Changes in the Environmental Features of the Zuari and Mandovi Estuaries in Relation to Tides by Padmakar V

CHANGES IN THE ENVIRONMENTAL FEATURES OF THE ZUARI AND MANDOVI ESTUARIES IN RELATION TO TIDES BY PADMAKAR V. DEHADRAI (Nationa/ Institute of Oceanography, Miramar, Panfim, Goa) Received November 6, 1969 (Communicated by Dr. N. K. Panikkar, F.A.SC.)

ABSTRACT Changes in the temperature, salinity, pH and dissolved oxygen in relation to tidal rhythm were found to be well marked in the Zuari and Mandovi estuaries. Seasonal differences in the hydrographical factors of the two estuaries are considerably influenced by the south-west monsoon.

In the Zuari estuary, during the post-monsoon months, almost marine conditions seem to prevail with a littlefreshwater drainage whereas in the Mandovi estuary the freshwater discharge continues throughout the year which influences the environmental features. The two estuaries are rich in inorganic phosphate-P, the source of which in the Zuari estuary appears to be largely from the neritic waters whereas the input of inorganic phosphate-P in the Mandovi estuary seems to be influenced by the river-water. The rate of production was high in the samples from the two estuaries during the monsoon month. Changes in the rate of production induced by the tides corresponded with those in the inorganic phosphate-P, chlorophyll a and the phytoplankton abundance. Probably the fora associated with the lower salinity dominates the crop of phytoplankton throughout the year in the Mandovi estuary whereas in the Zuari estuary the preponderance of the freshwater flora appears only during the monsoon month.

INTRODUCTION

AN important aspect of estuarine oceanography includes the study of dynanflc conditions induced by the tidal curronts and freshwater discharge 68 Environmental Features of the Zuari and Mandovi Estuaries 69 by the rivers in the estuary (Pritchard, 1967). Near Panjim (Goa) two large estuaries are found within 5 kin. along the rivers Zuari and Mandovi. A study of the environmental features of these estuaries is essential for understanding the hydrographical features of waters around Goa.

Earlier work on the estuaries of the west coast of India have been confined to Cochin Backwater (see Ramamirtham and Jayaraman, 1963; George and Kartha, 1963; George, 1958). More recently, some notable contributions on the Cochin Backwater hax(e appeared on plant pigments (Qasim and Reddy, 1967), light penetration (Qasim et aL, 1968), organic production (Qasim et al., 1969), tidal influence (Qasim and Gopinathan, 1969) and nutrients (Sankaranarayanan and Qasim, 1969). No previous account on the Zuari and Mandovi estuaries is available except the author's report on Marmugao Bay (Dehadrai, 1970).

The two rivers, Zuari and Mandovi, are situated along the southern and northern boundaries of the Panjim Island. The foimer flows into the Marmugao Bay and the latter into the Aguada Bay and thus two large estuaries are formed fairly closeby. The Cabo Ridge, which is the western extremity of the main Panjim Island, separates the Zuari and Mandovi estuaries (Fig. 1). The Mandovi river has a large number of tributaries whereas the river Zuari has none. The approach to the interior of Goa is through these ~wo rivers and thus they are at present intensively used for navigation. On the eastern side of the Panjim Island the Combarzua river connects the Zuari and Mandovi rivers.

The mouths of the two rivers are funnel-shaped and the two estuaries, though land-locked, are nevertheless exposed to the waves and tidal actions of the sea. The tides at Goa are of the semi-diurnal type with a range of about 2 metres (Fig. 2). These cause a periodic reversal of the currents which greatly influence the environmental features of the estuaries.

The freshwater discharge from the Zuari and Mandovi rivers vary greatly during monsoon and post-monsoon periods and the conditions in the two estuaries change to a large extent with the volume of water dis- charged by the rivers. The annual rainfall at Goa is about 3,000 mm. of which nearly 8070 occurs from June to August. The seasonal and daily fluctuations in atmospheric temperature are not large except during the monsoon months (July-August) and December-January when a notable decrease in daily temperature has been recorded. 7O PADMAKAR V. DEHADRAI

35'

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FIG. 1. Map showing the Zuari and Mandovi estuaries in co~¢¢tion with the Arabian Sea. Closed circles indicate the stations for observations,

3 2 = I ~O IJ ~2 ~,~'i .... ~'A~ l 1000 t800 oooo 0700 H~ WATER LOW WATER HIG H WAT[R LO w WATgR FIo. 2. Tidal changes in the Zuari and Mandovi estuaries during the observations.

MATERIAL AND METHODS Studies on the changes in the hydrographical features of the estuaries caused by the tides were conducted during the spring and neap tides as indicated in the Tide Tables for Goa. The observations were made on August 9-10, 1968 and on February 1-2, 1969 to determine the influence of mon- soonal changes. Environmental Features of the Zuari and Mandovi Estuaries 71

A plastic bucket was used to collect water samples from the surface and a van Dorn sampler from the 3 m. depth. All samples were transferred to suitable polythene bottles and analysed as soon as possible. The temperature of the samples was read off immediately and the pH from a Philips' pH meter. Salinity was measured by using Mohr's technique and dissolved oxygen by Winkler's method. Inorganic phosphate-P was analysed by the method of Wooster and Rackestraw (1951) using Lange's Photoelectric colorimeter. The values were not corrected for salt error. For the estimation of chlorophyll a, 500 ml. of water sample was filtered through HA Millipore filter and the pigments extracted according to the instructions given by Strickland and Parsons (1965). The optical density of the acetone extracts was measured on the Photoelectric colorimeter using a red glass filter and 90~ acetone with a dissolved HA Millipore filter-paper as blank. A factor 159 × Optical density/volume determined byusing pure chlorophyll a for the colorimeter was used to convert all values into chlorophyll a in mg/L. The rate of photosynthesis was measured by using the radiocarbon (C 14) method of Steeman Nielsen (1952). In each experiment, 50 mI. of water samples were incubated in 60 ml. (Corning) light and dark bottles with 1 ml. of 0.4 t~c/ml. C 14 (4.0 tzc/ml, ampules supplied by Bhabha Atomic Research Centre, Bombay, with ten times dilution)in an incubator. A panel of fluorescent tube-lights with an intensity of 2,000 foot candles was provided as a source of illumination during incubation for 5 h. After incubation, the samples were filtered through HA Millipore filters. The filters were dried and counted using a windowless gas flow proportional counter. The rate of photosynthesis in mgC/m3/h was calculated by the usual method. Phytoplankton and zooplankton crops were determined by filtering 75 litres of water from the surface through a net of Bolting nylon of 0"065 mm. mesh width and organisms collected Were preserved in 5~o foimalin solution. These were left undisturbed for about a week until fully settled. Later the substrate was made to 5 ml. and examined in parts. A quanti- tative estimation of plankton was made by counting the organisms contained in subsamples. A single station in each of the two estuaries (Sh and Stz)was fixed for observations (Fig. 1). The depth of water at the station in the Zuari estuary ranged from 3 to5 metres and in the Mandovi estuary it was about 5 to 7 metres. B-3 72 PADMAKAR V. DEHADRAI

RESULTS AND DISCUSSION Temperature.--Changes in temperature of the two estuaries in the monsoon and post-monsoon periods have been shown in Fig. 3. The two estuaries were somewhat warmer in August than in early February. The temperature in the Zuari estuary at the surface and at 3 m. depth which ranged from 26.2 ° C. to 28.0 ° C., showed a marked tide-controlled rhythm. In the Mandovi estuary during the monsoon month the temperature values (26.0 ° C. to 28.0 ° C.) on the surface and at 3 m. depth did not show any changes with the tides but were apparently influenced by the night cooling of the atmospheric temperature.

During the post-monsoon months, however, the effect of tide was notice- able both in the Zuari and Mandovi estuaries as the temrerature decreased at the flood and increased at the ebb. The range of temperature in the Zuari estuary at the surface and at 3 m. depth was alike from 24" 5 ° C. to 26- 5 ° C. But in the Mandovi estuary a lower range of temperature at the surface from 26.0 ° C. to 27"5 ° C. and a higher range of temperature at 3 m. depth from 26.5* C. to 27.5 ° C. was recorded.

~ MANDOVrDEPTH ~_+~

27 ...... 4- ...... -" '~ --~'" ~[

26 i i i L- I A I I I I I I I , . -- • A • "~" &O0~ NtGH WATER LOW WATER HIGN WATER LOW WATg~

Flo. 3. Changes m temperature at the surface and at 3 m. depth in the Zuari and Mandovi estuaries in relation to tides during monsoon and post-monsoon periods.

Salinity.--Salinity changes induced by the tides were well marked both at the surface and at 3 m. depth in the two estuaries (Fig. 4). During monsoon in the Zuari estuary, the salinity values remained high at the surface from 12"6%o to 22"6,%0o and at 3 m. from 12-3,%oo to 27.6~oo. But in the Mandovi estuary during monsoon the surface water showed the highest salinity of 12"0%o during the flood and the lowest 0"0%0 during the ebb. Similarly, at 3 m. depth the highest salinity during the flood reached 20.5Yoo while at the ebb the salinity was almost zero. This indicates that during the monsoon month a heavier freshwater discharge occurs in the Mandovi estuary than in the Zuari estuary. Environmental Features of the Zuari and Mandovi Estuaries 73 During the post-monsoon period in the Zuari estuary the fluctuations in the salinity were very little in between 31 "8%0 and 32.4%0 both at the surface and at 3 m. depth. But in the Mandovi estuary during the post, monsoon months the salinity fluctuations were much greater and ranged from 27"6%0 to 32"l/o;~--theo/ difference being larger at 3 m. depth. In the Zuari estuary, almost marine conditions prevailed during the p0st-monsoon months with a little freshwater discharge. In the Mandovi estuary, on the other hand, appreciable changes in the salinity continued to occur during the post-monsoon months also as a result of considerable freshwater discharge by the tributaries feeding the Mandovi river.

2 34 ...... I .... ~ I ...... 0 32 ¢,.-_.-27~+-_ ~÷ .... ~ l L-- ZUARI ~PTH --~-- MANDOVI SURFACE...... 2 6 MANDOVI DEPTH ---t--~ 2~

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I000 1800 0000 0070 HN3HWATER LOW WATER HIGH V~TI~ LOW WATER FIG. 4. Changes in salinity at the surface and at 3 m. depth in the Zuari and Mandovi estuaries in relation to tides during monsoon and post-monsoon periods.

pH.--The pH in the Zuari and Mandovi estuaries showed an increase at the flood and decrease at the ebb (Fig. 5). During the monsoon month in the Zuari estuary the range in pH at the surface was small from 7.6 to 8.0 whereas at the 3 m. depth the fluctuations were much greater (7.5 to 9.0). In the Mandovi estuary, on the other hand, the variation in the pH was similar both at the surface and at 3 m. depth but more pronounced (6.4to 8.2). Similarly, during the post-monsoon month in the Zuari and Mandovi estuaries the values of pH were from 7.2 to 9 "0 and 7' 1 to 9 '0 respectively, 74 PADMAKAR V. DEHADRAI

and no difference in the values were observed between those at the surface and at 3 m. depth. However, the effect of tides was marked on the pH in the Mandovi estuary.

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FIG. 5. Changes in pH at the surface and at 3 m. depth in the Zuari and Mandovi estuaries in relation to tides during monsoon and post-monsoon periods.

Dissolved oxygen.--The changes in the dissolved oxygen during the two seasons in both the estuaries have been shown in Fig. 6. During the monsoon month in the Zuari estuary the dissolved oxygen at the surface ranged from 4.1 ml./L to 4.7 ml./L whereas at the 3 m. depth the range was high from 2.8 ml./L to 4"7 ml./L. Similarly, in the Mandovi estuary during the monsoon month the range of dissolved oxygen was at the surface from 4"7 ml./L to 5.4 ml./L and at the 3 m. depth from 4-1 ml./L to 5.3 ml./L During the post-monsoon months the dissolved oxygen varied within a small range at the surface from 4.4 ml./L to 4.5 ml./L and at 3 m. depth from 4-3 ml./L to 4.5 ml./L. Similarly, a narrow range of dissolved oxygen was recorded in the Mandovi estuary at the surface from 4.6 ml./L to 4.8 ml./L and at the 3 m. depth from 4"5 ml./L to 4.8 ml./L. Rao and Rao (1962) reported the occurrence of high and low dissolved oxygen with the flood and ebb tides respectively off the Waltair coast while Rangarajan (1958) did not find any correlation between oxygen and the tide in the Vellar estuary. In the present study, high and low values of dissolved oxygen were obtained with the flood and ebb in the two estuaries and in the post-monsoon months the fluctuations were smaller than those during the monsoon month (Fig. 6). Moreover, during the monsoon month the dissolved oxygen values in the two estuaries showed a marked inverse r~lationship with salinity (Fig. 4) in the area. Durin~ the post-monsgon Environmental Features of the Zuari and Mandovi Estuaries 75 month also a similar inverse relationship between oxygen and salinity was observed in the Zuari estuary but in the Mandovi estuary the dissolved oxygen appeared to be influenced by the temperature (Fig. 3) and the pH (Fig. 5). The latter two factors were inversely related with the oxygen.

...... ' [ ' ' ' ' ' I ......

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Fie. 6. Changes in dissolved oxygen at the surface and at 3 m. depth in the Zuari and Mandovi estuaries i n relation to tides during monsoon and post-monsoon periods.

Inorganic phosphate-P.--The inorganic phosphate-P in relation to tides and seasons in the two estuaries has been illustrated in Fig. 7. During the monsoon month in the Zuari estuary the inorganic phosphate-P values increased with the flood and decreased with the ebb ranging at the surface from 0" 30 tzg-at/L to 1"00 t~g-at/L and at the 3 m. depth from 0"30 tzg-at/L to 1.75 t~g-at/L. But the range in variation at the depth was high. In the Mandovi estuary, on the other hand, while the fluctuations in the values of the inorganic phosphate-P were high at the surface (0.23 tzg-at/L to 1.00 t~g-at/L) and low at the 3 m. depth (0" 10 #g-at/L to 0.50 #g-at/L), the values were high at the ebb and low at the flood. During the post-monsoon period, in the Zuari estuary, though the range of variations in the values of the inorganic phosphate-P was small at the surface from 0" 17 #g-at/L to 0-23 t~g-at/L, an increase with the flood and decrease with the ebb tides were observed similar to that in the Zuari estuary during the south-west monsoon period, which indicates that the source of phosphorus is largely from the neritic waters. Qasim and Gopinathan (1969) found a similar relation of inorganic phosphate-P with the tidal cycle at the entrance channel of the Cochin Harbour. During the post-m0nsoon period in the Mondovi estuary, however, the values of inorganic phosphate-P were high during the ebb and low during the flood and the range of variation in the values of inorganic phosphate-P was lower at the surface from 0"00 t~g-at/I~ 76 PADMAKAR V. DEHADRAI to 0"23/zg-at/L and relatively higher at 3 m. depth from 0" 10/~g-at/L to 0.30/~g-at/L. In the Mandovi estuary, where the freshwater influx is dominant throughout the year, the increase in the inorganic phosphate-P which was observed during the ebb tides was probably brought in by the land drainage. Fresh- water generally enriches the estuary which may include organic as well as inorganic input of nutrients (Riley, 1967). In the west coast waters the organic phosphate is known to constitute upto 309/o of the total phosphorus (Sarma and Ganapati, 1958). Accordingly, the Zuari and Mandovi estuaries are rich in phosphate which compares favourably with the values reported by Santha Kumari (1968) from the Vellar estuary, Rao and Rao (1968) off the coast of Waltair, Suryanarayana Rao (1957) off Calicut and Sankaranarayanan and Qasim (1969) from the Cochin Backwater.

4 |F ...... i ..... I .... --,-r--~v,

zu~.P,~ SunFAC£ --, -- 7 ZUAAL DEPTH -- ~:- -- .J MANDOVl SURFACE --.--

0 ' A' .... a'A I .... ,jk .... 3 "t It"t"~"h]"~ k~rEIt 1800 CX~CK~ 0070 L OW WATER HIGH W ATEP, LOW WATE~

FiG. 7. Changes in the inorganic phosphate-P at the surface and at 3 m. depth in the Zuari and Mandovi estuaries in relation to tides during monsoon and post-monsoon periods.

Chlorophyll a.--In the two estuaries, the in situ fluctuations of chlorophyll a in relation to flood and ebb were well marked (Fig. 8). As compared to the Zuari estuary, the pigment concentration in the Mandovi estuary was high but the magnitude of variation in the quantity of chlorophyll a was considerable in both the estuaries during the monsoon period. In the Zuari estuary, the chlorophyll a varied from 1.9 mg./L to 3.8 mg./L during the monsoon month, but during the post-monsoon months it ranged between 0"6 mg./L and 2.5 mg./L. In the Mandovi estuary the chlorophyll a during the monsoon period ranged between 0.6 mg./L and Environmental Features of the Zuari and Mandovi Estuaries 77

5.7 mg./L while during the post-monsoon months it varied from 0.6mg./L to 3.5 mg./L. It is interesting to note that the concentration of chlorophyll a was inversely related to the salinity in the two estuaries as the latter changed with the flood and ebb tides. Probably the flora associated with lower salinity dominates the crop of phytoplankton in the Zuari and Mandovi estuaries. Diurnal changes in the chlorophyll content of natural phytoplankton populations due to endogenous rhythm have been discussed by Rabinowitch (1945, op. cit.; Yentsch and Scagel, 1958), Doty and Oguri (1957), while Yentsch and Ryther (1957) and Yentsch and Scagel (1958) have pointed out that light is the causative factor. However, in dynamic estuarine ecosystems such as the Zuari and Mandovi estuaries, the effect of day and night conditions seems to be overshadowed by the tidal changes ard thus the quantity of the pigments is probably influenced by the different stock of phytoplankton which is introduced by the flood and ebb tides.

I J .~ -----~~:-~Z.Z___2"--, _+

i ZuA~I SURFAC~ -- :-- ZUARI DEPTH , ---~--- .j6 ~5 / t %'%

+~ ~.~ ~Jr "-.. ~ ~ +

o,~ I AI I I I I A, I AI ,, I ,, , ,.'? IOCX~ 18 O0 oo 0070 141GHWAT~R L OW '#ATEF~ HK;H w^'r ER LOW WATER

Fzo. 8. Changes in chlorophyll a at the surface and at 3 m. depth in the Zuari and Mandovi estuaries in relation to tides during monsoon and post-monsoon periods.

Primary Productivity.--Primary production in the Zuarl and Mando,J estuaries during monsoon and post-monsoon periods (Fig. 9), which was studied by the rate of photosynthesis (C 1~ uptake) in an incubator exposed to constant illumination, was found closely related to changes in the inorganic phosphate-P and chlorophyll a which in turn varied markedly with the flood and ebb tid,~s. In the Zuari estuary, the rate of production during the monsoon month for surface samples ranged from 3 "4 to 4-0 mgC/m3/h and at the 3 m. depth between 1 "4 and 3.7 mg C/mZ/h. During the post-monsoon month the variations at the surface were from 0"3 to 1" 1 mgC/m3/h and at 3 m. depth samples from 0"7 to 1"6 mgC/m3/h. 78 PADMAKAR V. DEHADRAI

In the Mandovi estuary, on the other hand, during the monsoon month the rate of production varied from 0"4 to 3-5/mgC/m3/h and for the depth samples from 0-7 to 4.3/mgC/mS/h. During the post-monsoon period the variations in the surface and the depth samples were 1.3 to 1 "7 mgC/mS/h and 0.1 to 2"8 mgC/m3/h respectively.

Thus the rates of production in the two estuaries were higher during the monsoon month than those during the post-monsoon period. In con- trast, earlier observations on seasonal variations in gross ar, d r, et productivity determined from oxygen evolution in the Marmugao Bay (Dehadrai, 1970) indicated the lowest values during the south-west monsoon period. Experiments on C z4 assimilation of the natural phytoplankton suggested that in natural environment the south-west monsoon causing excessive turbu- lence, high turbidity and insufficient light penetration, probably impairs the photosynthetic activity of phytoplankton even in the presence of high concen- trations of nutrients. Thus the potential productivity was higher than the values observed in the natural environment.

0'_ ' • ' , , ~ , I , , , , , I , , , , , iooo ~8oo o~oo oo,"o I,IIGH ~TIER L OW WATER HL~H WATER IOW WATER

FIG. 9. , Changes in the rate of primary production (Ct4 uptake) at the surface and at 3 m. depth in the Zuari and Mandovi estuaries in relation to tides during monsoon and post-monsoon periods.

Phytoplankton.--Numerical abundance of diatoms at the surface in the two estuaries changed with the flood and ebb (Fig. 10). These changes were well marked during the post-monsoon period.

During the monsoon month, the diatom were more abundant in the Zuari estuary (336--474 per litre) than in the Mandovi estuary (25-112 per litre) whereas during the post-monsoon period the diatoms counts in the Zuari estuary were 25 to 380 per litre and in the Mandovi estuary 61 to 352 per litre. Environmental Features of the Zuari and Mandovi Estuaries 79

The pattern of variations in the number of diatems in the two estuaries in relation to tides during monsoon and post-monsoon months corresponded with that of chlorophyll a and the rate of photosynthesis (C 1. uptake).

41 3 ~2 /fizz ...... !

8_o MA~DOW SUPFACE .....

i 4

I O ..... " , a i I I I NII~'GOH~W(~AT[C~ ~ a i I i IO(30 H~t~ WATCF~ LOW WATER FIG. 10. Changes in the phytoplankton counts at the surface in the Zuari and Mandovi estuaries in relation to tides during monsoon and post-monsoon periods.

Thus the pigment quantity and the rate of production in the two estuaries were closely related with the natural phytoplankton population.

ACKNOWLEDGEMENTS

Author isg reatfully indebted to Dr. N, K. Panikkar, Director, National Institute of Oceanography, for encouragement and advice and to Dr. S. Z. Qasim for his valuable suggestions in the paper. Thanks are also due to Mr. P. M. A. Bhattathiri and Mr. R. M. S. Bhargava for their help in the course of the work.

REFERENCES

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Qasim, S.Z. and Reddy, .. '"lhe estimation of plant pigments cf Cochin Backwater C. V. G. during the monsoon months," Bull. mar. Sci., 1967, 17 (1), 95-110. Rabinowitch, E. I. .. Photosynthesis and Related Processes, Interscience, New York, 1945, 1, 355-571. Rangarajan, K. .. "Diurnal tidal cycle in Vellar Estuary," J. zool. Soc. India, 1958, 10, 54-67. Rao, T. S. S. and Rao, V.C. .. "Studies on diurnal variations in the hydrobio!ogieal conditions off the Waltair coast," J. mar. biol. Ass. India, 1962 4, 23-43. .. "Distribution of total phosphorus in the Bay of Bengal," Bull. nat. Inst. ScL India, 1968, No. 38, 93-102. Riley, G. A. .. "~Ihe plankton of estuaries," In Estuaries, Edited by Lauff, G. H., A.A.A.S., Washington, D.C., 1967, pp 316-26. Sankaranarayanan, V. N. and "Nutrients of the Cochin Backwater in relation to environ- Qasim, S. Z. mental characteristics," Mar. Biol., 1969, 2(3), 236-47. Santha Kumari, P. "Phosphorus fractions in Porto Novo waters (11°29'N - 79°49'E) during 1965-66," Bull. nat. hist. Sci. India, 1968, No. 38, 87-92.

Rama Sarma, D. V. and "Hydrography of the Kakinada Bay," Ibid., 19~8, No. 38, Ganapati, P. N. 49-79. Steeman Nielsen, E. '°The use of radioactive carbon (C 14) for measuring organic production in the sea," J. Cons. int. Explor. Mer., 1952, 18, 117-40. Strickland, J. D. H. and Parsons, A Manual of Sea- Water Analysis, Bull. Fish Res. Bd. Canada, T.R. 1965, No. 125. Suryanarayana Rao, S.V. .. "Preliminary observations on the total phosphorus content of the inshore waters of the Malabar Coast off Calicut," Proc. Ind. Acad. Sci., 1957, 45, 77-85. Wooster, W. S. and Rakestraw, "The estimation of dissolved phosphate in sea-water," J. mar. N.W. Res., 1951, 10, 91-100. Yentsch, C. S. and Ryther, J. H. '"Short-term variations in phytoplankton chlorophyll and their significance," Limnol. and Oeeanogr., 1957, 2 (2), 140--42.

--~ and Scagel, R.F. .. "Diurnal study in East Sound, Washington," J. mar. Res.. 1958, 17, 567-83.