be integrated into global biogeochemical models to provide robust constraints on present and
past fixed N losses in marine suboxic regions. It is
additionally helpful in evaluating the effects of
ODZ expansion on future climate (29), potential
shifts in average C/N ratios with changing atmospheric CO2 concentrations (30), and negative
feedbacks to global primary production via N loss.
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Acknowledgments: We thank the captain and crew of the
R/V Thomas G. Thompson for assistance in sampling;
C. Buchwald and A. Morello for nutrient measurements; and
B. X. Chang, D. Bianchi, and D. M. Sigman for useful discussions
of this work. F. M. M. Morel provided invaluable advice in honing
and improving this paper. The data presented in this paper are
available in the supplementary materials. This study was supported
by an NSF grant to A.H.D. and B.B. W. and a National Defense
Science and Engineering Graduate Fellowship to A.R.B. A.R.B.,
A.H.D., and B.B. W. designed and conducted the experiments;
R.G.K. collected and analyzed sediment trap material; and A.R.B.
and B.B. W. wrote the manuscript with input from all authors.
Materials and Methods
Figs. S1 to S3
Tables S1 and S2
11 November 2013; accepted 26 March 2014
Published online 10 April 2014;
Fig. 2. Theoretical famx
from OM stoichiometry.
Fractions of N loss attributed
to anammox (famx) are calculated by a modified version of Eq. 1 (supplementary
materials text) are shown in
color contours. Cas., casamino
acids; Suc., sucrose. Predicted
(circles) and measured (squares)
famx for each OM treatment are
overlaid. The inset shows these
values in comparison to the
expected 1:1 line.
C oxidation state
−1 −0.5 0 +0.5 +1 0
Fig. 3. Organic C-dependence of N loss.
(A) Organic C flux previously measured (gray symbols) by benthic lander
incubations (27) and collected in this study (black
symbols) by sediment traps.
The derived power law fit
(black line) for the benthic
lander study is included
for reference. (B) Total N
loss rates for both coastal
(solid symbols) and offshore (open symbols) stations from +15NO2–-only
experiments. The power
law best fit of these points
(dashed gray line) and
the theoretical rates (solid black line) driven by and derived from the organic C flux in (A) are also shown.
Asterisks denote the rate experiments closest to the sediment trap deployment depths. z, depth; R2,
C flux (mmol C m−2 d−1)
flux = 5 [ ] z 100 –0.3
0 15 30
N loss rate (nmol N L−1 −1)
[ ] rate = 11.2 z 100 –1.3
R = 0.88 2