Indian Journal of Marine SciencesVol. 5, June 1976,pp. 58-71

,I

Chemical Oceanography of the Arabian Sea: Part III- Studies on"Nutrient Fraction & Stoichiometric Relationships in the Northern &

. the Eastern Basins

R. SEN GUPTA, V.,N. SANKARANARAYANAN, S. N. :DE SOUSA & S. P. FONDEKAR

National Institute of Oceanography, Dona Paula 403004

Received 27 November 1975; revised received 23 March 1976

Phosphorus and inorganic nitrogen compounds have been divided into 'reserved' andoxidative fractions based on their relationships with apparent oxy~en utilization in the northernand the north-eastern basins of the Arabian Sea. Two oxygen minima are observed in thewater masses north of 200N latitude; The reserved nitrogen compound is negative in the deeperW\atersand this varies from .north to south. This deficit indicates a possible loss of nitrate togaseous nitnlgen through denitrification. Spatial variation of oxidative ratios' indicates changes

iq the rates of nutrient regeneration associated mainly with regional variations in photosyntheticproductivity. A stoichiometric relationship based on the ratios of change of nutrients in waterabd their ratios' of concentration in plankton indicates that organisms in the Arabian Sea may

'h~ve an atomic relation for' C:N:P of 105:15:~. Applying the oxidative ratios the loss of nitratethrough denitrification has been calculated to be about 2/3 of its total availability. Similarityof' conditions between this area and the eastern tropical North Pacific suggests denitrificationa~ a chara~teristic 011 the easter;ll sides of oceans which are poorly ventilated in the north.

LARG-E volume of chemical oceanographic data" . has been collected during the InternationalIndian Ocean Expedition 1960-65. Literature

records of studies on various· chemical parametersin the Arabian Sea are now available1-6. Almostall these studies are concerned with the distributiveaspects of di:fferel).t c!:temica.l cOIstituents. Noanalysis has so far been made on the quantitativerelations between nurtients and oxygen. Similar'studies have been carded out in other oceanic areasand semi-enclosed basins7-16. In this report anattempt Ihas been made to examine approximaterelationships between phosphorus, inorganic nitrogencompoun!ds and dissolved oxygen based on the data

collecte~ on board INS Darshak during its cruises(1973-74~1to the northern and north-eastern basinsof the Arabian Sea (Fig. 1). These relationshipscan be usefully applied to calculate the rate ofoxygen consumption by organic material duringtheir de<;omposition in studies of coastal pollutionand in areas with poor water exchange.Materials and Methods

Salinity, temperature, dissolved oxygen, inorganicphosphate, total phosphorus, nitrate-nitrogen,nitrite-ni~rogen, ammonia-nitrogen, pH and total,alkalinity were measured for water samples collectedfrom standard depths from hydrographic castsusing transparent TPN water samplers (type HydroBios, Kiel, West Germany). All the analyses,,except total phosphorus, were carried out on boardimmediately after the collection of samples follow­ing stannard methodsl7-25.

Plankton - Plankton samples were collected andanalysedl by a procedure described by Sen Guptaand Koroleffl6. In the beginning nitrogen was

58

analysed applying the micro-kjeldahl technique,but when chemicals 0.£ required purity were avail­able it ~s estimated by the same method as, des­cribed in the reference.

The samples were analysed by different analystson board and in the laboratory and as such errors,due to personal bias, in them cannot be uniform;:

.Hydrographic Featu.resThe Arabian Sea, exclusive of the Gulfs of Aden

. and Oman, occupies an area 'of about 7·5 X 106km2 and forms the north-western part of the IndianOcean. For the study of the oceanographic featuresSchoU26 bounded the Arabian Sea as follows: "thesouthern boundary runs from the Indian coastnear Goa, along the western side of the Laccadiveislands to the equator; thence it trends slightlysouth to the point on the east African coast nearMombasa, at approximately 50S latitude". TheArabian Sea is an area of negative water balance.Annual excess of evaporation over precipitationand runoff varies between 7 and 10 km3 with twomaxima during the north-east and the south-westmonsoons27. Due to the influence of surroundinglandmass the NE monsoon is quite weak in thisarea, while the SW monsoon is very intense withwind speeds reaching a maximum of 25-30 knot,causing strong upwelling along the coast of Somaliaon its western side and a comparatively weakerupwelling along the Indian coast on its eastern side.The water mass in the Arabian Sea is regulated bythe out-flowing high saline waters from the Per­sian Gulf through the 50 m deep sill at the HormuzStrait and from the Red Sea through the 125 mdeep sill at the Strait of Bab-el-Mandeb. Its watermasses are composed of a high saline water at the

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

6it~RIA N

ARABIAN SEA(NORTH EASTERN PORTION)

OCEANOGRAPHIC EXPEDITION INS DARSHAl< 1973-1974

Natural leal. -1:1,500,000 (at lat. 21 )0'1

Projection _ MtrCltor

-.- - .•.,--. --.

Fig. 1 - Area of observations in the AI;abi:l.n Sea (north-eastern portion) during o~eanographic expedition of INSDarshak (1913-74)

surface layers, called the Arabian Sea surface \-yatu,an intermediate layer consisting of both PersianGulf and Red Sea waters, and deep and bottcmwaters originating in the equatorial regie,n of theIndian Ocean and in the Antarctic circt:'mpolarwaters respectivelyl.

Because of high temperature in the surface layersand low oxygen contents in the intermediate watersof the equatorial Indian Ocean the oxygen Concen­trations are comparatively lower in the ArabianSea. There is always a minimum of oxygen at· anintermediate depth and north of 18cN latih:de 2minima can be detected (Fig. 2).

To illustrate this point further, distribution ofoxygen is shown along a transverse section in theNW-SE direction between 19°00'N and 24aCO'N

(section 1, Fig. 1) and another section nOlmalto the coast along 17°30'N (section 2, Fig. 1).It can be seen from Fig. 3 that there are 2 oxygenminima, the first one between 350 and 600 m ar:dthe other between 850 and 1100 m, the depths ofboth of which increase southwards. Decrease inoxygen concentration at the first minimum layeralso shows a southward .intensification.

Fig. 4 almost confirms the above statements,The second oxygen minimum is absent and thedepth of the oxygen minimum layer, with valueslower than those in Fig. 3 and appearing almostat the shelf-break, extends from about 200 Ill" to1100 m.

We shall, however, make one reservation here.It has been observed14,that at concentrations below

25 !lg-atjlitIe (•.•.•.0·3 mljJitre) Winkler oxygen values.are cc.nsishntly high by ,8-10 !lg-atjlitre (.....,0·1mljlitre). Eut due to practical difficulties thecolorirr.etric oxygen method could not te applied.Therefore, pres.ent kw oxygen values' might teslightly higher, but then the accuracy' of the Winklermethcd is no tetter than 0,05 mljlitre.

Two distinct features of the oxygen minimumzone are the apparent deficits in nitrate and theunusually high concentrations of nitrite. Nitrateion presumably appears in this layer as an inter­mediate pre duct of the process leading to free nitro­gen (denitrification). A secondary nitrite maximumhas, also, been observed occasiorally at the deeperoxygen minimum. Since nitrite concentrations arehigher than those at the upper oxygen minimumwith increasing nitIate ccnuntraiicns, nitrite inthis layer is suggested to te an inte] mediate productof the reverse precess, i.e. nitrifieatic·n28•

Photosynthetic prcductivity in the Arabian Sea,is higher nan that in other oceanic areas29-31. Ahigh concentraticn of organic matter, resultingfrQ1nhigh photosynthetic prcductivity at the surfacerwhen gets accumulated deeper down, will depletethe oxygen concentration in that layer becauseof decay and degradation. This and also the slug­gish water circulation at an intermediate depthmay sometimes produce near-anoxic or sub-oxicconditions at that level, which, in the Arabian Sear.has been termed as an 'azoic zone'32. Verticalmixing processes from the oxygen-rich deep andbottom waters in the Arabian Sea do not appear

59

•

",

and the apparent oxygen utilization (AOU=0~-02'where O2 is the observed oxygen conc. and 0; isthe conc. of dissolved oxygen in water, having theobserved salinity and temperature when in equili­brium with a water-saturated atmosphere) givesalmost the same ratios. Redfield et al.7 have con­cluded that concurrent changes in the concentrationsof carbon, nitrogen and phosphorus in water wouldbe almost equal to their ratios of concentration inplankton.

We have divided the water masses of the ArabianSea chemically into 2 layers, an upper layer be­tween the surface and 125 m and a lower layer from125 m to the depths of the maximum AOU values.The upper layer, where photosynthetic producti­vity takes place, is well oxygenated down to thethermocline depth. At the thermal discontinuitylayer there is always a sharp decrease in oxygenconcentration and at about 125 m values as lowas 0·2 mlflitre O2 have often been observed. There­fore, it is apparent that oxidation processes startalmost at or just below the thermocline and therespiration rates may reach a pretty high intensityalmost immediately. The depth of the low­oxygenated layer stretches down to about 1000­1200 m. Possibility of advection of this oxygen­poor water upwards resulting in decrease in oxygenconcentration cannot be ruled out also.

The lower layer has been selected following theaccepted ideas about the oxidation of organicmatter in the oceans7•8• To start with we havepooled together values from all stations in the area

2500

Fig. 2 - Dissolved oxygen in fJ.g-at/litre versus depth-latitudinal variation

Soo

60

INDIAN J. MAR. 8C1., VOL. 5, JUNE 1976

t7°26'tN,70s6E t7"SS'7N,7001E tlhO'SN,d40'2E t9"00'SN.6!f.27E24°0(N,S9"4SE ).J.g-atflo 50 100 150200250 0 50 too ISO200250 300 0 50 100150200250300350 0 50 100150 0 50 100 150200250300350 400

tOO- ..

""'~'=,,±.'.,.,1.", .m

Nutrient Fractions in Relation to Oxygen Utilization

Phosphate (expressions phosphate, nitrate, nitriteand ammonia used in this paper stand for phos­phate-phosphorus, nitrate-nitrogen, nitrite-nitrogen,and ammonia-nitrogen respectively) present inoxygen-saturated water at the surface, just beforeit starts sinking, has been termed as 'preformed'phosphate by Redfield33. Because of practicaldifficulties in ascertaining 'preformed' phosphatein a water mass Sugiura34 introduced the term<reserved' which is the phosphate concentrationobtained byl subtracting phosphate of oxidativeorigin from the total inorganic phosphate. Inthis study we have used the term 'reserved' in :theabove sense.

Redfield et al.7 worked out the relationship be­tween phosphorus, nitrogen and carbon on thebasis of their concentrations in plankton. Theatomic ratios in organisms, obtained by them,for carbon, nitrogen and phosphorus are 106:16:1.The relationship between oxygen consumption and{;arbon dioxide production as calculated by them is276:106 by' atoms. These relations are obtainedfrom an examination of their composition inplankton, of water masses and theoretical considera­tions. The relationship in water obtained from theslope of the regression line between the nutrients

to be sufficiently strong to compensate for thisdeficiency in oxygen.

Results aJd Discussion

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

-.•.......... -------- ....-

o•i

o8.,..8.,...,..

----.--- ...----.--------,.., ..... -•....-.•.~....

>50

50-------------/",.-. < 50 ------~----.-­.'

-.SE

ARABIAN SEATRANSVERSE SECTION

(24°00N 59°45E TO 1906N 72'23E)

I.N.S;DARSHAK

OXYGEN-»g- at!l

ooI.I'l

oooN

ooI.I'lN

ooenN

,,,,", •• ,_--....------250---------.--

\ .-\ /, --\ ", /\ ,"',;'. /

~ •••••• <i'------ ...--

SCAlE:-32 MILES~

tem

Fig. 3 - Distribution of oxygen across a transverse section in NEXSE direction (section 1. Fig. 1)

61

INDIAN J. MAR. 5C1., VOL. 5, JUNE 1976

w- --E

102S40

ARABIAN SEALATITUDINAl SECTION

. I ALONG 17'30N ) .

I.N·S DARSHAK

.OXYGEN-'ug,. at/I

160 130 100 85 70 550151 ANCE FROM COAST (NAUTICAL MILES)

Fig. 4 - Distribution of oxygen along 17°30'N latitude in an east-west section

,,S600

......... -- .............•...,' ", .•...•---------

I f""'" 2 5 ,/'

. '.. /, ,...... ,,'

" •... "''' .., ,,.,,,'000 ~ "..•--'

I "50'.

I' : -.-1500

2

of observation assuming that regional variations

in thel oxidation intensity may not be very signi­ficant from the viewpoint of totality.Oxygen content in the surface waters of the

area studied varies from 10% super~aturation to10% undersaturation. This will cause errors inthe AOU values (as calculated from the nomogramof Gr~en and Carritt35) but the errors are ~mallcompared to the variations of AOU. Sinking ofsurfac~ waters in oceans usually takes place duringthe winter months resulting in ~eme"vl~at super­saturation of oxygen in the upper few metres. Butin thel equatorial and the tropical regions of theoceans' where the winter is always very mild and

where f0ften evaporation exceeds precipitatic.n andrunoff, it is not yet clearly understood to whatextentl this may influence the sub-surface oxygenconcentrations. However, in the pre~ent calcula­tions those values of nutrients where the surfacelayers were calculated to be super~aturated withoxygen are omitted.

Fig. is shows the relationship between tl:.e apparentoxygen utilization and inorganic pho~ptate in theupper Ilayer at all the stations. The slepe of theregression line gives a ratio of 286: 1 by atoms.The relation between nitrate and phosphate, calculat­ed in the same way, gives an atomic relation of 6·4:1between them (Fig. 6). Combining ~AOU :~p inFig. 5 with ~N:~P in Fig. 6 ~AOU:~N as 45:1 byatoms is obtained.

In marine environment inorganic nitrogen com­pounds may exist' in their oxidizing or reducingforms. The general cycle is N03~N02~NH3'The occurrence and predominance of different forms

depend on the stage of the prccess.·But phosphateoccurs mostly as· orthophospbate,- the highest oxida­tion state of pr.o~pl:orus compounds. Surplusphospl:ate together with the addition by advectionfrom the deeper water will cause an exce~s of thiscomronent in the upper layer. Deficiency inoxygen will cause He nitrcgen cempounds to under­go reduction, in deep waters. Moreover, additionof nitrate by advection to this layer is not verysignificant due to its low concentrations in thelayers just below. Thus it appears that the rela·­tion ~N :~P=6'4:1 atoms is the result ofthe presenceof excess phospbate and con~equently ~AOU :~N =45:1 atoms will not be repre~entative of the actualnitrate produced during the respiration processes.Moreover, zero or near-zero value~ of AOU at or nearthe mrface, cau~ed by the atmospheric exchange,.will alw affect the ratio.

Fig. 7 shows the relaticmhip tetween apI=arentoxygen utilization and inorganic phosphate at allstations frem the deeper layers. The slope of theregression line gives a ratio of oxygen consumptionto phospbate regeneration of 270:1 by atoms.From Fig. 8 we get ~N :~P ratio in deep waters of17:1 by atoms. Combination of these 2 ratiosgives an atomic relation of 16:1 for ~AOU:AN.For reasons mentioned above we decided to applythese relationships to examine the oxidation of

. organic matter in both the layers as, we believe,that would give -a more realistic picture of the realstat~ of affairs.

. HFollowing Sagi13 -who iritrodu~ed 'the concept'reserved nitrate' in the same way as 'reserved

phosphate' by Sugiura34 in his observations in the

62

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

6AQU

4001- ~=2a6otoms

FIG. :>

3

"

2

­.

• I

7

FIG. 6

6

9

5

8

3,5 03­

P04""P L.rJ-ot/t

3

'.

....

..

....

- ..-

.••,

o

100

200

~oo15I

~

Fig. 5 - Correlation between phosphate-phosphorus andapparent oxygen utilization in the upper layers (0-125 m)

Fig. 6 - Correla.tion between nitrate-nitrogen and phos­phate-phosphorus in the upper layers (0-125 m)

Pacific Ocean, Sen Gupta and, Koroleff16could studythe deficit of nitrate and denitrification in theoxygen-poor waters of the Baltic Sea. The sameanalysis is applied here in an attempt to studynitrification and/or eventual denitrificatiun in theoxygen-poor layers of the Arabian Sea. However,in the case of the nitrogen compounds nitrite andammonia must be considered.

The total phosphorus and' inorganic nitrogencompounds have been separated into their consti­tuent fractions according to Redfield et aU andthese are presented in Table 1.

To test constancy and validity of the ~AOU :~Pratio in the depth interval 150 m-AOU maximumTeserved phosphate (PrY was calculate a from' the,relations outlined in' Table 1 and plotted again:o:tsaEnity14, for allthe stations (Fig. 9). The relatidnbetween potential temperature and salinity at thisdepth interval is' linear. But Fig. 9 indicatesthree distinctly separate areas where individuallyPr is approximately linear 'with salinity. VaryingPr values will result from differen( rates of mixingand slower or faster rates of oxidation of organicmatter. Inorganic phosphitte that can be liberatedduring nitrate reduction is estimated to be nogreater than 0·17 [Lg-atflitre, assuming that the,ratio is ~N03: ~P04=94'5 (ref.' 9) during denitri­fication and that on average 16 [Lg-atllitre of nitrate-ion has,been reduced. No:attempt has' beel] made to~orrect for this contribution to'the phosphate pool.

TABLE 1 - DEFINITION OF VARIOUS FRACTIONS OFPHOSPHORUS AND NITROGEN

Fraction PhosphorusNitrogen

Total

AnalysisafterAnalysisafteroxi-oxidation (Pt)

dation (Nt)Inorganic

Direct analysisDirectanalysisforNO.

+(NO.+NH3)Oxidative

POl<= AOU/270NOl<= AOU/16Reserved

Pr=POi- P-POl<N~ == NO'-Nol<Organic +

pt-POi- PNt-(NO; + NO;;+particulate

NH3)

Following the demarcati.ons· obtained in Fig. 9the total area of observations was divided as(A) between 20°00' and 24°30'N west of. 66°30'Elongitude; (B) between the same latitudes east of66°30'E; and (C)between 17°30' and 20000'N latitudenormal to the Indian coast, and are marked in Fig. 1.Averages of values of all the variables yonsideredhave been calculated for the areas separately andare presented in Table 2.

Nutrients of oxiqative origin are usually thosefractions which have been regenerated in the w~terfrom the decomposition of organic and detntalmaterial and which are dir~ctly proportional to thedepletion of dissolved oxygen. The oxidative frac­tion can thus be calculated from the apparent oxygenutilization and the ~AOU :AP ratio.

03

lNDIAN J. MAR. set, VOL. 5, JUNE 1976 <

4

.................

., • 4

- ..... -.~.~.'

.................. --- ...-'

.. ,- .

<..-o,Ol

~ 2Q.

"

~'<tc..

3

o 100 200 300 400AOU ..ltg-at/'

soo 600

Fig. 7 - Correlation between phosphate-phosphorus and apparent oxygen utilization in the deeper layers (from 150 mto AOU maximum)

TABLE 2 - AVERAGE VALUES OF VARIABLES CONSIDERED

C* 125 to900 m

2·582·690·11 (4):1:7·081·561·2489

1·272·260·99 (45):1:3·050610·85

78

Area surfaceto 125 m

A* 125 to Area surface B* 125 to Area surface900 m to 125 m 900 m to 125 m

2·9 1·55 3·4 0·822·44 2·76 4·03 1·760,54 (16)t 1·21 (44)t 0,63 (16):1: 0,94 (5m7·62 2·07 9·28 0·910·48 0·38 0·99 0,40·85 0·51 0·42 1·22

46 42 459 480'64 0·96 0·81(0'17-1-59)§ (0'15-2-67)§ (0'11+34)§

*Area: A-West of 66°30'E; B-East of 66°30'E and North of 20030'N; and C-20030'N to 17°30'N.turlit gC m-a day·., and measured from surface to maximum 75 m.tFigures'denote percentages of organic P in relation to total P.§Figures are ranges of values over the areas.

Parameter [Lg-at{litre

POi-PTotal POrganic + particulate PNOs-NNO.-NNH~-NAOUProductivityt

.The different componehts cf nutrients help inunderstanding their influence on water maSEesmore clearly than their total inorganic concentra­

tions. IMoreover, for a quantitative study of theeffecti~e nutrient flux, the fractionation can l:;e_expected to give a better picture than that of theoverall nutrient transfer. Based on the ratiosgiven ill Table 1, various phosphorus fractions forthe Zllayers have been calculated applying theoxidative ratio to the average values of the relatedvariables and are presented in Fig. 10. An exampleof the calculations for both the layers in area (A)is presented in Table 3.

It can be seen from Table 3 that if all the phos­phorus were released from the oxidation of organicmatter, the oxygen saturation values to be expect-

ed would be 32 and -47% for the upper and the­def;per layers respectively. This is evidE.ntly notthe case, for both the layers snm to be E.nrichedby nutrients as a result of turbulE.nce, diffusicn andother physical procesEes. In the upper layer, theexcess of oxygen remaining aftu the oxidation oforganic matter could be the result of the limitingquantities of phosphorus or organic matter presentor it may be the effect of averaging of the valuesfrom all the stations. It is sef;n that in the upperlayer, oxygen is available in excess, while in thedeeper layers more oxygen is required, than isavailable, to complete the oxidation process.Similar condition is obseryed in the eastern tro­pical North Pacific also; whereas in the Atlanticoxygen is always available in large excess even

64

SEN GUPTA et at.: CHEMICAL OCEANOGRAPHY OF THE ARABIAN SEA: PART III

observed occasionally at around the same depthinterval ranging from 2·02 to 3·22 fLg-atflitre.

This raises the question concerning the amountof reduction of nitrate beyond nitrite, presumablyto gaseous nitrogen, and the rates of these pro­cesses.

The amount of nitrate reduction is estimatedfrom the following equations:

'ON = (N03)exp-(N03)obs-(N02)Obs ... (1)

where (N03)exp is the amount of nitrate expectedfrom both the oxidative and the reserved sources,which represents the concentrations expected tobe present before the onset of nitrate reduction andhas been calculated from the AOU nitrate and phos­phate relationships established earlier. The otherterms on the right hand side of Eq. (1) are observedquantities. Because of its low concentrationthroughout the water column « 1 fLg-atjlitre)except the near-surface layer, ammonia-nitrogenis not included in the calculation, assuming that theerror due to this would be negligible.

The first term on the right hand side of Eq. (1)for nitrate-anomaly is:

(N03)exp = (NO;;)ox± (N03)r ... (2)where (NO;;)ox has been calculated from the relationin Table 1.

Applying Eqs. (1) and (2) attempt has beenmade to study the nitrate deficit in the low-

oxygenated waters of the 3 areas using the averagevalues of nitrate and AOU (Fig. 11). Two maximain the anomaly are observed from the values fromareas A and B which coincide well with the oxygenminima. At the :first maximum in the anomaly ahigher value is noted in area B which might havebeen the result of higher quantity of organic matterin the area. It appears that the anomalies are dueto low oxygen values and are not quite related tothe nitrite maxima. This is evident from the valuesfrom area C where oxygen concentrations have beenobserved to be the lowest resulting in higher valuesin the nitrate anomaly.

Nitrate lost due to denitrification can now becalculated. The average of all the anomaly valuescomes to about 16 fl-g-atflitre while the averagenitrate concentraticn is about 8 fl-g-atflitre. Thus,it can be expected that on an average two-thirdsof the nitrate is lost due to denitrification in thearea of observations increasing in its magnitudefrom north to south.

Stoichiometric Relations hips

Redfields observed that the ratios of change incarbon, nitrogen and phosphorus in sea water wereapproximately similar to the ratios of concentrationof these elements in plankton, Redfield's C:N:Prelation, 140:20:1 by atoms, when corrected forsalt error in the analytical method for phosphorus38

SN ..Ltg-at/lII 2420

-;~ ....,...,,.' '0•." -'., ~"-

~q,

~...

.~

15

,/,,/,,

",,,,,-,(IIIfI

A

o100

tm

600

1100

2600

2100

1600

Fig. 11 - Nitrate deficiency with depth in areas A. Band C (Fig. 1 - clarification in the text) calculated from -theaverage values applying Equation 1

67

INDIAN J. MAR. seL, VOL. 5, JUNE 1976

... (4)

... (5)

Composition NPN: pa

NATURAL Mainly

phyto-0,890,0811: 1plankton Mainly

zoo-3-620'2912'5: 1plankton

Mainly

phyto-2-320·1812·9: 1plankton

1·240·112'4: 11'21

0,0815·0: 1z.86

0'2411'9: 1

do

1'440'1212,0: 10·92

0'0811'5: 11'55

0·1212'9: 1

do

86·254'5119,9: 177-50

3'9319,7: 1

CULTURE Chlorella sppt

29-631·9315,3: 1T etraselmis spp t

37'751·2829,4: 1T etraselmis gra-.

50·252'7718,0: 1cilist

Artemia

spp27539'1$7·0: 1(Californian strain) §

Artemia

spp452·2619'9: 1(Indian strain) §

452·2619'9: 1

*Values in fLg-at(g dry matter.tValues in fLg-atf45 m!.tBy courtesy of Mr V. P. Devassy.§Dr Joseph Royan of Biological Oceanography Division,NIO.aN: P average for natural, 13'8: 1; for culture, 18'2: 1

and for all values 15'4: 1.

hUg. 1975t

Aug. 1975

July 1975t

24 July

4 March 1975*

TABLE 4-ANALYSIS. OF PLANKTON (NATURAL AND CULTURE)

Da.te

18 March 1,975*

17 March 1975*

n(CH20) +n02=nC02+nH20

15NH3+30 °2=15HN03+15H20Adding, the overall reaction will be

(CH20)••(NH3)lSH3P04+ (n+ 30)02

=nC02+15HN03+H3P04+(n+15)H20 ... (6)It is calculated that the atomic ratio of consump­tion of oxygen to phosphorus regeneration in wateris 270 : 1. This would give(n+30)02 =2700=13502 or n=105 ... (7)

Fig. 12 shows the relation between CO2 and oxida­tion equivalents. Difference between the saturatiGnvalue and the titration value has been used as theapparent oxygen utilization. It has been observed41that the inclusion of ammonia in the calculationof oxidation equivalents gives a better representa­tion of the same assuming that all ammonia origi­nates from nitrate reduction in non-anoxic or sub­oxic waters and nitrate as an oxidant is equivalentto 4 atoms of oxygen (NH:t+202~NO;+2H++H20).Average concentrations of ammonia 'in the watersof this region varied from 0·51 to 1·22 (kg-at/litrefor upper layer and from 0·42 to 1·20 [Lg-atjlitrefor the deeper layer between the 3 areas. Thusit is apparent that inclusion of ammonia will notaffect the calculation of the oxidation equivalents

and basetl on new observation material was :finally106:16:1 by atoms for oceanic environments40•

Becaus)e of differing marine conditions, the stoi­chiometnc relationships cannot be the same every­where. The ratio may not hold good in coastalregions \fhere the proximity of land, river runoffand also Ithe influence of local flora and fauna willchange t,e situation. The only attempt to deducedifferent !stages in the oxidation of organic matterand to P1esent a stoichiometric model and to verify

it in different oceanic areas has taken into considera­tion the inherent limitations on the subject9-1l.The present attempt following the lines of Richards9aims at presenting a picture of conditions to; beexpected in the northern and the eastern basins

of the A~abian Sea resulting from the oxidation oforganic mlatter. The conclusions are based mostly on

observed falues with some theoretical assumptions.Compos'ition of organisms: A working hypothesis­Table 4 gives the result obtained from analysis ofplankton,' collected in the inshore waters off theGoa coast and some laboratory cultures. Theaverage :N:P ratio, combining all the values, comesto 15:1 by atoms.

It has been suggested earlier that denitrificationand residral phosphate will influence the ratios inthe upper layers. We have, therefore, used onlythe atomit relations calculated for the deep waters,which arJ 270:1 for ilAOU:AP (Fig. 7) and 17:1for AN:AP (Fig. 8).

It has peen observed from studies in many areaswith vari1ed hydrographic conditions that thoughthe AN :AP relation in water may vary, sometimessignificantly, from the oceanic values, N:P ratioin plankton remains essentially unchanged. Itis assumed that N:P ratio obtained from the analy­sis of plan:kton from the inshore region will not showsignificant difference from areas offshore. Further,the differ~nce between the average values obtainedin natural population and the overall average beingnot very :significant, it has been decided to applythe latterj in the present calculations.

It has been concluded before that the ratios inwater maly vary from place to place depending

mostly O%the stage of denitrification process. Butassumptio about the uniformity in composition ofplankton rganisms would, probably, indicate thatthe general pattern of oxidation of organic mattermay not !differ spatially, particularly at the firststage when only dissolved oxygen is consumed.

Thereforej an attempt has been made to presenta general picture of the overall process.

It is a sumed that the average oxidation stateof carbon is that of carbohydrate (CH20), the boundnitrogen i~ in the amino form, and the oxidationstate of phsphorus is that of orthophosphate. Theorganic matter can thus be tentatively suggestedas having'the composition (CH20)••(NH3)15H3P04·If the organic matter is completely oxidized byoxygen and the first step in organic decompostionis the hyd10lytic release of ammonia with concurrentbiochemical oxidation of carbohydate, differentstages in Ithe process can be represented by thefollowing equations:(CHP)" (NH3)15H3P04=n(CH20)+15NH3+H3P04

.•... (3)

68

j -

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

§..("\

AC02 174A AOU: 270 rc;:f()·70.. .'

:••••: 't :

1---------------

-----------.------------

t:. C0:2(biogenic) 1056. ox.equiv. = 270

(THEORETICAl. SLOPE I600 700

AOU .Ltg - at II

'.

••••• ;~•••:••: ~ ;.#,... '... '. " ..... \:,.: ..:\ .

., ~.":.:: .:.:':.'.'.~•. ~ :~---' 1••~,.·. ~" •.• _1-. :..:. " . _ 'T'~-~-. --'~T".-r lor ~ • ('.- ~.- ..., ". -:., ../ ..\,:. ,::., ..

a•••••• '.

," ._, .'t-

.g 0N '--400

500

.:::::::.­oI

~8UUl'NNoU

W

Fig. 12 - Relationship between the amount of oxygen consumed (from free oxygen only) and the total CO! computedfrom the alkalinity values in the deeper layers [Slope of the line indicates ratio between total inorganic carbon (bothfrom the decomposition of organic matter and the possible dissolution of calcium carbonate) and oxygen consumption.

Broken line is the slope for CO2 arising from the decomposition of organic matter only (1' = coefficient of correlation)]

significantly. Therefore, it is felt that leaving outof the ammonia values would not have muchinfluence in the calculations. The total carbondioxide values have been computed from the tablesgiven in Strickland and Parsons20 which is only areproduction from Harvey42, who expanded theoriginal work of Buch43to cover higher chlorinities­using measured values of chlorinity, temperature,pH and total alkalinity. The slope of the regres­sion line in Fig. 12 gives a ratio between total CO2production and oxidation equivalents as 174/270.Ratio between biogenically produced CO2 andoxidation equivalents, from Eq. (7) is only 105/270. The difference may simply be explained asanalytical and computational error, since it is arelation between two computed quantities only.But there can be some oceanographic explanationsas well. It has been observed44 that at an inter­mediate depth in the ocean conditions are idealfor lime-secreting organisms to exist, which is laterfound to be applicable to the northern IndianOcean including its 2 arms, the Bay of Bengal andthe Arabian Sea4S. This might have influencedthe ratio. Boric acid and borate ions are known toplaya significant role in the regulation of alkalinityin sea water and they might have also someinfluence. In the absence of data from direct CO2analyses by gasometric methods from the IndianOcean, this explanation may be accepted for thepurpose of the present study.

Thus the organisms in the northern and the-eastern basins of the Arabian Sea appear to havean approximate composition of (CH20)lOS(NH3)lS­H3P04. Ratio of oxygen consumption to CO2production becomes 2·6 in agreement with valuesfrom other oceanic areas. Therefore, in the regionsstudied, complete oxidation of organic matterassociated with 1 atom of phosphorus would require270 atoms of oxygen and will be accompanied by

the release into the water of 1 phosphate ion, 15nitrate ions and 105 molecules of CO2,

Denitrification - When all or nearly all of thefree di~solved oxygen has been consumed, nitrateions are the next most abundant source of freeenergy for the oxidation of organic matter9• Theprocess in which they serve as the reducible sub­strate is denitrification and evidence suggests thatthe process produces nitrite ion~ as intermediateand free molecular nitrogen as an end product.The limiting concentration of oxygen that can bepresent at the inception of denitrification is uncer­tain and probably depends on the nitrate concentra­tion, but in any event it is relatively low-probablyof the order of tenths of a ml/litre. Since very lowconcentrations of oxygen, around 0,1-0·2 ml/litreor less, have often been observed in the northernArabian Sea associated with low nitrate and maxi­mum of nitrite at intermediate depths it is quitepossible that denitrification takes place with theloss of inorganic nitrogen compounds to gaseousnitrogen. An attempt has been made to study thisprocess applying the stoichiometric relation deducedabove. Denitrification in this basin can berepresented by the equation

(CH20)los(NH3)15H3PO4+ 84HN03=105C02+15NH3+H3P04+42N2+147H20 ... (8)This reaction indicates that ammonia released

during denitrification is not oxidized. However,it is very possible that it does oxidize, since theconcentration of ammonia in this basin is almostinsignificant, according to the basic equation

5NH3+3HN03=4Nll+9H20 ... (9)If this were so, the overall reaction would be

(CH20)los(NH3)lSH3PO4+ 93HN03=105COll+54Nll+H3P04+174HllO •.. (10)

69

INDIAN J. MAR. 5CI., VOL. 5, JUNE 1976

The average of the observed values of nitrateconcehtrations in the deeper layer is about 8 {Lg­at/litre. Applying Eq. (10) it is observed thatabout 90% of the available nitrate should be lostby denitrification. It has been suggested earlier thatabout two-thirds of nitrate can be lost as gaseousnitrogen. The difference may be due to analyticalerrors, errors in the statistical treatment or due tospatial variations in nitrate concentration. Thepresence of nitrite indicates that the process isincomplete in this water mass, as nitrite can appearas an intermediate product both during nitrification

and ~nitrification (N03~N02~NHt). It is con­cluded! that denitrification proceeds at an inter­mediate depth in the northern Arabian Sea butthe prbcess does not reach its completion. A studyof thej dissolved gaseous nitrogen in this area willdefinitely help to find the 'missing link' in thechemistry of inorganic nitrogen in the Arabian Sea.

In contrast, in the northern Arabian Sea oxidativephosphorus forms a major part of the total inorganicphosphate while oxidative nitrogen is much higherthan the inorganic nitrogen present in water. This,difference results in negative values of the reservedfraction.

Low oxygen concentration gives a high apparentoxygen utilization value. It appears that becauseof denitrification the supply of nitrogenous organicmatter may not be sufficient or the low oxygencontent of the water inhibits the regeneration pro­cess and organic matter settles undissociated. Butvalues of organic phosphorus seem to be quite com­parable with general oceanic condition whereinorganic phosphates make up for about 90-95%of total phosphorus concentration in deep water.Therefore, it appears that insufficient supply oforganic matter is the cause of deficiency of inorganicnitrogen compounds in the Arabian Sea. This.may also be caused by poor mixing of oxygen-richbottom water with intermediate water massescausing the secondary oxygen minimum to appear.This minimum mav also be the effect of the mini­mum in the eql1atorial Indian Ocean. There itappears at around 1000 m and has an oxygen con­centration of about 1·5 mlflitre. This water massseems to spread northwards and the oxygen con­centration is depleted with its northerly transportdue to the oxidation of organic matter resultingfrom the productivity in the euphotic layers of theregions of its passage.

Appearance of oxygen minimum and nitritemaximum at intermediate depths, the cause of whichhas been attributed to denitrification, has also beenobserved in the eastern tropical North Pacific46,47'and in waters off the coast of Peru48,49. Consider­ing the hydrographic conditions in the ArabianSea and the Bay of Bengal and comparing theirsimilarities with the eastern tropical North PacificOcean it may be concluded that these are charac­teristics of oceanic areas, not well ventilated in thenorth and lying off the western coasts of continents.These are generally the areas of intense upwellingand the resultant high photosynthetic productivityseem to be a major cause for these phenomena.The Atlantic is well ventilated both in the northand in the south and that is why neither any oxygenminimum nor any nitrite maximum could he observ­ed along the western shores of the continents border­ing it.

It is concluded that the normal oceanic C:N:Prelation of 106:16:1 by atoms is also maintainedin the Arabian Sea by the normal mechanism inits deep water and by somewhat different mecha­nisms in its upper layers, which can be comparedto conditions in oxygen-poor semi-enclosed quasi­oceanic or brackish water basins considerablyinfluenced by river runoff where the relation is alsomaintained but by very different mechanisms1S,16.

52086

606

86%

0'99283

0'300'69323

53%

poi- P486 X pot P

Pox = AOUj-286Pr = poi- P-Pox100% O2-286 X PO~- P(323/606) X 100

NITROGEN

COmpjriSOn with Other AreasThe present data from the northern Arabian

Sea arf compared with data from the eastern NorthAtlant~c and the eastern tropical Pacific Ocean.Calculations for nutrient fractions from the un­published data collected on board· RRS Discoverydurin~ its cruise in June-July 1967 along a sectionextending from the Faro Bank Channel in the northto the' Bay of Biscay in the south in the easternNorth Atlantic by one of the authors (R.S.G.) are

presenf'ed in Table 5.The calculations show that oxidative phosphorusand nitrogen forms a minor portion of the totalconcenltrations of their inorganic forms. Beingan are~ which is well ventilated the oxygen concen­tratiorJ. never goes below 4 mlflitre which does not

give a ISignificant apparent oxygen utilization value.I

TABLE 5 - COMPUTED RELATIONSHIPS BETWEEN OXYGEN,PHOSPHORUS AND NITROGEN FRACTIONS BELOW 75 m IN THE

EASTERll' NORTH ATLANTIC BASED ON ATOMIC RELATIONSAl<\OU: AP = 286: 1 AND AAOU: AN = 22: 1

(Units are in flog-at/litre)OXYGEN

Oxygen saturation (from O~tOC and 5%0)

Av. oxygen cone. O2Apparent oxygen utili­

zation: O';:-OsOxygen saturation

InorganicEquivale!nt oxygen utili-

zation!P of oxi\:iative originReserved phosphorusExcess CDsOxygen saturation

PHOSPHORUS

NitrateEquivalent oxygen utili-

zationNO:;.of oxidative originReserved, nitrateExcess O2

Oxyg= rmxtioo70

---_:..-=---=-=--~.~_.

NO:;22 X NO.

Nox = AOU/22Nr = NOs-Nox100% O2''':22 X NO;(382/606) X 100

10'18224-

3·916'27382

63%

Complications and Limitations

In calculating the ratios of change by the leastsquare method, it was assumed that the variables.are linearly related. This assumption is uncertainand the regression lines may not always be theideal fit throlJgh the points. Also, components

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

for wihch oxidatjo:~ equivalents were sought wereassumed to be invariant. The reverse would,probably, result in somewhat different relations.Tests have however, shown that the differencewith the reverse rela~ions lie within the limits oferrors in the analytical and statistical methods.

The stoichiometric model has been suggestedfrom measurements on one occasion each year for2 yrs at only a few stations in a fjord-like area withpermanently anoxic conditions at the bottom 10.

Agreement, calculated from observations fromdosely spaced stations over a wide oceanic areawith near-anoxic or sub-oxic conditions at inter­mediate depths, suggests the universal validity ofthe model in the oceans.

The possible chemical complication have beenreviewed by Richardsg• The analytical methodsare also bound to influence the results and theultimate conclusions. The measurements are carriedout by different analysts and the errors due topersonal bias in them cannot be uniform.

In spite of all these complications and limitationsit may be concluded that the relationships suggestedhere would probably give an approximate ideaof the overall conditions which can be expectedfrom the decomposition of organic matter in thenorthern and the eastern basins of the ArabianSea. Also, because of the similarity of the ratiowith those prevailing in other oceanic areas therelationship, suggested here, can be expected tobe valid for the Indian Ocean as a whole.

AcknowledgementThe authors thank Dr S. Z. Qasim, Director, for

providing facilities and for going through the manu­script and suggesting valuable improvements.Thanks are due to Capt. K. L. Chopra and his officersand men for all help in the work on board INSDarshak and to their colleagues, Shri S. Y. S. Singbalfor computing the CO2 values, and Smt. ThresianlIDaand Smt. B. Das for carrying out the total phos­phorus and plankton analyses in the shore labora­tory. One of them (R.S.G.) extends his thanksto Prof. Francis A. Richards and his colleaguesof University of Washington, Seattle, USA, forvaluable suggestions and to Mr James c. Crease,Institute of Oceanographic Sciences, Wormley,England, for allowing him to participate in thecruise of RRS Discovery in June-July 1976.

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71