An analysis of the horizontal regime of denitrification
in the eastern tropical North Pacific1
L. A. Codispoti
Department
and F. A. Richards
of Oceanography,
University
of Washington,
Scattle
08105
Abstract
A method using isentropic
analysis to cstimatc nitrate removals by denitrification
in
the oxygen deficient waters of the eastern tropical North Pacific agreed reasonably well
with a technique
based on vertical
distributions.
Dynamic
computations
permitted
an
estimate of the horizontal
advective
transport of these nitrate deficits out of the study
region; such transport was a large fraction of the total, indicating
that horizontal
processes
are important.
The horizontal
transports used in combination
with estimates for vertical
losses and
horizontal
diffusive
losses yielded a denitrification
rate of -2 x 1013 g N yr-’ for the
portion of the eastern tropical North Pacific east of =112’W.
This value was in good
agreement with estimates based on the application
of respiration
rates to the appropriate
volumes of oxygen deficient water and with an estimate bnscd on a vertical model.
Goering et al. (1973) found two tcchniques useful in a preliminary study of denitrification
in the oxygen deficient ( O2
< eO.1 ml liter-l)
waters of the eastern
tropical North Pacific (Fig. 1) : the application of respiration rate estimates and a
horizontal approach involving
cr,-nutrient
relationships
and dynamic
calculations.
Codispoti (1973a) later used an improved
version of these transports and a more
thorough isentropic analysis. This study
produced a new method for estimating nitrate removals by denitrification
(nitrate
deficits) and yielded a value for the denitrification
rate in the study region that
agreed well with one based on respiration
rates estimated
by Packard’s
( 1971)
method.
Dynamic
calculations
and iscntropic
analyses emphasize horizontal
processes
and are classic oceanographic techniques.
Despite the many successful applications of
these methods, chemical oceanographic
studies tend to emphasize vertical profiles
and vertical models, particularly
in low
environments
oxygen
(e.g. Brandhorst
1959). In a more general study of the oxygen minimum layer, Wyrtki (1962) also
emphasized vertical processes. Such in’ Contribution
877 from the Department
of
Oceanography,
University
of Washington.
This
research was sponsored by Office of Naval Rcsearch contract N-00014-67-A-0103-0014.
LIMNOLOGY
AND
OCEANOGRAPHY
vestigations produced useful results and
some statdd that the horizontal mode could
be significant, but their emphasis on the
vertical tends to imply that horizontal processes are unimportant.
We here examine the horizontal regime
in the eastern tropical North Pacific and
provide estimates for the denitrification
rate in the portion of this region east of
112OW, extending from the equator to Baja
California, and including the Gulf of California. Our results suggest that horizontal
advection is a major mechanism in the removal of the products of denitrification,
and they demonstrate the utility of dynamic computations for estimating dcnitrification rates.
We thank J. J. Anderson, J. D. Cline, A.
H. Devol, T. T. Packard, and W. Smethie
for their constructive suggestions and for
their special contributions to the data used
in this study.
Methods and results
The isentropic analysis was based on
data from 22 cruises. The data were avcraged by 1” squares and charts for nine (Jo
surfaces (Table 1) were produced, Codispoti (1973a) discussed the criteria for selection of data; Codispoti (1973b) listed
the 22 cruises and presented the latest series of charts. Data extending over more
379
MAY
1976,
V.
21(3)
Codispoti
and Richards
Table 1. Surfaces chosen for isentropic analysis and the a,-cxpectcd
nitrate relationship.
_-- ____ __-.___ ---__I_-_-_
_ _---____----_--p_I______ ___ _ *.-..--- _. - ._.-_._-- - .- 0
Expected
Approximate Depth Range
T
Nitrate
in the Study Region (m)
(ug-atoms
liter-l)
_I__.-_____-_-25.5
25.8
26.1
26.4
26.6
26.8
27.0
27.3
27.5
32.75
33.65
34.61
35.37
36.41
38.73
42.52
48.60
50.70
40 50 50 -
100 - 300
1.75 - 375
300 - 475
450 - 560
770 - 880
1100 - 1350
----_---
Fig. 1. Rpprotimatc
horizontal
extent of the
oxygen deficient region in the eastern tropical Pacific. The hatched arca indicates oxygen conccntrations of less than 0.25 ml liter-’ on the 26.81
ur surface. In the study region this surface is at
-400 m (after Reid 1965).
than 10 years wcrc sufficiently invariant to
permit contouring even the nitrite concentrations on some surfaces, and the entire
set of charts reveals a number of interesting
fcaturcs including the possibility of a shift
in the geographic location of the denitrification zone with depth. However, charts
For two surfaces near the core of the oxygen deficikt
stratum (Figs. 2 ancl 3) will
illustrate the features pertinent to this discussion.
Our initial purpose in the isentropic
analysis was to estimate the nitrate deficit
without
relying on reactive phosphorus
data. We also wished to evaluate a prcviOLIS method
(Cline ancl Richards 1972)
based on vertical distributions.
The new
method was developed as follows.
nitrate”
A property called “equivalent
was dcfincd as
NOR-T~:quiv
= ‘Non- + NO, + O2
X ( ANO,-/AOh,
(1)
whcrc NOZ-, NOZ-, and O2 arc thti observed
concentrations, and hN03-/~02 is the ratio
of nitrate-N produced to oxygen consumed
during aerobic respiration,
16 : 276 (by
120
140
200
-----_-
atoms) as suggcstcd by Rcdficld ct al.
(1963).
This property should bc insensitive to in
situ changes in oxygenated waters, and it
was designed to help tistimate the amount
of nitrate present in the source water for
the oxygen deficient zone when dissolvtid
oxygen concentrations were essentially zero
and just before dellitrification.
It was contoured on selected cr7 surfaces ( Table 1,
Figs. 2 and 3) along with nitrite and salinity. As expcctcd, equivalent nitrate decreased in and near regions whcrc high
nitrite concentrations inclicatecl active denitrification
( Figs, 2 and :3) . To calculate
nitrate deficits it was necessary to define
)-the
appro“expcctcd nitrate” ( NO:I-141,1,
priate initial equivalent nitrate concentration for each (Jo surface. Calculations,
based on salinity values and the assumption
that mixing occurred only along a7 surfaces
( Codispoti 1973a), indicated that most of
the water on the 25.5 to 27.0 uT surf::ices
came from the south, so only values from
the southern source arca were used to estimate expected nitrate on thcsc surfaces, In
the deep layers (27.3 and 27.5 err surfaces)
most of the water also appeared to be of
southern origin but salinity diffcrcnces were
slight and cquivalcnt nitrate values in both
the northern and southern source regions
IIorizontal
denitrification
liter-’ on the 26.6 1~~surface. B. Salinity in % and
nitrite in pg-atoms liter-’ on the 26.6 u7 surface.
Expcctcd nitrate on this surface is 36.4 pg-atoms
liter-l.
were similar, so data from both arcas were
used.
Nitratq deficits might be estimated by
simply subtracting cquivalcnt nitrate valucs from expected nitrate, but a correction
term was introduced to allow for two possiDuring denitrification
blc complications.
organic nitrogen could be converted to ammonia which might subscqllcntly
be oxidized, and some organic nitrogen might be
converted to fret nitrogen. Richards (1965)
presented equations indicating that, in the
first case, simply subtracting
equivalent
from expected nitrate would result in an
undkestimatc
for the nitrate deficit of
-18.9% and, in the second, nitrate removal
would bc correctly estimated but free nitrogen production could bc undercs timated
381
regime
Fig. 3. A.
Equivalent
nitrate
>n pg-atoms
litc? on the 26.8 a7 surface. B. Salinity in go and
nitrate in pg-atoms liter-’ on the 26.8 c7 surface.
Expected nitrate on this surface is 38.7 pg-atoms
litfd.
by about 12%. Because both situations are
merely possibilities, a correction factor of
9.4% was used so that the calculated values
would fall bctwcen
the extremes. Thcrefore, the final equation for calculating nitratc deficits is
NOx-nnom Tr = NOcm
T x 1.094,
(2)
NQr~nonn TI is the nitrate deficit,
is NOSeEsDminus NOR-I,:quiv, and
1.094 is the correction factor. Expected
nitrate values for the applicable c7. range
are listed in Table 1 to facilitate use of the
method. At times, we have extrapolated to
u7 values within 0.1 of the actual range,
whcrc
NO:~-~nom I
382
Codispoti
and Richards
20.00
8
c
T
g
0
‘3
I
15.00
s
&
10.00
6
h
i!
”
5.00
3v -
t
E
z
:
- 30
!O” -
0.00 -
SLOPE
0
I
5.00
- 5.00
- 5.00
NITRATE
0.00
DEFICIT
(Noi
ANOM
I
10.00
= 1.28
.
R2
0.93
R = 0.97
I
15.00
JIWg-atoms/liter
Al7
l
.
Al8
IO”
*20.
A21
-
1
. .-
.
.
10”
a33
Fig. 4. Nitrate deficit
( NO~A,,~~ II ) vs. Cline
and Richards’ ( 1972 ) nitrate deficits.
but using the method at depths much farther above the 25.5 cr7 surface is probably
not valid because of the greater possibilities
for oxygen exchange with the atmosphere
and for significant temporal changes in expectcd nitrate concentrations.
When the above method was applied to
the data used for the vertical method of
Cline and Richards (1972), agreement
was good ( Fig. 4). Reasons for the agrecment will be discussed below. At present,
we merely indicate that the method appeared to give reasonable answers and was
especially useful in cases where reactive
phosphorus data were not of the quality
used by Cline and Richards ( 1972).
The new method used in combination
with dynamic computations permits estima tion of horizontal,
advective, nitrate
deficit transports. Most of the evidence
(Cline 1973; Cline and Kaplan 1975; Codispoti 1973a; Fiadeiro and Strickland 1968;
Richards and Benson 1961; Thomas 1966)
indicates that the nitrate deficits do result
from denitrification
and are reasonable approximations of the free nitrogen produced.
By assuming that nitrate deficit transports
are good estimates of the transport of the
free nitrogen resulting from denitrification
we can use the transport results to help
estimate d&nitrification
rates. An initial
.
1200
110"
1000
II
Fig. 5. Location
chart for sections I, II, and
III. Triangles indicate stations taken in December
1969 during RV T. G. Thompson cruise 46. Dots
Thcmpson cruise 66, January-Febrnary
indicate
1972.
study ( Codispoti 1973a) based on section
I (Fig. 5) indicated a nitrate deficit transport of about 0.9 X 10’” g N yr-l between
-100 and 1,000 m out of the eastern tropical
North Pacific. By assuming that nitrate
deficits decreased linearly from the uppermost value that could be calculated ( cr7
~25.5) to the surface, we estimated the
transport in the upper layer ( -100 to the
surface) as 0.3 X 101” g N yr-l. This was
assumed to bc: a reasonable estimate of nitrate deficit losses through the upper
boundary of the oxygen deficient waters
east of 112”W.
To estimate the horizontal
diffusive
losses we assumed that cross-stream and
upstream diffusive losses were negligible
because they would tend to be carried
back into the region or prevented by the
coast. The average downstream nitrate defi-
Horizontal
Table
2.
cs
-r
Intervals?
Denitrification
denitrification
rates east of 112”W
based on respiration
Approximate
Thickness of
the o
T
Intervals
Cm>
Estimated Volumes of
Low Oxygen Water with-l
NO; > 0.2 ug-atom liter
383
regime
rate estimates.*
Denitrification
Rate
(g N year-1x1012)
ETS Respiration
Rate
(g N m'3year'1x10'2)
(m3 x 1013)
25.8-26.1
25
0.8
X
9.4
=
0.8
26.1-26.4
100
7.5
X
3.8
=
2.8
26.4-26.6
26.6-26.8
100
100
12.3
16.6
X
2.4
2.3
=
=
3.0
3.8
26.8-27.0
125
24.2
X
2.1
=
5.1
27.0-27.3
250
25.0
X
1.5
=
3.8
Totals
700
86
X
1.9 x 10;;
g N year
*
Some minor corrections
arc incorporated
in this table, but the total denitrification
is the same as reported
in Codispoti
(1973a).
.
' There was little
or no low oxygen water in the 25.5-25.8 and 27.3-27.5 intervals
nitrite
concentrations
> 0.2 &atom
liter-l.
tit gradient in the North Equatorial Current region was estimated as -2 x 1CV rugatoms liter-’ km-l from an examination of
the data. A value of 10” cm2 s-r was selected for the horizontal diffusion coefficient, based on the 4/3 law ( Brooks 1959)
and on the investigations of Sverdrup and
Fleming ( 1941). With these values and
an estimated cross section of 8 X 1Ol2 cm2,
the downstream diffusive loss was only
about loll g N yr-l.
Although the -1,000-m lower boundary
for the dynamic computations may have
extended below the water column in which
denitrification
occurs (see Tables I and 2))
significant nitrate deficits were still present
at depths of ~1,000 m. Consequently, a
nitrate deficit loss through the bottom
boundary ( area -3 x 10” km2) was estimated using a vertical approach. Information presented by Sholkovitz and Gieskcs
( 1971) and Munk ( 1966) indicated that
at the lower boundary reasonable estimates
of the vertical diffusion coefficient and velocity would bc 3 cm2 s-l and 3 x lo-‘) cm
s-l (upward).
The estimated vertical
gradient was 1 X 10m7 pg-atoms cm-4 and
the estimated concentration was 4 pg-atoms
rate
with
liter-l. We now feel that the concentration
selected is too high, but Neumann and
Pierson ( 1966) present information
indicating that the estimated diffusion coefficent could also bc generous. Consequently,
the 0.2 x 1Ol3 g N yr-’ obtained for the
loss through the lower boundary still seems
re‘asonable.
Because section I samples the polcward
California
Undercurrent
inadequately,
a
correction term (0.2 x lOI g N yr-’ ) based
on the nitrate deficits and on literature valucs for the volume transport was included.
With the above .terms, the total estimated
denitrification
rate east of 112”W in the
castcrn tropical North Pacific was 1.6 X
101” g N yr-‘, An independent estimate,
based on respiration rates determined by
Packards (1971) method, was used to judge
the accuracy of this value. The isentropic
analysis charts were used to determine the
volume of water in which nitrite concentrations below the primary maximum excccded 0.2 pg-atoms liter-l
(Table 2);
denitrification
was assumed to be the dominant respiratory process in these volumes,
and the rate estimates were applied yielding an answer of 1.9 X lot3 g N yr-l (Codi-
384
Codispoti
Table 3. Horizontal
g N yr-1 x 1013.
advective
Sections
I
nitrate
deficit
and Richards
losses indicated
II
by sections I, II, and III.
III
III
Sta. 3335 only
All values in
Totals*
p + (C - D) 4 g + D
2
2
-___.---
1.
0 - 100 m"
0.26
0.02
(Gain)
0.29
0.12
0.32
2.
100 m to Reference level
0.78
0.90
1.27
0.65
1.80
3.
Statement
1.04
0.88
1.56
0.77
2.12
4.
200 - 700 m
0.45
0.61
0.94
0.56
1.28
* See text
1 + 2
for
an explanation
of how the data were combined.
' We consider these transports
to estimate losses through the upper boundary oE the oxygen
deficit
zone in the study region.
All surface layer transports
assume that nitrate
deficits
decrease linearly
from the last value which could be calculated
(Ok 2 25.5) to
zero at the surface.
spoti 1973a). The enzyme actvitics measured by Packard’s ( 1971) method were
converted to dentrification
rates using a
preliminary
formula; Devol ( 1975) obtaincd a similar answer ( ~2 x 10’” g N
yr-’ ) with a more refined tcchniquc.
These values neglect the possible contribution from the scdimcnt-water
interface,
but only about 2 x 10’ I m2 of sediment contact the oxygen deficient waters in the
study region. We know little about nitrate
removal rates at the interface, but Codispoti
(1973a) argued that they should be no
greater than a tenth of Richards and Brocnkow’s ( 1971) cstimatc (18 g N m-2 yr-l)
for the more shallow and warmer environment of Darwin Bay. A value of 1.8 g N
m-e2yr-1 gives a denitrification
rate for the
interface
of only 0.04 x 10”’ g N yr-‘. Recently, Smith ( 1974) cstimatcd bacterial
oxygen
consumption rates at the interface
in the San Diego Trough, a relatively product ive region. He believed his estimate
was a minimum and obtaine’d a value of
0.21 ml O2 me2 he’. Total community oxygen respiration was 1.31 ml 02 m-” h-l. Although the sediments of interest here arc
at shallower depths, Pam&mat’s (1971)
data indicate that the rates could be similar.
If we assume that the denitriFying bacteria
in our study region respire at a rate equivnlent to 0.5 ml OZ rrp2 h--l, the annual nitrate
removal at the sediment-water
interface
would bc 2.2 g N m2 yr-I, close to Codispoti’s ( 1973a) earlier estimate. Thus, the
available cvidencc on bottom processes
does not indicate that the denitrification
rate estimated from water column respiratio11 should bd significantly
raised.
When the uncertainties arc considered,
the figures after the decimal point may bc
meaningless and the close agreement bctwccn thd two estimates of denitrification
rate for the study region is probably fortuitous. These estimates arc, however, of
the order of magnitude required for dcnitrification
in the eastern tropical North
Pacific, and the analogous regions in the
South Pacific and the Arabian Sea, to balancc the combined
nitrogen
additions
( Emery et al. 1955; Eriksson 1959; Tsunogai and Ikeuchi 1968; Tsunogai 1971) that
could not be compensated without denitrification. The point is that the initial calcu-
Horizontal
&nitrification
lations appeared to yield a reasonable answer, in which horizontal advection was a
major factor.
Since the initial analysis, we have calculated the nitrate deficit transports for sections II and III (Fig. 5) and recalculated
section I. Thcsc results (Table 3) indicate
that our original estimatk of the horizontal
advection of nitrate deficits may have been
low. The calculations were carried out by
dividing
each section into quadrilaterals
with depth intervals of 50-200 m and horizontal distances equal to the difference
between adjacent stations. Average volume
transports and nitrate dkficits were calculated for each quadrilateral.
Multiplying
these values produced a nitrate deficit
transport for each quadrilateral, and these
values were algebraically summed to produce a nitrate deficit transport for each
section. Current speeds and volume transports were computed using the dynamic
method (Sverdrup et al. 1942). Differences
between the new and original calculations
included the USCof Hclland-Hansen’s (1934)
shallow water method so that stations 3
and 4 could bc added to s&ion I and stations IO and 11 included in II, varying interpolation
procedures
for the surface
layer and using slightly different refcrcnce
kvcls. The l,OOO-db surface was used for
I, but 1,200 db was used for II, and a variable reference lcvcl between 1,000 and
1,200 db was used for III. The ratio of the
l,OOO- over the’ 1,200-db reference lcvcl
transport for section HI was 0.8; between
the l,OOO-db and variable rcferencc l&e1
casts it was 1.0. IIorizontal nitrate deficit
transports below 1,000 db for section I and
below 1,200 db for sections II and III were
assumed to be zero,
The nitrate deficit transport o;lt of the
region indicated by I (including O-100 m)
is now 1.0 X 1013 g N yr-l partly because
of the inclusion of the shallow stations (3
and 4). These stations indicated an eastward transport of 0.1 X lOI g N yr-l, 80%
of which was in the’ upper 100 m. Section
II was designed to estimate the transport
in the region of the California Undcrcurrent, a narrow poleward flow normally
regime
385
found close to the western coast of Baja
California.
The’ net .transport out of the
study region indicated by this section was
0.9 x lOI g N yr-l. Of this total, 0.4 X 10’”
g N yr-l were in the transport between the
two stations ( 10 and 11) with depths less
than the refer&cc level. Because of the
added complications of applying dynamic
calculations to shallow environments, WC
are somewhat suspicious of this value.
EIowevcr, the transport between stations 9
and 10 was in the reverse direction and
even larger than betwekn 10 and 11, so
elimination of all values involving depths
less than the rcfcrcncc level would have
caused the total to increase.
While 0.9 x 1013 g N yr-l may be a reasonable 6stimate of the transport in the rcgion of the California Undercurrent,
it is
difficult
to decide how to combine this
value with the results of the other sections.
For example, a westward transport of 1.1
x lOI g N yr-l was indicated between stations 46 and 48 in section III ( Fig. 5)) and
this flow could turn north and be included
in section II. On the other hand, if the
sections were’ not perpendicular to the flow
or did not extend far enough, estimates
could have been low. WC feel it best to
include only half of section II’s 0.9 x 10’”
g N yr-l transport in our total (Table 3).
Fortunately,
a difference of 20.45 X 10’”
g N yr-l should not alter our main conclusions.
Sections I and II have been prcscntcd
clskwherc ( Codispoti 1973a,b), so we have
only shown section III (Fig. 6). Its nitrate
deficit transport is somewhat greater than
section I’s (Table 3) bccalise of the station
33-35 contribution.
These stations extend
beyond the southern limit of section I and
both sections (I and III)
might show
higher transports if they extended farther
south. The transport north of station 35
is less in section III than in section I. This
could bc an indication of temporal changes,
the precision of the values, or additional nitrate deficit production
west of 1lO”W.
There is some cvidcncc for offshore production (Codispoti 1973a), but to bc conscrvativc we have averaged the section I and III
386
Coclispoti
and Richards
SThTlONS
46
I
45
I
.
.
44
I
.
43
I4
.
42
I
_
.
41
I
---
l
40
I
39
I
.
.
38
I
.
37
I
.=-’
36
-,--+!+c-*
?
33
t
-3
.
r 3r4
‘!=I(). i”:
.
.
.
.
.
.
.
\
\
_.,
.
.
--
.
-.
.
-’
;,
.
l \
33
-,
.
.-.
-,
.
-
in ,ug-atoms liter-’ for section III. Lower-Fig. 6. Upper-nitrate
deficit ( NO:IA,,~~ II ) distribution
rclntivc baroclinic currents in section III using a variable rcfcrence level (either 1,000 or 1,200 db). Contour lines arc in cm s-l. Solid lines indicate eastward flow and dashed lines indicate westward flow.
transports north of -lOoN and used the station 33 and 35 values for the .transport
south of --lOoN to estimate the total wcstward nitrate deficit transport out of the
study region.
Temporal changes, the various assumptions, etc. all introduce uncertain ties, but,
dcspitc these’ problems, all of the sections
indicate a similar nitrate deficit loss. Using
the new values of the horizontal, advective
nilrate deficit transports (Table 3) and the
original estimates ( Codispoti 1973a) for
the horizontal diffusive losses ( insignificant) and for the loss through the lower
boundary (0.2 x 1Ol3 g N yrl ), we get a
denitrification
rate for the eastern tropical
North Pacific ( east of ~5112~W) of 2.3 X
10’” g N yr-I. That is somewhat higher than
the original value of 1.6 X 1Ol3 g N yrl,
but still in good agreement with the estimates based on respiration rates. Although
we have been somewhat arbitrary in combining the transport data, any reasonabllc
summation will still be within -1 X 1O1-3g
N yr-l of the respiratory denitrification
estimates.
Discussion
We have shown that a method for estimating nitrate deficits based on an isen-
Horizontal
denitrification
tropic analysis ( Codispoti 1973a,b) agrees
fairly well with a procedure based on vertical distributions
(Cline
and Richards
1972). Cline ( 1973) calculated a denitrification rate of 1.6 x 1Ol3 g N yr-l for the
eastern tropical North Pacific ( east of
115’W) using a vertical diffusion-advcction model; considering, the uncertainties,
his value is in good agreement with the
above estimates. Explanations for the surprising agreement b&vecn the results of
the two methods for calculating
nitrate
deficits include the possibility that much
of the mixing contributing
to the vertical
distributions examined by Cline and Richards (1972) occurre’d outside the study region. For example, horizontal mixing in
regions where water masses sink could produce vertical gradients, because at some
distance from such a source rtigion the flow
may become horizontal or nearly so and
the gradients will tend to be rotated -90”
(Is&n 1939). Another possible contributing factor is the apparent insknsitivity
of
the cr,-expected nitrate relationship to vertical mixing. Table 1 gives the relationship
bctwcein these parameters.
Calculations,
assuming equal volumes in each of the
listed c7. intervals, indicate that even complete homogenization
would have only a
modest effect. If we neglect the minor cffeet of caballing, which would reduce the
error, the resulting a7 is 26.56 and the expected nitrate value is 38.95 p.cg-atoms
liter-l, within 3 pug-atoms liter-l of the’ actual value for a (Jo of 26.56.
All of the methods used to estimate the
denitrification
rate involve simplifications
an d assumptions, so the relatively
close
agreement could be fortuitous,
The order
of magnitude of the vertical and horizontal
transports may be similar (Packard 1969;
Cline 1973), and differbnces in factors
such as the western boundary of the study
region ( our 112”W vs. Cline’s 115’W)
could contribute to the similarity of the results.
In addition, the vertical diffusion-advection model could include the effects of horizontal processes (Munk 1966; Cline 1973;
Cline and Kaplan 1975)) and vertical losses
regime
387
could be included in the horizontal transports. An example would be a case in which
nitrate deficit production is confine’d to a
very thin stratum, and horizontal currents
are measured from top to bottom downstream from this production
region. In
these circumstances, horizontal losses of nitrate deficits me’asured above and below
the depths of the producing volume would
initially be removed by vertical processes.
Because we have’ excluded the upper 100-m
transports from our estimates of horizontal
losses (Table 3) and the calculated transports below 400 m are small (Fig. S),
our vertical boundaries are not much differ&t than the -700-m maximum depth
range of the denitrification
zone (Table 2).
The denitrification
zone is not this thick
everywhere, however, and v&tical losses
from a stratum with a high nitrate deficit
to those with lower concentrations could
occur within the d&nitrification
zone. Consequently, it could be argued that we should
assign horizontal losses to a smaller vertical
interval.
However, even within the 200700-m level, horizontal
losses still stem
dominant and it appe’ars that the vertical
thickness would have to be quite restricted
before horizontal advection would be insignifican t. Horizontal
diffusion
could increase the horizontal losses, but Codispoti’s
( 1973a) approximation indicates that this
pro&s is insignificant.
Although the transport calculations and
the partitioning
of vertical and horizontal
losses are not very precise, the data indicate clearly that horizontal processes could
be important in the’ oxygen minimum zone
of the eastern tropical North Pacific. Indeed, the 40-200-m
upper boundary of
the oxygen minimum zone is so shallow
that it would be surprising not to find significant horizontal advection,
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Suhmittetl:
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Accepted: 10 December 1975