Hydrobiologia vol. 50, 2, pag. I23-127, 1976

DENITRIFICATION IN BAHIA DARWIN (GALAPAGOS ISLANDS)AN INTERMITTENT ANOXIC BASIN IN THE TROPICS'

James COX* & Peter H. WIEBE**

* Hopkins Marine station, Pacific Grove, California, 93950** Woods Hole Oceanographic Institution Woods Hole, Massachusetts 02543

Contribution No. 3193 from the Woods Hole Oceanographic Institution. Woods Hole, Massachusetts 02543. This study wassupported in part by the National Science Foundation Grants GA 29303 and GB 6870. Reprints should be requested from the second author.

Received December 14, 1974

Keywords: Denitrification, Anoxic Basin, Galapagos Islands

Abstract

Changes in oxygen and nitrate in Bahia Darwin between July1968 and March 1969 indicate that denitrification occurred in thedeeper waters of this tropical, intermittently anoxic basin. As-suming constant rates of oxidation of organic matter in equallyspaced depth intervals below the pycnoclifie depth, the rate ofdenitrification was estimated to be about 62.0 mg-atom NOD-N/liter/year. This rate is attributable to denitrification in the watercolumn because a smaller rate was estimated for strata closer tothe sediment water interface.

Introduction

Bahia Darwin of the Galapagos Archipelago is one offew intermittently anoxic basins of the tropical type(Richards, 1965), in which the pycnocline that isolates thedeeper waters from vertical exchange is due largely tothermal stratification. Since processes which occur in thebay are likely to be more applicable to tropical oceansthan processes occurring in halocline-type basins in tem-perate regions, it has been the subject of rather frequentstudy over the past six years.

Bahia Darwin is a deep roughly circular bay on IslaGenovesa (o°18'N, 890 57'W). The basin (Fig. I) is ap-proximately 2 km in diameter, has a maximum depthof 220 m, and is isolated from surrounding waters exceptfor a shallow sill usually extending to within four to six

Dr. W. Junk b. v. Publishers - The Hague, The Netherlands

meters of the surface. The sill is cut by several channels;the deepest is reported to be 40 meters (Barber & Norton,

unpublished data). Vertical exchange of water is pre-vented by strong thermal stratification, and lateral ex-change with water outside the basin is blocked by the sill.

The entrapment of deeper water of the bay causes anintermittently anoxic condition resulting from biochem-

21'N

0

20'

18

17'

90 w 59 58 57 56

Fig. . Position of two hydrocasts taken in Bahia Darwin on July31, 1968 and the single hydrocast taken outside the basin.

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68

II T

T _

.

-1-1/00 ,

ical oxygen utilization. Goering & Dugdale (966) foundthe basin to be anoxic in April 966. Hydrogen sulfidewas present in the water column, nitrite was unmeasur-able and high ammonia values were observed. Richards& Broenkow (1971) reported on chemical changes in thebay between March and May 1969. During this interval,oxygen values below 40 m were constant and extremelylow (ca. 2 g-atom/l). By May, significant quantities ofnitrite had accumulated in the water column above 130meters. A minimum rate of 50-70 jig-atom/l/yr ofnitrate reduction was estimated for the water column be-tween March and May 1969.

In March 969, Richards & Broenkow (I97) foundthat nitrate concentrations decreased from about 20 jig-

atom/l just below the sill depth (40 meters) to about o

.. .

200

jig-atom/l at the bottom of the basin (Figure 2). Theyassumed that prior to March 1969, the basin was flushedwith fully oxygenated surface water of zero nitrate con-

tent. Thus they predicted that since oxygen had declinedto near zero, there should be present about 26 jg-atom/

of nitrate of oxidative origin in the basin below sill depth,based on an assumed stoichiometry of 16 atoms of ni-trate-N to 276 atoms of dissolved 02 (Redfield et al.,

I963). This yields an apparent nitrate disappearance of 6

Mg-atom/l at 40 meters, increasing to 6 Ajg-atom/l nearthe bottom. This gradient of apparent nitrate disappear-

ance was interpreted by Richards & Broenkow as eviden-ce that '... nitrate had been lost from the water column by

diffusion into the bottom'.The difference between the amount of nitrate expected

o0 -o og.o //)

20 40 60 1 00

b

VER o0 ,~/A .1969 x ,

iT' ?W

{~~J

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. 1 16 0 P1 I

- - -- -- -- - I

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4_ 8 12 16 20 24 20

NHj-N (Op-o//)

2 0 4 06 06 0 12 14

no .. ... - ... dI - - ----- t

_.. C '- - .

zo I *

SIO,-Si {goM//)

;I%

I4

Fig. 2. Profiles of physical-chemical properties in Bahia Darwin. a. Comparison of July 968 density struc-ture inside (...a, A - - A) and outside (* *) the bay. b-f. Comparison of chemical properties ob-

tained in July 1968 (..., - - - ) with values (* .) given by Richards & Broenkow (I971).

124

C

1;

0

III

BO

120

200

O .

160

-oo

based on apparent oxygen utilization and the amountactually observed may be termed 'negative preformednitrate' and in such calculations is assumed to result fromthe conversion of nitrate to N2 by denitrifying bacteria.

We observed water column chemical properties in Julyi968; comparison of our observations with those ofRichards and Broenkow (I97I) from March 969 pro-vides additional evidence on denitrification processes andpermits an alternative method for calculating denitrifi-cation rates.

Methods

On July 31, 1968, two hydrocasts (Stations 64, 66) in thedeepest part of the basin and one (Station 68) outside thebasin in oceanic waters were made from the R/ V Te Vega(Figure I). Temperature, salinity, phosphate, ammonia,nitrate, nitrite, and silicate were measured. Nitrate valueswere lost from Station 66. All procedures for chemicalanalyses were those of Strickland and Parsons (1968). Inthe results and discussion sections, concentrations areexpressed as gram atomic weights of nutrient elementsper unit volume of seawater.

Results

In July 1968, we found substantial quantities of oxygen,nitrate, silicate, ammonia, and phosphate at all depths(Figure 2). A sharp oxycline occurred between 20 and 40meters, decreasing from approximately saturation val-ues in the surface water (491-391 Atg-atom/l) to less thango gtg-atom/l. Below 40 meters there was a distinct 02

maximum zone between 80 and Ioo meters and oxygendeclined to low values near the bottom (Figure 2b). Acomparison of the density structure at Station 68 with thestructure inside the basin shows that it was sealed off fromlateral water change blow the sill depth (Figure 2a).

Discussion

During the 195 day interval between our observations ofJuly 1968 and Richards' & Broenkow's observations ofMarch 1969, only slight changes of conservative proper-ties occurred, suggesting that the basin remained stagnantduring this period. Salinity averages for all depths below40 meters differed by 0.005%o. This magnitude of salinity

change is less than the salinity difference for the intervalbetween March 1969 and May 1969 (Richards & Broen-kow, 971), when the basin was quite obviously stagnant.Temperature change during this period was an average ofo. 4° C for depths below 40 m, a difference which can beattributed to the downward flux of heat from surfacewaters.

By the end of the interval, in March 969, it is clear thatsubstantial denitrification had occurred. This is evidentfrom the March 969 data of Richards & Broenkow(1971) and the fact that nitrate actually declined from theJuly 968 values (Figure 2c).

The denitrification rate can be calculated by estimatingthe rate of biochemical oxidation of organic matterfalling from the surface waters. Since nitrite did not ac-cumulate during the decline of nitrate, we assume thatnitrate was used as a terminal electron acceptor afteroxygen declined to low values, and that all of the reducednitrate was converted to N2. Furthermore, the non-ac-cumulation of ammonia during the period suggests thatorganic nitrogen was remineralized to nitrate until allavailable oxygen was utilized.2

The total amounts of oxygen and nitrate consumedwere calculated in terms of oxygen equivalents accordingto the following equation:

Oe = Ao + 2.92 x (ANO3) R + 2.92x(NO3 ) x (I)

where Oe = total oxygen equivalents consumed duringthe 195 day period, Ao = difference in oxygen concen-trations from July 968 and March 969, (ANO3)R = dif-ference in nitrate concentrations from July 968 andMarch 969, and (NO 3)ox nitrate produced coincidentwith oxygen utilization ((NO 3)ox (6/276)xA0). Thefactor of 2.92 for converting NO 3 to oxygen equivalentscomes from an equation given by Richards et al. (1965,p. R85) for biochemical oxidation of organic materialby nitrate, which indicates that 94.4 moles of HNO 3 arerequired for the oxidation of 106 moles of the organic

The reason for non-accumulation of ammonia under partialor total anoxic conditions is not well understood. According toRichards & Broenkow (197I, p. 763), the lack of accumulationof ammonia in the presence of extremely low oxygen concen-tration is possibly due to oxidation to free nitrogen. However.they state that '... this process has never been conclusively de-monstrated in seawater'. Cline & Richards (972, p. 897) alter-natively propose that bacteria responsible for denitrificationassimilate the ammonia, thus preventing it from accumulating.Since it represents such a small fraction of the total nitrogenbudget, we have ignored it in our calculations.

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carbon, and the equation () of Richards et al., 1965 (p.R 85) indicating that 276 atoms of oxygen are required tooxidize the same amount of the organic carbon. Oe wasdetermined for to meter intervals between 50 and 200

meters (Table ). The mean value of Oe was 98.1 jug-atom/I oxygen equivalents. The total amount of nitrateconsumed (NO 3) is equivalent to the bracketed term inequation (I), and had an average value of 32.2 /g-atom/loxygen equivalents.

Changes in reactive silicate in the water column overthe same period amounted to an increase of 5.3 ug-atom/liter. According to Richards et al. (965), the ratio A oxi-dation equivalents: ASiO4 is approximately 6:. Usingthis ratio, changes in silicate should amount to a oxida-tion equivalent of 85.3 pug-atom/liter in the water columnbetween 50 and 200 m. This is reasonably close to ourestimate of 98.2 p/g-atom/liter derived from the utiliza-tion of oxygen and nitrate.

In estimating these rates in Table I, we have assumed aconstant rate of oxidation of organic matter by 02 andNO 3 in any particular depth interval. We have also as-sumed that in situ consumption of oxygen and nitrate israpid compared to mixing rates, hence the rate esti-

mates represent minima.Figure 3 shows the relationship between O and

ANO 3 at the same iO meter depth intervals used in thecalculations in Table I. The line on the graph represents a

Table I. Estimated rate of denitrification calculated assuming auniform rate of oxidation of organic matter by 02 and 03 at agiven depth interval.

(a) (b)

Total oxygenequivalents Estimated days Rate of NO 3

Depth consumed over % of total of NO3 utiliza- utilization(m) 195 day period due to NO3 tion* (Mg-atom/liter/day)

(Oe)

50 88 35 68 .15660 100 38 74 .17470 108 30 58 .18880 103 19 37 .18190 102 19 37 .172

100 113 21 41 .198110 113 24 47 .194120 103 27 53 .181130 99 34 66 .179140 90 39 76 .159150 88 39 76 .153160 84 38 74 .147170 84 43 84 .146180 88 50 98 .155190 84 56 109 .148

X = .169

* The time estimate for NO3 utilization is computed by multiplying the fraction indicatedin column (a) times 195 days. The result appears in column (b).

t2

P

ZZ

.40 g- at//j

Fig. 3. Plot of oxygen (o) consumed in the interval July 31, 968to March 9, 1969 versus nitrate (NO 3) consumed (depths ofto m interval points indicated). The line represents the com-bination of values of aNO 3 and ao required to produce the watercolumn average of 98.2 Mg-atoms/liter of oxidation equivalents.

calculated relationship between ANO3 and AO assuminga constant rate of oxidation of organic matter in the en-tire water column below 50 meters of 98.2 g-atom/loxygen equivalents over the 195 day period.

The relationship of the points to the line in Figure 3shows that the rate of oxidation was not constant withdepth. There were significantly fewer (P = o.o5, Mann-Whitney U-test based on measured values, not extra-poled ones) oxygen equivalents consumed in the interval140 meters to 19o meters than in the interval 50 meters to

130 meters. This is due to the lower Or values in the watercolumn below 130 meters (Table 1).

The average rate for complete nitrate utilization waso.169 ug-atom/l/day (Table I). With steady state con-ditions this would correspond to a yearly rate of 6i.6 ,ug-atom NO3/1. This yearly estimate falls within Richards &Broenkow's (I971, p. 763) estimates of 50 to 70 ,ug-atom/

I/yr. for nitrate reduction in the 40 to 120 meters depthinterval of Bahia Darwin from March 969 to May 1969.Our method of calculation for denitrification prior toMarch I969, however, shows no evidence for nitrate dif-fusion into the sediment as a denitrification mechanism(Table 1), inasmuch as there does not seem to be a gra-dient adjacent to the sediment water interface. One possi-ble cause for the discrepancy in our results might havebeen the presence of negative preformed nitrate due to

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r11

19

some denitrification event prior to July 968, combinedwith incomplete flushing of the basin. The mean cal-culated nitrate concentration for July 968 for depthsbelow 50 meters based on apparent oxygen utilization is23.05. However, the actual average observed nitrate con-centration below 50 meters is 23.04, so there was nonegative preformed nitrate at the beginning of the 195 dayperiod (Table II).

The discrepancy between our interpretation of the rateof denitrification as a function of depth as compared tothat of Richards & Broenkow (197 I) is due to the differentmethods of calculation. We observed differences betweentwo sets of observations; they used a single set of obser-vations combined with an assumption of prior condi-tions. They were led to conclude that the rate of nitratedisappearance prior to March 1969 below I30 meters wasgreater simply because the total amount of nitrate dis-appearance they calculated was greater than the calcu-lated nitrate disappearance higher in the water column.Their conclusion derives from their assumption of a spa-tially uniform distribution of oxygen concentrationthroughout the water column due to a complete flushingevent prior to March 969. Our data show clearly thatprior to the onset of denitrification, the vertical distribu-tion of oxygen was in fact not uniform. Other evidence(Wiebe, Cox, & Malone, unpublished data; Barber &Norton, unpublished data) suggests that this uneven ver-tical distribution of oxygen was due to a partial basinflushing prior to July 968.

Acknowledgements

We wish to thank Mr. Ralph Vaccaro and Dr. EdwardCarpenter for critically reading the manuscript and Dr.Stanley Watson for helpful discussion. Mr. Peter Davollprovided the technical assistance in making water chem-istry determinations.

References

Barber, R. T. & Norton, J. I968. Hydrography of Bahia DarwinIsla genovesa Expedition 17. Stanford Oceanographic Ex-pedition I7: 78-84 (unpublished manuscript).

Cline, J. D. & Richards, F. A. I972. Oxygen deficient conditionsand nitrate reduction in the eastern tropical North PacificOcean. Limnol. Oceanogr. I7 (6): 885-900.

Goering, J. J. & Dugdale, R. C. 966. Denitrification rates in anisland bay in the equatorial Pacific Ocean. Science 154:505-506.

Redfield, A. C., Ketchum, B. H. & Richards, F. A. 963. Theinfluence of organisms on the composition of seawater, p.26-77. In M. N. Hill (ed.), The Sea, V. 2. Interscience.

Richards, F. A. & Broenkow, W. W. I97I. Chemical changes, in-cluding nitrate reduction in Darwin Bay Galapagos Archipela-go, over a 2-month periode, 1969. Limnol. Oceanogr. I6 (5):758-765.

Richards, F. A., Cline, J. D. Broenkow, W. W. & Atkinson, L. P.1965. Some consequences of the decomposition of organicmatter in Lake Nitanat, an anoxic fjord. Limnol. Oceanogr.10 (suppl.): RI85-R2o1.

Strickland, J. D. H. & Parsons, T. I968. A practical handbook ofseawater analysis. Fish. Res. Bd. Can. Ottawa, 310 p.

Wiebe, P. H., Cox, J. & Malone, T. 1968. A hydrographic surveyof Darwin Bay. Stanford Oceanographic Expedition i9:44-63 (unpublished manuscript).

Table II. Calculation of Preformed NO3 below 50 meters in Bahia Darwin,July 1968.

(a) (b) (aHb)

Apparent Oxygen Observed Calculated Calculatedutilization NO 3 -N NO3- N of preformed

Depth /pg-atom/l* pg-atom/l oxidative ori- NO3 -N(M) gin jug-atom/l pg-atom/1

(16/276) x A.O.U.

56 395 25.6 22.9 -2.774 377 20.0 21.9 -1.993 379 18.4 22.0 -3.611 378 27.5 21.9 +5.6

130 400 24.0 23.2 +0.8149 409 21.7 23.7 -2.0167 414 21.6 24.0 -2.4189 428 25.5 24.8 +0.7

X= 23.04 X= 23.05 X= -0.67

* Based on a saturation value of 0, of 466 for initial water.

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