Relationships between reduced gases, nutrients, and fluorescence in surface waters off Baja California

Deep-SeaResearch.Vol.29. No. IOA,pp. 1203to 1215,1982. Printedin Great Britain.

0198~)149/82/101203-13$03.00/0 L~1982PergamonPressLtd,

R e l a t i o n s h i p s b e t w e e n r e d u c e d g a s e s , n u t r i e n t s , a n d f l u o r e s c e n c e in surface waters off Baja California PATRICK J. SETSER,* JOHN L. BULLISTER,*~ ERNEST C . FRANK,* NORMAN L. GUINASSO, JR.* a n d DAVID R. SCHINK* (Received 8 October 198 ! ; in revisedform 8 March 1982; accepted 31 May 1982) Ab~ract--Temperature, in vipo fluorescence, silicate, phosphate, dissolved hydrogen, and carbon monoxide were measured in surface waters while crossing the California Current system into San Diego. The measurements were made to define surface water variability and to help understand the causes of variations in H 2 and CO concentrations. Striking changes were observed crossing the current system, with the most dramatic effects occurring within 100 km of the coast. In an attempt to separate the elT~-ts of different water histories, the data were subdivided according to 'water type,' with four types identifaxl on the basis of temperature. Relations between parameters were distinctly different in the different waters. Over the "oceanic' part of the track all variables were relatively constant. Hydrogen was not correlated with CO or in vivo fluorescence except in the warm, colWofl water, where the correlation between hydrogen and fluorescence was strong. Carbon monoxide showed a distinct daily cycle when data from different water types were examined separately. The mum concentration Of CO for each water type covaried with the mean in vivo fluorescence. Likewise the amplitude of daily CO variation was positively correlated with mean fluorescence. This lends support to the hypothesis that the CO abundance is related both to light intensity and to organic content of the water.

INTRODUCTION

TEMPERATURE, in vivo fluorescence, silicate, phosphate, dissolved hydrogen, and dissolved carbon monoxide were measured in surface waters of the Pacific during Cruise 79-G-6 of the R.V. Gyre; both gases are biologically mediated in the marine environment. In vivo fluorescence was used as a rough measure of biological activity in the waters. Our measurements were intended to define better the surface variability of the gases and to understand the causes of natural variations. Dissolved H 2 concentrations have been measured in the tropical North Atlantic (S~LFJt and SCHMWT, 1974; H ~ and BAt~OER, 1978), tropical Pacific (BuLLmTn, GmNASSO and SCmNK, 1979), and in the Norwegian Sea (HERR, SCRANTONand BAROEIt, 1981). Tropical surface waters arc commonly supersaturated in H 2 with respect to atmospheric equilibrium, but HEaR et al. (1981) reported values below saturation in the Norwegian Sea. S ~ L ~ and SCtlMmT (1974) and StaLER (1978) reviewed the marine chemistry of H 2 and CO. BULUSTER,GmNASSOand SCmNK (1982a) described some of the biological relationships

* Department of Oceanography, Texas A&M University, College Station, TX 77843, U.S.A. ~" Present address: Scripps I n ~ o n of Oceanography, University of California, San Diego, La Jolla, CA 92093, U.S.A. 1203

1204

PATRICKJ. SETSERe!al.

affecting H 2 concentrations. They found the H2 inventory to increase greatly in the few days following a chlorophyll bloom at the Controlled Ecosystem Populations Experiment (CEPEX) site in British Columbia, but the relationship between fluorescence and H 2 appeared to be indirect. A daily cycle in the concentration of dissolved CO has been reported (e.g., SFJLE~ and SCHMIDT, 1974), and its existence now seems well established. SeIJUSR(1978) and BULLXSTERet al. (1982a) reported a strong correlation between CO concentration and the intensity of sunlight. SWINNERTON, LINNENSOM and L^MONTAGNE (1970) and BULUSTER et al. (1982a) provide evidence that CO is more abundant in more productive waters than in waters of low fertility. BAUER, CONRAD and S~Jloes (1980) found C O production did not correlate with net photosynthesis. STUDY AREA

The most intereating patterns in the data were observed as the ship passed through the California Current system. During this part (15 to 17 September) of the third leg of Cruise 79-

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Gases. nutrients,and fluorescenceoff Baja California

1205

G-6 (Fig. 1), the R.V. Gyre sailed from 25°30'N, 12 I°30'W into San Diego, California on a course of approximately 025 ° from North Pacific Central Water across the California Current system. The California Current, an extension of the Subarctic Current, flows southward between latitudes 48 and 23°N. The current is extremely variable and has been described by KNAUSS (1978) as a series of large eddies superimposed on a broad, weak equatorward movement; the speed and direction of the current at any given time may be quite different from the average flow. SVegDRUP and FLEUIN(3(1941) reported that a north or northwest-flowing Southern California Countercurrent is present east of the California Current:(fu~ing most of the year (June to March). Shoreward of the countercurrent, a southeastward flow may be present. LVNNE(1967) described an isolated region of warm surface water that develops off San Diego throughout the year but most clearly in July and August. This region lies on the east side of a permanent counterclockwise eddy found among the Channel Islands off southern California (ReiD, RODm~and WYLLle, 1958). A northeasterly flow feeds water into this eddy, and part of the northeasterly flow branches to the southeast along the coast. The branching coincides with the center of the isolated warm water, where the flow is sluggish. The slow advection, in conjunction with a net heat gain and the mixing of upwelled waters in the immediate vicinity of the coast, produces the isolated region of warm surface water. METHODS

Continual surface measuremeats of inorganic nutrients, temperature, in vivo fluorescence, and dissolved H 2 and CO were made on surface seawater supplied to analytical equipment via a towed vehicle ('fish'). The towed fish was attached to a stainless steel cable and lowered into the water from a boom placed amidship. While underway, the fmh was towed about 2 m away from the side ofthe ship, at a depth of 0.3 to 3 m, but usually between 0.3 and Im. The exact depth was controlled by the length of cable and by the speed of the ship. An opaque hose was run up from the forward tip of the fish along the length of cable, drawing seawater that had not had previous contact with the ship or fish body. The hose ran to the deck, where a Vanton (TM) Fiexiliner pump pulled seawater up to deck level, then pushed the water to the upper deck and into the laboratories at a flow rate of 61 rain -~. The water was not exposed to any metal or light after collection. Residence time of water in the hose was approximately 100 s. The stream was subdivided and directed to several separate laboratories for analysis. The measuroment of H 2 and CO was based on a technique involving the reduction of mercuric oxide (HgO) as described by McCuLLOUGH, CRANE and BECKMAN(1947) and further refined by SCHM~Tand SmLEg(1970). A gas chromatographic column was added to the system to allow separation of H 2, CO, and other gases before they entered the heated chamber and passed across the HgO. The H 2 and CO analyzer and the methods used are descn~oed by B ~ et al. (1982a). Gases were extracted from solution by a continuous flow equilibration method. Approximately 80 ml rain-t of air (free of H 2 and CO) was bubbled into a water stream of ,,-1200 ml min-t. The bubbles and the water passed through a long coil in a water bath, then were separated. The partially equilibrated air was dried, then passed through the sample loop of the H2--CO analyzer. The time required for the chromatographic column to separate the H 2 and CO peaks results in a minimum sampling interval of 10 min. The ship maintained a speed of ,,-5 m s-~, giving a distance of ,,,3 krn between samples. Evaluation of the equilibrator showed 65 and 56%, respectively, for the

1206

PATRICK J. S F r s E R e t at.

concentration of H 2 and CO in the final air stream as compared to the concentrations at complete equilibrium with the water. Hydrogen and CO standards were run once each hour. The nature of the standards and their calibrations are described by BULUSTERet al. (1982b). The absolute accuracy of this entire system is hard to establish with confidence. Several possibilities of sample contamination or modification are difficult to eliminate, or even evaluate; these include generation of an H, halo around the ship or in front of the sample intake; generation or consumption of H 2 or CO in the pump or hose; generation or consumption of H 2 or CO in the gas equilibrator; and variations in the efficiency of the gas equilibrator. The absolute accuracy of the dissolved gas analyses will be discussed elsewhere. In this work we focus on the relative magnitude of dissolved gas concentrations; we note however, that the lower values reported here are considered accurate to + 35% or better, while higher values are more accurate. We consider the consistency of values in the oceanic region as evidence for the stability of the gas equilibration system. Therefore, we feel that our precision is better than the estimated accuracy. Moreover, variations in the gas equilibrator efficiency could not affect the fluorescence values that correlate so well with H 2 concentrations in the warm coastal waters (see below), but they should cause better-than-observed correlations between H 2 and CO. Continuous measurements of fluorescence in vivo were made using a Turner Designs Model 10 fluorometer with a flow-through cuvette (STRICKLANDand PARSONS, 1968). Further details of our sampling system and other methods are given in SE'rsr~, GUINASSOand SCmNK (1982). Temperature of the fluorometer effluent was measured. I n situ temperature was also continuously recorded using a Leeds & Northrop thermograph with a platinum resistance thermometer. Nutrient concentrations were determined photometrically using the Technicon AutoAnalyzer ® II system, silicate by the reduction of silico-molybdic acid to the blue silicomolybdous form (BREWER and RILEY, 1966), and phosphate by the formation of a phosphomolybdenum blue complex (MURPHYand RILEY, 1962). MEASUREMENTS

ALONG THE TRACK

Measured values of dissolved H 2 and CO, in vivo fluorescence, temperature, and inorganic nutrients are shown in Fig. 2. Over parts of the track dramatic changes were observed. The observed pattern of surface temperature is consistent with the long-term monthly mean map for September (ROBINSON, 1976), although the absolute values of temperature appear to be higher in 1979 than the long-term September mean. Changes in surface temperature along the track suggest distinct water types. Unfortunately, the salinograph was not working at this time, so no salinity data were taken. From 1700L on 15 September until 0600L on 16 September, at the seaward end of the data set, all parameters were relatively stable. Dissolved H 2 concentrations were 'normal', between 0.3 and 0.6 nM* or between about 1.4- and 2.5-fold supersaturated with respect to atmospheric equilibrium. For comparison, HERR and B^ROEg (1978) found surface values of dissolved H z mostly between about 0.3 and 1.3 nM in tropical North Atlantic surface waters, with some values as high as 3.3 nM. HERR et al. (1981) reported values between about 0.2 and 0.4 nM in Norwegian Sea surface waters; there they mapped surface concentrations with a 4kin sample spacing and found relatively smooth distributions. Our seaward measurements,

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too, show relatively smooth, orderly changes. In this part of the track we observed CO concentrations between 0.07 and 1.0 riM. By comparison, SEIL~ and SCHM[DT0 9 7 4 ) reported surface CO values in the Norwegian Sea ranging from about 0.5 to 5.0 nM and in the South Atlantic from about 0.1 to 10.0 nM. The relatively constant concentrations typical of the open ocean began to change as the ship entered colder waters at 0800L. A slight increase in fluorescence in oivo marked the thermal gradient. Associated was an increase (less than two-fold) in dissolved H~. Once across the thermal boundary and into 'cool' water (20.5°C) the fluorescence returned to oceanic values, but H2 and CO concentrations began a series of changes. Carbon monoxide followed the daily pattern that has been observed before; the increase was greater than the previous day, but well within the range observed by others. Hydrogen concentrations oscillated upwards, reaching 3.7 nM, a value higher than any reported by HERR and BARGER (1978) for tropical Atlantic surface waters. The oscillations did not correlate with temperature variations during the same period. Just before midnight on 16 September surface water temperatures dropped abruptly from 20.5°C to as low as 19.0°C. Again a distinct increase in fluorescence in vi~o marked the gradient zone. Dissolved H 2, however, had returned to the low concentrations and stable distribution pattern seen cartier in oceanic waters. Carbon monoxide was quite different; the concentration jumped to substantially higher values as we entered cold water, then declined as the night wore on. This increase in CO was concurrent with a rise in fluorescence, but the sample-to-sample changes in CO and fluorescence were not correlated. Although the above changes were more distinct than any surface water variations obl~rved before, they seemed modest when compared to the observations during the final 100 km of the cruise. At 0600L on 17 September the ship entered warmer 'coastal' water. At the same time in rive fluorescence, CO, and H, increased dramatically and the dissolved silica concentration dropped. In Fig. 3 the data from this part of the cruise are expanded to show the striking variability. Measured values of fluorescence, H,, and CO were substantially greater than before and less regular. The CO values fall within the range reported by others, but H 2 concentrations up to 21.4 nM stand well above any previously reported values [except for values up to the equivalent of 87 nM reported by WILLIAMSand B^mmUDG~ (1973), values that often are attributed to contamination]. The above variations occurred at a considerable distance from the coast, far from any shallow water or contact with continental sources of pollution. Contamination due to crossing of other ship's tracks cannot be completely ruled out. However, on an earlier part of the cruise the ship steamed in a tight circle for 2 h, more than once passing directly over floating markers left on an earlier pass. During this exercise the measured values of dissolved H 2 and CO showed no effect, remaining at normal oceanic values. Thus, we consider contamination by unseen ships to be an unlikely source of the H 2 excursions. The relative constancy over most of the cruise seems to preclude any source of contamination from normal operation of the Gyre. Changes in CO and H~ concentrations cannot be attributed to their solubii/ties over the observed temperature range. Although atmospheric concentrations rose sharply as we approached the continent (BULUSn~R et al., 1982b), the equilibrium solubilities under such conditions would range only from 0.26 to 0.45 nM for H 2 and from 0.05 to 0.75 nM for CO (WIESENBURG and GUINASSO, 1979). The higher values reported here are far above atmospheric solubility.

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DISCUSSION

The observed temperature gradients probably represent boundaries of eddies in the California Current. TP.,AOANZA, Nesrog and MCDONALD (1980) and TP.AOANZA, CONRAD and BREAKER (1981) described and mapped thermal and chemical fronts associated with the California Current and coastal upwelling north of our study area. They found blooms associated with such boundaries and our observations seem to show the same. Changes observed in the H 2 concentrations are related to differences in water masses. Carbon monoxide concentrations vary with the time of day, but other factors appear to be important. Surveys of this sort are necessary to discover horizontal variabilities, but they are not effective for discovering causes of variations. Better explanations might be obtained by lopping to explore further, but that is often not practical and was not possible here. Given the available data, it seems useful to look for relationships between parameters, not because they demonstrate cause and effe,~ but because they offer clues to focus future surveys and to design more extended observations and experiments. Two lines of statistical analysis were used: multiple analysis of variance and correlation to test the strength of association between variables. Analyses were performed using SAS (Statistical Analysis System, H ~ w l o and COUNCIL, 1979). Simple correlation analyses using a finear model between H2, CO, fluorescence, and the additional variables S i O 2 and PO4 over the entire temperature range showed many significant

P ATRICK J. S ETSERet al.

i 210

Table I.

Correlation coefficients and number o f observations between parameters measured along ship's track shown in Fig. I

H2

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~ 2 206 166 71 73

CO 0.56 0

6 206 166 71 73

Correlation coefficient (r) T IVF 0.57 O.37 ~ . . ~ ~ 91 90

0.90 O. 72 0.46 5 55

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relationships and some high correlation coefficients (Table 1). But mean values of the various parameters changed substantially after the ship entered warm coutal waters. This change in scale was the major moment in pulling data into line to produce the calculated correlations. The various parameters behave quite differently at differem places along the track. Thus, it appears that the variations in properties may be due to differences in water types, differences either in their histories or their biota, or both. Accordingly, the data were subdivided into four classes based on water temlm'ature: cold <20.3; 20.3 ~< cool ~< 21.3; 21.3 < 'oceanic' ~< 22.1; and warm >22.1°C. The divisions were first selected on the basis of observed differences in water composition, then adjusted slightly to increase the statistical significance of the differences between water types. Table 2 summarizes the data as subdivided. Differences between water types were confirmed by multiple analysis of variance. The null hypotheses--that there were no differences between water types in regard to the dependent variables H,, CO, and fluorescence---were rejected at the 0.0001 probsbir~ level. Daily cycles in the CO concmtration tend to confuse the search for relationships between CO and other parameters. In our data set, the changes between water tended to obscure such daily cycling. To show this, CO concentration was plotted against sin [2n(t- 12)/24] for each water type (Fig. 4), where t is the local hour of the day. The function models production during the day and consumption at night. The argument of the sine function is denoted as T, representing time from noon in radians. Figure 4 shows the different CO content and diffm'cnt range of cycling for each water type. Regression analysis of CO vs sine T showed a high dqrn¢ of con,elation (Table 3). Slopes and intercepts were all positive and the eoufatence intervals about slope and intercept parameters indicatte that they all differ from each other. In Fig. 4 the ~ (at sine T = 0) represems a time-weighted mean CO concentration. This type of plot should be useful in sma~ing for patterns of dissolved CO because it is seldom practical to run analyses in one water type for an entire 24-h period; using the sine T approach a few hours of data should determine the daily cycle. The high correlation between CO and the sine function does not prove that the buildup and decay of CO are truly sine functions, but they do resemble such functions. The slopes of the

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lines in Fig. 4 represent the amplitude of the sinusoidal function, in effect the magnitude of daily production and decay processes. These differ in each water type. Not surl~singly, the amplitudes are positively related to the mean values (intercepts), although this need not necessarily be true. Also not surprisingly, the CO functions are positively related to the mean fluorescence in vivo in each water type (see Table 3). Illumination levels are the primary factor in determining the extent of CO buildup. StaLER (1978) reported that the daily cycle in CO concentration in seawater was notably less in amplitude on cloudy days than on sunny days. Our data support the conclusion that solar

Table 3.

Relationships between dissolved CO, a sinefunction of the time of day, and in vivo fluorescence Oceanic

Correlation coefficient CO vs sine T Intercept Weighted mean CO (nM) Slope Amplitude of daily CO (nM) N Number o f observation pairs Average fluorescence Relative units

Water type Cool Cold

Warm

0.88

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Gases. nutrients, and fluorescenceoff Baja California

1213

radiation increases CO concentrations until about sunset; during the night consumption causes the concentration to follow a function resembling the sine. Unfortunately we did not collect good data on illumination to compare with the CO measurements. This leaves open the possibility that the differences in CO content between water types were actually due to differences in cloud cover. If we neglect this possible coincidence, we can propose a relationship between the amount of fluorescence in vivo and the mean concentration and amplitude of the daily CO cycles. As high values of fluorescence are likely to be associated with high levels of dissolved organic compounds, our data seem to support CONRAD and SELLER (1980), who attributed CO production to chemical photo-oxidation of dissolved organic matter. Earlier workers postulated the metabolic production of CO (L^N~DON, 1916; RADLER, GREESE, BOCK and SFaLER, 1974). Metabolic production should be temperature sensitive, and such production might be indicated by a CO-temperature correlation. Bacterial consumption rates, too, might be temperature sensitive. Both effects would be obscured by daily cycling and by differences in population. Therefore, we examined the relations between temperature and CO residuals (i.e., the departure of CO concentrations from those predicted by the sine relation) for each water type. However, the narrow temperature range within each water type offered only minimal opportunity to find temperature-CO relationships. In two water types, cool and oceanic, statistically significant (0.02 probability level) correlations appeared. However, each correlation accounted for less than 9% of the variation in the CO residuals indicating that some other factors have more importance, Correlations between CO residuals and other parameters were also sought. Using the 5% probability level as a criterion for significance, correlations were found in the warm water region between CO residuals and fluorescence, PO 4, and SiO 2 (r = 0.57, --0.50, and -0.50, respectively). However, we are reluctant to attach any specific physical significance to correlations at this level, particularly when the correlation patterns are not consistent from one water type to the next. BUI.taSrER et al. (1982a) reported a daily cycle of dissolved H 2 concentration in the C E P E X containers, but there was no cycle in the surrounding inlet waters. Therefore, the dissolved H2 data were.tested against sine T, but showed no evidence for sinnsoidal fluctuation. The 12-h phase lag used in defining T w a s then replaced by every other possible phase shift (at l-h steps), but in no ease was a noteworthy linear regression found. We concluded that there was no positive evidence for daily cycling of H 2 in the data. However, dissolved H , and in vivo fluorescence showed a striking correlation in the warm coastal water (Fig. 5). The correlation coefficient for H, vs fluorescence was r = 0.97*. The strong relationship did not extend to the other water classes, which produced r = -0.49, -0.16, and 0.50 for oceanic, cool, and cold waters, respectively. A strong negative correlation ~ c u r r e d in warm waters between silica and H, ( r = -0.75*) or fluorescence. Again, the relationship was not observed in our other waters; in cold and oceanic regions there were too few paired measurements; in cool water the correlation is weakly positive rather than negative. The association of H, and fluorescence is reminiscent of the H 2 enrichment reported by BtJLUSTERet al. (1982a). At CEPEX, H, values were never more than 3.5 nM as contrasted with concentrations of up to 21.4 nM reported here. Also, at CEPEX high H 2 in surface

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Statisticallysignificantat a probabilitylevelof 0.001.

PATRICK J. SETSERet al.

1214

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Dissolved H 2 vs in vim fluorescence in the w a r m water region. Similar correlations were not preheat in the other water classes.

water was associated with low silica values. However, the chlorophyll bloom at CEPEX had sunk and would not have been measured in a surface water survey such as described here. Clearly, sometimes H 2 and fluorescence are related, but the exact nature of the relation can only be determined by further work. CONCLUSIONS

Our measurements in the California Current system off the coast of Baja California showed dissolved CO concentrations to vary sinusoidally with the time of day; the amplitude of the variation is related to the water type and probably to the oqlanic conteat of the seawater. Dissolved hydrogen is often very con~Jmt in mxrfac¢ waters at concentrations not far above atmospheric solubility. However, the concentrations vary dramatically in a warm water mass that extends approximately 100 km out from the c o ~ well removed from the continental shelf or direct contact with land. Premmmbly a chan~ in biota is principally respons~le for the change in H 2 d ~ ' b u t i o n . Hydrogen concentrations are not directly related to in vivo fluorescence in most surface ocean waters, but in the warm water, where concentrations are greatest, the two variables are strongly correlated. actno~Wc wire to t h ~ S ~ U . S ~ , a d D,m, A, w m for t l ~ ~ ~ tinm ~ thepmINw-.aioaoftht, ~ , ~ ~ DONA ~ uct the ¢nw ~ e ~ LV. Gyre~ ~ and ~ duringthe Ions 1979 ~ ~ T h i s work w a s s u ~ tractsN00014-75-C-0537and N00014-80-C-0113.

by (NF~ of Naval Reset-ohCon-

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Gases, nutrients, and fluorescence off Baja California

!2 !5

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