Remotely driven thermocline oscillations and denitrification in the eastern South Pacific: The potential for high denitrification rates during weak coastal upwelling

The Science of the Total Environment, 75 (1988) 301 318 E l s e v i e r S c i e n c e P u b l i s h e r s B.V., A m s t e r d a m - - P r i n t e d in T h e N e t h e r l a n d s

301

REMOTELY DRIVEN THERMOCLINE OSCILLATIONS AND D E N I T R I F I C A T I O N IN THE E A S T E R N S O U T H PACIFIC: THE P O T E N T I A L FOR HIGH D E N I T R I F I C A T I O N R A T E S D U R I N G W E A K COASTAL UPWELLING

L.A. CODISPOTI, G.E. F R I E D E R I C H a n d T.T. P A C K A R D

Bigelow Laboratory for Ocean Sciences, W. Boothbay Harbor, ME 04575 (U.S.A.) R.T. B A R B E R

Duke University Marine Laboratory, Beaufort, NC 28516 (U.S.A.)

ABSTRACT D u r i n g F e b r u a r y - M a r c h 1985 d e n i t r i f i c a t i o n in t h e o c e a n off P e r u w a s e n h a n c e d in four e n v i r o n m e n t s d e s p i t e r e l a t i v e l y w e a k c o a s t a l u p w e l l i n g winds. T h e y were (i) b o t t o m w a t e r s over t h e shelf, (ii) a deep l a y e r ( ~ 3 0 0 m ) n o r t h of ~11°S, (iii) a n e a r s u r f a c e l a y e r ( ~ 4 0 m ) w i t h e x c e p t i o n a l n i t r i t e c o n c e n t r a t i o n s , a n d (iv) at r e l a t i v e l y s h a l l o w d e p t h s s o u t h of ~ l l ° S . T h e e x c e p t i o n a l 1985 o b s e r v a t i o n s m a y h a v e b e e n r e l a t e d to a n u n u s u a l l y s h a l l o w t h e r m o c l i n e t h a t in t u r n c o u l d h a v e r e p r e s e n t e d a large-scale r e s p o n s e to t h e 1982-83 el Nifio, w h i c h w a s t h e l a r g e s t in over 100 years. T h e s u p e r - e l e v a t i o n of t h e t h e r m o c l i n e n o t e d in 1985, h o w e v e r , r e p r e s e n t e d only a b o u t a 10% i n c r e a s e in t h e r a n g e of t h e r m o c l i n e d e p t h o b s e r v a t i o n s in t h e p r e v i o u s d a t a , s u g g e s t i n g t h a t t h e 1985 d a t a m a y r e p r e s e n t a n e x t r e m e d e v e l o p m e n t of a r e c u r r i n g s i t u a t i o n . D u r i n g F e b r u a r y M a r c h 1985 t h e a v e r a g e p r i m a r y p r o d u c t i o n r a t e w a s ~ 2 g C m - 2 d a y 1, d e s p i t e t h e e x t r e m e l y w e a k u p w e l l i n g w i n d s ( a v e r a g e speed < 4 m - i s ) , p r e s u m a b l y b e c a u s e t h e e l e v a t e d t h e r m o c l i n e placed h i g h n u t r i e n t w a t e r s in close p r o x i m i t y to t h e p h o t i c zone. T h e t h e r m o c l i n e w a t e r s off P e r u a r e t y p i c a l l y low in o x y g e n , a n d t h e low w i n d speeds a p p a r e n t l y r e s t r i c t e d t h e flux of o x y g e n from t h e a t m o s p h e r e into t h e ocean, s i n c e w a t e r s w i t h o x y g e n c o n c e n t r a t i o n s of a l m o s t zero ( < 0.1 ml 1 1 ~ < 4.5 × 10-6M) were s o m e t i m e s f o u n d w i t h i n 20 m of t h e s e a surface. S i n c e r e s p i r a t i o n r a t e s d e c r e a s e w i t h d e p t h a n d s i n c e d e n i t r i f i c a t i o n is f a v o r e d by low o x y g e n t e n s i o n s , t h i s s i t u a t i o n could lead to e x t r e m e l y h i g h d e n i t r i f i c a t i o n r a t e s in t h e w a t e r c o l u m n e v e n t h o u g h p r i m a r y p r o d u c t i o n m i g h t be lower t h a n t h e a v e r a g e v a l u e ( ~ 3 g C m 2day 1) d u e to t h e w e a k u p w e l l i n g winds. T h e r m o c l i n e s h o a l i n g s like t h o s e o b s e r v e d in 1985 a r e believed to be r e m o t e l y forced by c h a n g e s in e q u a t o r i a l winds, a n d s h a l l o w t h e r m o c l i n e d e p t h s m a y be a c o m m o n c o n d i t i o n d u r i n g a u s t r a l s u m m e r . T h u s , t h e F e b r u a r y M a r c h 1985 d a t a m a y s h e d l i g h t on a n i m p o r t a n t a n d r e c u r r i n g r e m o t e i n f l u e n c e on d e n i t r i f i c a t i o n r a t e s in t h e e a s t e r n S o u t h Pacific. A m a j o r p o i n t t h a t e m e r g e s from t h e 1985 o b s e r v a t i o n s is t h a t r e m o t e l y forced t h e r m o c l i n e o s c i l l a t i o n s m a y h a v e a s i g n i f i c a n t effect on d e n i t r i f i c a t i o n in t h e e a s t e r n S o u t h Pacific O c e a n a n d may, at times, c o u n t e r a c t t h e effect of w e a k u p w e l l i n g winds. T h e s e d a t a also lend f u r t h e r s u p p o r t to t h e idea t h a t l a r g e t e m p o r a l v a r i a t i o n s in t h e m a r i n e d e n i t r i f i c a t i o n r a t e c a n o c c u r in r e s p o n s e to r e l a t i v e l y s m a l l c h a n g e s in c i r c u l a t i o n a n d s t r a t i f i c a t i o n .

0048-9697/88/$03.50

~C! 1988 E l s e v i e r Science P u b l i s h e r s B.V.

302 INTRODUCTION Denitrification or the microbial reduction of oxidized forms of nitrogen (mainly nitrate and nitrite) to gaseous end-products (nitrous oxide and free nitrogen) is a globally significant process because it helps to regulate the O:N ratio in our atmosphere and is the most important sink for combined nitrogen in the ocean. Denitrification under marine conditions tends to produce free nitrogen as the major end-product and to consume nitrous oxide at rates t hat may be appreciable with respect to the build-up of nitrous oxide in the atmosphere (e.g., Codispoti and Christensen, 1985). When scaled against a marine denitrification rate of ~ 120 Tg N year 1, the residence time for atmospheric nitrogen is ~ 40 million years (Bolin and Cook, 1983; Codispoti and Christensen, 1985), but this process could affect the fertility of the ocean over much shorter periods. This is because such a rate could remove all of the n i t r ate in the ocean in ~ 1 0 4 years if it were not compensated by other processes, and because the nitrate supply is a major det erm i nant of new biological production (DUgdale and Goering, 1967) in the photic zone. Since the rate of nitrous oxide consumption ( ~ 1 Tg N y e a r - 1) by marine denitrification is similar to the build-up rate of this gas in our atmosphere (Codispoti and Christensen, 1985), changes in marine denitrification may also have a significant effect on atmospheric chemistry over several-decade time-scales (Fig. 1). Previous studies (Codispoti and Packard, 1980; Codispoti et al., 1986) have demonstrated th at the low-oxygen waters of the eastern tropical South Pacific represent a globally significant site of marine denitrification and t hat the denitrification in this region appears to vary considerably. This paper discusses the potential for variability in the E as t e r n South Pacific's denitrification rate caused by remotely driven oscillations in the depth of the thermocline and associated oxycline and nutracline. Such oscillations are common (F. Chavez, personal communication) and frequently are opposite in direction to what would be expected from seasonal variations in the strength of the coastal winds and associated coastal upwelling off Peru. One remote process t hat has been suggested as a cause for some of these oscillations is seasonal changes in wind patterns and in the position of the Intertropical Convergence Zone along the e q uato r (Meyers, 1979; O'Brien et al., 1981). Oscillations which lead to high thermocline stands during austral summer when the upwelling winds off Peru are weak may help to maintain high denitrification rates, and the exceptionally shallow thermocline observed in F e b r u a r y March 1985 appeared to be associated with a globally significant increase in denitrification. This situation may, in turn, have been a reaction to the el Nifio of 1982-83 (Cane and Zebiak, 1985; Codispoti et al., 1986), which was the strongest in over 100 years. BACKGROUND Denitrification becomes a prominent respiratory process only when oxygen has been depleted. Therefore, a supply of organic material is required to

303 TABLE 1 M e c h a n i s m s for d o u b l i n g t h e d e n i t r i f i c a t i o n r a t e off P e r u a n d C h i l i ~ A. 02 d e c r e a s e in s o u r c e w a t e r s 1. Vol. of M S N M ~ 1.4 × 10'4m 3 2. D e n i t r i f i c a t i o n r a t e in M S N M ~ 2 × 1013gN y e a r ' 3. N i t r a t e --* N z r a t e ~ 10ttg-atoms 1-1 y e a r 1 4. 10 N i t r a t e x 5/2 - 2 5 p g - a t o m O21 ' y e a r a 5. Therefore, a 2 5ttg-atoms O21 1 ( 0 . 3 r o l l - i ) d e c r e a s e i n s o u r c e w a t e r s c o u l d d o u b l e t h e r a t e B. I n c r e a s e in c a r b o n flux 1. A r e a of M N S M ~ 1.0 × 1012m2 (see Fig. 2) 2. " N e w " p r o d u c t i v i t y of ~ 400 g C m 2 y e a r 1 3. D e n i t r i f i c a t i o n r a t e of 20 g N m -2 y e a r - 1 ( C o d i s p o t i a n d P a c k a r d , 1980) 4. T h i s r e q u i r e s ~ 20 g C m -2 y e a r -1 (see Fig. 1) 5. T h e r e f o r e a n a d d i t i o n a l ~ 5% o f " n e w " p r i m a r y p r o d u c t i v i t y is a l l t h a t is r e q u i r e d to d o u b l e the denitrification rate C. D e c r e a s e in d o w n w a r d 02 flux 1. R a n g e of p o s s i b l e d o w n w a r d 02 fluxes t h r o u g h t h e b o t t o m of t h e p h o t i c zone u s i n g k z = 0.3 cm2s 1 for w e a k w i n d s a n d 1.0 cm2s 1 for s t r o n g w i n d s a n d a n a v e r a g e 0 2 g r a d i e n t min: 15 g - a t o m s O2m -2 y e a r -~ max: 5 0 g - a t o m s 0 2 m 2 year-× 2. D i f f e r e n c e m a x - m i n = 3 5 g - a t o m s O2m 2 y e a r ' - 1 3 g - a t o m s of C m 2 y e a r ' w i t h a Redfield r a t i o of O 2 to C of 276:106 by a t o m s 3. A C : N r a t i o of 1:1 by a t o m s d u r i n g d e n i t r i f i c a t i o n 4. T h erefore, 1 3 g - a t o m s C m 2 y e a r - l ~ 1 3 g . a t o m s N m 2or 182gNm 2year 1 5. T h erefore, c h a n g e s in d o w n w a r d flux of O z o n l y 10% as l a r g e a s t h e one e s t i m a t e d a b o v e c o u l d d o u b l e t h e d e n i t r i f i c a t i o n rate. T h e s e c a l c u l a t i o n s a s s u m e a d o u b l i n g of t h e r a t e t h a t C o d i s p o t i a n d P a c k a r d (1980) e s t i m a t e d for t h e m a i n s e c o n d a r y n i t r i t e m a x i m u m (MSNM).

consume oxygen and then to fuel respiration in the form of denitrification. A supply of nitrate, nitrite or nitrous oxide as terminal electron acceptor is also required as indicated by the following equations (taken from Codispoti and Christensen, 1985). They suggest a possible stoichiometry when denitrification and nitrification are coupled by N20: (CH20)~o~(NH3)16H3PO4 + 81.6HNO3 = 102CO2 + 40.8N2 + 142.8H20 + 16NH3 + H3PO4 + 4CH20

(I)

16Oz + 16NH3 = 8N20 + 24H20 (nitrification step)

(2)

8N20 + 4CH20 = 8N2 + 4CO2 + 4H20

(3)

(1) + (2) + (3) = (CH20)a06(NH3)~H3PO4 + 81.6HNO3 + 1602 = 106CO2 + 48.8N2 + 170.8H20 + H3PO4

(4)

Because of the three requirements for high denitrification rates (abundant organic matter, low oxygen, and abundant combined nitrogen in oxidized forms

304 ~

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RNH2 +02

6C02~'3N2+t"--6NO~ +RNH2 6C02÷N0~" RNH2+02

Fig. 1. C o n c e p t u a l model of (left) a n o c e a n w i t h a b a l a n c e d c o m b i n e d - n i t r o g e n b u d g e t (combined-N i n p u t s = o u t p u t s ) a n d (right) a n o c e a n in w h i c h e x c e s s d e n i t r i f i c a t i o n leads to a n e t loss of c a r b o n dioxide from t h e ocean. T h e c a r b o n dioxide loss will o c c u r o n l y u n t i l a n e w b a l a n c e is e s t a b l i s h e d for c o m b i n e d - n i t r o g e n , a n d t h i s w o u l d e v e n t u a l l y h a v e to o c c u r as r e d u c e d p r i m a r y p r o d u c t i o n a r i s i n g from d e c r e a s e d n i t r a t e levels c a u s e d d e c r e a s e s in s u b s u r f a c e r e s p i r a t i o n (Piper a n d Codispoti, 1975; B r o e c k e r a n d P e n g , 1982). A b a l a n c e m i g h t o c c u r f a s t e r if n i t r o g e n fixation r a t e s could i n c r e a s e r a p i d l y in t h e face of i n c r e a s e d d e n i t r i f i c a t i o n which, at p r e s e n t , is a n open a n d very interesting question.

s u c h as n i t r a t e a n d nitrite), m o s t m a r i n e d e n i t r i f i c a t i o n o c c u r s in r e s t r i c t e d localities. T h e y a r e s h a l l o w a n d h e m i p e l a g i c sediments, a n d o x y g e n - d e f i c i e n t p o r t i o n s of t h e w a t e r c o l u m n of the o p e n o c e a n w h i c h o c c u r m a i n l y in the A r a b i a n Sea a n d in t h e e a s t e r n t r o p i c a l Pacific O c e a n (e.g., H a t t o r i , 1983). T h e w a t e r c o l u m n sites r e p r e s e n t o n l y ~ 0.1% of the t o t a l m a r i n e v o l u m e , y e t t h e y a p p e a r to s u p p o r t a s i g n i f i c a n t f r a c t i o n ( ~ 60 Tg N y e a r ]) of t h e t o t a l m a r i n e d e n i t r i f i c a t i o n r a t e of ~ 1 2 0 T g N y e a r 1 (Codispoti a n d C h r i s t e n s e n , 1985). T h i s fact, alone, s u g g e s t s t h a t s i g n i f i c a n t t e m p o r a l c h a n g e s in the m a r i n e d e n i t r i f i c a t i o n r a t e are a d i s t i n c t possibility. C h r i s t e n s e n et al. (1987) s u g g e s t t h a t the s e d i m e n t a r y d e n i t r i f i c a t i o n r a t e also v a r i e s g r e a t l y as s h a l l o w s e d i m e n t s a r e a l t e r n a t e l y exposed a n d flooded d u r i n g g l a c i a l cycles. T a b l e 1 p r e s e n t s c a l c u l a t i o n s t h a t d e m o n s t r a t e t h a t small c h a n g e s in t h e o v e r a l l c a r b o n a n d o x y g e n supplies to t h e d e n i t r i f i c a t i o n r e g i m e in the w a t e r c o l u m n off P e r u c a n c a u s e a d o u b l i n g of t h e r e g i o n a l d e n i t r i f i c a t i o n rate. S o m e a d d i t i o n a l s i g n i f i c a n t a s p e c t s of m a r i n e d e n i t r i f i c a t i o n a r e i n d i c a t e d s c h e m a t i c a l l y in Fig. 1, w h i c h depicts a n o c e a n w i t h o u t d e n i t r i f i c a t i o n a n d a n o c e a n in a t r a n s i e n t s t a t e in w h i c h the d e n i t r i f i c a t i o n r a t e exceeds t h e i n p u t s of c o m b i n e d n i t r o g e n due to c h a n g e s in c i r c u l a t i o n , s t r a t i f i c a t i o n , or o t h e r f a c t o r s t h a t c a u s e a n i n c r e a s e in zones t h a t a r e depleted in oxygen. In this figure, we h a v e used g r e a t l y simplified e q u a t i o n s w h i c h a s s u m e t h a t t h e r a t i o of c a r b o n to n i t r o g e n in " f r e s h " o r g a n i c m a t e r i a l p r o d u c e d in the sea is ~ 6:1 by atoms. It is i m p o r t a n t to r e c o g n i z e t h a t w h i l e t h i s r a t i o is ,~ 6:1 d u r i n g the

305 90" I

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Fig. 2. Approximate horizontal extent of the MSNM with the position of the "C-line" indicated (from Codispoti and Packard, 1980). The three bands (inner 175 km, etc.) mentioned in the text and their approximate areas are indicated.

production of organic material, it is only ~ 1:1 when organic m at t er is oxidized during denitrification. This means t hat only ~ 18% of the organic material supported by a given amount of ni t r a t e uptake would be required to destroy an equivalent amo unt of ni t r a t e during denitrification! This figure also demonstrates how an imbalance in the marine combined-nitrogen budget caused by

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306 STATIONS 434

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excess denitrification would have the effect of increasing the carbon dioxide content of the atmosphere (McElroy, 1983). A possible imbalance in the marine combined-nitrogen budget of the presentday ocean has been suggested by Codispoti and Christensen (1985). In their budget, the present-day ocean is losing combined-nitrogen at a rate of 70 Tg N year -1. This loss could reduce the production and sinking rates of organic matter which, in turn, would cause a significant increase in the rate of carbon dioxide build-up in the atmosphere. For example, the anthropogenic production rate of carbon dioxide has increased from ~ 2 to 6 × 1015gC year 1 between 1880 and 1980 (Broecker and Peng, 1982) and about half of this accumulates in the atmosphere (Mackta et al., 1977) giving an increase of ~ 13 × 101~gC year -1. Since the carbon-to-nitrogen ratio in marine organic material is ~ 5:1 to 10:1 by weight, an imbalance of 70 Tg N year i arising from increased denitrification could cause an annual increase in atmospheric carbon dioxide of ~ 0.3-0.7 × 1015gC year -1. Other oceanic processes such as the changes in the inorganic carbon system arising from increases in the partial pressure of carbon dioxide (e.g. Broeker and Peng, 1982) and changes in preformed nutrients (Sarmiento and Toggweiler, 1984; Siegenthaler and Wenk, 1984) may be even more important. Changes in denitrification should, however, not be neglected when considering the regulation of atmospheric carbon dioxide (and nitrous oxide). Denitrification in the water column is generally signaled by the co-occurrence of low dissolved oxygen concentrations and high nitrite levels, and the major water column site of denitrification off Peru has been called the "main secondary nitrite maximum" or MSNM (Codispoti and Packard, 1980). This feature occurs between depths of ~ 75 and 400 m in the area shown in Fig. 2. Nitrite concentrations in the nearshore portion of the MSNM appeared to increase markedly after the 1972-3 el Nifio, an event that occurred at about the same time as the collapse of the anchoveta fishery. Codispoti and Packard (1980) suggested that this condition signaled an approximate doubling of the denitrification rate in the water column of the eastern tropical South Pacific. For the post-1972 period, they estimated a total denitrification rate off Peru and Chile of ~ 25 Tg N year 1 with most of this (~ 15 Tg N year- 1) occurring within the 175 km wide band of the MSNM that occurs closest to the coast (Fig. 2). They estimated rates of 2 and 3Tg N year 1 for the portions of the MSNM between 175 and 300 km offshore and > 300 km offshore, respectively. Codispoti and Packard suggested t h a t other significant sites of denitrification were transient oxygen-deficient zones t h a t occur at shallow (< 150m) depths (~ 2 Tg N year 1), the bottom waters over the shelf and upper slope (~ 1 Tg N year-1), a deep nitrite maximum found near 5°S (~ 1 Tg N year -1) and the shelf and upper slope sediments (~ 2 Tg N year-1).

F i g . 4. N i t r i t e a n d o x y g e n c o n c e n t r a t i o n s f r o m a s e c t i o n t a k e n a t ~ 10°C o f f t h e P e r u v i a n d u r i n g M a y 1977 ( m l l -~ o f O 2 ~ 4.5 × 1 0 - S M , t~g-at. N O 2 = 10 6 M ) .

coast

308 $TATIO N 0

15

12

II

I0

9

8

7

6

5

4

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I

100

200

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500

60C

Fig. 5. A nitrite section taken along 10°S during the NITROP-85 experiment. The dotted line indicates the upper 0.2 ml 1 1dissolved oxygen isopleth. Values were lower than this below this line everywhere within the depth range shown in this figure. The inset shows the vertical profile of nitrous oxide measured at station I (data provided by J.W. Elkins).

RESULTS O u r a n a l y s i s shall rely h e a v i l y on t h e r e s u l t s of the NITROP-85 e x p e r i m e n t t h a t was c o n d u c t e d d u r i n g F e b r u a r y - M a r c h 1985 at t h e l o c a t i o n s s h o w n in Fig. 3. T h e m e t h o d s e m p l o y e d are d e s c r i b e d in F r i e d e r i c h et al. (1985), Codispoti et al. (1986) a n d F r i e d e r i c h a n d Codispoti (1987). D e n i t r i f i c a t i o n a p p e a r e d to be u n u s u a l l y s t r o n g w h e n c o m p a r e d w i t h p r e v i o u s results, p a r t i c u l a r l y in the n o r t h e r n p o r t i o n of the region. F o r example, the r e g i o n n e a r 10°S lies n e a r t h e n o r t h e r n b o r d e r of t h e M S N M a n d is u s u a l l y typified by r e l a t i v e l y low n i t r i t e levels in the w a t e r s o v e r t h e s h e l f (Fig. 4). In c o n t r a s t , a s e c t i o n (Fig. 5) t a k e n a l o n g 10°S d u r i n g F e b r u a r y 1985 i n d i c a t e d t h a t h i g h n i t r i t e c o n c e n t r a t i o n s existed o v e r a c o n s i d e r a b l e p o r t i o n of the s h e l f a n d t h a t low o x y g e n c o n c e n t r a t i o n s e x t e n d e d o v e r the e n t i r e s h e l f a n d a l m o s t r e a c h e d t h e s e a - s u r f a c e n e a r t h e coast. A l o n g s h o r e s e c t i o n (Fig. 6) s h o w s t h a t b o t h s h a l l o w a n d d e e p e r n i t r i t e m a x i m a existed e q u a t o r w a r d of t h e n o r m a l b o u n d a r y of the M S N M . A s e c t i o n t a k e n n e a r 15°S a l o n g the wellstudied "C-line" (Figs 2 a n d 7) seemed m o r e or less t y p i c a l w i t h r e s p e c t to n i t r i t e c o n c e n t r a t i o n s w i t h i n the M S N M , b u t c o m p l e t e d e n i t r i f i c a t i o n was o b s e r v e d at the i n n e r m o s t s t a t i o n s w h e r e the o d o r of h y d r o g e n sulfide could be detected. T h i s is n o t the first t i m e t h a t c o m p l e t e d e n i t r i f i c a t i o n h a s b e e n o b s e r v e d n e a r 15°S ( D u g d a l e et al., 1977), b u t t h e d a t a r e v i e w e d by Codispoti

309

64 65 66

67

68/69 70

71

i'..,..-..!~

72

75

~

74

75

76

" "

:

98

I

99

I00

I01

102

105

;

A

I

J

i

I00

200

300 0-

:~.~.~ :"~"i~'"? )/ l" ~ ~:~_~

.

.

.

.

2

~ ~ _ ~

'

~o;~ ~'=-'-

400

NOz- ( ~ g - a t . ~-I )

/

L(:)NGSI-~RESEZCTIO'N I 50C

I

/ ,/ •

.....'

...

IOOKm ......

r'--.J

..." "

/

•

Fig. 6. The longshore nitrite section taken during NITROP-85. The dotted lines indicate the u p p e r lower 0.2ml 1-1 dissolved oxygen isopleths. The numbers at the top correspond to the stations in Fig. 3. and

and Packard (1980) plus more recent data reported by Gagosian et al. (1980, 1983) suggest that these occurrences are atypical near 15°S. A comparison of nitrate deficits (which estimate the free nitrogen produced by denitrification) from September 1976, May 1977, and February 1985 suggests higher levels over t h e shelf and upper slope o n t h e "C-line" during 1985 (Fig. 8). 117

116

115

1

.

.

•

IO0

. '

".......

•

STATION 114

113

I

.

[12

[11

IK:) 109 [ ~ 107 I

.

.

L

I

~

• . . . . . . ~.............................

[i!

,. 2 ~ ~

°N~O

......

•

.~ . . . . . . . . . . .

fi~ ~

:~5:~ii~::~i?::

Fig. 7. The nitrite section taken along the "C-line" (see Fig. 2) during NITROP-85.

310 Station ~4

,3

,

N -° ,z

l

Lo9

L

i

s

J

~

76543z~ i

L A ,

=,

•

!

I001

•

~

""

. __~5.

¢-~2o .--:'.~.. "

200 ~ E 2:300

IJ.I "~ 4OO <5

500

,~o

' 378

377

'

NO-~ Deficit (JJg atoms/J.) I0-11 Sept. 76

,~o

576

e~

575

'

574

-" I

;o

6'

37t

365

I00 200 E "1- 5 0 0 I-EL hi O 4O0 NO~ Deficit (ug atoms/C) 5 - 8 May 77

500 L

2~

L

L

L

2~

L

L

I

160

'

117

'

'

116

t

~'o'

' 115

'8o' 114

'~'

' 113

112

Ill

I10

108

I00 200 E i

50O

w Q 400 •

500

~ o ~ '

,6o

(~g atoms/£) 22- 23 Feb. 85 ' ,io 1o ' 4'0 Distance from Shore(kin)

o

Fig. 8. A comparison of nitrate deficits along the "C-line" sections taken in September 1976, May 1977 and February 1985. Nitrate deficits estimate the amount of nitrate originally present in a water parcel that has been converted to free nitrogen by denitrification. Nitrate deficits are in 10-6 g.atoms N 1 1.

311 ETS Activity within 175 Km of thecoast 1.0 0.8 06 /985

&

0.4

\ J r-,'s

T '~0.2 o

!~ ~

OJ

.= > =

.08

m

.04

i'%.\

.OE

m .....

.02 I 50

[ I I00 I'~

"~-o---":<---ct

I I I I ] I~ I 200 250 300 350 400 4 5 0 " k ~

"\ I

~ 600

I

Fig. 9. Activity of the respiratory electron transport system (ETS) versus depth in 1977 and 1985. The 1985 data were taken using a more sensitive technique and have been multiplied by 0.635 to be comparable to the results reported by Codispoti and Packard (1980). ETS activities in pl 02 or ~lOl-~h -1 ~ 4.5 x 10-8 moles 1-1h-L Activities are converted to estimates of in situ rates by multiplying by appropriate factors (see Fig. 15). F i g u r e 9 c o m p a r e s m e a s u r e m e n t s of the a c t i v i t y of t h e r e s p i r a t o r y e l e c t r o n t r a n s p o r t system (ETS) m a d e d u r i n g 1977 and 1985. T h e r a t i o n a l e for using these d a t a to e s t i m a t e d e n i t r i f i c a t i o n r a t e s has been established by Codispoti and R i c h a r d s (1976) and Codispoti and P a c k a r d (1980). T h e ETS d a t a suggest t h a t , w i t h i n the M S N M , a c t i v i t i e s in F e b r u a r y 1985 were slightly lower t h a n in M a y ~ I u n e 1977 (ratio ~ 0.75), b u t in 1985 no E T S d a t a were t a k e n o v e r the shelf and u p p e r slope (sonic d e p t h < 1000 m) n e a r ~ 15°S w h e r e h i g h e r v a l u e s m i g h t be expected. In addition, the s c a t t e r in the d a t a from a n y given period and the t e m p o r a l and spatial v a r i a b i l i t y of the M S N M suggest t h a t the differences b e t w e e n the 1977 and 1985 results are not s t a t i s t i c a l l y significant. E T S d a t a t a k e n from t h e r e g i o n to the n o r t h of the M S N M d u r i n g 1985 are also s h o w n in this comparison. Values from depths t h a t c o r r e s p o n d to the shallow n i t r i t e m a x i m a f o u n d to the n o r t h of the M S N M are high, w h i c h should be e x p e c t e d from t h e g e n e r a l t e n d e n c y for r e s p i r a t i o n r a t e s to d e c r e a s e m o r e or less e x p o n e n t i a l l y with d e p t h in the u p p e r ~ 1000 m (e.g., Suess, 1980; P a c k a r d et al., 1983; P a c e et al., 1987), and from the g e n e r a l l y high c h l o r o p h y l l levels in this r e g i o n (see below). In all t h r e e E T S lines a s u b s u r f a c e m a x i m u m o c c u r s at ~ 2 0 0 - 3 0 0 m , w h i c h is m o r e or less c o i n c i d e n t w i t h the " c o r e d e p t h s " of the M S N M and t h e d e e p e r m a x i m u m f o u n d to the n o r t h . O b s e r v a t i o n s and models of p r i m a r y p r o d u c t i o n (e.g., Chavez and Barber, 1987; F. Chavez, p e r s o n a l c o m m u n i c a t i o n ) suggest a seasonal a l t e r a t i o n in the r e g i o n a l p r i m a r y p r o d u c t i o n r a t e off P e r u with an a v e r a g e a n n u a l r a t e of 2 - 4 g C m -2 day -1, and w i t h the r a t i o of " n e w " to t o t a l p r o d u c t i o n (sensu

312 86 °

83 °

74°

77 °

80 °

70 °

i

S°

~ /

~ , .

"

-

JAN

- MAR

APR

- JUN

JUL-SEP ' olt0

-

-

OCT-DEC

6°

,,

.:... • 9°

i•.211:la

12°

o

~-

"~

Pisco

~

uon ~ O i ! endO

I 86 =

~5 °

18 °

18=

20

12 °

83 °

20 °

i 80 °

77 °

74 °

70 °

Fig. 10. E x t e n t of the l # g - a t o m l 1 (10 6M) p h o s p h a t e isopleth in the surface w a t e r s off Peru d u r i n g different s e a s o n s (re-drawn from Zuta and Guill~n, 1970).

Dugdale and Goering, 1967) being ~ 0.5~).75. Although the 1985 observations were made during the season of weakest upwelling winds when historical data (Zuta and Guill~n, 1970) suggest t ha t high surface nut ri ent concentrations are usually confined to a narrow coastal band (Fig. 10), a Coastal Zone Color Scanner satellite image (provided by G. Feldman) suggested t hat a large zone of high chlorophyll with " t o n g u e s " extending more t han 250km offshore existed in the region nor t h of ~ ll°S. The average primary production rate for F e b r u a r y - M a r c h 1985 was ~ 2 g C m 2day 1(j. Kogelshatz, personal communication, 1988) even though most of the observations were made offshore of the shelf and upper slope. DISCUSSION Codispoti et al. (1986) suggested t hat the denitrification rate in the eastern South Pacific during F e b r u a r y March 1985 may have been 3 - 1 0 T g N year 1 higher th an the rate estimated by Codispoti and Packard (1980), and t hat this condition was driven largely by the relatively shallow depth of the thermocline. Codispoti et al. noted t ha t the thermocline was unusually shallow in the n o r t h e r n portion of the study region and emphasized the possibility t hat this situation may have been a response to the 1982-83 el Nifio. This suggestion was supported by a model of Cane and Zebiak (1985) t hat predicts a thermocline ~overshoot" in the year following an el Nifio (with its associated deep thermocline) and by data presented by Guillen and Calienes (1981) t hat suggest cold

313 8O

120

160

xX

~

200

240

280

320

36O

0

x

x

•

m• J= ~L

•

I1~

•

•

10C •m

•

I 125

Distance from Coast ( km )

Fig. 11. D e p t h of t h e 26.0 a t s u r f a c e v e r s u s depth b e t w e e n 7°30'S and 10°30'S. Crosses = data from F e b r u a r y - M a r c h 1985. S q u a r e s = h i s t o r i c a l d a t a (see Codispoti et al., 1986). (7t is t h e c o m m o n t e r m used by o c e a n o g r a p h e r s to d e n o t e t h e d e n s i t y of a g i v e n parcel of s e a w a t e r at a t m o s p h e r i c pressure. It is m o r e properly a specific g r a v i t y s i n c e t h e v a l u e s are referred to distilled w a t e r at 4°C. A 0 t of 26.0 ~ a d e n s i t y of 1 . 0 2 6 g c m -3 (Knudsen, 1901 and 1931).

O~t

25.5

....__~.....~.:,

26.0

26.5

.....

• '~--FEB

85

',, ., Shelf Breok

~lsos

o

I00

,-,,

, ~" .% ,t

150

°,%

20C Fig. 12. at versus depth at the shelf break along the "C-line, M a r c h - M a y 1977 versus February 1985.

314 PHA at Eastern Boundry

30-

- 3O 2O

20-

- 10

1 0 - ~

¢n

.-10

-10-

- 20

-20 -

-30

30 12

1

2

3

4

5

6

7

8

9

10

11

Months

Fig. 13. Variations in the pycnocline height anomaly (PHA) from a nonlinear model solution (thin line) compared with smoothed observations (thick line). This figure was re-drawn from O'Brien et al. (1981) and is based on the work of Meyers, and the data are from the equatorial Pacific. The PHA is essentially the depth variation in the 14°C isotherm, which is in the lower part of the thermocline.

anomalies can occur for up to 3 years after an el Nifio (see below). In the region of the shallow nitrite maximum north of ll°S (Fig. 6), data for the depth of the 26.0 sigma-t surface ( ~ a specific gravity of 1.0260 and a temperature of ~ 15°C), which is typified by high nutrient and low oxygen concentrations, are in agreement with the concept of a thermocline "overshoot" (Fig. 11). Analysis of data from the shelf-break on the "C-line" (Fig. 12) suggests t h a t the thermocline was also elevated there, indicating t h a t this effect was widespread as the "overshoot" idea requires. While the shallow depth of the upper thermocline was extreme in F e b r u a r y March 1985, there are indications that this condition may be only a modest enhancement of a signal t h a t frequently occurs during the season of relatively weak upwelling winds. For example, Fig. 3, taken from O'Brien et al. (1981) and based on the work of Meyers (1979), suggests t h a t there is a normal seasonal signal in the equatorial thermocline depth in the eastern Pacific. Two peaks occur, one during the season of weakest local upwelling winds and the other during a period of relatively strong local winds in September-October. Although the latter elevation may be higher, strong winds in SeptemberOctober may reduce its effect on the regional denitrification rate due to enhanced vertical mixing that increases the downward oxygen flux. A preliminary analysis of some data from the "C-line" (Fig. 2), for example, suggests that the upper portion of the oxygen-deficient layer was "eroded" during the season of strong winds in September 1976 (Fig. 14). Because respiratory activity decreases with depth (Fig. 9), the erosion of the "top" of the oxygen-deficient zone could have a significant repressive effect on the denitrification rate within the MSNM. Additional support for the possibility of a more or less regular shoaling of the thermocline during the season of weakest upwelling winds comes from the temperature record collected at a shore station near 7° 30'S that was reported by Guillen and Calienes (1981). In this record, cold anomalies occurred in southern winter for 3 years after the 1972-73 el Nifio. That

315 Feb 1985

~

~

Sept 1976 vs Feb 1985

i

2OO

i

.

160

.

.

~

.

120

0

4

.

0

0

.

80

40

O i s l o n c e f r o m Shore

'~

0

(kin)

Fig. 14. Comparisons of the upper boundary of the MSNM (as denoted by the 0.2 ml 1 102 isopleth in September 1976 and February 1985. The 1976 data are from Codispoti and Christensen (1985)). i.O

I

I

I

i

I

2.42

vs DEPTH INNER I00 k m x SEP 76

ETS

{

(6)

~

t6)

2

)

•

MAY7 7 .242

.E (6)

'E

0

4 bJ .01

.001

0

.0242 W

I ~00

I ZOO

I

300

I

400

I

500

,00242

600

DEPTH ( m )

Fig. 15. A comparison of ETS activities in the inner ~ 100 km of the MSNM, September 1976 versus May 1977. The values in parentheses give the number of observations (from Codispoti and Packard, 1980). Converting activities to estimates of in situ denitrification rates was accomplished by multiplying the activities (left) by 2,42 and expressing the results in g N m -3 year -1 (right).

i n c r e a s e d d e n i t r i f i c a t i o n m a y be a s s o c i a t e d w i t h t h e s e e v e n t s is s u g g e s t e d by t h e w o r k o f Z u t a a n d G u i l l e n (1970) a n d S o r o k i n (1978) w h o s e w o r k i n d i c a t e s t h a t e p i s o d i c d e n i t r i f i c a t i o n d o e s o c c u r n o r t h o f ~ 10°S. I n s u m m a r y , t h e 1985 d a t a s u g g e s t a s i g n i f i c a n t i n c r e a s e i n t h e d e n i t r i f i c a t i o n r a t e off P e r u d e s p i t e w e a k u p w e l l i n g w i n d s . T r a n s i e n t s i t e s o f denitrific a t i o n in t h e n o r t h e r n p o r t i o n o f t h e s t u d y r e g i o n p l u s a n i n c r e a s e in denitrific a t i o n in t h e w a t e r s o v e r t h e s h e l f p r o d u c e d 3 - 1 0 T g N y e a r -1 o f " a d d i t i o n a l

316

denitrification". Although the differences in ETS data in the inner ~ 175 km of the MSNM may not be significant, activities in 1985 were ~ 25% lower t han in May 1977 and similar to September 1976. Since the upper portion of the MSNM appears to be eroded during September, which is within the season of strong winds, the data would suggest t ha t denitrification within the inner ~ 175 km of the MSNM during the 1985 experiment was intermediate between the yearly maxima and minima even if we assume t hat the differences in ETS are significant. Codispoti and P a c k a r d (1980) estimate a rate of ~ 15 Tg N year 1 for the inner ~ 175 km of the MSNM, so it would take a decrease of ~ 10-70% at this denitrification site to compensate for the "additional denitrification" t hat was observed during F e b r u a r y - M a r c h 1985. A question tha t cannot be answered with the presently available data is w h e t h e r denitrification in the offshore portions of the MSNM increases dramatically during the season of strong upwelling winds when high n u t r i e n t c on cen tr atio n s extend far from the coast (Fig. 10). Codispoti and Packard (1980) estimate a total rate for the offshore portion of the MSNM of ~ 5 Tg N y ear 1, so a large percentage increase in the rate at this denitrification site would be required to have a significant effect on the regional denitrification rate. Pak et al. (1980) and Codispoti and Pa c kard (1980) have suggested t hat a major source of organic material for the entire MSNM may consist of material produced over the shelf which sinks to the bottom layer and is supplied to the offshore portions of the MSNM by a "quasi-horizontal" flow. Since the satellite image mentioned above suggested high nearshore chlorophyll levels over the entire region sampled in 1985 (Fig. 3), denitrification within the inner ~ 175 km of the MSNM may have continued at a high rate despite weak upwelling winds. The denitrification rate in the offshore portion of the MSNM and its temporal variability is an outstanding question t hat has not yet been addressed. Similarly, seasonal effects on the sedimentary denitrification rat e off Peru have not yet been addressed. Our present view is t h a t denitrification in the eastern tropical South Pacific may, at times, be relatively high during the season of weakest upwelling winds. Since denitrification is driven by a supply of organic material which should in t u r n be governed by the n u t r i e n t supply, this conclusion may seem counterintuitive. It becomes a plausible hypothesis, however, when one considers the erosion of the upper portion of the MSNM t hat may occur during the season of strongest upwelling winds, and the possibility of a regional elevation of the upper portion of the thermocline driven by remote processes during the season of weakest upwelling winds. Such a thermocline elevation can (i) lead to high n u t r i e n t fluxes into the photic zone despite weak winds, as suggested by a vertical model presented by Friederich and Codispoti (1987), and (ii) produce extensive shallow regions that are oxygen-deficient since the 26.0 surface is typified by low oxygen concentrations. Since respiration rates increase towards the sea surface and since high primary production rates can be supported by the n u t r i e n t flux, the shallow thermocline leads to the co-occur-

317 r e n c e of h i g h n i t r a t e a n d / o r n i t r i t e , l o w o x y g e n a n d h i g h r e s p i r a t i o n r a t e s , t h e r e b y s a t i s f y i n g t h e r e q u i r e m e n t s for h i g h d e n i t r i f i c a t i o n r a t e s . ACKNOWLEDGEMENTS W e t h a n k o u r c o l l e a g u e s i n t h e N I T R O P p r o j e c t for t h e i r c o m m e n t s a n d for t h e i r h e l p d u r i n g t h e field p r o g r a m . G. F e l d m a n of N A S A g r a c i o u s l y p r o v i d e d u s w i t h t h e C o a s t a l Z o n e C o l o r S c a n n e r d a t a t h a t we e m p l o y e d i n t h i s a n a l y s i s , a n d f u n d i n g w a s p r o v i d e d b y N S F g r a n t s OCE-83-16610 a n d 16611. W e a l s o t h a n k t h e c a p t a i n , c r e w a n d s h o r e - b a s e d s u p p o r t g r o u p o f t h e R / V W e c o m a for t h e i r h i g h l y p r o f e s s i o n a l s e r v i c e s . P. C o l b y , K. K n o w l t o n a n d J. R o l l i n s p r o v i d e d e x c e l l e n t s e c r e t a r i a l a n d d r a f t i n g s u p p o r t d e s p i t e o u r u s u a l lastminute changes!

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