Marine Chemistry, 12 (1983) 25--42 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

25

COPPER AND pH IN THE SURFACE WATERS OF THE EASTERN TROPICAL PACIFIC OCEAN AND THE PERUVIAN UPWELLING SYSTEM

A. ZIRINO, C. CLAVELL and P.F. SELIGMAN

Naval Ocean Systems Center, San Diego, California 92152 (U.S.A.)

R.T. BARBER

Duke University Marine Laboratory, Beaufort, North Carolina 28156 (U.S.A.)

(Received June 23, 1981;revision accepted September 6, 1982)

ABSTRACT

Zirino, A., Clavell, C., Seligman, P.F. and Barber, R.T., 1983. Copper and pH in the surface waters of the eastern tropical Pacific Ocean and the Peruvian upwelling system. Mar. Chem., 12: 25--42.

In April--May 1976, investigations were conducted of the relationships between Cu, planktonic growth and other oceanographic variables. Continuous measurements of "Cu" (by ion~elective electrode), pH and chlorophyll-a (by fluorescence) were made from San Diego, California to Pisco, Peru. Addit ional measurements of Cu, by automated anodic stripping voltammetry (AASV), were made in a 30 x 30 km area directly off Pisco, a site of pronounced upwelling.

The pH and chlorophyll-a increased toward the equator and were exceptionally high in the Peruvian upwelling zone. No "pools" of Cu 2+ were detected within the sensitivity of the Cu electrode (0.2--0.3pg1-1 ). Off coastal Peru, the electrode showed a sharp decrease in potential , which coincided with the pH and chlorophylt-a maxima.

Off Pisco, Cu measurements by AASV indicate that a temporal decrease of surface Cu occurred during the sampling period. However, the hypothesis of the Cu2+-induced toxici ty of upwelled waters could not be verified because of (1) a lack of adequate analytical sensitivity, (2) possible riverine Cu input and (3)because the water had earlier contained S 2-.

INTRODUCTION

The distribution and speciation of copper in the surface waters of the ocean are important because copper is both an essential micronutrient for living organisms (Bowen, 1966) and a powerful phytoplankton growth inhibitor, when present in certain forms above a critical concentration. Recently, several investigators (Steeman-Nielsen and Wium-Andersen, 1970; Sunda and Guillard, 1976; Anderson and Morel, 1978; Jackson and Morgan, 1978) have shown that growth inhibition by Cu in seawater is proportional to the concentration and/or activity of the hydrated ion Cu 2 ÷ (the Cu ion activity ACu 2+ and the Cu ion concentration Cu 2+ are related by a constant, the activity coefficient). Copper speciation models (Zirino and Yamamoto, 1972; Dyrssen and Wedborg, 1974) indicate that the activity and/or concen- tration of the hydrated ion is dependent on the pH of seawater. In upwelling areas of the ocean, where the surface pH may vary greatly, it is possible that Cu activity may vary enough to affect phytoplankton growth by being sufficiently high to inhibit growth, or alternatively, sufficiently low to limit it.

03044203/83/0000--0000/$03.00 © 1983 Elsevier Scientific Publishing Company

26

In waters of the south Pacific, north of the Antarctic Convergence, Boyle and Edmond (1975a) showed that Cu covaried with inorganic nitrogen and phosphorus. Similarly, Bruland (1980) and Bruland et al., (11978 a,b) have shown that several metals covary with inorganic nitrogen, phosphorus and silicon, i.e. the "nutrient" depleted surface waters are depleted in metals as well. Earlier, Barber and Ryther (1969) found that newly upwetled, nutrient- rich water supported less rapid phytoplankton growth than water horizontally removed from its upweUing site at the equator. Subsequently, they observed that the growth rate limiting property of the newly upwelled water could be removed by the addition of ethylenediamine tetraacetic acid (EDTA). Since EDTA combines strongly with the divalent ions of transition metals, they suggested that the improvement in growth rate could be attributed to an EDTA-induced reduction in the free metal ion content of the newly up- welled water. Later, Barber and Huntsman (1977) indicated that the growth rate limiting property in nutrient rich, deep ocean water was related to Cu ion activity.

In coastal upwelling zones, growth rate experiments have always yielded more complicated results than experiments with open-ocean deep water because of the complex trajectory of the upwelling circulation (Barber et al., 1971; Barber and Smith, 1980). However, in addition to the need to achieve a complete understanding of the physical circulation, it is apparent that any investigation of the relationships between phytoplankton growth and Cu must include some measure of Cu 2+ made at the natural pH of seawater, as well as a measurement of the pH of the water. Such determina- tions allow for a better estimate of ACu 2+ and Cu 2+ than the customary measurements made after acidification of the sample. Unfortunately, Cu analyses at the natural pH of seawater, performed without a sizeable disrup- tion of the solution equilibria, are difficult, if not impossible. This difficulty stems from the marginal sensitivity of the trace metal methodology presently available and from the tendency of Cu 2+ to adsorb on virtually all surfaces.

Though cognizant of these limitations, we nevertheless attempted to investigate some of the relationships between Cu, pH and planktonic growth as part of the JOINT-II expedition of the Coastal Upwelling Ecosystem Analysis (CUEA) program. We measured Cu (at various pH values and by two different methods), pH, chlorophyll-a (by fluorescence) and other variables in seawater pumped aboard ship while underway from San Diego, CA, to Pisco, Peru, and in a 30 by 30km area adjoining Cabo Nazca, Peru. This paper presents and interprets this work.

METHODS

Sampling

Seawater was collected with a variable-speed Masterflex peristaltic pump (Cole-Parmer No. 7545--15) with silicone tubing in the pump head. The

27

OCEAN

TO A A S V REFERENCE . . . . ] _ pH COMBINATION OVERF'O~I V - - ~ n '7" ELECTRODE

I ~ ~ TEMP V ', ~--~r~'t l I [SENSOR I I I

// II CELL H ~ IJ " TEFLON OEL" // ] PUMP A ~ Jl k~ ~ , /3-WAYSTOPCOCK // II y ~ , h L t ~ ~ ~ O,SOHARGE

PUMP ] ) ~ ( ~ , J 3 - W A Y ~ /~ 12cm "1 II f ~ " ~ . ~ STOPCOCK f//l~ .Cu2 + il

~ TEFLON RESERVOIR

STANDARD SALTWATER SYSTEM DISCHARGE

OCEAN

Fig. I. Schematic diagram of the system used for underway sampling and measurement of pH, Cu, temperature and chlorophyll-a.

pump was placed in the ship's laboratory and drew water through approxi- mately 50 m of heavy-waUed, 1/4 in. i.d., tygon tubing. The tubing was sup- ported by a block attached to the end of a boom and was towed along the starboard side of the ship. A specially-built depressor maintained the intake of the tubing below the surface at speeds up to 13 knots. Seawater drawn into the ship's laboratory was pumped into a small reservoir and allowed to overflow into a sink. A second peristaltic pump, identical to the first, was used to draw water from the reservoir and move it through a specially- machined, Teflon manifold containing pH and Cu ion-selective electrodes (ISE's) and a temperature sensor (Fig. 1). The use of a second pump insured that seawater would flow through the manifold at a constant rate. Samples for Cu analysis b y automated anodic stripping vol tammetry (AASV) were collected with a third peristaltic pump, which also drew water from the reservoir and circulated it through the instrument.

Seawater for the measurement of in vivo fluorescence was obtained from the salt-water system of the R/V 'Alpha Helix', which has an intake 3 m below the surface. Thus, fluorescence was measured at a different depth than the other variables.

Cu by ion specific electrode (ISE)

An Orion Model 94-29 Cu 2+ ISE was placed in the Teflon manifold with its ion-sensitive surface upward into the flow channel in the manner de- scribed by Rice and Jasinski (1976) and Zirino and Seligraan (1981). An Orion (Model 90-20) double-junction reference electrode was placed opposite

28

the Cu 2+ electrode (Fig. 1). The distance between the face of the reference electrode and the ion-sensitive surface was approximately I mm. Seawater was used to fill the inner and outer chambers of the reference electrode. Under these conditions the potential developed by the reference electrode is similar to that o f a Ag/AgC1 electrode (Gilbert and Hume, 1973). Electrode potentials were measured with either a Coming Model 101, or an Orion Model 801 digital voltmeter, and were recorded with a Soltec multipen recorder. Since no buffer system has been developed for Cu 2÷ in seawater, a rigorous calibration of ISE potentials versus Cu 2+ was not made. However, checks were made on the responsiveness of the sensor. Twice a day, flow from the sampling stream was interrupted and fresh seawater was re~irculated from a 125ml Teflon bottle through the manifold. After a constant potential was reestablished (generally within l mV of the potential generated by the stream water), additions of Cu(NO3)2 standard were made into the Teflon bottle with a micropipette. Each addition nominally increased Cu 2÷ in the recirculated water by 0.3pg 1 -~ The electrode potential was allowed to stabilize after each increment (Fig. 2). This procedure demonstrated that the system could resolve approximately 0.2 t~g l- ~ of Cu 2+. However, because of adsorption on the walls and of other factors, the actual Cu 2+ in solution could not be known with certainty.

Cu by automated anodic stripping voltammetry (AASV)

The specially built instrument used has been fully described by Zirino and Lieberman (1975) and Zirino et al. (1978). This device, which features a wax-impregnated, tubular mercury--graphite electrode (TMGE), periodically samples water from a continuous stream and carries out complete p la t ing- stripping analyses automatically.

Measurements at pH 4.9 were made using CO2 as the purging gas. Measure- ments at pH 2 were made by manually adding concentrated HNO3 (ULTREX Grade, J.T. Baker and Co.) to the samples with a micropipette. Potential scans were made in the differential pulse mode from an initial potential of - -1.0V vs. Ag/AgC1. The scan rate was 10mV s - I and the current range was 100pA.

}a3 I I0 ix{ {0.3 ~g t l)

~' N BASELINE POTENTIAL

> BEFORE STD

128 RETURN l O / " ~ f .a~. 30 ,u.t ( i. 8 ,u.g L_ ( ) ADDITION ~- '~ F LOVVIN FLOWING ~ ] [

STR£AMG W 50 /zL (3 .5 A~g L - r ) 5

ADDITIONS OF Cu STANDARD (FINAL ADOED CONCENTRATIONS

L~ IN PARENTHESIS)

3 2 1 c,

TIME {hr}

Fig. 2. Changes in Cu ISE potential with additions of Cu standard.

29

The instrument was set to perform a complete plating--stripping cycle every 11 minutes, including 4 minutes for pre-electrolysis. An addition of Cu(NO3)2 standard was made automatically every third analysis. Concentra- tions were computed by comparing the peak height of the second determina- tion with the peak height of the third, assuming that the peak height of the second would have been identical to the peak height of the third had no Cu addition been made. While this procedure causes some error in regions where large, closely spaced variations in Cu occur, this error was ignored. The relative uniformity encountered between succeeding peaks justified this procedure. A complete analysis, requiring three separate plating ~stripping cycles, could be completed every 33 minutes. Thus, assuming an average speed of 10 knots for the R/V 'Alpha Helix', a Cu measurement could be made every 8 kilometers.

Depending on the condition of the electrode, the limit of detection of the AASV system ranged from 0.02 to 0.05pg 1-1. The precision of these determinations is estimated to be 10--15% (Zirino and Lieberman, 1975). Cu determinations by AASV at pH 4.9 were found to give values indis- tinguishable from those obtained by atomic absorption after preconcentra- tion on Chelex 100 (Zirino et al., 1978). A recent intercalibration exercise, which compared Cu values obtained with a computer-controlled AASV instrument (Clavell, 1979) to those obtained by a coprecipitation technique (Boyle and Edmond, 1975b), showed that both methods yielded concentra- tions which were, on average, within 50% of each other at the 0.06 pg 1-1 level (Zirino et al., 1980).

pH

pH was measured in the flowing stream with a No. 476055 Coming com- bination microelectrode (with Ag/AgC1 reference) mounted in the Teflon manifold by means of a specially designed adapter (Fig. 3). Two electrodes were used alternately for the prolonged continuous measurement of sea- water. While the first was in the manifold, the second was stored in a buffer solution, or in deionized water. The electrode was replaced in the manifold approximately every two days. Prior to making measurements of the surface seawater, the electrode was calibrated with Beckman buffers 3007 (pH = 7.00 at 25°C) and 3009 (pH = 9.18 at 25°C). The buffers were placed in the Teflon reservoir and re-circulated through the manifold until a constant millivolt reading was obtained. The calibrations were performed at the tem- perature of the ship's laboratory which was 25 + 2°C. Measurements were carried out at the temperature of the seawater in the 'manifold, which varied from 19 to 28°C. Electrode response, in millivolts, was recorded on one of the channels of the Soltec multipen recorder.

In order to calculate the pH of the surface seawater, it is first necessary to correct the raw electrode potentials, obtained at the temperature of the manifold, to 25°C. This may be accomplished with the millivolt vs. tem- perature calibration plot obtained by Flynn (1980}, who found that, in the

30

0 125 in

L T

- ~ .... :I 28[

! \, \ SEAT f (:)i~ ,' q RING

0 510 m ~

HOLLOWED

~ P0 LUG

500 m

0 750 in i

[ CORt,,j NG

t i / / ' E L ~ ,C g !:? ('){:)F

{ E i

"~ . . . . ~ RUBBER / O " RINGS

/

SCALE 1 1

Fig. 3. Detail of pH electrode assembly.

10--35°C range, the temperature response of the potential developed by the Coming semi-micro combination electrode in 34.3°/00 seawater was + 0.27 m V ° C -1 . Since this correction is not significant for this discussion, the pH values reported herein remain uncorrected and good to only + 0.02 pH units, pH values determined underway are believed to have significant accuracy for the following reasons: (1) there is a high degree of correlation (R 2 = 0.91) between pH and chlorophyll-a values (Simpson and Zirino, 1980), even though they were measured in waters pumped by different pumps, through separate conduits of unequal diameters and through separate inlets. Thus, little CO2 was lost by the sampling. (2) pH values measured in the manner described herein were within -+ 0.04 pH units of those measured by a discrete technique (Zirino, 1975). (3) Off the coast of northwest Africa, Weichart (1974) measured the pH of seawater with a glass electrode im- mersed in a stream of seawater. His results show pH values and spatial distributions of pH similar to ours.

31

Chlorophyll-a

Water for the determination of chlorophyll-a was passed through a high volume flow-through cell on a Turner Model 10 fluorometer. The fluoro- meter readings were registered on a strip-chart recorder. Chlorophyll-a concentrations were calculated by calibrating the flow-through in vivo fluorescence with discrete, acetone extracted fluorescence measurements on water samples that passed through the in vivo system. The discrete fluorescence system was calibrated at frequent intervals, at least once a week, by the SCOR-UNESCO method. The flow-through in vivo fluore- scence method is based upon the procedure originally described by Lorenzen (1966); a description and data from the calibration are given in Barber et al. (1978).

Temperature

Temperature in the manifold was measured with a thermistor coupled to a Yellow Springs Instrument (YSI) Company Model 47 scanning tele- thermometer. Bucket samples taken en route, and measured for temperature with a glass-mercury thermometer, showed that temperatures of the water at the manifold were approximately two degrees higher that at the ocean's surface. Temperatures were also recorded on the Soltec multipen recorder.

RESULTS

Two sets of measurements were made in JOINT-II in the March--May, 1976 period. Each set of observations corresponded to a "leg" of the JOINT- II cruise of R/V 'Alpha Helix'. The first leg, referred to as Leg 0, left San Diego, California, on March 5th and arrived at Pisco, Peru on March 20th. During Leg 0, the vessel followed essentially a direct course to Pisco, initially transiting through the coastal waters of Baja California into the eastern tropical Pacific Ocean. Following an equatorial crossing, approximately equidistant between the coast of Ecuador and the Galapagos Islands, the vessel traveled through highly productive coastal waters off Ecuador and Peru, finally making land at Pisco.

The second set of observations was made on Leg 2, which consisted of several underway grids in the 30 x 30km area off Cabo Nazca, Peru, at 15°S, where the intensive JOINT-II studies concentrated. The 15°S region of the Peru coast, bounded on the north by the Ica River and on the south by Punta Santa Ana, is a region of persistent and strong coastal upwelling.

The grid that was sampled is shown in Fig. 4; in addition, discrete stations were occupied (Barber et al., 1978; McIsaac et al., 1979}. At these stations water for copper and pH analyses was pumped into the laboratory by lowering the intake from the surface to a depth of 40 m. Two extended transects were made along the "C" line shown in Fig. 4, extending 90 and 150 km offshore.

32

10=N

3{)' N

20 ~ N

10°S

;ZOOS

120°W 110°W lO0°W 90°W ~ ) °W 70~W

Fig. 4. Cruise track for Leg 0 (San Diego to Pisco) and sampling grid for Leg 2.

Leg 0

Continuous strip chart recordings of potentials generated by the glass electrode (pH), temperature, Cu ISE potentials and in vivo fluorescence were made from San Diego to Pisco. A portion of the continuous record obtained in Baja California water is shown in Fig. 5; the temperature and

T

30 CHLOROPHYLL

20 (X 10 SCALE~

10

84 25.5 , pH

pH/'~ 61 PH'I /i pH

- - ~ . . . . . J J 1015 1315 1615

TtME (hr)

Fig. 5. Recorder trace of an upweUed "patch" of water in the vicinity of Bahia Magdalena, Baja California, Mexico, showing variations in chlorophyll-a, Cu, temperature and pH electrode response.

33

pH record clearly indicate an upwelled "patch" of water in the vicinity of Bahia Magdalena. pH was found to decrease sharply, from about 8.35 at the frontal edges, to a low of approximately 8.1 at the center of the parcel. Since the ship's speed at this point was about 12 knots, the "patch" was approximately 72km long. Temperatures at the manifold were found to decrease, from 24°C to a low of 22.5°C, reflecting the lower temperature of the upwelled water. Potentials generated by the Cu ISE reference pair became slightly more negative, following the patterns of temperature and pH.

Also of interest, is the fine structure of the pH, which showed a great deal of spatial variability. Since these features coincided closely with the patterns shown by the chlorophyll-a trace, it follows that the fine structure was of biological origin.

Despite the patchiness associated with Bahia Magdalena, from San Diego to Pisco, we did not encounter much variability on the 100 km scale except in regions of upweUing. This is shown in Fig. 6, which was prepared from the continuous record by taking values obtained every three hours and replotting them as functions of time and ship's position. The figure shows pH, temperature, chlorophyll-a and raw Cu ISE response, uncorrected for changes in temperature. Two regions of pronounced divergence can be seen. The first of these, off the Baja California coast, is characterized by decreased temperature, low pH, and a chlorophyll-a content > 1 pg 1-1. On this scale,

148

140

c~ 132

28

o. 24 F-

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8 8 8.6 8 . 4

8 . 2

2 0

T ~0 9

O

POSITION

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i I

,I

I '

i I

I

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o ~ ~

I "-~ :-'2 V ,~

I 'ij I i

I ( I

37 38 39 310 311 312 3.13 314 315 316 317 3.18 319 320 321 DATE

Fig. 6. Plot o f chlorophyll-a, pH, temperature and Cu (ISE response) values taken from the cont inuous record at 3 h intervals, from San Diego to Pisco, Peru.

34

the region shown in Fig. 5 shows up only as a pronounced dip in temperature and pH.

Southward of the Baja California coastal region the surface waters of the oligotrophic, eastern tropical Pacific Ocean showed little lateral chemical alteration in structure, pH and temperature increased equatorward, while the Cu ISE potentials became somewhat negative. Chlorophyll-a also in- creased southward, f romapproximate ly0.2 pg l-l to about 1 pg l-~. Between 5 and 6°N, pH dropped rather sharply, from 8.4 to 8.3, indicating another divergence. South of 4°N, from 3--14 to 3 17, no pH, temperature and Cu data were taken and chlorophyll-a was determined only intermittently. In the figure, an open space is used to indicate the absence of values. Simi- larly, a broken line is used to show absence of data in the plot of ISE values. These gaps in the record indicate that the electrode was temporarily off- line for calibration.

Highly productive water was encountered near the South American conti- nent. This water supported a massive bloom of Gymnodynium splendens, which was first observed just south of the Galapagos Islands and appeared to be continuous toward the coast (Packard, 1976). While the temperature declined rapidly, from 27°C offshore to 20°C at the coast, chlorophyll-a values rose sharply, up to 22/~g1-1 . pH showed corresponding increases, rising from 8.4 to a high of 8.9 and decreasing again to 8.2 at the coast. The Cu ISE potential showed an abrupt negative change in potential, correspond- ing to high pH and chlorophyll-a values and then turned in a positive direc- tion near Pisco.

Fig. 7. Surface concentrations of Cu measured at pH 4.9 in the JOINT-II area during April 1976. Position of numbers indicates approximate sampling location; solid Nos. 1814/76, boxed area 24/4/76.

35

Leg 2

The Peruvian upwelling area, centered around Cabo Nazca, was measured for surface Cu on April 18 and May 4, 1976 using the AASV instrument. Water was sampled along a zig-zag course which joined the CUEA reference stations. Additional excursions were also made along the C transect, extend- ing offshore for approximately 90 and 150 km. During the experimental period, several shallow stations (to 40m) were also occupied and vertical profiles taken. Initially, several measurements were made at pH 8, but yielded no peak currents. Thereafter, all determinations were made at pH 4.9, until it became apparent that the measured concentrations at the coast had decreased to the point where they were approaching the limit of detection of the instrument. Similarly, the concentrations measured westward of the 30 by 30 km area were also at the limit of detection: Thus, after April 28, the pH of the seawater was lowered to 2 for all analyses. Data collected on the zig-zag transects of April 28 and May 4, are shown in Figs. 7 and 8. Other measurements made underway and on station during the survey period are listed in Table I.

Measurements of pH, chlorophyll-a and temperature at the manifold were also made in the upwelling area. However, while some Cu ISE measurements were made on April 18, use of the electrode was discontinued thereafter because of problems with the instrumentation. In this region of divergence, we found that the pH of the surface water increased sharply to the north- west, along with increasing temperature and chlorophyll-a. Figure 9 shows a strip chart recording of pH, temperature and Cu ISE potentials made during the zig-zag survey of April 28.

\ "~ m~ oo,,a M~.,. \ \ \ ~, ~m, ,,,s o 1o i

" ~ , ~ Pta Lom,~as k,,ome,e s • HYDRO STATIONS 440

" \ " ~ c

' 8 g

5/4/76 e,. '\~ j ' . -~ ~ \

, , ° , ° , v ° . . . . . . . ' " "

Fig. 8. Cu concentrations measured at pH 2 in the JOINT-II area 4/5/76.

36

T A B L E I

Cu, pH and NOB at several s t a t ions in the Peruvian upwel l ing zone

Date S t a t ion Dep th (m) Cu(pg 1 - l )b pH b NO~" (DiM 1 ] )a,d

2 0 / 4 / 7 6 15 ° 5 .5 'S 2 0 .39 c 7 .77 1.66 75 ° 30.4 'W 2 0 .10 7.77 t . 6 6 (No. 43) 10 0.08 7 .76 5.20

20 O,04 7.75 40 0 .02 7 .70 0.14 50 0.08 90 0.05

2 1 / 4 / 7 6 15 ° 9 .4 'S 2 0 .03 7 .76 75 ° 39 .7 'W 2 0.02 7 .76

10 0 .02 7 .76 30 0 .02 7.72

2 4 / 4 / 7 6 15 ° 14 'S 300 0 .02 a' 75 ° 49 .3 'W

a Sample col lec ted f rom Niskin bo t t l e . b Sample co l lec ted b y lower ing hose . c Possible c o n t a m i n a t i o n b y sh ip ' s hull . d F r o m McIsaac et al., 1979.

" / i ' ~ i

i i ; i

A5 A B.'

' I } G 22/ | ~ k [

A o 2~1

I ; ~ - T J

A5 A1 2 CASO B1

<? !

i i

8 I ~ CA

" D~: : :~<T ,~r ,, ,

' - - . 2 6 + - 30 - j - - 30 .i- 30 -~- 29 - { - 29 - t

DISTANCE {kin /

Fig. 9. Reco rde r t races of the Cu ISE, pH e lec t rode and t h e r m i s t o r r e sponse for t h e JOINT-I I area m a d e on 18 /4 /76 .

37

The lowest pH was found at Station C-l, indicating that it was the center of the upwelling dome. The low pH values at all the coastal stations, and the high values at the offshore turning points (As, Bs, etc.), indicate that the plume spread from C1 and extended northwestward. High values of chloro- phyll-a offshore, and a strong correlation between chlorophyll-a and pH, indicated that vigorous planktonic growth had taken place as the plume traveled offshore. The relationships between pH, temperature and chloro- phyll-a in this area are discussed in another paper (Simpson and Zirino, 1980).

DISCUSSION

Leg 0

From San Diego to Pisco, temperature, pH, ISE potential and chloro- phyU-a vary in phase. Logically, the independent variable is temperature, which affects the response of the electrodes and indicates chemical changes in the water itself. Chlorophyll-a also varies in relation to temperature, albeit indirectly, since phytoplankton growth is enhanced by increased nutrients available in the cooler water that has recently reached the surface. Between the Baja California and Peru coastal upweUing zones, the sea surface temperature, measured at the electrode manifold, traces an arc about the equator, reflecting the increased insolation of this region (Fig. 6). This arc is also seen in the pH. From the tip of Baja California at 23°N to about 7°N pH increases gradually from 8.2 to about 8.5, while the temperature at the manifold increases from 20 to 27°C. If a constant value of Pco2 in the atmosphere is assumed, along with a salinity of 35 + 1%o and a specific alkalinity of 0.123, equilibrium considerations (Skirrow, 1975) indicate that the observed pH change is much too large to be solely the result of the change in temperature. Thus, the increase in the pH may be due to the production of phytoplankton removing inorganic carbon from the water. This conclusion is supported by the increasing concentration of chlorophyll-a towards the equator.

The effect of primary production on the surface pH is apparent in the upweUing areas. Off Baja California, chlorophyll~a values as high as 3pg 1-1 coincide with an increase in pH from 8.20 to 8.35. Off Pisco, Peru, where the standing stock of a dinoflagellate was exceptionally high in 1976 (Barber et al., 1978), with chlorophyU-a values greater than 50 pg1-1 , the pH values were correspondingly high, reaching a value of 8.91 at one location. Here the high coastal production is responsible for driving the pH beyond 8.2 + 0.2, the "normal" pH value of semitropical coastal water in equili- brium with atmospheric CO:.

Cu ISE potential is found to trace the same arc as temperature and pH, reaching more negative values at warmer temperatures. Rice and Jasinski (1976) have shown that the temperature response of the Cu 2+ ISE<louble

3~

junction reference pair in natural seawater is .... 1.5 mV °C -~ . Thus, if the potential is corrected for temperature changes, it can be seen that much of the equatorward decrease is removed and that the overall ISE potential can be normalized to 25°C with a reading of approximately -- 140 mV. There- fore, for the area surveyed, but short of the Peruvian upwelling zone, marked changes in the Cu 2 ÷ content of the surface waters are not indicated° We do not imply, however, that temperature induced changes are not occurring in the water. Temperature and pH-related changes in Cu 2+ would still occur, since Cu 2+ participates in acid-base type reactions (Williams and Baldwin, 1976). However, they could be masked by the large temperature coefficient of the electrode.

In the Peruvian area, the response of the Cu probe is highly negative, reaching a low of -- 156 mV. This value, which coincides with the pH maxi- mum of 8.91, is above the correction for temperature and may indicate removal of Cu from the water. Such removal would be associated with the high productivity and chlorophyll-a content of the water. Possible mechan- isms for removal would include (1) biological uptake, (2) complexation with hydroxide or carbonate ions, (3) adsorption on plant surfaces and (4) complexation by exuded organic chelators.

For the upwelled patch near Bahia Magdalena, we find that Cu removal is suggested even after the correction for temperature is applied. Again, we find that negative increases in electrode potential coincide with chlorophyll-a maxima, despite the overall lower pH of the patch. Biological removal, whether direct or indirect is again suggested. This suggestion is made in light of the fact that the reactions and mechanisms which control the elec- t rode potential are not fully understood (Zirino and Seligman, 1981, Westall et al., 1979).

Throughout Leg 0, no abrupt, positive shifts in electrode potential were observed. Thus, within the sensitivity of the measurement (0.2 pgl-1 ), it may be concluded that no pools of Cu 2+ were encountered, even in up- welling areas. This conclusion is supported by the fact that during the early port ion of Leg 0, individual samples were collected from the flowing stream and analyzed aboard ship by AASV at the hanging mercury drop electrode (Zirino and Kounaves, 1980). While the current-voltage traces were noisy, most were interpretable. Eleven measurements made at pH 8, and 2 yielded mean Cu concentrations of 0.11-+ 0.07#g1-1 and 0.29-+ 0 .16pg l - I , re- spectively. While these values contain too much experimental error to be taken as "oceanic surface concentrations", they confirm that any variability in the surface Cu 2 + at pH 8 was below the sensitivity of the ISE.

Leg 2

In spite of the large onshore-offshore pH gradient, which existed at the Cabo Nazca site, our a t tempt to measure Cu ion gradients in these waters was unsuccessful because of the premature failure o f the ISE system, and a

39

lack of adequate sensitivity and spatial coverage (at pH 8) with the voltam- metric system.

The ISE trace obtained up to the failure point (Fig. 9) shows an average potential of approximately -- 120 + 5mV at T = 20.5 + 1.5°C, a value somewhat more positive than that predicted by the temperature correction, but probably within our present understanding of the ISE. On the other hand, we would have predicted that the electrode potential be most positive in the vicinity of station A1, where low pH, low chlorophyll-a water was found, and most negative near station B5, where the opposite conditions existed. In fact, the ISE trace is out of phase with pH (and chlorophyll-a) and may indicate sources and sinks of Cu beyond those predicted from a simple upwelling mechanism. The voltammetric measurements of acidified samples support this hypothesis.

Cu concentrations measured at the Cabo Nazca site at pH 4.9, ranged from 0.39gg 1 -~ at the surface, to 0.02~g 1 -~ at 40m. Surface Cu measured during the grid map operation of April 18 averaged 0.25 + 0.6#g 1- ' and did not show significant variability in the 30 x 30km area. However, measure- ments made along the "C" line on April 24th showed that Cu decreased with distance from the coast (Fig. 7), while chlorophyll-a and pH increased (Simpson and Zirino, 1980). A depth profile to 40m at station C-3 and to 30m at station C-7, indicated that the subsurface water contained less than 0.03pg 1-1 Cu. The high values at the surface and the relative decrease of Cu with distance offshore, indicate that there was an inshore source for this element. A plausible source is the local river water, which carries the mine tailings of several Andean mines. Separate analyses of the river, estuarine and coastal waters have shown these to be high in heavy metals, including Cu (Zirino and Guillen, 1977, unpublished). A "pulse" of riverine Cu may have been introduced by the decennial rains of the late March and early April, 1976 which covered the Ica and Nazca regions of Peru, just inland of the JOINT-II study site.

AASV measurements made (at pH 2) on May 4, 1976 indicated that the overall levels of Cu in the surface waters had decreased since the determina- tions in mid-April. This decrease was greater than a simple comparison of concentrations would suggest, since seawater samples yield higher Cu con- centrations with increased acidification (Nilsen and Lund, 1982; Zirino, 1981). Reasons for the observed decrease include dispersion out of the 30 x 30km area and planktonic uptake, direct or by adsorption. Also, for riverine Cu, adsorption on flocculating organic and inorganic matter may have led to precipitation (Sholkovitz, 1976; Sholkovitz et al., 1978).

Overall, Cu concentrations measured in the Peruvian upwelling area in 1976 were not significantly different from those of coastal waters off the southeastern U.S. (Windom and Smith, 1979), and of some of the open ocean surface waters of the Pacific (Boyle and Edmond, 1975a; Boyle et al., 1977). They are lower than the values reported for North Atlantic surface waters (Chester and Stoner, 1976; Alberts et al., 1976; Bewers et

40

al., 1976). However, the low Cu concentrations reported here may be atypical. During the April 1976 period, there was denitrification at 15°S and H2S was detected in the sub-surface waters of the poleward-flowing Peru-Chile undercurrent, which is the source of the water upwelled along the Peru coast (Dugdale et al., 1977). Thus, any labile Cu would have been precipitated from the ,source water, as sulfide, before transport to the surface.

CONCLUSION

Our at tempts to detect Cu 2+ with the Cu 2+ ISE in the surface waters of the eastern, tropical Pacific Ocean, indicate that the Cu 2+ concentration is relatively uniform and that no "pools" of Cu 2+ exist which are greater than 0.2--0.3pg 1-1. By taking the baseline electrode response as a measure of Cu 2+, we conclude that Cu 2+ in a highly productive coastal area is controlled by the biology, either through direct uptake, organic complexing, or through the increase in pH which accompanies primary production.

In the Peruvian upwelling zone, Cu measurements by AASV indicate that a temporal decrease of surface Cu of possible riverine origin occurred during April--May 1976. We presume that this decrease occurred as a consequence of the massive b loom of Gymnodinium splendens, although co-precipitation of Cu with flocculating materials may also have occurred. During this period, the hypothesis o f the Cu :+ -induced toxici ty of upweUed waters could not be verified because of the possible riverine input and because the waters had earlier contained S 2 -. However, even without such complications, it is unlikely that Cu 2 + gradients could have been detected by us, since measur- able Cu 2+ was at or near the limit of detect ion of our instrumentation, even after acidification.

ACKNOWLEDGEMENTS

We wish to thank Jane Kogelschatz for technical assistance and encour- agement throughout this work. Typing b y Beverly Patch, Yolanda Jimenez and Nicki Kell is gratefully acknowledged. This work was funded by the U.S. Office of Naval Research {Contract N.R. 083-301) and b y the National Science Foundat ion IDOE Grant OCE 75-23722.

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