Deep-Sea Research, Vol. 34, Nos 5/6, pp. 785-805, 1987.
[/198NII4W87 $3.00 + 0.00
© 1987 Pergamon Journals Ltd
Printed in Great Britain.
Nitrogen transformations in the Southern California Bight
B. B. WARD*
(Received 10 October 1985; in revised.form 10 January 1986; accepted 11tJune 1986)
Abstract--Oxidation and assimilation of ammonium and nitrate were measured in profilcs from
the surface to 1000 m at four stations in the Southern California Bight. Assimilative processes
dominated the turnover of these nitrogen compounds in the nutrient depleted photic zone:
however, at the bottom of the photic zone, nitrification and assimilation were equally important
in the turnover of both compounds. Although ammonium oxidation wits strongly inhibited by
light in the surface samples, nitrification probably contributed to rapid nitrogen cycling within the
photic zone. Assimilation of ammonium and nitrite, integrated over the photic zone, was about 3fold greater than calculated regenerated production, estimated from 14C(-) 2 uptake measurements. Integrated nitrification rates below the photic zone usually exceeded new primary
production estimates. Most ammonium oxidation occurred in a discrete maximum around the
depth of 0.2% surface light intensity; rates within the maximum exceedcd rates in shallower and
deeper samples by up to 80-fold. Nitrite oxidation rates exceeded ammonium oxidation rates, and
did not display the prominent subsurface maximum found in the former. Abundance of both
ammonium and nitrite oxidizing bacteria varied less with depth than did oxidation rates.
Autotrophic activity of the nitrifiers, measured by ~4C-autoradiography combined with immunofluorescent counts, was minimal in the surface samples, and increased with depth, but was not
directly correlated with nitrification rates, measured by ~SN techniques.
INTRODUCTION
AMMONIUM a n d nitrite b o t h occm: as i n t e r m e d i a t e s in the n i t r o g e n cycle in s e a w a t e r .
T h e s e c o m p o u n d s are i m p o r t a n t as the s u b s t r a t e s for a s s i m i l a t i o n by p h y t o p l a n k t o n a n d
b a c t e r i a , a n d for o x i d a t i o n by nitrifying b a c t e r i a . N i t r a t e , the e n d p r o d u c t of s e q u e n t i a l
a m m o n i u m a n d nitrite o x i d a t i o n , r e p r e s e n t s the m a i n flux of new n i t r o g e n into the m i x e d
layer. In the s e m i - c l o s e d n i t r o g e n cycle of o l i g o t r o p h i c o c e a n i c w a t e r s , the rate o f n i t r a t e
s u p p l y to the p h o t i c z o n e , the r a t e o f n e w p r i m a r y p r o d u c t i o n (DuGDALE a n d GOERING,
1967; EPPLEY a n d PETERSON, 1979) a n d t h e net d e p t h i n t e g r a t e d nitrification rate all
s h o u l d b e r o u g h l y e q u i v a l e n t . T h e o b j e c t i v e s o f this study w e r e to d e t e r m i n e the
d i s t r i b u t i o n a n d m a g n i t u d e of nitrification in the S o u t h e r n C a l i f o r n i a Bight, a n d to assess
t h e role o f a m m o n i u m a n d nitrite o x i d a t i o n in n i t r o g e n cycling a n d c o n t r o l of n u t r i e n t
d i s t r i b u t i o n s in t h e s e o l i g o t r o p h i c w a t e r s . M e a s u r e m e n t s of nitrification r a t e s a n d
n i t r o g e n a s s i m i l a t i o n r a t e s a r e c o m p a r e d to e s t i m a t e s of new p r i m a r y p r o d u c t i o n .
R e c e n t m e a s u r e m e n t s of nitrification in m a r i n e e n v i r o n m e n t s h a v e s h o w n that the
d i s t r i b u t i o n a n d m a g n i t u d e of t h e r a t e s a r e i n f l u e n c e d b y light (OLsoN, 1981a; WARD et
al., 1984; LWSCHULTZ et aI., 1985; WARD, 1985) a n d by a m m o n i u m c o n c e n t r a t i o n
(HAsI-tlMOTO et at., 1983; WARD et al., 1984). L a b o r a t o r y w o r k on c u l t u r e s of nitrifying
* Institute of Marine Resources, Scripps Institution of Oceanography, University of California, San Diego,
La Jolla, CA 92093, U.S.A.
785
786
B.B. W,~RD
bacteria have demonstrated an effect of oxygen concentration on nitrification rates and
the ratio of product formation that may be important in oxygen deficient envimnmems
(GoREAU et al., 1980; Lwscnt3L.TZet al., 1981). All three of these environmental variables
may be important factors in influencing nitrification rates in the Southern California
Bight.
The research described herein was motivated by several questions regarding nitrific~tion and its role in the nitrogen cycle of the Southern California Bight: What is lllc
magnitude and distribution of nitrification in surface and deep waters of this region? ltm~
do measured nitrification rates compare with other rate steps in the nitrogen cycle.
including assimilation of ammonium and nitrite, and the overall balance betwecJ~
nitrification and new primary production? Does most nitrate regeneration occur in the
water column or must deep ocean sediments be considered as potential major sites ~f
nitrification? Does significant nitrification occur within the photic zone, and if so what
are the implications for nitrate production within the nitracline as a supply of rapidly
recirculating nitrate to the photic zone?
1 approached these questions with experiments at three deep stations (samples ~,
850 m) in the Southern California Bight by: (1) measuring the oxidation and assimilation;
of both ammonium and nitrite; (2) determining the distribution of ammonium and
nitrite-oxidizing bacteria by immunofluorescent enumeration; and (3) assessing by
autoradiography the relative autotrophic assimilation of carbon by nitrifying bacteria.
This approach yields several kinds of information on the distribution and activity of
nitrifying bacteria, and permits the determination and comparison of rates of several
steps in the nitrogen cycle.
M~. ] l l ( ) t ) s
Results from 2 cruises on the R.V. N e w H o r i z o n in the Southern California Bight arc
presented here. Stations 205 (33°18.7'N, 118°09.6'W) and TRPI (33°33.8'N.
118°27.3'W) were occupied in November 1982 and Stas 206 (33°07.7'N, 118°32,0'W) and
TRP3 (33°30.8'N, 118°25.0'W) in May 1983. All 4 stations are located over senu.
enclosed basins in the California borderland in depths of 800-1250 m (Fig. 1). Samples
were collected at wide depth intervals in 30 t Niskin bottles, At each depth, aliquots for
15N and 14C incubation experiments, nutrients, oxygen and immunofluorescent e n u m c
ration were obtained from the same Niskin sampler. Nutrient samples were filtered
through GF/F flters, frozen and analyzed by standard methods (SI"mCKLA~D and
PARSONS, 1972) after the cruise. Chlorophyll a was measured at sea using the fluor~,
metric method of HOLM-HANsEN et al. (1965). Dissolved oxygen concentrations were
measured using the Winkler titration method of BRYANet al. (1976). Particulate nitrogen
and carbon were determined on a Hewlett Packard C/H/N analyzer (SHAm', 1974)~
Nonparametric statistical methods (TATE and CLELI,AND,1957) were used to analyze the
immunofluorescence and autoradiography results where noted in the results section
below.
lSN Tracer experiments"
Four Pyrex incubation bottles were filled with 4 1 of water from each depth. Two
bottles were supplemented with 0.2/aM (JSNH4)2SO~ (99 at.-% 15N) and 0.25 jaM
Na~4NO2, and two bottles were supplemented with 0.2 ~tM Na~SNO~ and 0.25 I-tM
Nitrogen transformationsin the Southern California Bight
121°W
120 °
119°
118°
787
117 °
34°N
33 °
Fig. 1. Map of the Southern California Bight, showing stations locations occupied during
SCBS-21 (205 and TRP) and SCBS-22 (206 and TRP3).
KI4NO3 (except for deep water where NO3 was plentiful). Samples were incubated in
running seawater incubators at surface seawater temperature at simulated in situ light
intensities. Incubations were terminated after 24 h by filtration through baked GF/F
filters. Procedures for isotope ratio analysis of particulate and soluble (nitrite and nitrate)
nitrogen fractions were described previously (OLSON, 1981a; WARD et al., 1984). Nitrite
was extracted at sea while filtrates for nitrate analysis were frozen and returned to the
lab. Thawed samples were first run through a large Cd-Cu reduction column and the
resulting nitrite extracted as described previously (OLSON, 1981a; WARD et at., 1984). The
15N:~4N ratios of nitrite and nitrate in the resulting extracts were determined on a
JASCO emission spectrometer after combustion using a modified micro-Dumas procedure (BARSDATEand DUGDALE,1965).
All samples were incubated in replicate and 2 4 replicate isotope ratio determinations
were made of subsamples of each filter or extract. The measured ratios from subsamples
of the same filter were generally within 2-10% ; differences between replicate incubations
were small for nitrogen assimilation into the particulate fraction, but varied by more than
100% in a few nitrate samples analyzed for nitrite oxidation (see Results). Standards
were prepared from the tracer solutions and treated identically to the samples; standard
deviations of replicate isotope ratio determinations were 2-5% of the mean when the
isotope ratio was 10% or less. Samples filtered and processed immediately after tracer
additions were used as blanks for calculating subsequent at.-% enrichments in the
particulate and soluble pools after incubation. The equations of DUGDALE and GOERING
(1967) were used for rate calculations except where mentioned below (see Results).
Immunofluorescent enumeration and autoradiography
Antisera produced in response to immunization with the following bacterial strains
were used in this study: ammonium oxidizing species Nitrosococcus oceanus and
Nitrosomonas sp. (marine) nitrite oxidizing speciesNitrobactersp. (marine) andNitrococcus
mobilis. Although cells identified as reacting with these sera cannot be unequivocally
788
B.B. WARD
identified to species, specificity testing of these s e r a (WARD and CArLtJucl, !9851
indicates that the sera are genus specific. Thus, the cells enumerated in this study z~re
referred to as belonging to a particular genus, rather than a species. Samples were
preserved in formalin (2% final concentration) and stained with immunofluoresccnt
reagents by methods previously described (WARD, 1982: WARD and CA~<I_t~:l, 1985i,
Cells were enumerated with a Zeiss fluorescent microscope at × 125(L using a 100 W
tungsten halogen bulb and Zeiss filter set No. 487709. Three or more replicates at each
depth were stained with each antiserum so that each genus was enumerated on separate
sets of replicate slides.
At Sta. 205, 14CO 2 tracer experiments were performed in parallel to the ~N
experiments described above. Tracer solution (25.6 laCi of NaHI4CO3) was added ~t~
250 ml samples, which were incubated under the same conditions as the ~SN samples.
Incubations were terminated after 24 h by the addition of formalin (2% linal concentration). Five days later, the preserved samples were filtered, washed twice with filtered
seawater and twice with NH4COOH (0.6 M), and stored in desiccators until analysi.~.
Autoradiograms were prepared from the filters using the emulsion dipping method
described by WArD (1984). Minor modifications in the method included the use of gelatin
to precoat the slides before attaching the filters and drying the emulsion after devclupment at room temperature. Exposure of 3 weeks was required to obtain optimal signaito-noise ratio in the developed silver grains. Autoradiograms were microscopically
analyzed using the system described above, except that transmitted light was used
simultaneously with epifluorescent illumination (WARD, 1984). Each nitrifying genus was
enumerated on separate replicate slides and background development was determined
for each individual slide.
R E S t I.1 S
The sampling scheme under which these data were collected consisted of: 8 or 9 depths
at each station, four in the photic zone and the rest spaced at 100-200 m interwds ~,
within 50 or 100 m of the bottom. Additional samples collected during CTD-rosette c~tsts
have been included in the nutrient and chlorophyll data presented here.
Distribution Of"nutrients and biological parameters
Nutrient and oxygen distributions are presented in Fig. 2. All stations were character-.
ized by a nutrient depleted surface layer, defined most clearly in the nitrate distributioll.
The nitracline occurred between the 35 and 80 m samples during both cruises. Temper~,
ture (not plotted) is predictably correlated with nitrate concentration, and thus the
nitracline generally defines the thermocline in these waters (CuLI.EN e t al., 1983:
STRICKLAND, 1970). Ammonium concentrations were low; highest concentrations, aboul
0.5 taM, were found at Sta. TRP. At the other three stations, ammonium was usually
<0.1 taM (Fig. 2A), and was undetectable at Stas 206 and TRP3 except at a few depths
(35 m at 206, 80 m at TRP3). Nitrite concentrations were also low; subsurface maxima of
about 0.1 taM were clearly seen at Stas 205 and TRP, but were not observed at the May
stations, 206 and TRP3. Oxygen was saturated in the surface layer and decreased with
depth to <1 ml 1-~ in the deepest samples.
Chlorophyll a and particulate nitrogen were largely restricted to the photic zone
(Fig. 3). Chlorophyll a was usually <1 lag-at. 1-~ and exhibited a subsurface maximum ~t
Nitrogen transformations in the Southern California Bight
o[,~..j .:".~:~
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Fig. 2. (A) NH~ and NO~ (concentrations in laM) distributions at Stas 205 and TRP. Q, NO2,
©, NH4L (B) NO~ (laM) and oxygen (ml 14) at 4 stations; NO2 (laM) at Stas 206 and TRP3.
789
790
g.B. WARD
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PARTICULATE NITROGEN (ffM) ---a--[
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CHLOROPHYLL
Fig.
3.
('hlorophyll
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(PN:
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~tg-at. 1 ~) distributiCm~;
the 1.0% light level. A broader, but coincident, subsurface maximum m p a m c u l a t c
nitrogen was often observed: particulate nitrogen concentrations in the maximum were
about 1.5 gM decreasing to a constant concentration of 0.5 I,tM in the deep watc~.
D e p t h distributions of a m m o n i u m oxidizing bacteria (2 genera) and nitrite oxidizing
bacter,a (2 genera) are shown for Stas 206 and T R P 3 in Fig. 4. (Results for Sla. 205 can
be found in WARD and (7,.'\RLtJ('('I, 1985.) The coefficient of variation for rcplica~v
immunofluorescent counts averaged 35%: thus many of the apparent variations m
nitrifying bacterial abundances with depth are not significant. A m m o n i u m oxidizers were
enriched in the surface samples at Stas 205 and 206. Aside from the surface enrichmen!
total a m m o n i u m oxidizer abundances were low in the photic zone; maximum ccli
numbers were found in the 80-150 m (1-(I. 1% light depth) samples at all three stations,
At Sta. 21t6, the distributions of Nitrosomonas and Nitrosococcus were parallel through
out the water column, except in the shallowest photic zone samples. At Sta. TRP3.
Nitrosomonas was nearly constant over a large part of the water column and Nitrosoco~
cus varied by a factor of 9 from the m a x i m u m at 8(>-200 m to the minimum at 600 m.
Average abundance of a m m o n i u m oxidizing cells was highest at Sta. TRP3 (average ovc~
the entire water column = 4.48 __+ 1.55 × 105 cells l-J). The average of a m m o n i u m
oxidizers at Sta. 206, 1.88 _+ 0.53 × 105 , was significantly lower than at Sta. TRP3
(P < 0.115). The average abundance at Sta. 205, 3.51t _+ 0.86 × 105, was intermediate and
not significantly different from Sta. TRP3.
Nitrite oxidizing bacterial abundances varied little with depth at Stas 205, 2116 and
TRP3, but there was a trend of decreasing numbers with increasing depth at Stas TRP3
and 206. At these two stations, Nitrococcus was more variable than Nitrobacter (data for
Nitrobacter less complete), and significant abundance differences (P < 0./15) we,re
Nitrogen transformationsin the Southern CaliforniaBight
791
AMMONIUM-OXIDIZING BACTERIA
200I ,,,~/./.)
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NITRITE-OXIDIZING BACTERIA
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CELLS-LITER-~ (x 105)
Fig. 4. Abundances of nitrifying bacteria [cells I L ( x 105)] at Stas 206 and TRP3. Top panel,
ammonium oxidizers; bottom panel, nitrite oxidizers.
detected between Nitrobacter and Nitrococcus at the depths denoted in Fig. 4. Nitrite
oxidizers were significantly less abundant than ammonium oxidizers at Sta. TRP3, while
at Stas 206 and 205, the average numbers of ammonium and nitrite oxidizers were very
similar. Both ammonium and nitrite oxidizers were significantly less abundant
(P < 0.05) at Sta. 206 than at 205.
Nitrification rates
Kinetic experiments. Experiments designed to investigate the effect of increased
substrate concentrations on nitrification rates were carried out on the May 1983 cruise at
Sta. TRP1 for ammonium and TRP5 for nitrite (Fig. 1). For both ammonium and nitrite,
4 tracer additions varying from 0.02 to 0.8 pM were made in replicate to 4 I bottles filled
with samples collected from 200 m. Ammonium oxidation rate showed no correlation
792
B.B. WARD
with added tracer concentration and averaged 0.243 (S.D. = 0.108) nmol l t d '
Similarly, nitrite oxidation rate did not correlate with added ~sNO: concentration, and
averaged 0.319 (S.D. = (). 158) nmol 1 ~ d -j. These experiments were incubated for only
6 h, which contributes to the high variability (i.e. at.-% enrichments near the limi~ ~,
detection). The standard tracer addition of 0.2 tam for the profile experiments described
bek)w was chosen as being near the half-saturation constant of natural populations
reported by other investigators (HAs~UMOTOeta/., 1983) and was a compromise minimum
amount needed to detect the signal in 24 h or less. Results from these kinetic experiments imply that the measured oxidation rate was insensitive to tracer addition concernrations over more than a 10-fold range. In previous work in the same region, O~(),~
(1981a) also observed no relationship between ammonium oxidation rate and substrataadditions up to 1.0 pM.
A m m o n i u m oxidation rates. Three reproducible features are evident in the ammonitm~
oxidation profiles presented in Fig. 5. Minimal, but nonzero, rates were found in the
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AMMONIUM OXIDATION RATE n M ' d - I )
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O X I D A T I O N RATE ( n M - d - I )
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Distribution of a m m o n i u m oxidation rates (top panel) and nitrite oxidation rates
( b o t t o m panel).
Nitrogen transformations in the Southern California Bight
793
surface samples. A subsurface maximum was observed at all stations. Although the
maximum always occurred in the 0.2% surface light bottle, sample spacing was inadequate to precisely define the shape of the distribution in this region. The rate maximum
occurred at the same depth as the primary nitrite maximum at the two stations where this
feature was evident. Below the maximum, ammonium oxidation rates decreased sharply
with depth to a low, fairly constant rate. The maximum ranged from about 9 times (at
Stas 205 and 206) to 80 times (Sta. TRP3) greater than the average rate of all depths
below the maximum. Considerable variation in absolute rates was observed among
stations; the rates in the subsurface maximum at Stas TRP3 and 205 were 36 and 45 nmol
1 i d i respectively, while at Sta. 206, the maximum rate was only 2.3 nmol 1 1 d i. A
seasonal effect does not explain the difference, since 206 and TRP3 were sampled within
a few days of each other in May, while 205 was sampled in November. The sample
distribution at 206 may have missed the real maximum, given the apparent sharpness of
this feature at other stations.
Ammonium oxidation rates varied over the water column much more than the
abundance of ammonium oxidizing bacteria. The largest depth variation in bacterial
abundance was found at Sta. TRP3, where Nitrosococcus (and therefore total ammonium oxidizers) exhibited a subphotic zone maximum that exceeded the minimum
abundance at 600 m by a factor of 9. While the two maxima coincide, the maximum in
Nitrosococcus abundance was broader than the rate maximum, and the factor by which
the rate maximum exceeded the average deep rate was 80. The differences in ammonium
oxidation rates among stations can only partially be attributed to differences in bacterial
numbers. Highest rates were found at Sta. 205; highest numbers were found at Sta.
TRP3, and Sta. 206 had both the lowest oxidation rates and the lowest abundances of
ammonium oxidizing bacteria. At depths where detectable ammonium oxidation
occurred, per cell rates varied from <1.0 to over 1000 × 10 i~, tool cell -I d -I . Highest per
cell rates occurred in the subsurface oxidation rate maximum at Sta. 205. Lowest per cell
rates always occurred in the surface samples.
Turnover of the ammonium pool due to its oxidation had a depth distribution (not
shown) very similar to the distribution of the ammonium oxidation rate. In the vicinity of
the subsurface maximum, oxidation accounted for 20-60% of the daily turnover.
Ammonium oxidation accounted for nitrite production equivalent to 10-60% of the
nitrite concentration in the subsurface nitrite maximum per day.
Nitrite oxidation. Two approaches were used to calculate nitrite oxidation rates (see
Discussion). The results plotted in Fig. 5 were obtained using equations analogous to
DUGDALE and GOERING (1967) for consistency with the ammonium oxidation results.
Profiles of nitrite oxidation rates are less complete and do not show consistent patterns
with depth. At the two stations where results were obtained from surface samples, high
rates were measured in these samples. High nitrite oxidation rates were found deeper in
the water column but no distinct subsurface maximum analogous to that for ammonium
oxidation was found. Measured nitrite oxidation rates generally exceeded measured
ammonium oxidation rates in the deep water, by 2-40 times. As discussed below, rates
calculated by the equations of GLmERT et al. (1982a) give even higher results.
Although nitrite oxidation rates exhibited less consistency among stations than did
ammonium oxidation rates, values at depths where detectable rates were measured
varied by a factor of 10, compared to a factor of 80 for ammonium oxidation. Nitrite
oxidation rates did not correlate directly with nitrite oxidizing bacterial abundance, and
794
B.B. WARD
as in the case for ammonium oxidation, the variability in rates exceeded that in bacterial
numbers. Per cell nitrite oxidation rates varied from 15 to 5000 x 10 " ' M cell ~ d
Omitting the surface samples where photo-oxidation is implied (see Discussion), highest
per cell rates coincided with highest per volume rates and were found in samples from
400 m and deeper.
Measured nitrite oxidation rates could account for turnover of the entire nitrite pool
daily at some depths. The largest relative impact of nitrite oxidation on the nitrate pool
was observed near the surface, where low nitrate concentrations could be greatly
influenced by in situ nitrate production. In the nitracline, nitrite oxidation produced up to
33% of the standing nitrate concentration daily. At depth, nitrite oxidation contributed
generally <0.05% per day to the deep nitrate reservoir.
A utotrophic activity ofnitrifving bacteria. ~4C-Carbon dioxide assimilation activity wa~
found associated with cells of both strains of ammonium oxidizers at tnost, but not all.
depths (Fig. 6A). Due to loss of samples during processing, it was not always possible to
count replicates. Where more than one replicate slide was analyzed for each strain, the
range of replicate samples is plotted. The smoothed curves of activity distributions of the
two strains both show near-surface minima; Nitrosornonas activity increased with depth
while Nitrosococcus activity was nearly constant below 150 m.
It is possible to compute the product of relative activity per cell and cell number h~r
each strain, to obtain a relative measure of total autotrophic activity associated with each
strain. The sum total activities for Nitrosomonas and Nitrosococcus (Fig. 6B) i5
analogous to the total ammonium oxidation rate in that it should represent the activity ~t
the total population. The distribution of ammonium oxidation rates (Fig. 5) ~md
autotrophic activity (Fig. 6) by ammonium oxidizing bacteria have certain similarities but
are not parallel. Both distributions are lowest in the near-surface waters, and increase t~
high rates below the mixed layer. However, the subsurface maximum in ammonium
oxidation rate is not found in the total autotrophic activity, because the autotrophic
RELATIVE ACTIVITY PER CELL
r~
i/
2 0 0 i- #
L
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TOTAL ACTIVITY
i
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GRAINS PER CELL
Fig. 6.
Autotrophic activity per Cell at Sta. 205 in arbitrary units of grains per cell: total activil5
in units of'(grains per cell) × (cells per liter × 105).
795
Nitrogen transformations in the Southern California Bight
activity remains high with increasing depth. The subsurface increases in ammonium
oxidation rate and autotrophic activity are offset by at least 70 m, autotrophic activity
being highest just below the maximum in oxidation rate. Although the general shape of
the two distributions is similar, the two are not directly correlated because of the depth
offset.
Data for autotrophic activity of nitrite oxidizers are less complete (not shown).
Activity minima for both Nitrobacter and Nitrococcus were found in the surface waters,
while activity comparable to that of the ammonium oxidizers was found both in a
subsurface maximum and in the deeper samples.
A m m o n i u m and nitrite assimilation
Ammonium and nitrite assimilation patterns were similar among stations (Fig. 7).
Highest assimilation rates were found in the three surface samples (i.e. at 12% or more
surface light). Assimilation rates were more highly correlated with light intensity than
with biomass (Chl a or particulate nitrogen) since the rate distributions did not exhibit
subsurface maxima at the bottom of the photic zone. The vertical spacing of samples was
not sufficient to precisely describe the distribution on small scales, but the general
features of the profiles are evident.
Ammonium assimilation rates of 50 to over 100 nmol 1 ~ d -l were observed in surface
samples, and significant rates (4-10 nmol 1-1 d -l) were measured in the 0.2% light bottles
as well. In the dark samples, ammonium assimilation was fairly constant with depth, on
the order of 1 nmol 1-I d -1 or less. Measured ammonium assimilation rates imply that in
situ processes could recycle the ambient ammonium pool up to several times per day in
the photic zone.
Nitrite assimilation rates were lower than ammonium assimilation rates, and although
they showed a similar photic zone distribution, nitrite assimilation had decreased to its
low deep water level in the 0.2% light sample, Nitrite assimilation below the photic zone
0
200
"E 400
"c
I-(3_
UJ
c~ 600
800
205
I
'4~0
8'0
I
TRP
o
~
4'0
I
8'o
1
206
o
i
20
I
8'o
TRP3
I
I0
2
I
1
40
I
NITROGEN ASSIMILATION RATE (nM.d -I)
Fig. 7.
Distribution of ammonium assimilation rates (©) and nitrite assimilation rates (O).
796
B.B. War.
O[
..... ~
i •
b--"S
r
zoo~\l" ~
~T--~-
~- ",NO-
1:
~
i
~
:
L
......
L ,--+J
•
i
~!~!-
,
~'\
i
E 400
t
l--n
IJ.J
j
-
NO 2
+
-
H4
i
NO 2
+
i
4
I
-
+
NO 2
H4
600
~4
H
soo)
--I-
i
!"'
i
!',
205 ~
l O 0 0 1 ~ . t k . J . _~__.L _J
-2
0
2
\
i
0
L.-- i _
2
--2
i/
7
TRP ,
t __L J
-2
~ ,"
\
/
/
206i
J _.;
0
± _.L_ _ t ' . . ~ _
2
-2
/
TRP3
~ .i
0
;
E
J .....
,4
tOgloRATI 0 ( assimilation rate )
oxidation rate
Fig. 8.
R e l a t i v e i m p o r t a n c e o l assimilalion a n d o x i d a t i o n of a m m o n i u m a n d nitrite e x p r e s s e d
as ih¢ I o g w ratio ( a s s i m i l a t i o n t a l c / o x i d a t i o n r a t e ) .
was undetectable at Sta. 21)6 but was low and nonzero, about 0.1 nmol I t d t or less, at
the other three stations. Turnover of nitrite due to its assimilation was significant only m
the photic zone, and occurred there at a rnaximum rate of less than once per day.
Nutrient turnover and nitrogen fluxes
The relative contributions of assimilation and oxidation processes to fluxes into and
out of the ammonium and nitrite pools can be estimated by computing the ratio
(assimilation rate/oxidation rate) for each nutrient at each station (Fig. 8). The distribution of these ratios with depth is similar at all stations, showing predominance of the
assimilatory process in the photic zone, and increasing importance of oxidation with
depth. Plotted as the logm of the ratio, the values below zero indicate oxidation rates
greater than assimilation rates. For ammonium, assimilation and oxidation were of the
same order below the photic zone except at Sta. 205, where oxidation became significantly more important between the 12% and the 0.2% light samples. For nitrite, this
crossover occurred within the photic zone, where oxidation, either photochemical o~
biological, predominated. In the vicinity of the nitracline, nitrite oxidation and nitrite
assimilation are on the same order. The oxidation of nitrite in this depth range implies an
in situ production term for nitrate that may be significant to the nitrogenous nutrition ol
phytoplankton at the bottom of the photic zone.
DISCUSSION
Tracer experiments and rate calculations
The absolute values obtained for rate measurements made using kSN tracer techniques
are highly dependent upon the equations and assumptions involved in the calculations
Nitrogen transformationsin the Southern CaliforniaBight
797
once the isotope data are obtained (CAPERONet al., 1979; GLIBERTet al., 1982a, 1985;
GARSIDE and GLmERT, 1984; GARSIDE, 1984; LAWS, 1984). I did not measure the isotopic
composition of the ammonium pool, and therefore was constrained to use the equations
of DUGDALEand GOERIN6 (1967) for calculation of ammonium oxidation and ammonium
assimilation rates. The potential for error and underestimation of rates due to this
procedure has been discussed (see above references). However, the error in ammonium
concentration measurement alone in the oceanic waters where these experiments were
performed would introduce as large an error into the regeneration calculations, and
propagation of this error in the calculations is such that the precision of the rate estimates
would not be substantially improved, Thus, these rates may be considered as underestimates; GLIBERTet al. (1982a) suggested that this error would lead to estimates about 2fold lower than assimilation and oxidation rates measured by methods including the
consideration of ammonium regeneration.
For the 15NO~ oxidation experiments on the May 1983 cruise, however, I did measure
the isotopic content of both the nitrite and nitrate pools. This allowed me to calculate
nitrite oxidation rates by two different methods using equations from (l) DV~DAL~:and
GOERING (1967) and (2) GUBERT et al. (1982a). In the former equation, the isotopic
content of the substrate pool is considered to be constant with time and the initial ratio is
used as the divisor in the equation. In the latter case, the isotopic ratio of the substrate
pool is assumed to decrease exponentially from the initial to the final value during the
course of the experiment and an exponential mean is used for the isotopic content of the
substrate pool. Calculations based on the hitter assumptions produced consistently higher
results, the difference being a factor of 2.65 (S.D. = 0.33) at Sta. TRP3 and 2.89
(S.D. = 0.64) at Sta. 206. From other considerations (see below) it appears that rates
based on both calculations are overestimates of the overall nitrification rate.
Computation of mass balances was not attempted with these experiments. The sum of
measured rates was never sufficient to result in complete consumption of the added
tracer, and in all but a few cases, depleted the initial substrate concentration (ambient
plus tracer) by <:50%. The relative magnitude of oxidation and assimilation implies that
both must be included when making mass balance calculations in tracer experiments
(LAws, 1984; GLmERTet al., 1985).
Distribution o f rate processes
Ammonium oxidation exhibited consistent distribution patterns at the 3 stations where
rate measurements were made. The three features of the pattern were negligible rates at
the surface, increasing within the photic zone to a sharp maximum near the bottom of the
photic zone, and low fairly constant rates below the maximum. Closer sample spacing
would add detail, but the main features appear reasonable and consistent with both the
limited number of previous observations and with the general picture of the distribution
and controls on ammonium oxidizing bacterial activity. The minimum in ammonium
oxidation rate at the surface is probably due to light inhibition. Ammonium oxidizing
bacteria are light sensitive in culture (MULLER-NEUGLUCKand ENGEL, 1961; BOCK, 1965;
HORRIGANet al., 1981; YOSHIOKAand SAIJO, 1984) and similar rate distributions in these
waters (OLsoN, 1981a) and Washington coastal waters also show surface minima
consistent with light inhibition. Substrate limitation may contribute to low rates;
however, correlation analysis on data from these waters (WARD, 1985) indicated a strong
negative correlation with light (in the photic zone) but no significant correlation with
798
B.B. WARD
ammonium concentration. In addition, the maximum in ammonium oxidation at the
bottom of the photic zone is not consistently associated with a maximum m ammonmm
concentration and no increase in ammonium concentration with depth parallels the
increase in rate through the photic zone, ammonium being generally very low throughoui
the water column.
Rather than locally higher ammonium concentration being responsible for the subsurface maximum, the maximum can be interpreted as occurring at a depth where ligh|
inhibition is no longer significant and ammonium supply rate is enhanced. At this level u
switch from net nitrogen assimilation to net nitrogen dissimilation must occur, and tile
rate of organic matter remineralization is greatest. An exponential decay in the supply of
organic matter (WYRXKI, 1962) and therefore in the ammonium oxidation rate (below thc
maximum) might be expected with increasing depth (distance from the supply term in the
photic zone). The rate distribution at Sta. 205 did not fit any analytical curve well. A1
Stas 206 and TRP3, the subphotic rates fit a power distribution better than they fit an
exponential curve: Station 206: (n = 5) exponential; r 2 = 0.63: power: r= = ~).88: Sta.
TRP3: (n = 6) exponential: r 2 = 0.39; power: r 2 = 0.68. The power curve implies a
decrease in rate with depth that is much faster than an exponential decrease.
OLSON'S (1981a) ammonium oxidation data from the Southern California Bight shiny
similar patterns in photic zone samples, and there is evidence at some stations of u
subsurface maximum although a general increase with depth was more often reported.
These data contain very few measurements deeper than 100 m and thus cannot bc
compared with the shape of the distribution in deep water reported here.
The existence of a narrow subsurface maximum in ammonium oxidation rate in the
present data implies that most ammonium oxidation, and perhaps most organic matter
decomposition/nutrient regeneration occurred in this relatively small depth interval close
to the photic zone. The maximum occurred in only one or two of the profile samplcs~
implying that (1) it may be an easily missed feature, and (2) the true shape and actual
magnitude of the maximum may not be accurately represented in widely spaced vertical
profile samples. Nevertheless, an indication of the importance of this feature relative t~
the entire water column is that all three stations, the integrated ammonium oxidation
rate in the 200 m interval around the maximum included 50% or more of the ammonium
oxidation in the entire 800-1000 m water column. If this subsurface maximum feature i-~
really a few to 100 m thick, the estimate of its relative contribution might be revised
significantly upward by slightly different sample spacing. There is no evidence in these
profiles for the existence of another sharp maximum elsewhere in the water column
(although I did not sample within the benthic boundary layer).
Both ammonium and nitrite oxidation rates are expected to be influenced by oxygen
concentration, since both are obligately aerobic processes favored at low oxygen
concentration. The partial depletion of oxygen in the deep basins studied here mighl
influence & situ nitrification rates; however, no attempt was made to control oxygen
concentration in the incubation bottles, and no independent correlation between
measured rates and ambient oxygen concentration was detected in these data.
The magnitude of ammonium oxidation rates reported here is similar to those reported
by OLSON (1981a) for the same region and the range described here is encompassed by
the results of WAm~ et al. (1984) for the Washington coast.
Nitrite oxidation rates reported here are less reproducible, less consistent and less
easily interpreted than the ammonium oxidation rate results. Although nitrite oxidizing
Nitrogen transformationsin the Southern CaliforniaBight
799
bacteria are also inhibited by light, perhaps more strongly than ammonium oxidizers
(OLSON, 1981b; HORRIGAN et al., 1981), substantial nitrite oxidation was detected in
surface samples from some stations. OesoN (1981a) attributed the high surface nitrite
oxidation rates in his data to photo-oxidation. Abiological oxidation of nitrite and
ammonium does not occur under natural seawater conditions, with the only known
exception being photo-oxidation of nitrite in surface waters (ZAvmIOV et al., 1980). The
methods used here cannot distinguish between biological and chemical oxidation and the
latter is assumed to be negligible. However, photochemical oxidation of nitrite in surface
waters is consistent with the distribution pattern of nitrite oxidation observed here. A
minimum rate, where neither photochemical nor biological nitrite oxidation was occurring, was found just below the surface minimum. Biological oxidation begins only at very
low light intensities, separated by the depth of the photic zone from the region of
photochemical oxidation.
While highest ammonium oxidation rates were found near the photic zone, nitrite
oxidation rates were maximal much deeper in the water column. Nitrite and ammonium
oxidation are conventionally assumed to function as coupled processes, accurately
represented by the net reaction, oxidation of ammonium to nitrate. The data reported
here indicate that the two steps of nitrification were not tightly coupled in these
experiments, and imply that the two processes may be partially controlled by different
forces. The lack of a subsurface peak in nitrite oxidation corresponding to the ammonium
oxidation maximum may imply the existence of another sink for nitrite in this layer.
However, the nitrite oxidation rate in the depth interval of the ammonium oxidation rate
maximum usually equalled or exceeded the integrated ammonium oxidation rate. The
nitrite oxidation rate greatly exceeded the ammonium oxidation rate when both were
integrated over the water column (Table 1). At Stas 205 and TRP3, integrated nitrite
oxidation exceeded integrated oxidation rates by about 4-fold. However, at Sta. 206, the
difference was 19-fold, further evidence that the ammonium oxidation rates measured at
Sta. 206 were anomalously low. From mass balance considerations, this apparent
uncoupling cannot persist. Another source of nitrite, e.g. from partial denitrification, is
not likely in these waters. Temporal and spatial variations in substrate supply, as well as
methodological uncertainty in the nitrite oxidation rates (see below) can account for
these observations.
The discussion of nitrite oxidation rates thus far was based on values computed using
the equation of DUGDALE and GOERING (1967). If the exponential decrease in isotope
ratio of the substrate pool is considered (GLmER~~et al., 1982a) even higher values are
obtained. Given that either method yields rates in excess of a.mmonium oxidation rates
and of new primary production, possible errors in the measurement of nitrite oxidation
rates must be considered. Nitrite oxidation measurements are subject to the same
methodological constraints of containment, substrate enhancement and subsequent
depletion, etc., as ammonium oxidation. Less is known, however, about the nitrite
oxidizing bacteria and how they respond to such factors. Nitrite oxidizing bacteria are
apparently more substrate dependent, although this relationship was not observed in the
present data set. Nitrite oxidation in sgmples from the Southern California Bight
(OLsoN, 1981a) and off Baja California (B. B. WARD, unpublished data) showed marked
substrate dependence at nitrite concentrations <1 ~tM, unlike ammonium oxidation,
which was unaffected up to 2 ~tM. Addition of tracer would therefore enhance measured
rates out of proportion to the ammonium oxidation rates. This substrate effect means
800
B. B. WARD
.Table 1.
Comparison of measured and calculated production and nitrification rute_~
Total primary production
Ncw production
Method a
Station
21-205
21-TRP
22-206
22-TRP3
(gCm-+d I) ( g C m Z y l ) , ( m m o l N m e d ~)
11.253
0.419
0.651
(%)
92
i53
2.92
4.51
23
38
238
5.87
59
Regenerated production
(retool N m e d t)
NH~ + N O ,
21-2115
21-TRP
22-206
22-TRP3
a
0,67
t. 72
10S
I (+7
3.49
2.17
Nitrilication
below 80 m
Total/New
Station
Method b
(mmolNm+'d ~)(mmolNm ~d +?
(mmol N m 2 d i)
b
Nttj ~,N(),
N O , ~. NO~
Assimilation
3.75
4. t5
3.85
2.69
Intcgratcd
[N()~+-,NO~ i
[Ntt~ -, NOj
2.25
2.79
1.84
2.84
2.38
3.711
t0+79
tl.38
2.46
52.5b
25.90
7.4tl
8.85
4.87
1')+47
3. (~{I
[Integrated nitrilication ratc]
[New production (mcthod a)]
Station
21-205
2 I-TRP
22-2116
22-TRP3
Nt-t~ + NO,
NO,--* NO~,
16.01
77.97
15.02
tl. 71
2.54
*New production: a = new production calculated according to EPPLEY and PETkRSON 119791, (new
productioil/total production) = 0.11025 xtotat production; b = new production calculated as 37% of total
production (= average percent for the Southern California Bight; EPPLt:Yand PH'ERSON, 1979). Regencrated
production: NH4~ + NO~ assimilation = sum of ammonium and nitrite assimilation integrated over the photic
zone; (Total/New) = regenerated production calculated as (total production
new production): nc~
production calculated in two ways, a and b, as above.
that the a m m o n i u m o x i d a t i o n rates r e p o r t e d here are m o r e likely to r e p r e s e n t in sire
rates than are the nitrite o x i d a t i o n rates. T h e p o t e n t i a l for differential impac! ot
e n v i r o n m e n t a l factors such as i n c u b a t i o n t e m p e r a t u r e a n d oxygen p e r t u r b a t i o n c a n n o t bc
assessed in these e x p e r i m e n t s .
T h e use of ~SN tracers to m e a s u r e nitrite o x i d a t i o n is m a d e difficult in d e e p waters by
the fact that a very small e n r i c h m e n t in a very large pool must be m e a s u r e d to d e t e r m i n e
the rate. A slight e r r o r in e n r i c h m e n t multiplies up to a large e r r o r in rate. This is a
g r e a t e r source of variability in the n i t r i t e o x i d a t i o n data t h a n for a m m o n i u m o x i d a t i o n .
H o w e v e r , high rates were f o u n d in a n d a b o v e the n i t r a c l i n e w h e r e this source of error
w o u l d be m i n i m i z e d ,
T h e d e p t h v a r i a t i o n in m e a s u r e d nitrification rates a n d in the c o r r e l a t i o n s with
e n v i r o n m e n t a l factors p r e s e n t e d here a n d elsewhere (WARD, 1985) suggest that i n s t a n t a n e o u s rates are highly r e s p o n s i v e to e n v i r o n m e n t a l factors. T h e overall growth rate a n d
a b u n d a n c e s of the nitrifying bacterial p o p u l a t i o n is, h o w e v e r , less directly influenced.
F o r e x a m p l e , the m a x i m u m in a m m o n i u m o x i d a t i o n rates is m o r e likely a result of a
Nitrogen transformations in the Southern California Bight
801
short-term response of the cells to a dynamic balance between substrate availability and
light intensity than to a long-term strategy of nitrifying bacteria concentrating in a certain
depth or density interval. It is also evident from these data that autotrophic activity as
measured by autoradiography and nitrogen oxidation as measured by 15N tracers follow
the same general pattern in the water column, but are not necessarily closely coupled
over the scales of experimental incubations. Laboratory studies (BELsER, 1984; GLOVER,
1985) indicate that the ratio of carbon fixed to nitrogen oxidized varies somewhat as a
function of growth rate, substrate concentration or pH. The possible influence of oxygen
concentration was not quantified in these studies. The oxygen conditions of these
samples prior to collection and incubation varied from saturated to about 0.25 ml 1 r over
the water column. However, due to sample manipulation prior to incubation, oxygen
concentrations during incubation were near saturation, so oxygen could not have directly
limited the measured rates.
Laboratory studies on the growth and physiology of nitrifying bacteria indicate that
they are capable of sustained inactivity over long periods of substrate depletion (BooK
and HEINR1CH, 1971). Long generation times and long lag times preceding growth in
enrichment cultures are not unusual (CAaLUCCl and STRICKLAND, 1968; WATSON, 1965),
and environmental stress can result in cessation of activity independent of cell death
(YostnOKA and SMJo, 1984). The field observations reported here in combination with
findings from previous culture studies yield the not unexpected conclusion that bacterial
numbers are not necessarily good indicators of in situ activity. This may be particularly
true of marine nitrifying bacteria. Their numbers vary much less with depth and
environmental conditions than does the total bacterial population. The low abundances
of nitrifiers, <1% of the tothl bacterial population in the Southern California Bight
(WARD and CARLUCCI, 1985) makes it difficult to detect significant differences in
abundances in oligotrophic waters. In contrast, significant population variations were
found in the much more productive upwelling regime off Peru (B. B, WARD, unpublished data).
Assimilation of ammonium by natural populations has been widely studied (MACISAAC
and DUGDALE, 1969; EPPLEY et al., 1979; WHEELER et al., 1982; GLIBERTet al., 1982b).
The assimilation data reported here are consistent with those reported by EPPLE¥ et al.
(1979) and OLSON (1981a) for the same geographic area, and are discussed here only in
comparison to the distribution of oxidation rates. Nitrite is not usually considered an
important nitrogen source for phytoplankton compared to ammonium and nitrate, and
its assimilation is less well known (McCARTHY et al., 1984). In this study, significant
nitrite assimilation rates were much lower than ammonium assimilation rates in parallel
incubations and were restricted to shallower depths.
New production considerations
In oligotrophic waters, it is useful to consider the mixed layer and the underlying deep
ocean as separate but linked nitrogen cycling systems. While the net flux of nitrogen in
the mixed layer is from the dissolved pool into the particulate pool, the net flux in deep
water is in the other direction. The two reservoirs are linked by the sinking of particulate
nitrogen from the mixed layer and mixing and upwelling of the dissolved nitrogen
resulting from decomposition of particulate nitrogen in the deep layer. Over the time
scale of years, the two major fluxes represented by new production (equivalent to the
sinking flux of particulate nitrogen out of the mixed layer; EPPLEY et al., 1983) and
802
B.B.
WArD
nitrification must roughly balance each other. For typical marine systems, it is generally
assumed that new production is supported by nitrate (DUGDALEand Gor~rlN(;, 1967). l'he
data presented above will be compared with estimates of new production to assess thi~
relationship.
Total primary production (E. H. RENGER and R. W. EPI,I..EY, unpublished data) was
measured using 24 h incubations with NaHHCO3 (E~,pt,t.:Y et at., 1979) t~t 3 of the 4
stations for which nitrification and assimilation rate data are presented. Integrated
primary production (computed by the polygon method) in the upper 51r-60 m of the
water column (down to the 1% surface light level) is given in Table 1. Total carbon
production was converted to nitrogen production by multiplying by the N:C ratio of the
particulate matter at each depth (EPPH:'~ et al., 1983). Since nitrate assimilation was not
measured directly, we estimated new production as a fraction of the total. One estimate
was obtained using the algorithm of EPPLEY and PE'IERSON (1979) for regions with total
production <200 g C m 2 y-r: new/total (in percent) = 0.0025 total. Using an average
fraction of 37% for Southern California Bight stations, (EPPkeV and PETErSON, 1979)
yielded a second estimate.
Assimilation of ammonium and nitrite, urea and organic nitrogen (to a much smaller
degree) presumably accounts for "regenerated production" (Du(;DAL~ and G(~ErI~;.
1967). Measured rates of ammonium and nitrite assimilation integrated over the uppel
80 m (down to the 0.1% light depth) are presented in Table 1. Nitrite assimilation is a
minor process: in these data it comprises 1.1-13.8% of the total assimilation ~)i
ammonium plus nitrite. Ammonium and nitrite assimilation together represent from 4(~
to 129% (average = 89%) of the total nitrogen production estimated from the ~4C data.
The combined assimilation of ammonium and nitrite averaged 143% of the regenerated
production calculated from the new production algorithm of EPPLEY and PFl~so:',
(1979). If the measured assimilation rates are 2-fold underestimates, based on methodological considerations (see above), then true assimilation rates might exceed the regenerated production estimate by 2.25-3.34 times, It is interesting to note that sediment trap
experiments on the same cruise (NEt,SON et al., 1987) found that only 5% of primary
production (about 10% of calculated new production) reached 100 m. Both the measured assimilation rates and measured particulate fluxes imply that regenerated produc-tion was a large fraction of the total at the time these data were collected.
Measured nitrification rates, integrated between 80 m and the deepest depth sampled.
are also presented in Table 1. Nitrification shallower than 80 m was ommitted from the
calculation because, strictly speaking, it occurs within the mixed layer and could bc
considered to support in situ regenerated production. Nitrification above 80 m contributed an average of 18.3% of total water column rates for ammonium oxidation and an
average of 13.5% for nitrite oxidation. Integrated nitrification rates, both ammonium
and nitrite oxidation, exceeded new production in the overlying photic zone at most
stations, sometimes by a very large amount. Since measured nitrite oxidation always
exceeded measured ammonium oxidation, integrated nitrite oxidation overestimates
new production more extremely than does ammonium oxidation.
There are several possible explanations for the lack of agreement between nitrification
and new production estimates. (1) Measured nitrification rates seriously overestimate in
situ rates. Three lines of reasoning do not support this possibility, at least in the case oi
ammonium oxidation. The rates reported here are very similar in magnitude and
distribution to those reported by OLSON (1981a) and WArD et al. (1982) for the same
Nitrogen transformations in the Southern California Bight
803
region. Although the nitrifying bacterial abundances reported here exceed earlier
estimates (WARD, 1982), the computed per cell oxidiation rates presented here are well
within the range reported for cultures at low substrate levels or extrapolated to
environmental conditions from culture work (WATSON,1965; CARLUCCland STRICKLAND,
1968; GLOVER, 1985; WARD, unpublished data). An overwhelming systematic error
derived from the incubation and 15N-tracer procedure cannot be ruled out. However,
enhancement of in situ rates due to added tracer substrate is not supported by the lack of
substrate concentration response in the measured rates. Lastly, the relationship between
oxidation and assimilation of ammonium and nitrite observed here is similar to that
found in this region previously (OLSON, 1981a) and off the coast of Washington (WARD et
al., 1984). Although this relationship principally addresses nitrification above the 80 m
depth used as the upper limit for integration in Table 1, this consistent relationship
argues against a systematic order of magnitude error in the nitrification measurements.
(2) New primary production estimates are low. Again, the total and new primary
production rates used in these comparisons are lower than the average reported by
EPPLEY and PETERSON (1979) but are nevertheless consistent with previously reported
data for this area (EPPLEY and PETERSON, 1979: EPPLEY et al., 1979). The fractional
contribution of ammonium and nitrite assimilation to total production usually decreases
as the total amount of primary production increases (Table 1), and this trend is consistent
with the trend in percent new production calculated using the algorithm of EPPLE~ and
PETERSON (1979).
(3) Sample spacing was inadequate to accurately represent the distribution of the rate
processes. Wide sample spacing could result in underestimates of integrated rates by
missing narrow features. Overestimates could be obtained by integrating the effect of
narrow features over the distance between samples. The latter error is a strong possibility
for both the ammonium and nitrite oxidation distributions reported here. Since narrow
maxima were observed in the ammonium oxidation rate profiles, and high variability in
the nitrite oxidation profiles, much closer sample spacing is necessary to realistically
describe the distribution of these processes, and to accurately estimate their integrated
values.
(4) Steady-state assumptions do not apply over the time scale of these measurements.
The data presented here were collected over a few days and significant differences in the
magnitude of rate processes were noted among stations (e.g. ammonium oxidation at
Stas TRP3 and 206, both in May), while the nitrogen balance we seek is necessarily a longterm phenomenon. While each of the data sets presented here (primary production,
nitrogen assimilation and nitrogen oxidation) is internally consistent, and is consistent
with previously reported data, normal seasonal and annual variations in each may
minimize the differences we see here. The temporal relationship between primary
production, sinking particulate flux and regeneration of soluble nitrogen is quite variable
(EPPLEY et al., 1983) and we do not know the magnitude of the temporal or spatial
uncouplings that may be expected between sinking flux and in situ nitrification rates.
The assimilatory and oxidation processes are both evidently influenced by light, in
opposite ways. The ultimate light dependence of phytoplankton and light inhibition of
nitrifying bacteria produce rate distributions that cross near the base of the photic zone.
In this depth interval, the interactions of micro-organisms in the nitrogen cycle become
quite complex. Nitrification consumes ammonium and produces nitrate, and nitrite often
functions as a short half-life intermediate which can accumulate when the two steps of
804
B . B . WARD
n i t r i f i c a t i o n a r e u n c o u p l e d [ e . g . d u e t o d i f f e r e n t i a l l i g h t i n h i b i t i o n in f o r m i n g t h e p r i m a r y
n i t r i t e m a x i m u m (OLSON, 1 9 8 1 b ) ] . W h i l e all o f t h e i n o r g a n i c n i t r o g e n f o r m s a r c u t i l i z a b l e
by phytoplankton,
the relative concentrations
of each may influence their uptake rates
and the growth efficiency of phytoplankton. The rate data presented here imply that the
b o t t o m o f t h e p h o t i c z o n e / n i t r a c l i n e r e g i o n is a n i m p o r t a n t a r e a f o r f u t u r e i n v e s t i g a t i o n s
of microbial nitrogen cycling.
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