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Indian Journal of Marine Sciences Vol. 9, December 1980, pp. 240-245

Chemical Oceanography of the Arabian Sea: Part V-Hydrochemical Characteristics Off the Central West Coast of India

R SEN GUPTA, ANALIA BRAGANCA, R J NORONHA &S Y S SINGBAL

National Institute of Oceanography, Dona Paula, Goa 403004 Received 12 June 1980; revised received 21 August 1980

Data from Arabian Sea cruises 46 (Dec. 1978) and 59 (Oct. 1979) of R V Gaveshani have ~en used to calculate rates of denitrification at intermediate depths (72-1200 mi. About 34/0 of the available nitrate-nitrogen is depleted by denitrification.

The 'standing crop' of denitrified nitrogen is 70 g m-2Combining the rates of denitrification at an intermediate depth with the photosynthetic productivity at the euphotic zone, the mean residence time of the watermass is between 13 and 54 yr.

Fig. 1--Network of observation stations during 46 cruise of R V Gavc.I'hani

2 oxygen minima, stations on a transect along 14°55' N have been studied (Figs 3a and b). In this section, as in s~ction I, surface oxygen increases to 4.5 ml/litre to a distance of about 60 nautical miles offshore. The 2nd oxygen minimum is not discernible at the farthest station .. To examine this feature in more detail, isopycnals have been plotted to characterise different watermasses. Watermasses have been classified by their 0',values according to the criterion laid down for the Arabian Sea by Wooster et al.3.Comparing Figs 3a and 3b, it can be seen that the 2nd oxygen minimum normally appears in the lower limit of the Red Sea watermass, superposed on the Indian Ocean equatorial intermediate water with a higher oxygen concentration. It appears that mixing between these 2 watermasses becomes more and more intense as the Red Sea water flows southwards and gradually gains oxygen.

To check on this characteristic, oxygen con- centrations on a transect along 150N lat. extending from the coast to 68 E long. and worked during cruise 59, have also been studied (Fig. 3c). The presence of the 2nd oxygen minimum extends to about 71CE long. (st 1230). Beyond this, it gets diffused and its presence is hardly identifiable.

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Sen Gupta el af.I have studied denitrification in the Arabian sea, using stoichiometric relations hctween dissolved oxygen and nutrients. In the present paper.

their methods have been adopted to estimate rates of denitrification at 75-1200 m off central west coast of India (71-75' E, 14-16 N). The calculations are based on data collected during cruises 46 (Dec. 1978) and 59 (Oct. 1979) of R V Gal'eshani.

Results and Discussion

Hydrographical characteristics-Data from 3 sections (I, II & III; Fig. I) have been considered for detailed examination of watermasses.

Oxygen concentration in the surface layer increases offshore depending on the depth of the mixed layer (Fig. 2a). There are 2 oxygen minima which appear at stations away from the shore and deeper than 300 m.

The 1st minimum appears at about 125 m and extends down to' 500 m. The magnitude of oxygen concentration at this minimum layer decreases with increasing distance from the shore. The 2nd minimum extends from about 700 to 1400 m. The 2 oxygen minima correspond to 2 maxima in phosphate- phosphorus (Fig. 2b).

To test the southward extension of the waters having Materials and Methods

Water samples were collected from standard hydrographic casts using metallic Nansen reversing bottles up to a depth of 2000 m from stations (Fig. I).

Temperature, salinity, dissolved oxygen, PO~ --P, N03-N, N02-N and NH~-N, were measured on board. Temperature was read off reversing thermo- meters, salinity was measured by an inductive salinometer and the other parameters were estimated spectrophotometrically using standard analytical techniques2.

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SEN GUPTA et af.:CHEMICAL OCEANOGRAPHY OF ARABIAN SEA: PART V

... (2) ... (I) It had been suggested earlier4 that the 1st oxygen

minimum occurs due to biodegradation of organic matter and the 2nd minimum, wherever present, is mainly due to watermass effect. It now appears that this watermass effect gets weakened due to the increasing intensity of mixing in the 2 overlying watermasses as we proceed further offshore.

To examine further, the above fmdings, values for the same components at the deepest and farthest station in each transect were plotted (Figs 4a-c).

Though all the stations were not located on the same meridian they were assumed to lie on one transect for the purpose of generalisation. The longitudinal distance between the stations at the 2 extremes on this NW-SE transect is 50 nautical miles. However, it has been suggestedS•3 that the Persian Gulf water, and probably, Red Sea water, after reaching their density levels in the Arabian Sea, flow south-easterly almost parallel to and close by the Indian coast. Fig. 4a shows the isopycnals corresponding to different defined watermasses in the Arabian Sea. Depth levels of the isopycnals indicating Persian Gulf and Red Sea watermasses hardly appear to indicate any change from NW to SE, confirming the above suggestion.

However, the North Indian Deep and Bottom watermasses appear to lose depth as they flow northwards. This agrees well with an earlier observation 1.

Isopleths of oxygen (Fig. 4b) show that oxygen c9ncentration in the surface layer relatively increases from NW to SE. The presence of 2 minima can easily be detected to about 15°N, south of which the deeper minimum gradually diffuses into 1 thick layer of low oxygen concentration at an intermediate depth. The same can be observed for phosphate-phosphorus (Fig. 4c) where there is always a sharp maximum corresponding to the 1st oxygen minimum. The 2nd phosphate maximum follows the trend of the 2nd oxygen minimum and gradually gets diffused southwards from 15°N.

Thus, it can be concluded that in the region studied, 2 oxygen minima associated with corresponding maxima in phosphate-phosphorus are present along the offshore region off India at depths> 300m. The 1st oxygen minimum is caused by the biodegradation of organic matter while the deeper one is the result of mixing between watermasses. These phenomena are very prominent at depth levels where watermasses originating in the Persian Gulf and the Red Sea are expected to flow, identified on the basis of the slopes of isopycnals 26.5 and 27.1 respectively.

Oxygen consumption and denitrification- Denitrification was studied by the method of Sen Gupta et al.1.Apparent Oxygen Utilization (AOD) is defined as the difference between the concentration of

oxygen in water at the observed salinity and temperature when in equilibrium with water-saturated.

atmosphere and the observed oxygen concentration in the sample. 'Reserved' fractions of phosphate and nitrate (Prand Nr) are defined as the amounts present in th~ oxygen-saturated water at the surface. just before it starts sinking, and have been calculated as differences between the measured values and the computed oxidative fractions. Oxidative fractions are .obtained by converting AOU values with the oxidative ratios (obtained by the linear least-square regression method between AOU and the measured values of phosphate and nitrate respectively). The oxidative ratios in the Arabian Sea have been calculated for

AAOU/AP04=280 and AAOU/AN03=17 (by

atoms) for this depth intervaP.

As an initial step. amount of nitrate reduction is estimated at stations on the transect along 15°N during both the cruises from the following equation6:

(jN =(N03)"p-(N03)Obs-(N02)obs exp=experimental; obs=observed.

where

(N03)"p =(N03)ox ±(N°3)r ox = oxidative; r = reserved.

Ammonia-nitrogen was not included in the mass balance because of its low concentration in the water column (av. 0.76 jlg-atjlitre).

Applying the above oxidative ratios1, Pr and Nr values in the depth interval of a sharp increase in AOU to the depth where it is maximum can be calculated.

Using the values for the slope and the negative intercept obtained from the relation between these Nr and p ••reserved fractions of nitrate-nitrogen have been computed and used jn Eq. 2.

Combining the computed (N03)exp values with the measured values of nitrate and nitrite in Eq. I and 2 nitrate deficits at the stations have been calculated.

The range of denitrification in this region of the Arabian Sea extends approximately from 75 m to 1200m (Fig. 5). It shows 2 maxima of (jN which correspond well with the depths of oxygen minima described earlier. It has been calculated from Fig. 5 that the 'standing crop' of denitrified nitrogen in the Arabian Sea is 70 g m -2. This value is about an order higher than that reported by Deuseret al.7who applied the relation between nitrate and salinity obtained in the core of the Persian Gulf water at 2 stations, separated by 750 nautical miles, to calculate the nitrate deficit, instead of the stoichiometric relations.

The average of all values of (jN at all depths is 4.72 jlg-atjlitre. The area of the Arabian Sea (lat. 0-25°N and long. 50°-75°E) is 6.225 x 106 km2 (ref. 8). The volume between 75 and 1200m, the depth range for denitrification, will be about 7 x 106km3. Extending

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SEN GUPTA et al.: CHEMICAL OCEANOGRAPHY OF ARABIAN SEA: PART V

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Fig. 5-Reconstructed nitrate-anomaly calculated from depthwise average values at the stations in Fig. I

the average values to cover the entire Arabian Sea, it can be calculated that at any time 462 x106tonnes of denitrified nitrogen will be present at this depth range.

It has been assumed that stoichiometrically a maximum of 14 tlg-at/litre ofnitrate-N can be reduced during denitrification9. This amount of nitrate-N would go to accumulate 2.5 tlg-at/litre of ammonia6.

The amount of denitrified nitrogen, 4.72 tlg-atjlitre, will, thus, accumulate 0.86 tlg-atjlitre of ammonia. The average concentration of ammonia in the depth interval studied is 0.76 tlg-atjlitre (0 to 2.8 tlg-at/litre).

Thus the process of denitrification in the Arabian Sea

"is present at the intermediate stage of ammonia without proceeding to its completion. Based on the above values and relations, it can be calculated that about 34% of the available nitrate can be lost due to denitrification at this intermediate depth of the Arabian Sea. Earlier calculations1o,11 gave figures for denitrification in the Arabian Sea as 41-45%. The corresponding range for the Bay of BengaPO,12is 30- 40%. It can, therefore, be concluded that about 1/3rd of the available nitrate-N in the seas around India is lost due to the process of denitrification at an intermediate depth interval.

Total photosynthetic production in the Arabian Sea has been calculated as 1064 x 106 tonnes C yr-1 (ref. 8). It has been observed that about 10% of the photosynthetic productivitv getS below the euphotic zone13 and about 1%of the total production should result in' denitrified nitrogen7. Thus, at any time an amount of 462 x106tonnes of denitrified nitrogen can

be expected to be available between 75 and 1200m in the Arabian Sea. Assuming that about I% of total photosynthetic productivity ends up as denitrified nitrogen, it can be calculated that about 43 yr (mean residence time) would be needed to accumulate this amount. Calculating from the data given by Qasim8 the rate of photosynthetic productivity in the euphotic zone of the Arabian Sea comes to vary from 90 to 360 gC m -2 yr -1. With the assumption that about 1% of this productivity would be equal to the amount of denitrified nitrogen below the euphotic zone, about 0.9-3.6 gN m -2 yr -1 would be added to the pool of denitrified nitrogen. Following Deuser et al.7 this would give a mean residence time varying from 13to 54 yr, A residence time of 30 yr has been suggested by Hartmann et al.14 based on the data on the rate of outflow from the Persian Gulf, which is of the order of 3 x 103 km3 yr -1. Deuser et al.7 obtained a mean residence time for the watermass between 300 and 1500m in the Arabian Sea varying from 3 to 30 yr.

Considering the limitations and uncertainties inherent in such calculation, this agreement is quite good.

References

I Sen Gupta R, Rajagopal M D&Qasim S Z, Indian J mar Sci, 5 (1976) 201.

2 GrassholT K, Methods of seawater analysis (Verlag Chemie, Weinheim) 1976, 317pp.

3 Wooster W S, Schaefer M B& Robinson M K, Institute of Marine Resources, Calif, ref 67-12, 1967 (mimeo).

4 Sen Gupta R, Sankaranarayanan V N, De Sousa S N &

Fondekar S P, Indian J mar Sci, 5 (1976) 58.

5 Rochford D J, Aust J Mar Freshwat Res, 15 (1964) I.

6 Cline J D&Richards F A, LimnolOceanogr, 17 (1972) 885.

7 Deuser W G, Ross E H &Mlodzinska Z J, Deep-Sea Res, 25 (1978) 431.

8 Qasim S Z, Indian Jmar Sci, 6 (1977) 122.

9 Richards F A, in Chemical Oceanography, Vol. I, edited by J.P.

Riley and G. Skirrow(Academic Press, London) 1965,611.

10 Sen Gupta R, De Sousa S N &Joseph T, Indian J mar Sci, 6 (1977) 102.

II Sen Gupta R, Moraes C, Kureishy T W, Sankaranarayanan V N, Jana T K, ,Naqvi S W A&Rajagopal M D, Indian J mar Sci, 8 (1979) 215.

12 Naqvi S W A, De Sousa S N&Reddy C V G, Indian J mar Sci, 7 (1978) 15.

13 Deuser W G, Deep-Sea Res, 18 (1971) 995.

14 Hartmann M, Lange H, Seibold E & Walger E, METEOR Forschungsergebnisse, C4 (1971) 76pp.

Acknowledgement

The authors are grateful to Dr S.Z. Qasim for his valued comments. Help rendered by Captain R, Varma and his officers and men of R V Gaveshani during shipboard work and Mr C.V.G. Reddy in the preparation of the manuscript is deeply appreciated.

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