Oxygen supersaturations in the Chukchi and East Siberian seas

Deep-Sea Research, 1971, Vol. 18, pp. 341 to 351. Pergamon Press. Printed in Great Britain.

SHORTER

CONTRIBUTION

Oxygen supersaturations in the Chukehi and East Siberian seas* L. A. CODISPOTrt"and F. A. RtcrtaRos, (Received 2 September 1969; accepted 2 October 1970) Abstract--Unusually high oxygen concentrations (sometimes greater than 150 % of saturation values) are often observed in the Chukchi and East Siberian seas between Bering Strait and approximately 170°E. These high concentrations appear to arise primarily from in situ photosynthetic production. The highest concentrations were observed in stratified waters that probably left the surface in winter when the water was nearly saturated with air and contained high nutrient concentrations. These highly oxygenated strata generally were separated from the surface by shallow pycnoclines. High oxygen concentrations have also been observed in the relatively warm waters of the Bering Strait inflow. Nutrient enrichment of the surface layers by turbulence in the Strait may contribute to these high concentrations. INTRODUCTION

IN SUMMER,oxygen concentrations in parts of the Chukchi and East Siberian seas are often exceptionally high, attaining more than 150 ~ saturation and 13 ml/l. Such concentrations in these shallow seas (Fig. 1) and adjacent regions have been reported previously (BARNESand TrlOMVSON, 1938; CODISPOTI, 1965; GOODMAN,LINCOLN,THOMPSONand ZEUSLER, 1942; IvAr~I,ncov, 1964; LOCICEaMAN, 1968; and SVEP.ORUV,1929), but there has been little discussion of their causes. In this paper, data from recent U.S. Naval Oceanographic Office cruises are used to examine this feature. DATA

The observations were made from U.S.S. Burton Island in July-September 1964 and August 1966 and from U.S.C.G.C. Northwind in October 1962, August 1963, and August-September 1966. The sampling programs and methods of analysis have already been described (CAR and GSELL, 1967; CODISPOTI, 1965; CODISPOTIand RICnARDS, 1968; DAWSON, 1965; GLADFELTER,1964; LOCKEa_MAN, 1968; WARGELIN, 1967). CARPENTER'S(1966) oxygen solubility data and DOUGLAS' (1964, 1965) nitrogen solubility data were used to calculate gas saturations with respect to an atmospheric pressure of 760 mm including the vapor pressure of the water. The accuracy of the 1964 data was estimated by comparing Winkler and gas chromatographic (SwINNERTON,LINNENBOMand CHEEK, 1962) determinations made during the cruise. This comparison (Fig. 2) yielded a 95% confidence limit (4-2 S) for the differences of --0.67 to 0"65 ml/l. Most of the concentrations of interest were greater than 6-7 ml/l. so the agreement between these results was better than 4- 10 YD. No comparisons were made on the other surveys; hence average dissolved oxygen surface saturation values were calculated (Table 1) to judge the data. Appreciable departures from equilibrium can occur in surface waters, but they should equilibrate with the atmosphere relatively quickly, and it was assumed that their average saturation should be close to 100% if the data were free from large systematic errors. The highest departure among the acceptable data was a value of 110 ~ from the 1966 Burton Island data. These observations were in a region and at a time favorable to the photosynthetic production of dissolved oxygen, and it is concluded that the systematic errors in the five suites of oxygen data were generally less than I0 % of the equilibrium surface saturation values. *Contribution No. 576 from the Department of Oceanography, University of Washington. "{'U.S. Naval Oceanographic Office, Washington, D.C.; present address, Department of Oceanography, University of Washington,rseattle. :~Department of Oceanography, University of Washington, Seattle. 341

342

Shorter Contribution

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= 315 0.24 ml/L -0.01 ml/l.

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STANDARD DEVIATION OF THE DIFFERENCES 95 PERCENT CONFIDENCE INTERVAL FOR THE DIFFERENCES

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Fig. 2. Comparison of Winkler and gas chromatographic dissolved oxygen results: Burton Island, 1964.

343

Shorter Contribution

Table 1. Average dissolved oxygen saturations for surface samples. Acceptable surface samples Cruise

All surface samples

No. samp&s

Av. Sat. (%)

No. samp&s

Av. Sat. (%)

110 166 79 38 103

98 106 100 110 96

110 177 83 61 121

98 106 99 111 96

Northwind, October 1962 Northwind, August 1963 Burton Island, July-Sept. 1964 Burton Island, August 1966 Northwind, Aug.-Sept. 1966

No satisfactory evaluation could be made of the analyses o f the supersaturated samples, in which systematic errors could arise from degassing. Degassing errors might be in excess o f 10 % and they would systematically yield low results. DISCUSSION

The major features o f the areal distribution o f the dissolved oxygen supersaturations are exemplified by the 1963 Northwind data (Fig. 3). The occurrence of the highest values in the relatively deep region near Wrangel Island is of particular interest. High values were common during the 1966 Burton Island survey, which was concentrated in the deeper areas near Wrangel Island, and the highest saturation values in these data ( > 160 ~ ) were observed at that time. High values were less prevalent during the 1964 Burton Island and 1966 Northwind surveys, but saturations greater than 125 700 were observed near Wrangel Island and at one station in Bering Strait. No appreciable supersaturation values were observed in these areas during the 1962 Northwind survey. The distributions of dissolved oxygen as functions o f depth, time of year, and nutrient concentrations indicate that the high supersaturations resulted from the in situ photosynthetic production o f oxygen. Most values over 110% occurred in the 0-20-m layer, presumably in the photic zone, and

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165 °

Maximum dissolved oxygen saturations regardless of depth, in percentage: Northwind, August 1963.

344

Shorter Contribution

w e r e a l s o a s s o c i a t e d w i t h r e d u c e d nutrient c o n c e n t r a t i o n s . T h e o n l y recent o b s e r v a t i o n s ( N o r t h w i n d 1962) that s h o w e d n o h i g h s u p e r s a t u r a t i o n v a l u e s w e r e m a d e in O c t o b e r w h e n light intensities w e r e l o w ( D A w s o n , 1965). A l l the N o r t h w i n d a n d B u r t o n I s l a n d d a t a a n d d a t a f r o m s o m e earlier cruises

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D i s s o l v e d o x y g e n s a t u r a t i o n s vs. t e m p e r a t u r e a n d salinity: u p p e r 20 m .

B.

A.

Temp. (°C)

Salinity (%0)

----------

0"10 1.49 1"52 1"58 1"63 1"72 1.74 1.72 1.73

-- 0.20 -- 1.42 - - 1.48 -- 1"66 -- 1"74 -- 1"78 -- 1"76 -- 1"75 -- 1'76

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18-77 24.90 25.36 26"23 26-65 26"68 26-68 26-68 26"68

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2 0 64 10 22 20 I0 12 6

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106" Stability*

---

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7"22

135 132 135 116 115 99 74 67

86$ 85 85

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113t 160 162 121 90

106 143 144 132 114 80 74 73 76

02 ( % Sat.)

9"94t 13.67 13-80 10.29 7-66

10-46 12.29 12.31 11-38 9"68 6-77 6"32 6"18 6.45

02 (ml/l.)

*Stability was calculated from the practical equation for the u p p e r 100 m suggested by SVERDRUP, JOHNSON and FLEMING (1942). tQuestionable values.

0 5 10 15 20 25 30 35 40 45

Bering Strait Inflow 1. Northwind, August 1963, Station 14

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10

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2. Burton Island, A u g u s t 1966, Station 24

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Data ]rom selected stations with high dissolved oxygen supersaturations.

High supersaturations separated from the surface by strong pycnocHnes 1. Northwind, August 1963, Station 43

Depth (m)

Table 2.

t,~

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B.

A.

Averages

Colder Waters Northwind, 1963 Burton Island, 1964 Burton Island, 1966

Averages

Burton Island, 1964

39 6 19 20 21

1 2 17 18 19

• Sta. No.

Bering Strait Inflow Northwind, 1963

Cruise

Table 3.

40 40 30 40 40

------

1-80 1.76 1.75 1.74 1"76

2"65 2"65 3"32 3"08 3"01

(°C)

(m)

40 42 30 35 20

Temp.

Depth

33-21 33-38 32.94 32-95 33.13

32-73 32"68 32"85 32"81 32-78

(%0)

Salinity

2.50 2.43 2"18 2"27 2"13 2.30

85

2.06

93 88 84 85 84 85

2"22 2"05 1"86 2"06 2" 11

(/Lg-atoms/l.)

Reactive phosphorus

91 92 92 96 95

( % Sat.)

02

19.2

15-0 16"8 20"0 22-1 22.1

18.5

52

54 58 50 50 50

30

34 34 28 32 33

(/~g-atoms/'l.)

(tzg-atoms/l.)

17"3 16"4 16-8 20-4 21 "7

Reactive silicate

Nitrate

Examples of nearly saturated oxygen values in waters with high nutrient concentrations.

0

t~

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o

Shorter Contribution

347

(BARNESand THOMPSON,1938; ~DMAN, LINCOLN,THOMPSONand ZEUSLER,1942; SVERDRUP,1929) indicate that the highest supersaturation values occur only during the summer and early fall. Plots of oxygen saturation versus salinity and temperature (Fig. 4) indicate that the highest supersaturations were associated with higher salinities and that there were at least two major hydrographic regimes associated with the high oxygen Concentrations. The highest values were associated with near-freezing waters, and a second group of high values was in the relatively warm water flowing in through Bering Strait. This bimodality might have been more evident if additional samples had been taken in the Bering Strait region, especially since there are earlier reports of high dissolved oxygen concentrations in this area (BARNES and THOMPSON, 1938; GOODMAN, LINCOLN, THOMPSON and ZEUSLER, 1942). The temperatures of the waters are not directly responsible for the observed distributions, which depend on factors to be discussed later, but the temperature differences of the two regimes make them readily distinguishable. Turbulence in Bering Strait may be a major cause of the high oxygen concentrations in the Strait and nearby areas. Even in summer, the deeper waters in the Strait are often nutrient-rich (Table 3). Consequently, mixing could cause substantial enrichment of the surface layers. Thus, as long as there is sufficient light, phytoplankton blooms may continuously occur in the Bering Strait region. Because some time must elapse between nutrient enrichment and a phytoplankton bloom and because strong turbulence in the Strait could inhibit growth and dissipate oxygen maxima rapidly, the highest *C -5 0"? I0 % 6o o

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A o'tTEMPERATURE

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(F)

Fig. 5. Dissolved oxygen saturations, et, and temperature vs. depth (A)-(C) are stations where high supersaturations were observed; ( D ) s h o w s a station where warm Bering Strait inflow waters overlie the cold, relatively saline, bottom waters; (E) is a station taken near Bering Strait; and (F) shows conditions at a station occupied early in the navigation season.

IBO

B.

A.

(~)

(°C)

32.64 (13)

-

1.26 (36)

31-35 (36)

8.50 (36)

7.33 (13)

*Numbers in parentheses indicate the number of samples averaged.

-

Colder Water Samples with 02 Saturations ~ 140%

4.34 (13)

(ml/l.)

lOOK 02 sat. conc.

0.84 (27)

0.75 (12)

(f,g-atoms/l.)

Reactive phosphorus

1.1 (23)

0.5 (8)

(pg-atoms/l.)

Nitrate

Average characteristics of the waters with the highest supersaturations.*

Bering Strait Inflow Samples with 02 Saturations > 130~

Salinity

Temp.

Table 4.

15 (24)

5 (s)

(~,g-atoms/'l.)

Reactive silicate

o

~o oo

Estimated initial 0 2 c o n c e n t r a t i o n = 8.50 m l / l . (Table 4) x 85 ~o (Table 3) = 7-22 m l / l . Estimated reactive p h o s p h o r u s utilization = 2-30 t~g-atoms/l. (Table 3) -- 0.84 tLg-atoms/l. (Table 4) = 1.46 t~g-atoms/l. Estimated nitrate utilization = 19.2/~g-atoms/l. (Table 3) -- 1.1 tzg-atoms/l. (Table 4) = 18"1 tLg-atoms/l. O~ added b a s e d o n e s t i m a t e d p h o s p h o r u s utilization = 1.46 tzg-atoms/1, x 276 = 403 t~g-atoms/l. = 4"51 m l / l . 0 2 added b a s e d o n e s t i m a t e d nitrate utilization = 18.1 t~g-atoms/l, x 17.2 = 311 tzg-atoms/l. = 3-48 m l / l . Supersaturation b a s e d o n e s t i m a t e d p h o s p h o r u s utilization = (7.22 + 4.51)/(8.50) x 100 = 1 3 8 ~ Supersaturation b a s e d o n e s t i m a t e d nitrate utilization = (7-22 + 3.48)/(8.50) x 100 = 1 2 6 ~

1. 2. 3. 4. 5. 6. 7.

REDFIELD,KETCHUM a n d RICHARDS (1963) were used.

Colder Waters

B.

*To obtain these values t h e ratios AO : A N : A P = 276:16:1 suggested by

Bering Strait Inflow

Estimated initial O~ c o n c e n t r a t i o n = 7.33 m l / l . (Table 4) x 93 ~ (Table 3) = 6.83 ml/l. Estimated reactive p h o s p h o r u s utilization = 2-06 t~g-atoms/l. (Table 3) -- 0"75 ~ g - a t o m s / 1 . (Table 4) = 1-31 t~g-atoms/l. Estimated nitrate utilization = 18-5 t~g-atoms/l. (Table 3) -- 0"5 t~g-atoms/l. (Table 4) = 18"0 t~g-atoms/l. 0 2 a d d e d b a s e d o n e s t i m a t e d p h o s p h o r u s utilization = 1.31 t , g - a t o m s / l . × 276 = 362 t~g-atoms/l. = 4.05 m l / l . 0 2 a d d e d b a s e d o n estimated nitrate utilization = 18.0 ~ g - a t o m s / l . × 17-2 = 310 t~g-atoms/l. = 3.47 m l / l . Supersaturation b a s e d o n e s t i m a t e d p h o s p h o r u s utilization = (6-83 + 4.05)/(7.33) x 100 = 148 Supersaturation b a s e d o n e s t i m a t e d nitrate utilization = (6.83 q- 3"47)/(7.33) x 100 = 141

1. 2. 3. 4. 5. 6. 7.

Calculated supersaturations resulting from photosynthesis based on estimated reactive phosphorus and nitrate utilizations.*

A.

Table 5.

~D

L~

O"

O

350

Shorter Contribution

oxygen concentrations might be expected downstream from the Strait. In 1963, for example (Fig. 3), the highest saturations in the warm irtfiowing waters were a short distance north o f the Strait, and homogeneous layers with supersaturations greater than 1 2 0 ~ never exceeded 20 m in thickness. These high concentrations were frequently in the surface layers (Fig. 5, Table 2) and therefore reflect intense photosynthesis, since surface waters should lose excess oxygen rapidly. The highest values were in the cold, relatively saline waters west o f Bering Strait. All but two of the values of 1 3 0 ~ or more observed in these waters were in stratified layers overlain by pynoclines (Fig. 5, Table 2). (The two exceptions were in the surface layer.) Since the stability reduces vertical eddy diffusion and hydrostatic pressure increases oxygen solubility, these conditions favor the retention of oxygen produced in situ in thin layers and retard the erosion of maxima so produced. A n important factor contributing to the high oxygen saturation values appears to be the common occurrence, below the surface layers, of nutrient-rich waters that are nearly saturated with oxygen (Table 3). Apparently these waters were in contact with the atmosphere during winter when respiratory processes dominate, and their nutrient contents were increased to high levels while oxygen was maintained at near-equilibrium concentrations. I f such waters come into the photic zone during the warmer months, photosynthesis could produce additional oxygen and result in supersaturation values This mechanism seems most applicable to the colder waters. However, it may also be o f some importance in the Bering Strait inflow because even in surface layers, atmospheric exchange may not be sufficient to saturate the water before the in situ addition of oxygen begins. The above view is supported to some extent by observations from the northern Bering Sea during winter 1968 when waters with high nutrient concentrations and near-equilibrium oxygen values appeared to be forming (K. A. COV~CrRYMAN,personal communication). The changes that photosynthesis would produce were estimated to test the suggestion that photosynthesis was the primary cause o f the high oxygen concentrations. Initial nutrient and oxygen concentrations similar to some selected samples with high nutrient concentrations and near-saturation oxygen values were assumed (Table 3). Final micronutrient concentrations were based on values from samples with the highest oxygen concentrations (Table 4), and the atomic ratios of change during photosynthesis of AO: AN: AP = 276:16:1 (REDFIELD,KETCHUM and RICHAROS, 1963) were used. The estimates (Table 5) indicate that photosynthesis could cause the supersaturations observed in the Bering Strait inflow water without exhausting the available nutrients. The final nutrient concentrations selected for the colder waters were averages for samples with oxygen saturations of 140 ~o or more, so the calculated values of 138 and 126% seem to be low. Nevertheless, it is clear that photosynthesis can cause appreciable supersaturations in these waters without exhausting the available nutrients. Other mechanisms that suggest themselves cannot account for the observed supersaturation values. Mixing o f equal volumes of the extreme water types entering the area woutd cause supersaturation values o f 102% or less. Positive deviations in meart monthly barometric pressures seldom exceed 1% (U.S. Navy Hydrographic Office, 1954), and even if a warming o f 3°C is assumed, in situ heating would produce supersaturations of only about 108 %. Because gas chromatographic oxygen data were rejected when corresponding nitrogen saturations were < 90 or 110 %,* supersaturations caused by physical processes would tend to be eliminated from these data. However, even when all nitrogen saturation values were considered, significant correlations between the higher oxygen saturations and their corresponding nitrogen saturations were not evident. Only four nitrogen saturation values exceeded 120%. Although oxygen supersaturations have been attributed to an ice cover that prevents photosynthetically produced oxygen from escaping into the atmosphere (Swnxmve, 1929), only a few observations in these data exhibited a correlation between ice cover and high oxygen values. This mechanism should be more important in early summer and early winter when art extensive but relatively thin ice cover may be present. Acknowledgements--The assistance of many colleagues was necessary to complete this study successfully and is most gratefully acknowledged. We are especially indebted to J. BELIER, W. H. GLADFELTER and D r . . l . W . SWINr,mRTON. *The gas chromatographic method of SWn,a,,W.RTON, ~ N n O M and CHEEK (1962) was used exclusively for oxygen determination o n the 1963 and 1966 Northwind cruises and was the routine method employed during the 1964 Burton Island survey. Because dissolved nitrogen concentrations are generally close to surface equilibrium values, large departures from these values were considered to indicate unreliable results.

Shorter Contribution

351

This research was supported by a variety of sources, including the U.S. Naval Oceanographic Office, the U.S. Coast Guard, the Arctic Institute of North America, and the Office of Naval Research under Contract Nonr-477 (37). REFERENCES

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