Deep-Sea Research, 'Vol 30 No 7/8, pp 1329-1347, 1992 019941149/92 $5 00 + (J t)(J Pnnted m Great Britain Pergamon PressLtd Trace nitrite in oxic waters O C. ZAFIRIOU,* L. A BALL* and Q HANLEY* (Recelved 1 August 1990, m revtsed form 24 October 1991, accepted 21 November 1991) Abstract--We describe a modified (GARSIDE, 1982, Marme Chemistry, 11,159-167) mtnte method that permits measurements down to subnanomolar concentrations and present data from Atlanttc and Caribbean deepwater profiles for comparison with a pubhshed Pacific section This tmportant intermediate in the mtrogen cycle was detected m all samples Concentrations v, ere consxstently lowest (0 14 4 nM) m ohgotrophlc surface waters Below 1 km, Caribbean and Southwest Sargasso sea nitrite concentrations were 0 4-1 nM, decreasing with increasing depth, reported Pacific [NO2] averages are several times higher Profiles in the upper kilometer beneath the classical primary nitrite maximum (PNM) v~ere quahtatlvely similar, exhibiting a smooth supra- exponential drop with depth to values of -1-4 nM at 1 km The mtnte inventory m this "tall" of the PNM above 1 km with 1 nM -< [NO~-] -< 50 nM roughly equals that m the classical PNM Stgnlficant differences among profiles m the 0 1-1 km regxon are observed, consistent with mtnte pool turnover times of 3-7 days estimated from Redfield StOlch~ometry and trltlum-hehum ages Thus seasonal and/or regional variations in factors altering the nitrite production-consumption balance, rather than transport, seem to be responstble for mtnte variability Nitrite profiles w~th anomalous m~dwater or near-bottom fine structure, including multi-point maxima and minima, were found along the Venezuelan continental margin and at -13°N These features are tentatwely ascribed to boundary effects, as hydrographic and orcumstantlal evidence suggests that these waters interacted previously w~th the bottom INTRODUCTION NITRITE has long been recogmzed as a dynamic intermediate in the marine nitrogen cycle (VACCARO, 1965) and Its distribution and cychng have been studied intensively (for example, OLSON, 1981a,b, CARPENTER and CAPONE, 1983; CODISPOTI and CHRISTIANSEN, 1985, KAMYKOWSKI and ZENTARA, 1991) Phytoplankton production and uptake of nitrite occur in the euphotlc zone, but below the llmlt of photosynthetic activity nitrite cycling is thought to be controlled principally by microbial mtnficatlon (KAPLAN, 1983) as well as denltrification (HArrORI, 1983) in the water column and in surficial sediments The large oceanic nitrite data base for [NO2] > 100 nM has been interpreted (KAMYKOWSKI and ZENTARA, 1991) However, open-ocean distributions m OXlC waters outside the PNM remain poorly characterized (we are aware of only one section: WADA and HATI'ORI, 1972), because the standard colorlmetrlc method (STRICKLANDand PARSONS, 1972) is inaccurate below 50 nM (standard deviation -+20 nM, WARD et at., 1989) We find that nitrite concentrations are below 50 nM in more than 99% of the OXlCdeep water *Department of Chemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, U S A 1329
Transcript
Deep-Sea Research, 'Vol 30 No 7/8, pp 1329-1347, 1992 019941149/92 $5 00 + (J t)(J Pnnted m Great Britain Pergamon Press Ltd
T r a c e n i t r i t e in o x i c w a t e r s
O C. ZAFIRIOU,* L. A BALL* a n d Q HANLEY*
(Recelved 1 August 1990, m revtsed form 24 October 1991, accepted 21 November 1991)
Abstract--We describe a modified (GARSIDE, 1982, Marme Chemistry, 11,159-167) mtnte method that permits measurements down to subnanomolar concentrations and present data from Atlanttc and Caribbean deepwater profiles for comparison with a pubhshed Pacific section
This tmportant intermediate in the mtrogen cycle was detected m all samples Concentrations v, ere consxstently lowest (0 1 4 4 nM) m ohgotrophlc surface waters Below 1 km, Caribbean and Southwest Sargasso sea nitrite concentrations were 0 4-1 nM, decreasing with increasing depth, reported Pacific [NO2] averages are several times higher Profiles in the upper kilometer beneath the classical primary nitrite maximum (PNM) v~ere quahtatlvely similar, exhibiting a smooth supra- exponential drop with depth to values of - 1 - 4 nM at 1 km
The mtnte inventory m this "tall" of the PNM above 1 km with 1 nM -< [NO~-] -< 50 nM roughly equals that m the classical PNM Stgnlficant differences among profiles m the 0 1-1 km regxon are observed, consistent with mtnte pool turnover times of 3-7 days estimated from Redfield StOlch~ometry and trl t lum-hehum ages Thus seasonal and/or regional variations in factors altering the nitrite production-consumption balance, rather than transport, seem to be responstble for mtnte variability
Nitrite profiles w~th anomalous m~dwater or near-bottom fine structure, including multi-point maxima and minima, were found along the Venezuelan continental margin and at -13°N These features are tentatwely ascribed to boundary effects, as hydrographic and orcumstantlal evidence suggests that these waters interacted previously w~th the bottom
I N T R O D U C T I O N
NITRITE has long been recogmzed as a dynamic intermediate in the marine nitrogen cycle (VACCARO, 1965) and Its distribution and cychng have been studied intensively (for example, OLSON, 1981a,b, CARPENTER and CAPONE, 1983; CODISPOTI and CHRISTIANSEN, 1985, KAMYKOWSKI and ZENTARA, 1991) Phytoplankton production and uptake of nitrite occur in the euphotlc zone, but below the llmlt of photosynthetic activity nitrite cycling is thought to be controlled principally by microbial mtnficatlon (KAPLAN, 1983) as well as denltrification (HArrORI, 1983) in the water column and in surficial sediments
The large oceanic nitrite data base for [NO2] > 100 nM has been interpreted (KAMYKOWSKI and ZENTARA, 1991) However, open-ocean distributions m OXlC waters outside the PNM remain poorly characterized (we are aware of only one section: WADA and HATI'ORI, 1972), because the standard colorlmetrlc method (STRICKLAND and PARSONS, 1972) is inaccurate below 50 nM (standard deviation - + 2 0 nM, WARD et at . , 1989) We find that nitrite concentrations are below 50 nM in more than 99% of the OXlC deep water
*Department of Chemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, U S A
1329
1330 0 C ZAFIRIOU etal
column, Including the main thermochne in which it is an intermediate in the crucial biogeochemical process of nitrogen remlnerahzatlon (VACCARO, 1965; BROECRER, 1974).
METHODS
Chemiluminescence methods (GARSmE, 1982) enhance the convenience, precision and sensitivity of nitrite analyses We have refined th~s approach substantially Practices and precision differed within, and especially between, cruises as methods were refined and tested Erratic air contamination was found to be an insidious, occasionally large source of error [A recent lntercallbratlon by experienced analysts on a low-stacked ship (R V Cape Hatteras) showed standard deviations of - 1 0 0 % or more for rephcate 1000 m bottles (n = 6) for both analysts When these nitrite samples were drawn and analysed the odor of stack gases was erratically strong near the CTD and in the on-deck analysis laboratory ] Clearly, at low levels extreme care and frequent replication is essennal even with present procedures
General summary and recommendations
Method performance on individual cruises is detailed below. Detector sensitivity was always ->-0 1 nM nitrite The sampling plus analysis blank, very conservatively estimated as the lowest values found reproduclbly, was always <0 7 nM nitrite, and reproducibility was always better than 10% or 0 2 nM nitrite, whichever was larger The "'oxygen" method for low (nanomolar to subnanomolar) samples (Oceanus 189 only) was reproducible to _+ 1 nM or better, the "syringe" method to _+0.3 nM, and the "'direct" method to _+0 03 nM or 3%, whichever was larger
Based on our experience, for low-level (<10 nM) work we recommend "'direct" sampling on any unevaluated ship, drawing nitrite samples first or immediately after oxygens, frequently replicating samples and blanks, and extreme alertness to stack, galley, engine room and other gases (If possible continuously monitoring ambient air with the NO analyser for high NO/NO2) Such gases may contain high levels of "sticky" interferents such as NO~, HNO2(g), and possibly hydrolyseable organic nitrites in addition to NO As bottle effects may also occur, depths should be subsampled from rosettes at random, waters likely to harbor unusually high biological activity should be analysed immediately Replicate samples in unopened Nlskln bottles on R V Iselin sometimes increased as much as 2 nM per several hours (found previously by WADA and HArrORI, 1972), SO that analysis of entire CTD rosette samples must be monitored with care lest "old" samples be in error About three samples per hour can be run with "direct" sampling, more using syringes; the latter are not recommended for <20 nM nitrite
Samphng
On Oceanus 189 only, freshly sampled seawater ( - 3 0 0 ml) was drawn first from Nlskln bottles fitted with Teflon coated springs into Winkler bottles ("oxygen" style sampling) and stoppered. The samples were then poured into an N2-preflushed sample sparger in the hold lab and analysed within minutes to hours after sampling All sampling utilized a CTD- rosette system with 8-1 Teflon-coated Nlskin bottles with Teflon-coated springs Nitrite samples were drawn first, 0-5 h after arrival on deck, and analysed bottom-up at about
Trace mtrlte m ox~c waters 1331
three per hour except as otherwise indicated We confirm (WADA and HATrORI, 1972) that after hours in Niskin bottles nitrite concentrations sometzmes increase, but studies early in the cruise showed no effect of random sampling as a function of depth and no change upon sampling replicates after - 5 h on deck. Decreases also have been observed over longer periods; we have been unable to preserve samples reliably by mlcrowavlng or chilling on ice,
On the Northwest Atlantic/Caribbean cruise (R V Columbus Iselin 8816), seawater was sampled" (1) from previously unopened 10-1 Niskln bottles with Teflon coated springs on the CTD rosette within minutes to hours by connecting the spigot via 1,, Teflon, Tygon and silicone tubing to an N2-preflushed glass sparger, overflowing - 1 min, resealing, removmg excess seawater by syringe withdrawal and N2 displacement, and analysing tmmediately ("direct" sampling); and (2) by "syringe" sampling after multiple rinses into N2-preflushed 60 ml all-plastic syringes (no rubber plunger), capping and inJecting them after a few minutes delay into excess preflushed reagent Surface water samples on this cruise were drawn from the continuously running Teflon-lined surface pumping system (CooPER et at., 1987).
On the May 1989 Oceanus 206 cruise, sampling was similar to that on the Isehn cruise: "direct" sampling into the sparging vessel from Ntskm bottles for low-nitrite waters, 60 and 10 ml "syringe" sampling for the higher nitrite samples Continuously running pumped surface water (1-2 m depth) from an on-deck water table was sampled by the "'direct" method to obtain surface nitrite concentrations
CTD precision
CTD sensor performance was evaluated throughout cruise CI 8816. Alternate rosette bottles' reversing thermometers were read and the contents sampled on every cast for salinity Below -300 m these T and S (Autosal) analyses were used as "'truth" by calculating the means and the standard deviations of (CTD-truth) Short-term jitter and stability of the CTD signals were evaluated from: (1) variability in the data streams during the 5-min presampling "soak"; and (2) comparing data from the downcast in regions of slowly changing T and S with data from the same pressure horizons upon sampling -5 -90 mln later Both sensors were stable and accurate (better than _+0 02°C, +_0 01%o).
Analyses
Selective, quantitative reduction of nitrite to nitric oxide with potassium iodide and subsequent chemiluminescence detection was modified to improve its preosion, +2 nM (GARSIDE, 1982) by. (1) sampling and sample transfer methods avoiding air contami- nation, (2) using a -300 ml magnetically stirred gas-scrubber/sparging system positwely pressurized with NO-free N 2 coupled to a more sensitive NO detector (as for trace NO analyses, ZAFIRIOU and MCFARLAND, 1981; WARD and ZAFIRIOU, 1988) (on Oceanus 206 a commercial NO detector was substituted successfully); (3) using large samples (50-250 ml) to increase signal/contamination ratios, (4) sparging samples prior to adding reagents to remove dissolved NO (NO above traces suggests air contamination); and (5) presparging reagents to remove contaminant NO.
"Oxygen" samples were analysed by adding 15 ml of glacial acetic acid and 5 ml of reductant (120 g 1-1 K1 plus 4 ml 1-1 glacial acetic acid per liter), sweeping NO into the
1332 0 C ZAFIRIOU et al
detector, and integrating the signal with a photon counter for 2-3 mln (>99% of signal). "Direct" samples were preflushed with N 2, adjusted to 250 ml and injected with pre- stripped reagent as above "Syringe" samples were injected into 150-200 ml of prestrlpped seawater plus reagent. On Columbus Isehn 8816 and Oceanus 206 cruises NO peaks were quantified with a Hewlet t Packard Model 3390 integrator
Prectslon, blanks, spectfictty
On R V Oceanus cruise 189 the range of differences in replicates for samples with <10 nM nitrite was 0 00-0.49 nM, median difference 0.03 (n = 13). We do not obtain zero [NO2] signals even after reagent blank correction for seawater or "'Mllli-Q" water samples Hence a conservative upper hmlt on system blank (sampling and analysis) is the ohgotrophic surface water signal, generally the lowest obtained_ For all 5 m casts southeast of 65°N, 37°W on R V Oceanus 189 the range of surface values was 0.12-2.13 nM, median 0 7 nM These can be taken as extreme upper hmlts on contamination, so that the median error on this crmse was at most <1 nM, occasional samples might be up to 2 nM high.
On the Iselin cruise, frequent analyses of samples from the Teflon surface water system in ollgotrophic waters showed [NO2] = 0 4 + 0.25 nM (range) In one test, pumped surface water (0 57 nM), on prolonged flushing through a 5-1Niskin, standing for 140 mln, and subsampling by the "direct" method gave 0 58 nM nltnte Hence, "direct" values appear to be correct and non-reagent blanks seem undetectable (_+0 03 nM) Reagent blanks of - 0 04 nM nitrite, determined by adding a second ahquot of reagent to prestrlpped sample/reagent solutions, have been subtracted
On the R V Oceanus 206 cruise a commercial detector (Momtor Labs Model 8840) was used It was sensltxve enough to measure nitrite concentrations in all samples, with <0.3 nM uncertainty for small signals. Surface water on two consecutive days averaged 0 32 + 0_06 nM, similar to values from the Isehn cruise
Standard curves were routinely prepared using four-seven added standard concen- trations of freshly diluted concentrated stock solution (STRICKLAND and PARSONS, 1972) No interferences or non-nitrite blanks were detected by the sulfanilamide-added test (GARSIDE, 1982) m either a shallow or a deep sample
Apparent rapzd change m a near-bottom sample
We confirm the observation of WADA and HATIORI (1972) that nitrite concentrations in Nlskln bottles may Increase, although m ohgotrophlc waters we observed no such effects (see above), rather, profiles were extremely regular despite depth-random sampling_ However , one near-bot tom CTD cast unequivocally documented rapid changes On CTD 25, two bottles tr ipped simultaneously at the deepest level were sampled and analysed first and last They differed (2 17 nM vs 3 2 nM), fur thermore, reanalysls of the first bottle just after the second showed an increase to 3 1 nM The most reasonable explanation is that concentrations agreed initially and that nltnte concentrations increased m both bottles at the same rate, - 0 9 nM 5 h - I or - 0 1 9 nM h -1 ( - 9 % h -1) Reanalysls of the next-shallower sample showed that it had also increased at -<0 41 nM h -1 or -<17% h -1 This result suggests that the current procedures may be inadequate for h~ghly accurate sampling/analysis of biologically very active waters_
Trace nitrite in OXlC waters 1 3 3 3
R E S U L T S
Open-ocean stations and cruise dates are shown m Fig. 1 and the data are summarized in Tables 1-3 Nitrite was detectable in all samples The lowest concentrations were in oligotrophlc surface waters (0 1-0 4 nM) and below 1 km, where concentrations were generally <1 nM The method checks discussed above suggest that the data are accurate (particularly for "&rect" sampling). However, in wew of the difficulties of trace analysis at sea and the fact that the samplers were not sterile, we wew mdlwdual subnanomolar values as tentative, and therefore focus on systematic, multi-point trends m the data.
Profiles In the upper kilometer beneath the classtcal primary mtnte maximum (PNM) were qualitatively similar, exhibiting a smooth supra-exponential drop from 50 nM to values of - 1 nM at 1 km Close to the bottom and very near the shelf/slope region, these s~mple patterns arc violated, presumably due to benthic or nepheloid-layer influences on [NO;-]. Open-ocean profiles are shown on an exponentml concentration scale to en- compass the data range, while nearshore profiles are reported on a linear concentration scale to illustrate anomahes clearly
0
y
%
0
0
fD O
[]
z °
u~85° • 75 ~ N 6 5 " N 55 °
LONG]TUOE
Fl 8 1 Station locations and cruise designations Circles, R V Oceanus 189, Sargasso Sea, June 1987 Triangles, R V Co/umbu~/sehn 88]6, Northwest Atlantic and Caribbean Sea, September 1988 Squares, R V Oceanus 206, Northwest Atlantic and Sargasso Sea, May 1989 Sohd symbols indicate C! 8816 stations showing boundary effect anomahes Sohcl circle--Site A, sohd triangle--
Site ]3, sohd square--Site C
1336 o c ZAFIRIOU et al
O E - 2 - P T H
K -3" M
-4"
/
/ ATLANTI0 STATION SEPT 19B8
0 - 5 KM
[] [] FACTOR OF 2
- 5 I I I 0 i ~. 0 %0 0 100 0
[ND2] (riM)
Fig 2 Deep mtnte profile R V Columbus Isehn 8816, 19 September 1986 at 19°26'N, 67°29'W (Puerto Rico Trench, z -6000 m) Note logarithmic concentration scale Below 500 m "direcr" sampling, mixed sampling methods above 500 m Arbitrary line through 1 nM at 1 km has a slope of
0 1 km -l, corresponding to a three-fold change per 5 km
Deep waters
Below 1 km the data set establishes a pattern of low (subnanomolar), nitrite concen- trations that decrease gradually with depth The most detailed profile is from the Puerto Rico Trench north of the Mona Passage (Fig 2, note logarithmic concentranon scale) Here depths below 500 m were sampled randomly by the "direct" method, and duplicate Nlskins at 4100 m, analysed first and last, gave 0 41 and 0 40 nM nitrite. This profile shows a continuous -50-fold decrease of [NO2] from the PNM down to 1 km, and a much slower, apparently logarithmic drop between 1 and 5 km The arbttrary line (Fig 2), passing through 1 nM with slope equal to a three-fold change per 5 km, fits the data below 1 km to +25%, suggesting high method precision and an absence of structure below 1 km With the exception of three values near 2 nM [NO~-] between 1 and 1.8 km (Table 3), the line of Fig 2 also fits all our other data below I km. This uniformity is consistent with the view that the deeper metabolism (hence nitrite turnover) is slow and/or in steady state on the mixing tlmescale
In the only other survey of nitrite with adequate sensitivity to define deepwater concentration-distributions, WADA and HATrORI (1972), using a method they estimate has +10% error at 5 nM [NO2], reported a section along 155°E. Excepting one "outher" (25 nM), their range of 37 measurements below 1 km was 0-12 nM (mode 0, median 2, average 3 6 nM); eight surface values averaged 1 5 nM Although their reported values were thus often near their detection limit, these data suggest that deep Pacific nitrite values are higher than our Atlantic and Caribbean data near Puerto Rico However, most of
1 3 3 8 O C ZAFIRIOU et al
Table 3 Continued
O c e a n u s 206 Sta No 8
D a t e 5/13/89 La t i t ude 32°16'N
L o n g i t u d e 64°36 'W
D e p t h [ N O 2 ] ( n M ) [ O 2 ] ( m l l t) T ( o c ) S ( p p t ) A O U ( m l l L)
these h igher values were in a cons is ten t deep one -po in t m a x i m u m , > 5 nM, at 3 -4 k m ([02] 3--4 ml l - a ) . W i t h o u t these values , the i r average be low 1 km, - 2 . 1 nM, is only 0.6 n M above the i r surface wa te r average . I f Pacific surface wa te r values were ac tual ly <1 n M (as usual ly found in the A t l a n t i c and C a r i b b e a n ) , then sub t rac t ing the i r surface f rom deep values ( to cor rec t for a p r e s u m e d u n d e t e c t e d b lank) gives " a d ju s t e d" d e e p w a t e r concen- t ra t ions <1 nM, in a g r e e m e n t with our da t a (except ing the 3 -4 km m a x i m u m , which requ i res fu r the r s tudy) . H o w e v e r , bo th this " co r rec t ion" and the a s sumpt ion beh ind it a re qui te a rb i t r a ry (and unreahs t i c at high la t i tudes and in the equa to r i a l upwel l ing reg ion) so tha t d e e p Pacific ni t r i te concen t r a t ions may well be sys temat ica l ly h igher than our A t l a n t i c / C a r i b b e a n values . A s in surface and i n t e r m e d i a t e waters , da t a o b t a i n e d with ident ica l s amp l ing /me thods are r e q m r e d to charac te r i ze bas in-sca le deep ni t r i te distr i-
bu t ions unequ ivoca l ly
Mtdwater nitrite dtstrtbutton and turnover
In the ma in t h e r m o c h n e ( - - P N M to 1 kin) ni t r i te concen t r a t ions typica l ly vary - 2 0 - f o l d or more ; never the less , pa t t e rn s are s imilar . F igu re 3 c o m p a r e s a nea r ly c o n t e m p o r a n e o u s prof i le in the C a r i b b e a n , abou t 200 naut ica l miles south of the M o n a Passage (sill dep th < 1 km) with the u p p e r po r t i on of the prof i le and t r e n d h n e of F ig 2. T h e y genera l ly differ
1340 O C ZAFIRIOU et al
Table 3 Continued
Oceanus 206 Sta No 13 Date 5/18/89 Latitude 36°51'N Longitude 69°50'W
Depth [NO2](nM ) [O2](mll l) T(oc) S(ppt) A O U ( m l l L)
145 181 2 4 97 2(I 04 36 6047 0 15
175 71 4 4 78 19 58 - - 0 39
186 244 5 09 19 33 - - (I 0q
200 21(I 5 04 19 28 36 5955 0 14
242 24 5 5 01 19 03 - - 11 2
3(1/) 12 4 89 - - 36 5725 11 32
393 6 11 4 72 - - - - 0 62
4(1(I 5 3 4 58 18 21 36 496 0 76
50(1 4 35 4 31 17 77 36 2159 1 I)3
507 4 97 4 39 17 67 - - 0 96
600 3 65 3 9 16 41 36 4435 1 59
683 2 92 3 78 14 7 35 9293 1 9
700 2 86 3 97 14 86 - - 1 68
700 3 21 3 77 - - - - 1 88
7(17 2 94 3 98 14 - - 1 79
8(1/I 1 97 3 17 13 - - 2 74
982 1 73 3 42 8 5(, 35 1498 3 1
993 l 88 3 56 - - 35 1455 3 3
1000 - - 3 3 - - 3 8
1181 1 79 4 98 5 52 - - 2 04
1200 2 04 5 (18 5 39 - - 1 94
1500 2 5 83 4 48 - - 1 34
1793 1 99 6 01 3 87 - - 1 27
1800 1 5 6 0 3 3 87 - - 1 25
Duplicate depths from different casts
by much less than a factor of two We infer that the balance of sources and smks controlling concentrations are stmilar at the two sttes, despite the fact that they represent different situations with respect to long-range advective/dlffUSlVe supplies Hence the local blogeo- chemical dynamics almost certainly control deep nitrite concentrations m these tropical waters. In contrast, profiles from farther away (and in spring/summer) differ from those of Fig. 3, as illustrated by the "envelopes" of Fig 4. At intermediate depths Sargasso Sea mtnte concentrations are about three times those m the Caribbean, and they are still higher m the Gulf Stream (Table 1). In the Sargasso over hours and a few miles, there was lesser though apparently detectable variation, about two years later, a nearby profile was quite similar (Fig 5). (A time series study Is under way at Sta S.)
In all mldwater regions examined a substantml fraction of the total n imte inventory occurred m waters with concentrations <50 nM (the limit of reliable quantification of the standard method) For example, this newly found "tail" of the PNM contains 57, 72 and 33% of the total nitrite mventory down to 900 m for three stations from Oceanus 189 (Table 1); the fraction is higher m the Caribbean, where a weaker PNM more than compensates for lower concentrations deeper m the water column.
Trace mmte m oxlc waters 1341
- 0 . 2 5
- 0 50'
E PUERTO R ICO/
P CARIBBEAN AND T - 0 . 7 5 H ATLANTIC STATIONS
K O- i 5 KM M
- 1 . 0 0 '
- 1 . 2 5
1 .0
! [] FACTOR OF 2 t
1 100 0
- 1 50 " ~ ' ~ - ' u : ~ - ~" 0 i lO_O
[NO2] (riM}
Fig 3 Superposltlon of upper 1500 m of Fig 1 (hne only) on profile m Canbbean S of Mona Passage (open squares) R V Columbus lsehn 88-16, 20 September 1986, 17°17'N, 66°53'W, z -4900 m, "direct" samphng and (sohd triangles) R V Columbus lsehn 888-16, 21 September
1986, 15°21'N, 65°28'W, <-4000 m, "direct" samphng Trend hne as m Fig 2
The nmescale of nitrite turnover is crucial for interpreting these distributions and differentiating the extreme cases of, (1) diurnal or seasonal vanablhty vs; (2) turnover slow enough that profiles represent quasi-steady-state features controlled by regional or basin- scale mean rates of biogeochemlcal cycling and transport Esnmates of mtn te turnover rates below the PNM have not been made m typical oxic waters previously because of the unavailabd~ty of the reqmslte nitrite d~stribunon data_ To estimate this rate, we adopt the classical v i e w (VACCARO, 1965, KAPLAN, 1983) that below the euphotic zone nitrite turnover in ox~c waters is effected only by nitnficanon (in contrast to the presumed role of denitrification in the low-oxygen Pacxfic thermochne, WADA and HATrORI, 1972) and further note that nitrite is an obligatory intermediate m nitrification (KAPLAN, 1983) Assuming a regional, annual average balance between nitrification and "new" (nitrate) N input to the euphotic zone (EPPLEY and PETERSON, 1979), we can then estimate the depth- integrated average nitrification rate for the northwest Sargasso Sea from our profiles, remmeral izat lon rate estimates, and Redfield ratio Stolchlometry
SPITZER and JENKINS (1989) estimated new oxygen production below the mixed layer at Sta. S near Bermuda of about 5 _+ 2 moles m -2 y 1 using a model that assumed that the (annual average) depth-integrated oxygen utilization rate below 100 m balances that production. By approximating the bot tom of the PNM as being at about 100 m, assuming that <15% of this consumption occurs deeper than 1 km (by analogy to sediment-trap results from the Pacific, MARTIN et a l , 1987), and using the ratio 176 02 16 N O r (BROECKER et al , 1985), we arnve at a nitnficatlon rate in the 0.1-1 0 km interval, of 0 26- 0 66 moles N m - 2 y - t (0.46 midrange). On Oceanus 189 the nitrite inventory from the
1342 O C ZAFIRIOU et al
- 0 . 4 "
0 E P T H
- 0 e- K H
- t 2 ¸
! i I
0 1 0 1 0 0
( N 0 2 1 (nM)
Fig 4 Data fields for all cruises, excepting Gulf Stream stations Field encompass -90% of data points (Tables 1-3) Only data with [NO~] <50 nM below the PNM are encompassed Key R V Oceanus 189 diagonal hatching, no data point R V Columbus Isehn 88-16 cross
hatching, no data point R V Oceanus 206 diamonds, no hatching
base of the PNM ([NO~-] < 50 nM) to 900 m was - 5 x 10 -3 M m -2 (range, 4.4-5 7) Hence the nitrite reservoir (neglecting nitrite from 900-1000 m, <10% of the total) divided by the required nitrification flux gives turnover times of 0.008--0.02 years, or - 3 - 7 daysW
This very rapid turnover in the 0.1-1 km interval implies that although diurnal variations in nitrite profiles below the PNM are not likely to be detectable, seasonal or shorter tlmescale ocean responses are quite plausible. Clearly any correlations of [NO~-] with longer-tlmescale properties, (such as A O U , nutrients, T and S) cannot be interpreted meaningfully until the [NOy] variations driven by shorter-timescale (e g. seasonally variable, DEUSER, 1978) processes are evaluated
The depth-dependent diurnal and seasonal variability in these rates IS unknown and their errors are hard to constrain However , it seems very unlikely that our inventories are in error by more than 25% (at the times/places taken), or that average nitrification rates could be more than three-fold lower, (1 e. below that required by lower bottle-incubation productivity estimates). Therefore, if nitrate from water-column nitrification, not D O N (SuzuKI et al. , 1985), is the dominant component of "new" nitrogen, the mean nitrite turnover t ime can hardly he outside the range 2-20 days, intermediate between the diurnal and seasonal tlmescales. Since these estimates are concentration- (not depth-) weighted averages, they do not constrain well the turnover time of the relatively small nitrite pool below 500 m We also emphasize that fast pool turnover time merely established the poten t ia l for rapid concentration changes producing observable variations requires significant local imbalances in the production and loss rates
Trace nitrite m oxlc waters 1343
F~g 5
-0 4 D E P T H
K M
-0 B-
- i 2 P 10 i_ O. % 0 0
[NO2] (riM)
Three profiles at one station m the northwest Sargasso Sea, 14-16 June 1987 (data from Table l) Also shown is a profile from the vicinity from Oceanus 206 (heavy line)
Anomalous nttrtte profiles near boundartes
Several profiles at CI-8816 s~tes A - C (Fig 1, solid symbols) showed strong anomalies which were not reproduced by second casts nearby, as illustrated for sites A and B in Figs 6 and 7 Given the high consistency of the open ocean data, such anomalies are unlikely to be explained as method artifacts Profiles at site A exhibit multipolnt anomalies below 800 m. the 9/24 profile shows a maximum at 900 m, with concentranons decreasing towards the bot tom (1135 m), the 9/30 profile ( - 3 miles away in 1175 m) shows a different pattern. "Sptky" fine structure and difference in T and S data near the bot tom of both casts strongly suggest recent boundary interactions, so that the water parcels' recent histories probably differed, at least below 1000 m At site B (upper continental shelf off the Orinoco's mouth) profiles measured <24 h and - 8 . 5 nm apart in 1440 and 1450 m of water differed On the first cast the CTD data showed sharp decreases in T and S gradients below -1225 m, suggestive of boundary-induced mixing; the second cast showed much lower, smoothly varying values
Site C further offshore in 1975 m showed a two-point midwater maximum at 900- 1100 m (Fig. 8) that may plausibly be explained by boundary interactions A tongue of Sub-Antarctic In termedmte Water (SAIW) advects into the Caribbean at 750-850 m through St Lucia Passage (sill depth -1100 m) - 1 2 0 miles to the northeast (WosT, 1962, STALCUP and METCaLF, 1972). Its T/S signature in this cast shows that SAIW overhes the 900-1100 m nitrite anomaly The SAIW tracer thus suggests that the underlying anomal- ous layer also may enter vm the passage, its properties being modified by bot tom interactions Fur thermore , the nearest boundaries (continental margin, Aves ridge) are
1344 O C ZAFIRIOU et al
Fig 6
O E - 4 0 0 P T H
M E T E R -BOO S
I,,/3o . o t , o . . t , ~ ,
-
- i 2 0 0 I I I I 0 t 2 3 4 5
[NO2] (riM)
Anomalous nitrite profiles at site A of Fig 1 CTD 16, 9/24/88, 11°49'N, 62°55'W, CTD 25, 9/30/88, l l°50'N, 62°57'W Note linear concentration scale
Fig 7
- 4 0 0 []
E P T H
M - 8 0 0 E T E R S
-i200 i >
Bottom lit -~440 m
I I I I i I I I
0 2 4 6 B iO
IN02] (riM) Anomalous nitrite profiles at site B of Fig 1 (CTD 19.9°57'N, 59°12'W, open squares.
Fig 8 Mxdwater mtnte anomaly at site C (12°23'N, 62°58'W) of Fig 1 Note hnear concen- tration scale Analyses showing elevated concentrahons after substantml delays on deck not
shown
only 20-30 km away at 1-2 km and span - 1 8 0 °, so that alternative sites for recent bot tom interaction abound
Thus these anomalous profiles may all result f rom samphng boluses of water recently per turbed by side/bottom boundary interactions which alter [NO~]]. Less plausible alternatives which we cannot exclude are anomahes driven indirectly by coastally enhanced primary production, or artifactual changes arising between bottle closure and sampling due to (conceivably rapid) turnover of nitrite in waters with greatly enhanced microbiological rates. If ohgotrophic turnover times of days are reasonable (previous section), then enhanced turnover on the scale of minutes to hours does not seem impossible a p r t o r i . Such perturbations of nitrite cycling might result f rom benthic inputs of nitrite or nitrite precursors, or by nitrite-metabolizing and nitrite-producing bacterial populations. These effects may involve nitrification and/or denltrification in the water column, on suspended particles, on sinking aggregates, and/or at the sediment-water interface
The plausibility of the simplest perturbation mechanism, direct benthic nitrite input, can be crudely evaluated Off continental shelves, near-surface pore water nitrite concen- trations of 0 -16~M have been reported (SUESS e t a l , 1980; JAHNKE e t a l , 1982, SHAW e t a l ,
1990). Assuming a gradient f rom 16ktM at 0.5 cm to zero at the sediment-water interface and molecular diffusion, this highest concentration could supply 2 nM nitrite to a 100 m thick layer in - 1 week. Indirect effects seem even more likely, for example, potentially much higher benthic ammonia effluxes (KLUMP and MARTENS, 1983; FURNEX e t a l , 1989) may strongly stimulate near-bot tom nitrification.
1346 o C ZAr-'mlou et al
Acknowledgements--We thank the Captains, crews, and chLef scientists of R V Oceanus cruises 189 and 206, and R V Columbus lsehn cruise CI 8816, and also Ms Momque Vlllars for the oxygen data in Table 3 and R Jones and J Bugden for nutrient data The late Manuel Aparlc~o maintained careful CTD sensor cahbratlon, permitting our interpretations of anomalies This work was funded by NSF grants OCE 86-1618 and 87-00576 This is WHOI contribution No 7481
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