Ecology of Mangroves in Mandovi & Zuari Es~Uaries & The

Indian Journal of Marine Sciences Vol. 1. June 1973. pp. 47-53 I I Ecology of Mangroves in Mandovi & Zuari Es~uaries & the Interconnecting Cumbarjua Canal of qoa*

A. G. UKTAWALE, S. N. DWIVEDI & S. Y. S. SINGBAL I National Institute of Oceanography,Panaji I

Received 5 May 1973 I

Mangroves are absent in the mouth regtons (6 to 7 km lon~) of both the e~ltuaril'S. On either bank they have sandy beaches or rocky shores. These ZOneshave high wa action. high salinity and low ternper-arure, The adjoining reaches of estuaries are the zones of mang rove vegetation. extending from the lower part of the middle reaches right up to t e upper reaches of these estuaries. Cumbarjua canal is also included in this ZOnewhere the ~ave action Is weak, the sandy beaches are replaced by mudftats and the estuarine water Is sll~htly warmer, with lower salinity and higher oxygen. The mangrove flora of these estuaries Includes 31 species of 29 genera belonging to 19 famfltes of angfosperrns. Succession of species, associatton, dominance and other aspects of vegetation like density, adaptation and height have also Ibeen discussed.

i ANGROVES are widespread all along the can be classified into 3 b~oad categories, viz. Man- Indian coastline and on the west coast of d0:ri estuary, Zuari estu~ry and Cumbarjua canal M India in the estuarine environment many (FIg. 2). i rivers form mangroves. M andovi estuary - Th~; estuary is about 3 km Macnae! has reviewed flora, fauna and other as- wide at the mouth while i narrows down to 0·5 km pects of mangrove swamps. These swamps are in width upstream. In t e region of the confluence highly productive", however, they are not properly the tidal action is high coupled with strong currents. utilized for fish production". Prain? has studied It has sandy beaches on i either banks at Miramar mangrove flora of Sunderbans. Estuarine man- and Verem. Pronounced influence of the wave groves on the west coast of India have been des- action and the current i$ seen up to the head of cribed earlier>", However, no work appears to Chorao Island (6·5 km). !The effect of these factors have been done on mangroves of Goa region. gradually decreases upstream towards Savaiverem. General aspects of coastal lagoons and mangroves Zuari estuary - The mouth region of the estuary have been discussed by Chapman'? and Rutzler-'. is approximately 5 km wide while upstream its width narrows down to less th~n 1 km. Marked tidal Description of Area amplitude with heavy wlve action is present up to In Goa, the Mandovi and Zuari rivers form an Siridao head. In this r~ion sandy beaches inter- important estuarine complex. They are 61·6 and spersed with rocky porti ns are present from Dona 62·4 km long respectively. The total catchment Paula to Siridao head a d from Vasco to near St areas of these estuaries cover two-thirds of the Goa Jacinto Island on the no them and southern banks territory. respectively. The estu~a.ne environment extends The average rainfall in Goa region is 2611 tutu] upstream up to Savarde (28 km). annum. This rainfall is not uniformly distributed Cumbarjua canal- Ma dovi and Zuari estuaries throughout the rainy season, but the major part of are interconnected by th s canal which is approxi- the rainfall occurs during June to August, resulting mately 15 km long an 0·5 to 0·75 km wide at in the heavy flooding and active erosion. It gradually decreases during the postmonsoon period. The 900 r premonsoon period is almost dry (Fig. 1). Both these estuaries are surrounded by a hilly 800 r terrain with dense vegetation, hence the lateritic 700 soil of these forests is rich in humus. The soil is heavily leached and become deficient in lime, potash 600 and phosphorus, whereas the soil in river deltas is ~500 comparatively richer in these nutrients-". The high ~ 400 nutrient content, along with low wave action and ...• absence of pronounced currents near the banks leads ;#.300 u, to a favourable environment for the growth of man- <{~200 groves. It is seen that in the areas where such Q: conditions prevail, the mangrove flora is rich 100 and present in varying density in different parts 50 O~~~~~-±-=~~ __~~~~ of the estuarine systems. The entire system . JUL. AUi. SEP. OCT. NOV. DEC. JAN. *This paper is a contribution under the project" Ecology of Mangroves around Goa ", No. 201.3 of the Biological Fig. 1 - Average annual rai fall in Goa during 1931 to 1960 Oceanography Division, NIO. (From Climatological Tab s of Observatories in India)

47 INDIAN J. MAR. SCL, VOL. 2, JUNE 1973

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74E 5 Fig. 2 - Map of Goa sho ing mangroves in Mandovi and Zuari estuaries and interconnecting Cumbarjua canal

different places during it co~rse. T~e in~u~nce is established resulting in the absence of man- of wave action and current n this canal is negligible. groves. On either side it has mud ats and the bottom has In the zone of mangrove abundance, the influence a mixture of clay and san in different parts. . of wave action and current is considerably low and This major estuarine s stem can be classified the substratum is rich in humus and soft silty mud. further in two different ones in relation to the All these factors together create a favourable environ- mangrove vegetation (Fig. 2). ment for mangroves (Plate I, Figs. 1-4). Zone of absence - In t e Mandovi estuary, the . zone from its confluence to he head of Chorao Island Results and Discussion (6 km) is devoid of mang ve vegetation, while in Zuari there are no mangr ves from the confluence Environmental Parameters region to the Siridao head ( km). The main factors These tropical estuaries undergo strong seasonal for the absence of mangro es in these parts of the changes in their environment. The variations are estuaries are the high in ensity of wave action, of different magnitude at all 5 stations located in strong currents and a roc y or sandy substratum. this estuarine system, but they follow a certain Zone of presence - In t e Mandovi estuary the generalized pattern. The changes in temperature, mangroves start from the h ad of Chorao. Island and salinity and oxygen are given in Fig. 3. extend for a distance of 34 k ,up to Savaiverem. In Temperature - The confluence regions of the two the Zuari estuary the zone f mangroves starts from estuaries are the zones where mangroves are absent. east of Siridao head and ontinues till Savardem They have slightly lower temperatures as compared for a distance of about 28 In Cumbarjua canal to the zones of mangrove vegetation. During differ- the mangroves are prese t on both the banks ent seasons, the difference in the mean seasonal throughout its length (PIa e I, Fig. 1). temperature is of the order of 1°C. Amongst the In both the estuaries b yond Savaiverem and zones of mangrove vegetation, in Cumbarjua canal Savardem a practically reshwater environment the waters are still warmer. The temperature 48 UNTAvVALE ei al.: ECOLOGY OF MANGIWVES

Plate 1- Mangroves in Zuari and Mandovi estuaries an d in Curnbarj ua canal [Fig. 1:Den e ar.d tall vegetation in Cum- barjua canal. Fig. 2: Mangrove vegetation in Zuari estuary. Fig. 3: Beginning of mangr ve zone in Man dovi estuary. Fig. 4:Beginning of mangrove zone in Zuari estuary.Fig. 5: Prop roots of Rhizophora muc onat a, Fig. 6: Knee roots of Bruguiera sp. Fig. 7: Pneumatophores of Atricennia officinalis. Fig. 8: A canthus iticiJoti s , a member in association] ranges from 27'7° to 30'2°C (mean temp., 2S·9°C). premonsoon. In the mon oon the salinity becomes It appears that there may not be significant tem- very low, because of he y rains (Fig. 3). perature differences amongst the areas where the Dissolved oxygen - Du ing the monsoon period, mangroves are present, but the zones where man- due to freshwater influx, he variation in dissolved groves are absent have distinctly lower temperatures oxygen increases (range, 3·3 to 5·5 mljlitre). In (Fig. 3). this season sharp differen es are noticeable between Salinity - Salinity is the most important factor the zones of mangrove v getation in lower reaches in the estuarine environments.The salinity at and the zones where ma groves are absent, which station No i, 1 and 5 is higher. In the zone correspond with the mo th areas of the estuary. of mangrove vegetation the salinity varied from The mouth zone has 1 wer dissolved oxygen as % , 1 to 34.5 0 Further, there are sharp seasonal compared to the zones of mangrove vegetation. differences during the monsoon, postmonsoon and In the former, dissolve oxygen varied around 49 INDIAN J. MAR. SCI.. VOL. 2. JUNE 1973

~ 6 A 5 ~ ;!! 5 -~- 1 e•.. 4 4 "2 UJ 3 l.:> >- x 0 2 3

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35 35

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32 c 31

30 .u.. 29 a ~ 28 •....UJ 27

26 25 FEB. MU. APR. MAY JUN. JUL. IIU6. SEP. OCT. NOV. DEC. JAN. 2 3 4 5 (STATIONS)

Fig. 3 - Seasonal variations n oxygen (A). salinity (H) and temperature (C) around mangrove swamps durir.g 1971·72 [Curves 1 to 5 represent data r station Nos. 1 to 5. a, band c represent generalized picture of oxygen, salinity and tern- perature respectively fo premonsoon (Prc.M), monsoon (M) and postmonsoon (Post.M) for station Nos. 1 to 5]

3·6 ml/Iitre and in the lat er from 4·5 to 5·5 ml/litre compared to monsoon months, however, the differ- (Fig. 3). ence is not very appreciable. During the premon- During the postmonso n period, the conditions soon period the values vary from 3·3 to 4·4 mljlitre. are almost uniform in all t e zones and the dissolved Dissolved oxygen in the zones of abundance at oxygen values fluctuate be ween 3·1 and 5·2 rnljlitre. station Nos. 1,3 and 4 is higher. Thus the dissolved It is thus seen that in the postmonsoon period oxygen values show seasonal variations in different amplitude of variation be omes slightly narrower as zones. 50 UNTAWALE et al.: ECOLOGY OF MANGROVES

Environmental data for oxygen, salinity and a few grasses like Sporobolu , Diplanchne and Chloris. temperature for monsoon, postmonsoon and pre- Apart from these associatio s, the individual species monsoon period indicate a generalized picture (Fig. sometimes form pure colo ies.Detailed enumera- 3a, band c respectively). In all these seasons, in tion of species found in t e mangrove swamps of the zones of mangrove vegetation, located in lower both the estuaries is as fo lows: reaches of the estuaries and in the canal, the temperature is slightly higher, the salinity lower and the Family SPecies dissolved oxygen higher. Sterculiaceae 1. Heriii ra littoralis Dry. Nutrients - Phosphates and nitrates have an im- Papilionaceae 2. Erythrina uarigeta var. orientalis portant role on plant growth in these environ- Merr, ments-". These are studied for a period of 1 yr at 3. Ponga ia pinnata Ben th. 4. Derris trifoliata Lour. each station. The mean values for different seasons Caesalpiniaceae 5. Caesal inia nuga (L.) Ait. are given in Fig. 4. Rhizophoraceae 6. Rhizo hora mucronate Lam. Phosphate concentration in both the zones during 7. Brugu 'era conjugata (L.) Merr. the monsoon and postmonsoon periods is of the same 8. B. pa viflora Wight & Arn, Combrctaceae 9. Lumn tzera recemosa Wi lld. order and the values are around 0·5 to 1·25 [Lgat/ Sonneratiaceae 10. Sonne atia caseolaris Engl. litre. Later during the premonsoon months their Turneraceae 11. Turne a ulmif olia var. angustifolia value increases and ranges between 1·2 and 1·5 [Lg Willd. at/litre. Goodeniaceae 12. scaev~a frutescens Krause syn, S. koeni ii Vahl Nitrate is present in low concentration at all the Myrsiniaccae 13. A egic ras corniculata Blanco stations during the monsoon months. The values Apocyanaceae 14. Cerbe a manghas L. (Cerbera odol- increase during the postmonsoon period and decrease lam aertn.) again in premonsoon period. Asclepiadaceae 15. Leptaifenia reticulate Wight. Further it is seen that throughout the year in Convolvulaceae 16. Brew ia cordata Bl. 17. I pom ea grandiflora Lam. the mouth region of these estuaries, where man- 18. Opere lina turpethum. (L.) Silva groves are absent, the nitrate values are lower, but Mans phosphates do not show such concentrations. Acanthaceae 19. A can IUS ilicifolius L. Verbenaceae 20. Prem a integrifotia L. Floral Composition 21. Vitex trifolia L. 22. Clero endron inerme (L.l Gaertn. Principal dominant species in the zone of man- 23. Avice nia officinalis var. tomentosa L. groves in the two estuaries are: A vicennia 0fficinalis, Rhizophora mucronata, Sonnertia caseolaris and Euphorbiaceae 24.Exco caria agallocha L. Palmae 25. Cocos nucifera L. Bruguiera sp. The common plants in association Cyperaceae 26. Cypefus malaccensis Lam. with the dominant species are Acanthus ilicifolius, 27. C. 1'0undus L. Aegiceros corniculata, Clerodendron inerme, Vitex 28. Fimb istylis polyt1'ichoides Vahl trifolia, Pongamia pinnata, Cocos nucifera along with Graminaeae 29. Spar bolus maderaspatanus Bor. 30. Dipl nclme fusca (L.l Beauv. 31. Chlo 's quinguesetica Bhide 8 Though the same speci s are found in both these 7 estuaries, they show rem rkable differences in their growth and density in som parts of Mandovi and Zuari 6 estuaries and Cumbarjua canal. In the canal the average "height and the gr wth of mangroves is more ~ 5 - than in any part of the stuaries (Plate I,Fig. 1). "0 4 Similarly the number of r ws of the plants on either banks is more in Cum arjua canal as compared Cl , 3 with the estuaries. z Succession oj spectes I 2 o(Tl R. mucronata, commonl known as red mangrove, z 1 is the species which est blishes itself first on the 0 banks. Usually the manfrove swamps are protected 2 3 4 5 by R. mucronata with supporting root system, which form the front row towarqs water-front. These trees STATIONS •• grow up to 6-8 m high a d have very dense canopy 2.0 of the fleshy leaves (Fig 5). - Thus the red mangro es create an essential en- PRE.M -"0 1.5 vironment for the blaCkti angroves - A. officinalis. ,Cl Avicennia, soon after t e seedling stage, develops 1.0 a horizontal root syste , which gives rise to an- 0.•.• other special type of aeri 1roots - pneumatophores. I ·5 o<:t These roots occupy a vas area helping in the forma- 0. 0 tion of a suitable sub tratum by trapping silt. 2 3 4 5 Meanwhile the shoot s stem also develops into a dense canopy. Decayed leaves of these plants also STATIONS play an important role i the enrichment of the sub- Fig. 4 - Distribution of phosphates and nitrates in man- stratum. The root sys ms of Rhizophora sp. and grove swamps of Goa during 1971-72 [Pre.M, premonsoon; Avicennia sp. give addi ional support to the plant. M, monsoon; and Pcst.M, postmonsoon] Thus the plants can su tain high wave action and

51 INDIAN J.MAR. SCl., VOL. 2, JUNE 1973

the presence of horizontal root system spread over an extensive area for mechanical support. It further gives rise to aerial roots or pneumatophores, to perform function of respiration (Fig. 5). Bru- guiera conjugata and B. parviflora develop typical knee roots which come out of the substratum (Plate I, Fig. 6). The root systems of all these species are sub- merged under water during high tide and exposed completely at low tide. These mangrove plants are characterized by vivi- HT parous germination. They fall in the soft mud and soon develop a root system. L------4------~~~LT Discussion Fig. 5 - Diagrammatic rep esentation of typical species In this study ecological conditions in different succession in a mangrove swamp parts of the two estuaries which provide favourable and un favourable environment for mangrove growth have been investigated. These two estuaries are ultimately prevent erosio. Diagrammatic represen- very different from each other. Mandovi is a posi- tation of typical species s ccession in these estuaries tive estuary where freshwater influence is greater is shown in Fig. 5. while Zuari is a negative estuary which has predo- Association - After t establishment of these minantly marine influence-v-". high trees, shrubby bush s, herbaceous species and The mouths of these estuaries lie in close proxi- climbers make their ap earance. A. ilicifolius, A. mity and are separated only by a wry narrow ridge corniculata, V. trifolia, C inerme are some of the but earlier studies have shown that they are very species which form associ tions. Also some of these different biologically and ecologically-". These two species form their own si gle species colonies in the estuaries differ considerably in the amount of fresh- mangrove swamps (Plate ,Fig, 8). Apart from these water run-off, the intensity of waves an (1 other associations, in the mud y substratum around the hydrological factors. In spite of these differences mangroves sometimes gr w patches of blue-green in the mouth zone of both the e estuaries conditions algae (Nostoc sp.). Rising pneumatophores also show are un favourable for mangrove vegetation. The the presence of diatoms while the pools beneath common adverse factors for mangrove growth are mangrove trees are rich n plankton, the type of substratum on the banks, intensive wave In different parts of t estuaries a cross-section action, strong current and predominantly marine from water-front side to rds the land show differ- characteristic of the waters. These prevent the ent species. A typical p ttern of zonation is shown settlement and growth of mangrove seedlings. in Fig. 5. _ Towards the lower reaches of both these estuaries In the places where angroves extend over a in station Nos. 2, 3 and 5 the environment changes. wide part of a bank, he Rhizophora are seen The wave action becomes weaker and modifies the towards the water-front. This species is compara- environment. The sandy beaches are replaced by tively shorter than Avicen ia and develops a typical mudflats and mangrove vegetation starts in this Toot system. The edge 0 their canopy correspond region.First Rhizophora sp. which can sustain high to the level of highest hi h water-mark. Towards wave action are seen in first row towards water-front. the landward side Rhizo hora are succeeded by Towards the upper region of the' e tuary the Avicennia. These are tier trees having a root influence of tide action decreases further and a system occupying an exte sive area, In succeeding second row of A vicennia sp. is seen along the banks. generations depending up n the favourable condi- Sometimes along with the A tncennia sp. some asso- tion they slowly encroach owards the land and also ciated species also grow (Plate I, Fig. 8). extend further towards t e water-front. Gradually towards upper reaches the mangrove However, in this area ong the banks, both the becomes more extensive and dense. These condi- estuaries have agricultur 1 fields and the bunds tions are further modified in Cumbarjua canal, where of these fields are under constant care and main- the wave action is comparatively less and the banks tenance. These act as ba iers for further landward have mudflats. Thus the Environment is most extension of mangroves. urther extension of these favourable for mangrove growth. Though the canal zones, as reported by Bat istini-", is not present in has most favourable environment for mangrove, these estuaries. it does undergo considerable seasonal fluctuations Adaptation - All mangr ve species have adopted in the environment. These are brought about the suitable root systems 0 grow in adverse condi- during monsoon months due to heavy rainfall, most tions. They show differe t type of root systems of which comes within a short period of two months in different plants. Here he tap root is not very during June and July (Fig. 2). deep and hence unable to upport the tree. There- The changes brought about due to the influx of fore, special root systems e developed which grow fresh waters in different parts of the estuaries are horizontally covering an xtensive area (Plate I, broadly characterized by salinity (Fig. 3). In spite Figs. 5-7). of these fluctuations it is seen that at any time of In R. mucronata the obli ue prop roots give addi- the year Cumbarjua canal, as compared to the tional support to the plant s a result of which these mouth region of both the estuaries, has lower salinity, species can grow in the zone of wave action. higher oxygen and lower temperature (Fig. 3). The A. officinalis and S. caseol ris are characterized by seasonal variations are also seen in the concentration 52 UNTAWALE et al.: ECOLOGY OF MANGROVES of phosphates and nitrates. Phosphates are rich They are grateful to r Donald S. McLusky for in Cumbarjua canal during premonsoon months going through the manu cript. and nitrates are rich in postmonsoon months. During monsoon months the concentration of both References phosphates and nitrates become low. It is thus 1. MACNAE, W., Adv. mar. Biol., 6 (1968), 74. seen that in spite of seasonal variations in hydro- 2. VANNUCCI, M., Loguno costeras, Un Simposio, Mem, graphy and nutrients that in any season they do Simpo. Intern., UNA -UNESCO, Mexico, 1969, 457. not become limiting factors. Thus Cumbarjua canal 3. TAMPI, P. R. 5., Logun s costeras, Un Simposio, Mem, presents most favourable environment for mangrove Simpo. Intern., UNA -UNESCO, Mexico, 1969, 479. 4. PRAIN, D., Rec. bot. S1I v. India, 2 (1903), 197. growth in this area. These get established and 5. BLATTER, E. sj., j. Bom ay nat, Hist. Soc., 16 (1905), 644. stabilized during non-monsoon months and therefore 6. MUDALIAR, SHRI C. RA SEKHARA & KAMAT, H. S., J. during monsoon months at least for short periods Bombay nat. Hist. Soc., 52 (1952), 69. mangroves can stand variations in salinity from 7. NAvALKAR,B.S.,j.Bom ay nat. Hist.Soc., 53 (1956),335. 8. NAvALKAR, B. S. & BHA UCHA,F. R., J. Univ. Bombay, almost fresh water to marine conditions. (1940), 58. Towards the upper reaches beyond Savardem in 9. VENKATESWARALU,J., . Bombay nat. Hist. Soc., 44 Zuari estuary and Savaiverem in Mandovi estuary (1944), 431. when freshwater conditions prevail almost through- 10. CHAPMAN, V. J., Logun s costeras, Un Simposio, Mem. Simpo. Irrtern., UNA -UNESCO, Mexico, 1969, 505. out the year the mangroves are absent. 11. RtiTzLER, K., Logunos costeras, Un Simposio, Mern. In both the estuaries and Cumbarjua canal in Simpo. Intern., UNA -UNESCO, Mexico, 1969, 515. spite of favourable environment, the landward 12. DHEPE, P. D., Geology of Goa, Doctoral thesis, Poona extension of mangrove is checked on either banks University, 1956. 13. McLuSKY, D. 5., Ecolog of estuaries (Heinemann Educa- due to constant building of bunds for agricultural tional Books Ltd, Lo don), 1971. purposes. 14. BATTISTINI, R., Memoi es de L'Institut Scientifique de Madagascar, Serie F, orne III, 1959, 121. Acknowledgement 15. PRITCHARD, D. W., An stuaries, edited by G. H. Lauff Authors are thankful to Dr N. K. Panikkar, (A.A.A.S. Publication, Washington), 1967, 3. 16. DEHADRAI, P. V. & BHA GAVA,R. M. S., Indian j. mar. Director, for constant encouragement and facilities. Sci., 1 (1972), 52.