Diurnal pattern, seasonal change and variability of oxygen in the water column of the oyster ground (North sea) in spring-summer 1981

Netherlands,Journal of Sea Research 18 (1/2).. 13-30 (1984)

13

DIURNAL PATTERN, SEASONAL CHANGE AND VARIABILITY OF OXYGEN IN THE WATER COLUMN OF THE OYSTER GROUND (NORTH SEA) IN SPRING-SUMMER 1981 by S.B. T I J S S E N and EJ. W E T S T E Y N Netherlands Institute of Sea Research, P.O. Box 59, 1790 AB Den Burg 7~xel, The Netherlands

CONTENTS 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 2. Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 3. Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 3.1. General aspects of oxygen distribution in space and time . . . . . . . . . . . . . . 15 3.2. Diurnal oxygen rhythm and primary production estimates . . . . . . . . . . . . 20 3.3. Oxygen flux to the atmosphere and vertically integrated oxygen content . 21 3.4. Oxygen variability in horizontal surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 6. References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 1. I N T R O D U C T I O N In the tropical Atlantic O c e a n a n d in the S o u t h e r n Bight of the N o r t h Sea, studies of p r i m a r y p r o d u c t i o n u s i n g in situ diurnal oxygen concentration variations have b e e n u n d e r t a k e n with e n c o u r a g i n g results (TIJSSEN, 1979; TIJSSEN & EYGENRAAM,1982). H i g h - p r e c i s i o n W i n k l e r oxygen d e t e r m i n a t i o n s (TIJSSEN, 1981), routinely carried out a b o a r d ship, gave a m e a n s t a n d a r d e r r o r of replicate d e t e r m i n a t i o n s of 0.06%. In the tropical N o r t h Atlantic the small a m p l i t u d e of the diurnal oxygen signal of 0.5 to 1 m m o l . m - 3 . ( l i g h t period) -1 could a d e q u a t e l y be d e t e r m i n e d . T h e s e results have c o n t r i b u t e d to the a c c u m u l a t i n g evidence for the existence of m u c h higher p r i m a r y p r o d u c t i v i t y in oligitrophic oceanic areas t h a n m e a s u r e d before with mac methods. In a spring b l o o m in the highly p r o d u c t i v e S o u t h e r n Bight of the N o r t h Sea we applied this m e t h o d in a f o u r - h o u r l y s a m p l i n g scheme. Sinusoidal oxygen curves were o b t a i n e d that e n a b l e d the calculation of p r o d u c t i o n rates over 4 h o u r periods: net p r i m a r y p r o d u c t i o n , gross p r i m a r y p r o d u c t i o n a n d 24 h c o m m u n i t y production. In the euphotic zone of these regions no density stratification is present due to the prevailing intense vertical mixing. T h e application of this m e t h o d to

14

S.B. TIJSSEN & F.J. WETSTEYN

primary production measurements in a seasonally stratified temperate region is new. In May, July an September 1981 a team of biologists and chemists studied the flow of carbon and of oxygen in the North Sea at a station near 54°30'N and 4°30'E. The water mass was marked with a subsurface drifter. The first cruise was made in May during the initial phase of the formation of the thermocline; in July and September the thermocline was fully developed. The hydrographic conditions, residual water movement and sampling strategy are discussed in another paper(TIJSSEN & WETSTEYN, 1984). The feasibility of the in situ oxygen method to investigate primary production in a region where the depth of the euphoric zone is part of the year greater than the depth of the thermocline was the primary aim of this study. Secondly, the spring-summer oxygen budget was studied. For this purpose the oxygen flux to the atmosphere, exchange fluxes and the trend in the vertically integrated oxygen content of the water column are important parameters. The oxygen variability in time and space plays a role that cannot be overlooked and a great deal of attention had to be given to this aspect. The results of the plankton and production studies of GIESKES & KRAAY (1984) were indispensable for understanding our results. Vertical profiles of oxygen concentrations and percentages versus depth together with the data are published in a NIOZ-report (TIJSSEN & WETSTEYN, 1983). Acknowledgements.--We are indebted to our colleagues W.W.C. Gieskes, W. Helder and H. Postma for critically reading the manuscript. 2. METHODS Samples were taken near the drifter during three cruises in the periods 30 April to 11 May, 22 to 31 July and 2 to 12 September with a rosette sampler (11 bottles of 30 litres) attached to a Guildline CTD. Per cast 6 to 11 depths were sampled. Samples for horizontal surveys were taken either from the water intake at the bow of the ship or with the rosette sampler at constant depth and slowly steaming ship; bottles were closed at predetermined time intervals (cf TIJSSEN & WETSTEYN, 1984). Several Winkler oxygen methods with photometric endpoint detection have been published (BRYANet al., 1976; TSCHUMI et al., 1977; HARTWIG & MICHAEL, 1978) and an automated version has been described recently (WILLIAMS & JENKINSON, 1982). Our modification with continuous slow addition of thiosulphate shortly before reaching

OXYGEN

IN THE

WATER

COLUMN

15

the endpoint (TIJSSEN, 1981) has been partly automated since 1981 and integrated with the associated depth, temperature and salinity data obtained from the C T D computer. An I T T 2020 microprocessor system and a multi-purpose Basic programme were used. C T D data and oxygen bottle numbers from a cast were manually fed in the system, processed and stored on disc. Before commencing a titration series, input of the cast number provided the list of bottle numbers and the first titration could start. The amplified photocell output from the titrator was digitized and read by the computer. A small series of buret readings and the associated mean photocell output from the very last part of the titration was used to compute the equivalence point. Lambert-Beer's law was obeyed and extinction values (calculated from the signal changes) versus added titrant gave a straight line with correlation coefficients of 0.999. In this way, quality control was easily performed. After finishing a titration series the results were stored on disc. Results were computed, stored on disc and printed on board ship. These files served as input for further processing, e.g. for interpretation and plotting. The mean standard error of 60 series triplicate or quadruplicate samples was 0.2 mmol.m -3, i.e. less than 0.1% of mean oxygen concentration. Results of constant high quality can only be obtained through stringent application of the rules of procedure of this high-precision determination. 3. RESULTS 3.1. G E N E R A I ,

ASPECTS

OF OXYGEN

DISTRIBUTION

IN SPACE AND TIME

The general outline of the oxygen concentration distributions during the three cruises is presented in Fig. 1. Temperature and salinity distributions from the same casts are given by TIJSSEN & WETSTEYN (1984; Fig. 3). The upper part of the water layer was supersaturated all the time. From 30 April to 2 May buoy 1 was followed. It drifted southwards under the influence of strong northerly winds (TIJSSEN & WETSTEYN, 1984). This region was supersaturated to the bottom (103%; Fig. la). From 4 to 11 May buoy 2 was followed. This one drifted more eastwards and much higher vertical gradients in temperature, density and oxygen were encountered there. The bottom water above which the second buoy moved northerly was undersaturated 5 to 7%. The station occupied on 2 May belonged to a transition region in accordance with its position in the bottom water field (TxJsSEN & WETSWEYN, 1984: fig. 4). Due to a lower equilibrium oxygen solubility, caused by a higher temperature in July, the surface concentrations were much lower than

16

S.B. T I J S S E N

& F.J. W E T S T E Y N

in May. An oxygen maximum layer was present at depths between 10 and 25 m. The concentration difference between surface and bottom increased to 80 #mob m - 3. In September, temperatures were 1°C higher at the surface, accompanied by slightly lower oxygen concentrations. The surface to bottom concentration difference reached a level of 130/zmol.m-3. 02 lO'3mol 30

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Fig. 1. Oxygen distributions (mmol.m -3) with depth and time near a subsurface drifter; isopleths based on casts taken at sunset, a. May. b. J u l y a n d c. September.

17

OXYGEN IN THE WATER COLUMN

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Fig. 2. Oxygen (retool.m-3; left side) and temperature (°C; right side) distributions during 5 periods of intensified sampling in May, July and September 1981.

18

S.B. T I J S S E N & F.J. W E T S T E Y N

Many casts were taken for the study of short-term oxygen variability near the drifter (Fig. 2). The periods with intensified sampling usually lasted 48 hours, the time schedule being one cast per 4 hours. The small concentration changes associated with diurnal rhythms in the upper water layer cannot be seen here (see Section 3.2). From 6 to 9 May vertical temperature gradients, although present, were small. Oxygen concentration and temperature gradients were strongest at intermediate depths (15 to 30 m). The oxygen concentration of the bottom water indicates preceding oxygen consumption, a well known phenomenon in water below a thermocline. The lowest oxygen percentage observed was 93 %. In the periods 22 to 25 July and 28 to 30 July, the firmly established thermocline had its greatest vertical temperature gradient at depths around 20 m at a temperature of 14°C . The upper part of the thermocline warmed 0.5°C in 5 days. Simultaneously, the bottom temperature and oxygen concentration decreased. The layer with maximum oxygen concentration was detected in the first period of July. After changing the sampling scheme from depth intervals of 5 to 10 m to intervals of 2.5 m the maximum layer could be described in more detail in the next period of July. Oxygen percentages up to 124 were observed. The thickness of the layer even exceeded 7 m on one occasion. The impression obtained from the first period that casts with hardly any maximum concentration were many times followed or preceeded by casts with high m a x i m u m concentrations, was strengthened and an example is presented in Fig. 3. The high concentrations were always found within or close to the thermocline. VarioSens chlorophyll fluorescence profiles (GIEsKES & KRAAY, 1984: fig. 2), POC profiles (PoSTMA & ROMMETS, 1984) and ETS-activity profiles (VANDER WERF & NIEUWLAND, 1984) peak at the same depth as the oxygen m a x i m u m layer. The difference in integrated oxygen content per square metre was up

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3100 ,

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m m o l 0 2 • m3 300 i

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Fig. 3. Vertical oxygen profiles ( m m o l . m 3) near the subsurface drifter with different oxygen concentration in the subsurface m a x i m u m layer a. 28 July 23.40 h. b. 29 July 02.30 h. c. 2 9 J u l y 06.40. d. 2 9 J u l y 11.50 h.

O X Y G E N IN T H E W A T E R C O L U M N

19

to 700 mmol between casts separated in time less than 5 hours. Even exceptionally high rates of production or consumption cannot explain such huge differences in the same water mass. In both September periods, no trace of an oxygen concentration maximum layer was left. The vertical temperature range was shifted to higher values compared with July. The main difference between July and September was the depth of the thermocline, which was 10 m greater. The highest 09 concentration gradients (15 mmol.m-4) coincided with the thermocline layer. The ratio between thermocline layer gradient and the mean gradient was 4, almost the same as for temperature.

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0 to 20 m (A), 0 to 30 m ( V ) and 0 to 50 m (©) during 48 h periods in May, July and

September 1981 (nights shaded).

20

S.B. T I J S S E N & F.J. W E T S T E Y N

3.2. DIURNAL OXYGEN RHYTHM AND PRIMARY PRODUCTION ESTIMATES

Oxygen concentrations were vertically integrated for several depth strata after linear interpolation. T he depth intervals chosen were 0 to 10 m ,0 to 20 m, 0 to 30 m, and 0 to 50 m and the curves for the strata 0 to 10 m and 0 to 50 m are drawn in Fig. 4. In the idealized, simple case, diurnal rhythms manifest themselves by increasing oxygen concentrations during daylight and decreasing concentrations during the night. T h e amplitude should be highest in the layer of m a x i m u m primary production and this amplitude should decrease when thicker strata, which include deeper layers with net oxygen consumption, are considered. In all cases, except 7 to 10 September, the changes in oxygen concentration observed in the 0 to 50 m stratum were largest but did not show a diurnal rhythm. On the other hand most curves for the 0 to 10 m depth layer were as expected. In many cases the curves for the strata 0 to 20 m and 0 to 30 m showed only partial diurnal characteristics. Th e observations for the period 7 to 10 September (Fig. 4e) are striking. It was the only example in which the diurnal rhythms were at least semi-quantitatively traced in all strata. Those parts of the curves obtained during daylight in the 0 to 10 m layer possessed the expected positive slope, whereas the corresponding parts of the curve for 7 and 8 September in the stratum 0 to 50 m were almost horizontal. Th e non-ideal form of the curves, even in the 0 to 10 m layer, made it sometimes difficult to quantify the amplitude of the diurnal curves (Table 1). Quantification of the oxygen amplitudes to a primary oxygen production per unit area made it necessary to estimate a mean depth for which the amplitude could be valid. O n the basis of the information given by GIESKES & KRAAY(1984: fig. 11) a depth of 14 m for May and TABLE 1 Components of net oxygen production in May, July and September 1981 in the Oyster Ground. Amplitude of diurnal rhythm in oxygen concentration in the stratum 0 to 10 m (mmol.m 3); n u m b e r of estimates per curve; dearth of production (m); net oxygen production in the water column ( m m o l . m - ~ . d - l ) ; oxygen flux correction ( m m o l . m - 2 . d - 1 ) ; temperature correction ( m m o l . m - 2 . d - I ) and corrected net oxygen production ( m m o l . m - 2 . d - I ) .

Date May 7-9 July 23-25 July 28-30 Sept 3-5 Sept 8-10

02 ampl.

Numb. est.

Depth prod.

Prod.

Flux corr.

3.8 1.7 3.0 9 5.5

3 3 2 3 4

14 30 30 10 10

53 51 90 90 55

19 24 15 8 3

7~mp. corr. + +

13 6 6 6 6

Corr. prod. 59 69 99 104 64

OXYGEN

IN

THE

WATER

COLUMN

21

of 10 m for September was selected . No exact value could be given for July due to the high variability in the production m a x i m u m at 20 m (Fig. 3). In this case a fictitious depth of 30 m was chosen for the calculations, on the basis of the ratio (3) between the total 14C production and 14C production in the 0 to 10 m stratum (GIESKES & KRAAY, 1984). The oxygen production per square metre per day has to be corrected for (1) the difference between day and night of the oxygen flux to the atmosphere (Section 3.3) and for (2) the different oxygen solubilities at different water temperatures (Table 1). The weighted, mean daily primary oxygen production, calculated on the basis of 15 oxygen amplitude estimates, was 77 mmol (2500 rag-m-2). 3.3. O X Y G E N

FI, U X T O T H E

ATMOSPHERE OXYGEN

AND VERT[CALLY

INTEGRATED

CONTENT

Supersaturation of oxygen was always observed in the upper layer of the sea (104 to 112%) during the three cruises (Fig. 5) . In July and in September the variations were within 2 %, but in May a steep increase from 104 to 112% occurred. Fluxes to the atmosphere were calculated using the flux equation: F = k.{[O2]ac t - [O2]eq} The k values were calculated from wind velocities (TIjSSEN & WETSTEYN, 1984: fig. 1) with the equation:

k = lO-6.W2(Wandkinm.s

1)

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I101 May

221

1241

1261

1281

1301 Jul

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181

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Fig. 5. M e a n oxygen saturation percentages in the 0 to 10 m depth layer at sunset in May, July and September 1981.

22

S.B. T I J S S E N & F.J. W E T S T E Y N

TABLE 2 Oxygen saturation solubility, mean oxygen percentage (0 to 10 m), mean exchange coefficient k and mean oxygen flux to the atmosphere near the drifter in the Oyster Ground in 1981.

Cruise May July Sept

[02]eq (mmol. m 3)

02 (%)

k (m.d 1)

02-JTUX (mrnol.m-2.d-1)

308 255 250

107.2 109.3 108.2

7.0 5.2 3.2

93 116 53

(T0ssEN & EYGENRAAM, 1982). In view of the diurnal rhythms the oxygen percentages at sunset are biased towards high values. Therefore, a correction was applied of -1%, - 0 . 5 % or - 2 % for the successive cruises to approach a more representative daily mean percentage (Table

2). Due to changing wind conditions the daily fluxes were highly variable. T h e y were in the range of 0 to 250 m m o l . m - 2 . d -1 . For longterm budget considerations cumulative fluxes are of course more important. These are much less variable; 28 days of observation provided a mean daily flux of 87 m m ol . m -2. If we assume that this was a representative value for the spring-summer period (May to October), the total oxygen flux to the atmosphere was 13.4 mol.m 2 (line A in Fig. 6). During the second half of the M ay cruise lower wind velocities were accompanied by increasing oxygen percentages. Calculations show that this effect is well within the scope of building up higher percentages in the water column. Low wind speeds caused the lower oxygen flux in September. Apart from the flux to the a t m o s p h e r e , the vertically integrated oxygen content of the water column is a main datum in the construction of a spring-summer budget. T he oxygen content of the water column was calculated for those casts with a uniform vertical distribution of samples. At least three casts per day were assumed to be needed for a realistic mean daily value. In May and J ul y these conditions were fulfilled on 2 days and in September on 5 days (line B in Fig. 6). Both the diminishing oxygen solubility at higher temperatures and the net oxygen consumption in the bottom water layer contributed to the decrease in oxygen content of the water, amounting to 6600 m m o l . m - 2 in 5 months (43 m m ol . m - 2- d- 1) . This is only half the cumulative oxygen flux to the atmosphere, the oxygen excess (C in Fig. 6 ) being 6800 m m o l.m 2 (44 mmol-m-Z.d-1). Without further considerations the

OXYGEN

IN T H E W A T E R

COLUMN

23

oxygen excess is an estimate of the difference between oxygen p r o d u c tion a n d oxygen c o n s u m p t i o n in the w a t e r m a s s concerned. S e p a r a t e b u d g e t s have also b e e n set up for the u p p e r w a t e r layer (0 to 30 m) a n d for the b o t t o m w a t e r layer (30 to 50 m). T h e b o t t o m w a t e r layer so defined was always below the t h e r m o c l i n e a n d showed the expected decrease of oxygen c o n c e n t r a t i o n in the productive season. For the sake of best c o m p a r i s o n the s a m e casts as considered in Fig. 6 have b e e n used. T h e m e a n c o n c e n t r a t i o n s were 295 m m o l - m - 3 in May, 204 in J u l y a n d 142 in S e p t e m b e r . T h e general trend was -1.2 m m o l . m - 3 . d -1. A stronger decrease is evident f r o m the S e p t e m b e r values (Fig. 6, dashed line). tool

m -z

rnol.m

z

-14

. . . . .

12

i

-10 8

166 144 122

10Moy

I

June

I

July

I

Aug.

I

Sept.

0

Fig. 6. O x y g e n flux to the a t m o s p h e r e (right side, A), vertically i n t e g r a t e d oxygen content of the water c o l u m n (left side, B) a n d oxygen excess (C), expressed in m o l . m 2

3.4. ()XYGEN V A R I A B I L I T Y IN H O R I Z O N T A l . S U R V E Y S

To get s o m e idea of horizontal oxygen c o n c e n t r a t i o n variations on length scales of tens to t h o u s a n d s of metres a few series of m e a s u r e m e n t s were taken (Fig. 7). U n f o r t u n a t e l y we h a d no access to an electrochemical oxygen sensor for c o n t i n u o u s m e a s u r e m e n t s , which is almost a necessity for this type of work. Two series of oxygen concen-

24

S.B. T I J S S E N & F.J. W E T S T E Y N

tration m e a s u r e m e n t s are shown. T h e first one is based on samples taken with the rosette s a m p l e r at constant d e p t h a n d free drifting ship. W i n d speed was only a few m - s - 1 a n d we h a d no m e a n s to deduce the speed of drifting. T h e s i m u l t a n e o u s l y o b t a i n e d t e m p e r a t u r e a n d salinity ranges were small, 6.88 to 6.96°C a n d 33.91 to 33.93 respectively. Q u a d r u p l i c a t e oxygen samples were taken. A slight tendency of increasing oxygen s a t u r a t i o n percentages f r o m 108.0 to 108.9 was present. Two w a t e r bottles, however, gave up to 20% higher values. In September, oxygen c o n c e n t r a t i o n a n d high f r e q u e n c y (1 Hz), high precision salinity m e a s u r e m e n t s were m a d e in w a t e r p u m p e d f r o m the bow of the ship d u r i n g very quiet weather. T h e vessel's speed was 4.5 m . s z a n d oxygen samples were taken each minute. For ease of r e p r o d u c t i o n m e a n salinity values for 10 s periods were drawn. Small, but no d o u b t real differences to the extent of 0.004 were found. In this case, over a m u c h g r e a t e r length scale, a definite t r e n d in oxygen with an increase of 1.5% was detected. T h e steepest g r a d i e n t was 0.8% k m -1. T h e small salinity changes were not correlated with oxygen changes. O n the other h a n d the chlorophyll a t r e n d (GIEsKEs & KRAAY, 1984: fig. 13) r e s e m b l e d the oxygen trend: a chlorophyll a increase of 0.4 m g - m -3 was a c c o m p a n i e d by an oxygen c o n c e n t r a t i o n increase of 4.5 /~mol.m -3. U s i n g the ratio p h y t o p l a n k t o n c a r b o n to chlorophyll a f r o m GIESKES & KRAA¥(1984: table 1), this a m o u n t of chlorophyll a corresponds to a p h y t o p l a n k t o n c a r b o n change of 38 m g (3.2 mmol); thus the oxygen to p h y t o p l a n k t o n c a r b o n tool ratio for the trend in both p a r a m e t e r s was 1.4. A n o t h e r source of i n f o r m a t i o n on variability in oxygen concentra02%

02 %

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t08-

110-

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107 0

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min

tO0

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15

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201 0 ql

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10 5500m

t5

min 20 b

Fig. 7. O x y g e n (% saturation) and salinity (S) surveys at constant depth, a. At i0 m

depth, sample distance unknown, 8 May 16 to 18 h. b. and c. At 2 m depth, sample distance 275 m, 6 September 17.16 to 17.35 h.

OXYGEN

IN

THE

WATER

COLUMN

25

tion was found in the changing oxygen concentration in the subsurface maxima encountered during the July cruise in the thermocline layer (Figs 2c and 3). The mean relative velocity of these oxygen rich layers compared with the upper layer will be of the order of half the drifter velocity, that is about 0.02 m.s -1. This means a distance of 300 m in the time span of 4 hours between casts in a 48 h series. No simultaneous chlorophyll fluorescence profiles have been obtained but a series of profiles taken 28 and 29 July by GIESKES & KRAAY(1984) is available. The combined sets of oxygen and fluorescence profiles were treated as a time series but the observed changes were erratic. 4. DISCUSSION The availability of processed and corrected oxygen results aboard ship a few minutes after finishing the titration of the oxygen bottles of a cast does not only serve to check the quality of the whole procedure, but enables first hand interpretation at the spot and adaptation of cast and depth schemes to unexpected results. Subsurface m a x i m u m oxygen concentrations have been observed in other temperate and mid-latitude regions (e.g. SHULENBERGER& REID, 1981). Coincidence of maximum phytoplankton abundance and maximum oxygen saturation has been described by GRAN(1933). In the upper non-stratified layer the net oxygen production escapes to the atmosphere. However, within the thermocline, vertical exchange is a very slow process and under the hydrographic and biological conditions (GIESKES & KRAAY,1984) prevailing in July, a long-term enrichment of oxygen could take place in the thermocline. Inspection of the time series of mean oxygen concentration in the surface layer leaves little doubt as to the reality of diurnal variations in oxygen, reflecting net oxygen production during daylight through photosynthesis. The range of amplitudes encountered was 0.6 to 3 % of surface oxygen concentration. At the low side of this range the use of a high-precision oxygen determination is indispensable. In situations with a surface oxygen gradient like the one in Fig. 7b, the nonreproducible sampling positions around the drifter will add accidental positive or negative contributions to the undisturbed diurnal signals. In July the shallow thermocline, with its variable oxygen concentration, hampered the application of the in situ method for primary production estimates. Wind velocity probably influences supersaturation; high wind velocities tend to lower supersaturation. Under the prevailing conditions the correction for oxygen flux to the atmosphere constituted a minor part of total oxygen production.

26

s.B. T I J S S E N &

F.J. W E T S T E Y N

The derived oxygen amplitudes are underestimates due to the effect of missed maxima and minima. The hydrographic conditions in this stratified region and the associated chemical variability are the causes that the diurnal signal, although obvious, cannot be converted easily to reliable primary production estimates. Gross production and net 24 h production (TIJSSEN & EYGENRAAM,1982) could not be derived. The estimate of mean primary oxygen production was 77 m m o l . m - 2 . d -1, which corresponds to a C production of 610 mg-m-2-d -1 assuming a photosynthetic quotient (PQ) of 1.5. Such a conversion of oxygen to carbon values depends on the PQused: 1 mmol O~ is equivalent to 9.6 mg C ( P Q 1.25) or 6 mg C ( P Q 2.0). POSTMA& ROMMETS(1984) observed a mean diurnal POC amplitude between 5 and 10 m of 49 mg-m -3. Our oxygen amplitude, corrected for exchange with the atmosphere, was 6.1 retool.m-3, equivalent to 37 mg carbon m-3 in case of nitrate assimilation or 59 mg. m -3 in case of ammonia assimilation (WILLIAMS et al., 1979). GIESKES & KRAAY(1984) arrived at primary production estimates 25% higher than the latter value, based on a PQ. of 1.25. To calculate a spring-summer oxygen budget for the area of investigation we have to discuss two transport terms: (1) horizontal exchange in the bottom water layer and (2) vertical exchange through the thermocline. The Oyster Ground is surrounded at 3 sides by almost unstratified water with 95% oxygen saturation in the bottom water. These oxygenated water masses are located at a m a x i m u m distance of 50 km from the sampling positions. The horizontal gradients can be estimated to be 0 (probably slightly negative) for May, 6.8.10-4 for July and 19.10 -4 mmol.m -4 for September. The eddy diffusion coefficient along a track that crosses the boundary between stratified and non-stratified waters is not known. Therefore, we use as a rough estimate the value 50 m2.s-1 given by OKUBO(1974) for a length scale of 50 kin. To estimate an upper value we assume the flux to come from three sides. The cross section of the region into which the flux is transported is taken as 80 kin, which is the radius of the Oyster Ground. With Fick's second law a mean contribution to the oxygen concentration over the 5 month period of less than 0.1 mmol-m-3.d-1 was calculated. Such a small amount can be neglected. Evidently the oxygen excess (Fig. 6) can now be interpreted as net oxygen community production. PINGREE & PENNYCUICK (1975) used the long-term mean vertical temperature distribution and the heat budget at station E1 in the English Channel to calculate vertical exchange during summer stratification. They obtained maximum values of the apparent vertical exchange coefficient K z of 10 4 m2.s-1. Long-term mean distributions are biased towards lower gradients, in other words they underestimate

O X Y G E N IN T H E W A T E R C O L U M N

27

vertical temperature gradients. Heat budget calculations for the study area (C. VETH, personal communication) arrived at a i0 times lower value. The vertical oxygen distributions (Figs 1 and 2) provided values for the m a x i m u m vertical gradients of 1.5, 15 and 15 mmol.m -4 for May, July and September. The mean vertical flux across the thermocline was 13 m m o l . m - 2 . d -1. The components of a two layer oxygen budget for the spring-summer period are summarized in a scheme (Fig. 8). The rate of oxygen loss in the upper layer was determined as the difference between the rate for the whole water column and the rate calculated for the lower layer. The oxygen consumption rate of the lower layer has two components: the decreasing oxygen concentration in the course of the period and the downward flux through the thermocline. Without the flux through the thermocline the oxygen saturation percentage of the bottom water would have decreased to 25. The oxygen consumption in the lower layer must have been caused by oxidation of 220 mg C . m - 2 . d -1 ( P Q = 2.0) or 33 g for the period of 5 months. The net oxygen production was 44 (87- 7 - 36) m m o l . m - 2 . d -a (Fig. 8). The corresponding amount of carbon production for the community is 260 m g . m - 2 - d -1 ( P Q = 2.0). For the period 1 May to 1 October the amount of net community carbon production is no less then 40 g.m-2. A major part of this quantity must be used as biomass increase of the pelagic and benthic communities of the second and higher trophic levels and this must be produced with a conversion factor of about 0.5, assuming that most of the conversion took place in the lower

87

I ( -7 "~

upper

~

O--

layer

f

13

lower layer

.... +

...... :_ <2

50-

Fig. 8. Oxygen budget ( m m o l - m - 2 . d - I ) in the layered water column of the Oyster Ground, North Sea for the period 1 May to 30 September 1981; exchange fluxes (arrows), internal rates (circles).

28

S.B. T I J S S E N & F.J. W E T S T E Y N

layer. Neither the P O C budget calculated by POSTMA & ROMMETS (1984), nor the oxygen production and consumption values given by GIESI
During three cruises in May, July and September 1981 the oxygen regime was studied in the Oyster Ground (North Sea) near a subsurface drifter. Diurnal rhythms were detected in the 0 to 10 m depth layer.

OXYGEN IN THE WATER COLUMN

29

C o n v e r s i o n o f t h e o x y g e n a m p l i t u d e s to p r i m a r y p r o d u c t i o n e s t i m a t e s was h a m p e r e d b y h o r i z o n t a l o x y g e n v a r i a b i l i t y , p r o b a b l y c a u s e d b y p l a n k t o n p a t c h i n e s s . T h e m e a n p r i m a r y 0 2 p r o d u c t i o n o f 77 mmol.m-2.d 1 h a s a c a r b o n e q u i v a l e n t o f 740 m g - m - 2 . d - 1 a s s u m i n g a P Q o f 1.25 a n d o f 460 m g . m - 2 . d -a a s s u m i n g a P Q o f 2.0. T h e f o r m e r v a l u e is a b o u t 2 5 % l o w e r t h a n e s t i m a t e s b y GIESKES 8Z KRAAY (1984) b a s e d o n 14C d a t a . A t w o - l a y e r o x y g e n b u d g e t was c o m p u t e d . T h e o x y g e n flux to t h e atmosphere deduced from oxygen saturation values and wind velocities was t w i c e t h e r a t e o f d e c r e a s e in o x y g e n c o n t e n t in t h e w a t e r c o l u m n . T h i s c o n f o r m s to a n e t c o m m u n i t y c a r b o n p r o d u c t i o n o f 40 g . m -2, w h i c h s h o u l d b e p r e s e n t as b i o m a s s i n c r e a s e o f t h e p e l a g i c a n d b e n t h i c f a u n a in t h e m o n t h s M a y to O c t o b e r . V a r i a b i l i t y l e n g t h scales o f a few h u n d r e d m e t e r s w e r e p r e s e n t in surface a n d s u b s u r f a c e layers. A s m o o t h g r a d i e n t in t h e o x y g e n c o n c e n t r a t i o n o f 0 . 4 % k m - l w a s d e t e c t e d in S e p t e m b e r at 2 m d e p t h .

6. R E F E R E N C E S BRYAN,J.R., J.R RILEY ~ZP.J. LEB. WILLIAMS,1976. A Winkler procedure for making precise measurements of oxygen concentrations for productivity and related stndies.--J, exp. mar. Biol. Ecol. 21: 191-197. GIESKES,W.W.C. • G.W. KRAAY,1984. Phytoplankton, its pigments, and primary production at a central North Sea station in May, July and September 1981.--Neth. J. Sea Res. 18 (1/2): 51-70. GRAN, H.H., 1933. Studies on the biology and chemistry of the Gulf of Maine, II. Distribution of phytoplankton in August 1932.--Biol. Bull. 64: 159-182. HARTWIG, E.O. & J.A. MICHAEL, 1978. A sensitive photoelectric Winkler titrator for respiration measurements.--Environ. Sci. Technol. 12 (6): 712-715. OKCBO, A., 1974. Some speculations on oceanic diffusion diagrams.--Rapp. R-v. R6un. Cons. perm. int. Explor. Mer 167: 77-85. PINGREE, R.D. & L. PENNYCUICK, 1975. Transfer of heat, fresh water and nutrients through the seasonal thermocline.--J, mar. biol. Ass. U.K. 55: 261-274. POSTMA,H. &J.W. ROMMETS,1984. Variations of particulate organic carbon in the central North Sea.--Neth. J. Sea Res. 18 (1/2): 31-50. SCHOTT,E ~z M. EHRHARDT,1969. On fluctuations and mean relations of chemical parameters in the north-western North Sea.--Kieler Meeresforsch. 25: 272-278. SHULENBERGER,E. ~zJ.L. REID, 1981. The Pacific shallow oxygen maximum, deep chlorophyll maximum, and primary productivity reconsidered.--Deep Sea Res. 28A (9): 901-919. TIJSSEN, S.B., 1979. Diurnal oxygen rhythm and primary production in the mixed layer of the Atlantic Ocean at 20°N.--Neth. J. Sea Res. 13 (1): 79-84. , 1981. Anmerkungen zur photometrischen Winkler-Sauerstofftitration und ihre Anwendung zur Schfitzung der Prim/irproduktion im Meet. In: I. DAUPNER.III. Internationales Hydromikrobiologisches Symposium. Smolenice, 3-6 Jnni 1980. Verlag der Slowakischen Akademie der Wissenschaften, Bratislawa: 343-353.

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TIJSSEN,S.B. & A. EYGENRAAM,1982. Primary and community production in the Southern Bight of the North Sea deduced from oxygen concentration variations in the spring of 1980.--Neth. J. Sea Res. 16: 247-259. TIJSSEN ,S.B. & F.J. WETSTEYN,1983. Temperature, salinity and oxygen graphs and data obtained in May, July and September 1981 near a subsurface drifter in the Central North Sea. Interne Verslagen Nederlands Instituut voor Onderzoek der Zee, Texel, 1983-4: 1-48. - - - - , 1984. Hydrographic observations near a subsurface drifter in the Oyster Ground, North Sea.--Neth. J. Sea Res. 18 (1/2): 1-12. TSCHUMI, RA., D. ZBXREN&J. ZBXREN, 1978. An improved oxygen method for measuring primary production in lakes.--Schweiz. Z. Hydrol. 39 (2): 306-313. WERF, B. VAN DER & G. NIEUWLAND, 1984. Bacterial biomass and respiratory electron transport system activity in the Oyster Ground area (North Sea) in 1981.--Neth. J. Sea Res. 18 (1/2): 71-81. WILLIAMS, P.J. LEB. & N.W. JENKINSON, 1982. A transportable microprocessorcontrolled precise Winkler titration suitable for field station and shipboard use.-Limnol. Oceanogr. 27 (3): 576-584. WILLIAMS, PJ. LEB., R.C.T. RAINE & J.R. BRYAN, 1979. Agreement between the 14C and oxygen methods of measuring phytoplankton production: reassessment of the photosynthetic quotient.--Oceanol. Acta 2: 411-416.