Indian Journal of Marine Sciences Vol. 12, September 1983, pp. 143-150,
Ecology of Indian Estuaries: Part I-Physico-Chemical Features of Water & Sediment Nutrients of Ashtamudi Estuary
N BALAKRISHNAN NA.IR, P K ABDUL AZIS, K DHARMARAJ, M ARUNACHALAM, K KRISHNA KUMAR & N K BALASUBRAMANIAN
Department of Aquatic Biology & Fisheries, University of Kerala, Trivandrum 695007
Received 21 March 1983; revised received 24 May 1983
Detailed analyses of physico-chemical characteristics and sediment nutrients of Ashtamudi estuary covering the entire biotope from the marine to the riverine zone are presented; this second biggest estuarine system in Kerala is found to be a zone of severe pollution caused by effiuents from a paper mill situated upstream. Seasonal variations in temperature and pH and in the concentrations of dissolved oxygen, salinity, phosphate-P, nitrate-N, nitrite-N, and silicate-Si in estuarine waters have been observed at 4 selected stations. The study has indicated a distinct degree of change in relation to the water quality at Kadapuzha. The nature, degree, and extent of ecological transformation undergone by this vast estuarine system has been briefly discussed. Seasonal variations of organic carbon (DC), total P, total N and total K in the sediments are studied in relation to certain physico-chemical characteristics. Correlations between DC, total P, total N and total K and the grain size reveal that fine grained elements contain more DC and P. Existence of organic phosphate in the sediments of the Ashtamudi estuary is evidenced by the correlation between DC and P in all the 4 stations. Results of the present study indicate the imperative need to keep the effects of pollution within manageable limits.
Many studies have been undertaken to elucidate the average annual run off of 75 x 109 m3 of freshwater hydrography of Indian estuaries and backwaters1 -7. into the estuary5. The study area is polluted due to An outstanding phenomenon of the coastal belt of industrial and sewage effiuents. Four representative Kerala is its extensive system of estuaries and stations were selected (Fig. 1)in the system for detailed brackishwater lakes lying roughly parallel to the investigations and they are described below: Arabian Sea. These water bodies, popularly known as Neendakara (st 1)is one of the biggest fish landing 'kayals', which exist in different sizes and shapes have harbours of the west coast of India at the mouth of the their bed levels at about 1.5 to 1.8 m below mean sea estuary. Ashtamudi (st 2) is the central portion of the level. The waters of a majority of the 41 rivers in Kerala vast expanse of the backwater which is a very drain into the 'kayals' before they empty into the sea important fishing zone. Kanjirakode (st 3) represents through a large number of perennial or temporary the interior segment of the estuary and it has 3 arms openings. and several crec!ks. Even though retting of coconut There are 30 'kayals' dotting the coast of Kerala. husks and industrial effiuents have created some Many investigators working on the hydrography of these 'kayals' in Kerala8 -25 have provided a fairly good picture of the highly dynamic environmental conditions prevailing in the Cochin and Korapuzha estuaries, the Kayamkulam, Edava - Nadayara, Paravur and Veli 'kayals'. Of the many 'kayals' that remain relatively unexplored26 -27, Ashtamudi is the most important one, being the 2nd largest estuarine system in Kerala. In this investigation seasonal changes in physico• chemical parameters of waters and in the nutrients of N the sediments of Ashtamudi estuary have been studied. f
Study Area Ashtamudi estuarine system covering an area of 32 km2 branches off into 8 creeks, known by different ~ e"S3~" StUdyIS;¥- J- .. names (Fig. 1). The Kallada river originating from the ~i 7641.~~ Western Ghats enters the Ashtamudi estuary after Fig. I-Ashtamudi estuarine system in Kuala [st I, Neendakara; traversing for about 120 km and this river carries an st 2, Ashtamudi; st 3, Kanjirakode; and st 4, Kadapuzha] 143 INDIAN 1. MAR. SCL VOL. 12, SEPTEMBER 1983 pollution in the area it is an excellent fishing zone. flow of water in the Kallada river that has saved the Kadapuzha (st 4) is near the confluence of the Kallada entire Ashtamudi estuarine system from total river into the estuary and heavily polluted by the pollution caused by the paper mill effiuents. Pt.lOalurpaper mill effiuents. Water characteristics Materials and Methods Temperature- There was variation in temperature Fortnightly sampling was done both for water and· following heavy rains during 2 monsoons and the sediments during Feb. 1980-Jan. 1981. Both surface consequent river discharge (Fig.2). The mean value and bottom water samples were collected invariably during the premonsoon period was the highest both at surface and bottom waters at all stations. Monsoon j during the forenoon using a locally designed water \ sampler. Light penetration was measured using a \ I Secchi disc. Dissolved oxygen, salinity, pH, phosphate, Table I-Monthly Variations in RainfaIl at Kallada River nitrate, nitrite and silicate were estimated following Basin and Monthly River Discharge into the Ashtamudi standard methods. Sediment samples were collected Estuary During 1980-81 • -j using a metal corer (6.6 cm internal diam. and 21 cm long). Each sample covered a surface area of approx. Month Rainfalldischarge18.40204.92291.9573.69146.84114.9115.2090.95138.8725.4221.94 River 1025.18125.45101.9853.39203.20446.37281.3197.45 I 34 cm.l and a sediment volume of 718 cm3 (ref.28). A 264.02(mm)168.34 (Mm3) ~J portion of sediment was oven dried at lOO-105°C 2.38 )\ overnight for chemical analysis. Dried samples were 1980 Feb. Premonsoon ground and sieved through 0.5 mm sieve and 0.5 g of March the material was taken for the estimation of organic April carbon29. Sample preparation for the estimation of May total phosphorus, total nitrogen and total potassium June Monsoon was done by double acid fusion method using July sulphuric acid and perchloric acid. Total phosphorus Aug. Sept. was determined by colorimetric method using Oct. Postmonsoon aminonaptha1 sulfonic acid - reduced mo1ybdophos• Nov. phoric blue colour method, total nitrogen was Dec. determined by modified Kjeldahl method30 and total 1981 potassium was determined by flame photometer3o. Jan. Postmonsoon Oven dried samples were used for the grain size analysis following the international pipette method31. Data on rainfall and river discharge were obtained from' the Irrigation Department, Government of Kerala.
Results and Discussion Rainfall and river discharge-Data on rainfall and river discharge show clear seasonal variations (Table 1). The SW monsoon over Kerala brings very heavy rains in this river basin during June-Aug. and the NE ~ JJ 'Uppukutu' in the Kallada river where further MONTHS incursion is prevented by a water fall. It is this plentiful Fig. 2-Fortnightly variations of temperature in the Ashtamudi rainfall in the river basin and the consequent heavy estuary during 1980-81 (SW = surface water; BW = bottom water) 144 NAIR et a/.: ECOLOGY OF INDIAN ESTUARIES -PART I period registered lowest value at all stations followed st I and at I% level at s182 and 4. The pattern of by a period of recovery during postmonsoon period. temperature variation at the Ashtamudi estuary was At sts 3 and 4, seasonal average values of the bottom found similar to the distribution nattem observed in water was always higher than that of the surface water. the Kayamkulam lake19, in the Cochin estuary32 and This situation is presumed to be caused by the in the Paravur lake24. discharge of cold fresh water from the Kallada river pH-Surface and bottom waters generally and due to the absence of large scale mixing in the remained on the alkaline side at all station~ (Fig-.3) locality. The water column here is stratified on the except for a brief acidic phase at sts 3 and 4 in July. basis of temperature. But at st I, which is closest to the Seasonal variations were highly significant. Variation Arabian Sea, the surface water temperature was between surface and bottom water pH was not always higher than that of the bottom water. significant at sts 2-4, but significant at 5% level at st 1. Variations of surface water values between different A clear decrease in pH from the marine to the stations were not significant whereas those of bottom freshwater zone was evident during all the seasons of. water were significant at. I% level. Seasonal variations the year. were highly significant both at the surface and at the Light-Light penetration .was minimum at all bottom. Variation between surface and bottom values stations during monsoon season with heavy rains and were highly significant at st 3, significant at 5% level at discharge of turbid waters from all drainage channels FIG.3 FIG.5 NEENOAKARA (511). ASHTAMUDI (st 2), KANJIRAKCDE (st 3), KADAPUZHA (st 4) 0--0•...• sw}BW NEENDAKARA 0--0e- -e BWsw} ICANJIfW - ~ ...... ••..•• ::l===o =:-\;:;:: ,.,~"". , --+- -•.....•.....•....•...-•.....•.....•,,'..."._..•".--.-- •...... •...... •...... •.....•.....•...... •.....+••.• +--+ 20 • _-0 ••••0..••0--0 ••_0__0..."'o--~- 0 ••...0' ,0..-0 ••-0--0-- ••0.•._0-••••..•.-"'0"" 0....•.0 •• _0••.•.0•..•.0- "'" 10----a ~WW KANJIRAKOOE ~ , Ie- 0 ro Go.'i. -.- .••.._•.•.••..••• _•.•.•••....•_..•.••...•••. _•• _ •••.....•..•..•.•....•~ ~O ....•.•....r ~..•--•....••-•... ~ )0 -6--a..-4- .•6-0--4-••4..••4--6--4 .••.••A.••.•.A--A--A--A- ••A.••••~ ••A__A••.••••..••A.••••r-r""en ~ 20 10 ~~: ASHTAMUOI • -A-- A-- •• -A--A- •..•••.""--A-_A-_A-_•.•.•.I__, __••._•.•.•.•••_& __ •• _ •.•••• r-"_"",. 0 40 10 /0 10~ ~~: NEENOAKARA o '0 • 75 • 4 25 uE 15 '0 '0 -- NEENOAI 75 200 8 • ~~•.J·V .•~;- .. _-~~ 25 F 1910 M A M JJ A 5 aND J 198" 0 F 80_:~,-- M A M J J A SON 0' J 11 MONTHS • MONTHS Fig 3 to 6- Fortnightly variations in pH (3), light penetration (4), salinity (5). and dissolved oxygen (6) during 1980-81 at different stations [SW=surface w~ter; BW= bottom water] 145 INDIAN 1. MAR. SCI.. VOL. 12, SEPTEMBER 1983 , ,I ! around and from the Kallada river (Fig.4). During oxygen depletion in the Edava-Nadayara backwater premonsoon period light penetration was modest at lying south of the Ashtamudi estuary. these stations but with the advent of the monsoon rain, Generally, concentration of dissolved oxygen was water became turbid and light penetration depth higher in the present area of investigation. Average naturally decreased. The present data reveal that the values at all stations were J:lighestduring premonsoon water at st 4 is extremely turbid throughout the year. period followed by lower values during monsoon and Continuous sand collection, heavy discharge of paper postmonsoon. Seasonal variations in the surface water mill effiuents and river discharge from the upper were significant at a very high order whereas in the reaches are reasons for highly turbid conditions bottom water they were not significant. A clear noticed at st 4. Variations in light penetration between stratification on the basis of oxygen values also could the 4 stations were highly significant. be observed in the estuary in that the surface and Salinity- Based on the trend of salinity variations 3 bottom water differences were highly significant at all broad patterns of distribution can be elicited, (i) a stations. period of high salinity with very little fluctuations Phosphate-phosphorus-At st 1 mean seasonal during Feb. to May-pre SW monsoon period, (ii) a surface and bottom water phosphate was maximum fairly long spell of comparatively low salinit)! with during premonsoon season and minimum during the greater fluctuations from June to Nov.-period ofthe monsoon (Fig. 7). At sts 2 to 4 phosphate was 2 monsoons, and (iii) a period of recovery during Dec. maximum during postmonsoon and minimum in and Jan.-post-NE monsoon period. Thus a period of premonsoon period both in surface and bottom great fluctuations is flanked by periods of lesser waters. As the seasonal variations have been found to fluctuations (Fig.5). The surface salinity progressively be significant it can be stated that the impact of rainfall diminished from the estuary mouth at st I to sts 2 and 3 and river discharge on phosphate-P changes is indeed and attains the lowest value at st 4 on the riverine zone. prominent. Rainfall resulting in heavy river discharge Salinity of the bottom water, exhibited the same trend leads to transportation of sediments in a big way. Thus '1 from the marine to the freshwater zone but maintained an increase in the value of phosphate was noticed with Ij higher values throughout. Thus the entire water body the onset of the monsoon rains. This ascending trend is virtually stratified on the basis of this parameter lasted for several months recording the peak value in I I alone. Seasonal variations in salinity was highly the postmon~oon period. Sediments in estuaries are \ significant. The salinity distribution in the Ashtamudi reported to trap 80-90% of phosphorus33.34.3s, estuary and the division of the year into 3 periods releasing the same to the overlying water36.37. Thus, closely agree with the pattern observed by earlier presence of total phosphorus in an estuary can be workers8 -10,16.19.23.24.32.Inall these studies salinity taken as an index of potential fertility of the ecosystem in the estuaries of Kerala varied from limnetic to as a whole38.39. mixohaline conditions during the course of an year. Nitrate-nitrogen- Mean seasonal value in the Dissolved oxygen-The impact of paper mill surface and bottom waters was maximum at st 4 and i effiuents, retting, eutrophication, rain fall, etc. are minimum at st 1(Fig.8). The concentration exhibited a Y1 reflected in the oxygen regime at the 4 stations in the clear upward trend from the marine to the riverine ~ estuary. There were rapid and wide fluctuations in the zone. During the monsoon and postmonsoon periods, dissolved oxygen concentration from fortnight to lowest value was observed at st 2and the highest at st 4. fortnight and there was even total oxygen depletion When the seasonal average was observed it was resulting in the occurrence of brief anoxic periods generally low during the premonsoon period at aU ranging from a fortnight to a month at all the stations stations, slightly higher during monsoon and highest except at st 1 (Fig.6). At st 4 premonsoon was during pOstmonsoon period (Fig.8). The postmonsoon particularly a period of intense pollution resulting in peak might be due to the cumulative impact of rainfall very low oxygen values. The values in the surface water and river discharge experienced in the area. The ranged from 1.7 to 8.81 mLl -I and that at the bottom variation between seasons at all stations was highly from 1.24 to 5.2 mLl-I. The paper mill effiuents may significant. The concentration of nitrate was always be cited as the reason for the significant level of oxygen high at the polluted st 4. The river water probably depletion at st 4. At st 3 also there is depletion of enriched the nitrate content of the water as observed in oxygen presumably due to the effiuents that pass into the Korapuzha estuary21 and in the Cochin this zone through the rivulets from the Kallada river. backwater17. Apart from the river water enrichment Similarly, oxygen depletion was noticed at all stations most of the nitrate may be derived from decomposition toward the close of the monsoon season and in early of organic wastes. Total depletion of nitrate at st 4 in postmonsoon which is believed to have been caused by July might have been caused by the abundant growth eutrophication. Abdul Azis and Nair23 observed total of macrophytes40.41. This nitrate depletion, however, 146 I" NAIR et 01.: ECOLOGY OF INDIAN ESTUARIES--'-PART I was not accompanied with oxygen depletion as noticed throughout the year and minimum at st 1 during at the Edava-Nadayara backwaters 24. A distinct monsoon and postmonsoon periods (Fig.9). The stratification was also absent in its spatial distribution bottom water, in general, contained more nitrite than in the estuary. the surface water. The seasonal variation was Nitrite-nitrogen-Nitrite-N distribution in surface significant at all stations. At all the zones in the and bottom waters exhibited a similar trend in that the estuary, concentration of nitrite-N was low during I average concentration was maxim1Jm at st 4 premonsoon period. In the monsoon period a sharp 20 r II~ flU NEENOAKARA 1st 11. ASI'jTAMUOI1st 2). 12 ; KANJIRAKOOE(st 3), KADAPUZHAIst 4) -SW 16 ••..••• IIW KADAPUZHA I 14 '2 '0 16 12 -- sw 0--0 8W KADAPUlHA -5W '- .: 2 0--0 8W KANJIRAKOQE 10 11' '" ts 3'2 -0 '" 6 ::> Z'6 aIX • ~'2 cr:'o K 4 t: :g 2 -- sw Z 4 :x: ~ BW KANJIRAKOOE Q. 0 2 "'1 W'12 !;i 410 cr: 5 :x: ... '"Q. 6 z,i !f • .2 Q. 4 -- sw 0--0 8W ASHTANJDI -SW 30 0-08w NEENOAKARA ' -- SW "~: . 0---<0 BW NEENllAKARA 400 .,. 200 'DO 040 020 30 005 400 --5W _ '40 "Y o--GBW KA ••.• AkOOE 100 ; 60 , _-'- 050 t 1i 030 a. 010 ~ 30 ;:t Sl ,... 000 ....• Z 400 ill '0 l!l 200 l&J 100 ~ 100 5 Z 050 ;;i 60 40 030 010 ::000 _ '.W NEENOAKAAA ~o _n---sw . ,.~ M A M s o N o J,9I1 FI980 M A M J J A SON D JIll' MOMTHS MONTHS Figs 7 to \0- Fortnightly variations in phosphate - phosphorus (1), nitrate-nitrogen (8), nitrite-nitrogen (9), and silicate-silicon (10)during \980-81 at different stations [SW = surface water; BW = bottom water] 147 INDIAN J. MAR. SCI.. VOL. 12, SEPTEMBER 1983 rise in concentration was evident, presumably due to respectively at Mandovi, Zuari and Vellar-Coleron heavy rainfall and river discharge. Although the values estuaries. declined slightly during the postmonsoon it was higher than that observed during the premonsoon. Generally, Sediment characteristics the trend of variation followed that of the nitrate• Textural-In the sediment at st I coarse sand nitrogen in the estuary. The pattern of distribution ranged from 6.05% in July to 15.29% in Feb., fine sand noticed in this estuary broadly agrees with the one from 22.94% in Feb. to 73.44% in July, clay from observed in the Cochin estuary! 7. Higher nitrite value 1.04% in May to 11.98~() in Jan. and silt from O~oin observed at st 4 indicates the influence of land July to 4.02% in April. The grain size was of same drainage, and industrial waste discharge. magnitude at st 2 sediments also. Though sandy Silicate-silicon-Silicate-Si showed high values material (coarse sand, 60.47-85.88~o) dominated all throughout the estuary (Fig. I0). Average values in through the year, other fractions showed some surface and bottom waters were maximum at st 4 and :i~i fluctuations. Percentage of clay varied between 2.6 in I{ " minimum at st I. Values showed an increasing trend April and 12.49% in Nov. and silt between O~~in June j~ from st I to st 4 both in surface and in bottom waters. and 6.67% in Oct. At st 3 coarse sand ranged between Surface water was always higher in silicate-Si 33.28% in March and 92.56~1" in June dominating the ~ 1 I concentration, maintaining a stratification similar to textural characteristics due to a fall in the fine sized I the one observed in the case of other parameters. material during monsoon (June-Sept.). Premonsoon \ Variations were highly significant. The low season (Feb.-May) showed the peak percentage of clay concentration of silicate-Si in the surface and bottom (26.04%) and silt (11.46~o)' At st 4 coarse sand ranged waters during premonsoon period attains highest between 33.34-76.45/0 in Aug. and Feb. Percentage of values during monsoon at all stations presumably due clay and silt varied between 4.17% in Nov. and 18.29~'o to the river water that brings in additional quantities of in March and 0% in March and 10.2~o in April this nutrient. Concentration declines during postmon• respectively. soon period. While the silicate concentration ranged Nutrients-Seasonal variations in organic carbon. from 9-172 j:ig-at.l'] in this estuary, it ranged from total phosphorus, total nitrogen and total potassium 17.48 to 134.30, 12.43 to 89.98 and 20 to 140 flg-at.l -] are shown in Fig.II. 24 _ 60 e---e NEENDAKARA(st \) ., "- 0---0 ASHTAMUDI(st 2) c:n I' •. - - ..•• KANJIRAI Fig. II-Seasonal fluctuations of organic carbon, total phosphorus. total nitrogen and total potassium 148 Ii. ;,I'tlllNm,~~'!iJ!l"'II·III"'!~"lIIli~~pl~I"'IIII" 1~.:llllilllU ILl NAIR et 01.: ECOLOGY OF INDIAN ESTUARIES ~PART I Organic matter in lake sediments is derived from system which alters the textural characteristics primary production within the aquatic ecosystem thoroughly. The destruction of algal, clam and oyster {autochthonous sources) and also from terrestrial beds by the intrusion of the turbid freshets into the biota {allochthonous sources) by transport of leached estuarine system supports the fact that N-release from and eroded material into the lake42. Higher organic the sediments is controlled to a large extent .by carbon {OC) during premonsoon at sts 1, 3 and 4 and biological processes S0. Though the minimum and during postmonsoon at st 2 as compared with that maximum values of N from st 4 sediments occurred during monsoon revealed that the allochthonous input during June and July respectively the seasonal values of organic matter into the system is less when showed a peak during the monsoon and this is mainly compared to that of Cochin backwaters I5.18.43-46. due to the heavy disposal of effiuents from the Punalur Increase of OC during premonsoon may be due to the paper mills. retting of coconut husk; the practice of dumping the The correlation between OC and K in the pith into the adjacent water after processing is Ashtamudi estuary except at st I suggests that the prevalent. Direct correlation between OC and clay organic detritus associated with K entered the system fraction in the sediments of sts 2 and 3 (significant at mainly by leaching and Serruya 51 states that K is 500 level) is in accordance with earlier reports4 7 .48 that generally associated with detritic silicates and their fine grained sediments always have higher content of paucity in lake sediments may indicate the organic matter. The amount of organic matter in predominance of leaching over erosion. sediments is usually inversely related to the medium The correlation noticeable between total P and OC at grain size48 and this is applicable to the inverse all stations of the estuary suggests that detrital P is correlation (significant at I ~o level) of OC and coarse another fraction of the total P in the sediments. sand at st 3. This may be due to the increased surface area per unit weight for adsorption of organic matter Acknowledgement as particle size decreases. In a normal estuarine system This work has been carried out under a UGC total P will be at the maximum in both freshwater and sponsored departmental research project entitled marine zones although their derivatives in these zones "Studies on the coastal ecosystem of Kerala in relation are quite different49 and accordingly as a marine zone to fisheries" as per sanction letter No.F.22-9j77 at st I sediments show higher values during (SR.II). The help rendered by Dr R S Iyyer, Professor premonsoon. Relationship of silicate concentration of of Soil Chemistry, College of Agriculture, Trivandrum the overlying water at st I to the total P content showed and his colleagues are gratefully acknowledged. that most of the P released from the sediments may precipitate with colloidal silicates. Less turbulent References mixing at Kanjirakode during the premonsoon with I Ramanadham R, Reddy M PM & Murty A V S, J Mar Bioi Ass higher percentage of silt (6.8 and 11.5) and clay (l8.3, India, 6 (1964) 183. 2 Dulta N. Malhotra J C & Bose B B. Symposium on marine and 14.3 and 26) may be attributed to the retention of freshwater plankton in the Indo-Pacific (Indo-Pacific Fish autochthonous P. The positive correlation (significant Coun. Bangkok) 1954, 35. at I~" level) with clay and negative correlation 3 Bose B B. Indian J Fish, .\ (1956) 101. (significant at I~" level) with coarser, sand fraction of 4 Ramanadham R & Varadarajalu R. in Recent reseOl'ches in the sediments with total P support this fact. Besides, estuarine biology. edited by R Natarajan (Hindustan Publishing Corporation, Delhi) 1975, 151. macroalgae and seagrass beds are abundant at st 3 5 Venugopalan V K. Bull Not Inst Sci India, 380969) 743. and they are capable of using and precipitating as 6 Qasim S Z, in Marine productioll mechanisms, edited by J Dunbar much of P available to them33 -35 leading to higher (Cambridge University Press, Great Britain) 1979. productivity and hence the large fraction of P in the 7 Estuarine research in India: S/llte of the art report (National sediment3~.3\1. At st 2 though the pre and Institute of Oceanography. Goa) 1982, 75 pp. postmonsoonal conditions are similar to those of st 3 8 George M J & Kartha K N K. J Mar Bioi Ass India, 50963) 178. the incidence of algal species is relatively poor. Higher 9 Nair N B, Int Rev Hydrobiol, 50 (1965) 411. 10 Nair N B, Hydrobiologia. 27 (1966) 248. 4 content of total P during monsoon in st sediments II Qasim S Z. Bhaltathiri P M A & Abidi S A H. J Exp Mar Bioi can be explained by the deposition of alluvium brought Ecol. 2 (1968) 87. into the area by the Kallada river and this is supported 12 Qasim S Z, Wellershaus S, Bhattathiri P M A & Abidi SA H, by the maximum river discharge during this period and Proc Indian Acad Sei, 59 (1969) 51. also the texture of the sediments shows that the fine 13 Qasim S Z & Gopinalhan C K, Proc Indian Acad Sci. 69 (1969) 336. grained elements always dominated particularly 14 Qasim S Z & Shankaranarayanan V N. Mar Bioi, 15(1972) 15. during the monsoon and they can bind P. 15 Shankaranarayanan V N & Qasim S Z, Mar Bioi, 2 (1969) 236. L~wer levels in total N content at sts I to 3 during 16 Haridas P, Madhupratap M & Rao T S S, Indian J Mar Sci. 2 monsoon was mainly due to the river discharge into the 0973) 94. 149 INDIAN J. MAR. 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