Int.J.Curr.Microbiol.App.Sci (2014) 3(7) 31-39

31

Original Research Article

Seasonal variations and diversity of marine diatoms of Pudukkotai and Thanjavur districts south east coast of India

T.Gopalakrishnan1*, S.Jeyachandran2 and N.Ramakrishnan1

1Department of Botany, Governments Arts College for men, Kumbakonam, Tamil Nadu, India 2PG & Research Department of Botany & Microbiology, A.V.V.M Sri Pushpam College,

Poondi, Thanjavur, Tamil Nadu, India *Corresponding author

A B S T R A C T

Introduction

Diatoms are a major group of algae, and are the most common types of phytoplankton. They are eukaryotic, autotrophic microorganisms with a cosmopolitan distribution. Most diatoms are unicellular, although they can exist as colonies in the shape of filaments or ribbons (e.g. Fragilaria), fans (e.g. Meridion), zigzags (e.g. Tabellaria), or stellate colonies (e.g. Asterionella). Most diatoms are non-motile,

although some move via flagellation. Diatoms belonging to the class Bacillariophyceae. Size of individual diatom ranges from ~ 2 to 2 mm, but some chains can be several millimeters in length. Diatoms are ubiquitous in both marine and freshwater and sediments. Diatoms have proven potential for diversity and abundance in marine ecosystem (Stoemer and Smot, 1990). It shows a high diversity and is

ISSN: 2319-7706 Volume 3 Number 7 (2014) pp. 31-39 http://www.ijcmas.com

K e y w o r d s

Biodiversity, Marine Diatoms, Physico-Chemical parameters, India.

The present study Seasonal variations and diversity of marine diatoms of Jegathapattinam (Lat. 09º 95 N: Long. 79º 18 E) Kattumavadi ( Lat. 10º 13 N ; Long.79º 22 E ), Sethubhavachathiram (Lat. 10º 24 N : 79 º 28 E ) and Manora (Lat. 10º 26 N; Long.79 º 30 E) of Pudukkotai and Thanjavur District, South east Coast of India was carried out for a period of one year (from June 2010 to May 2011). For this study, four stations were chosen from the both districts.The study focuses attention on the survey, systematics of marine diatom diversity and the influence of physico-chemical factors on their seasonal distribution. During the study period, a total of 78 species belonging to 42 genera of marine diatoms were recorded from the four stations. The most common genera were Actinocyclus, Amphora, Bacteriastrum, Biddulphia, Chaetoceros, Coscinodiscus, Cyclotella, Diploneis, Gyrosigma, Licmophora, Mastogloia, Melosira, Navicula, Nitzschia, Pleurosigma and Tropidoneis were present in all the stations. Higher values of diatom population density were found during summer at both stations. The seasonal distribution and abundance are discussed in relation to physico- chemical parmeters.

Int.J.Curr.Microbiol.App.Sci (2014) 3(7) 31-39

32

responsible for typical phytoplankton blooms.

Diatoms are an important indicator of environmental changes where individual species respond directly or indirectly and sensitively to changes in chemical, physical parameters such as nutrients silicate, phosphorous, nitrogen, pH, light and temperature (Stevenson and Pan, 1999).

The fact that each diatom species has a specific optimum and tolerance for some environmental parameters including pH, salinity, temperature, nutrients and light availability, makes them particularly useful indicators for biodiversity (Lim et al., 2001). Hence the present work was undertaken to study the seasonal variation and diversity of planktonic diatoms and hydrographical parameters in Jegathapattinam, Kattumavadi, Sethubhavachathiram and Manora of Pudukkotai and Thanjavur Districts,. Southeast Coast of India.

Materials and Methods

The planktonic diatoms were collected from the surface water of the study areas by towing plankton net (mouth diameter 0.35 m) made of bolting silk cloth (No.35 mesh size 48 m) for 20 minutes. Water samples were collected from the surface during the monthly intervals for a period of one year (from June 2010 to May 2011) at four stations in Jegathapattinam, Kattumavadi, Sethubhavachathiram and Manora, of Pudukkotai and Thanjavur Districts. Water samples were preserved in 4 % neutralized formalin and used for nutrient analysis. For qualitative analysis of diatoms, the settling method described by Sukhanova (1978). Cleaning of diatom sample by Nitric acid Method (Hendey,1964). Identification of diatoms by Standard Manuals (Boyer, 1926;

Hustedt , 1927; Subrahmanyan, 1946; Hendey,1964; Desikachary 1986,1988).

1. Station I. Jegathapattinam (Lat. 09º 95 N: Long. 79º 18 E)

2. Station II. Kattumavadi ( Lat. 10º 13 N ; Long.79º 22 E ) 3. Station III. Sethubhavachathiram (Lat. 10º 24 N : 79 º 28 E ) 4. Station IV. Manora (Lat. 10º 26 N; Long.79 º 30 E)

Results and Discussion

Environmental Parameters and their Influence

Monthly variations in physico-chemical parameters viz, rainfall, air and surface water temperature, salinity, pH, dissolved oxygen, reactive silicate, inorganic phosphate, organic phosphate, nitrate, and nitrite were recorded for period of one year from June 2010 to May 2011.The Physico-chemical parameters of the present study areas are subjected to wide spatial temporal variations. The Physico chemical parameters of the present study areas are subjected to wide spatial temporal variations.

Rainfall is the most important cyclic phenomenon in tropical countries as it brings about important changes in the physical and chemical characteristics of the coastal and estuarine systems. In the present study, the total annual rainfall recorded from the study areas (I, II, III and IV) varied from 0.0 mm to 350.0 mm. Maximum (300.0 mm) rainfall was recorded during November 2010. Rainfall was totally absent during the months of March and May 2011. Similar observation was reported by Kaliyaperumal, 1992; Mathevan 1994.

Int.J.Curr.Microbiol.App.Sci (2014) 3(7) 31-39

33

Temperature is another important factor in the coastal and estuarine environments, which influences the physico- chemical characters. In general, air and water temperatures were recorded during the summer months. During the study period varied from 25 to 38ºC. The minimum (25ºC) was recorded during the months of August and December 2010 at station.IV and also in the month of January 2011 at station I. The surface water temperature varied from 22 to 37ºC. The minimum surface temperature was recorded during December 2010 at station II and maximum was recorded during (summer) April 2011 at station III. In general all the four station showed similar monthly changes. Similar observations were reported by Ramalingam Manikannan et al., (2011) from Vedaranyam Swamp of the Point Calimere, South-east coast of India, Sushanth and Rajashekhar (2012) from the coastal ecosystem.

Salinity is one of the key factor that determines the distributions of diatoms. In the present study, salinity range varied from 20 to 36ppt. All the stations showed similar monthly pattern in salinity distribution registering low value (20 ppt) during December 2010 (monsoon) at stations IV, and high value (38ppt) during the month of Sep 2010(summer) at station.IV. This could be due to the continues evaporation of water from the all study area especially during the summer seasons as observed by Palpandi( 2011) from Vellar estuary, South east coast of India. Similar trend in the salinity values were also observed from various parts in southeast coast of India (Seenivasan, 1998; Palanichamy and Rajendran, 2000; Sulochana and Muniyandi, 2005; Prabu et al., 2008; Soundarapandian et al., 2009; Damotharan et al., 2010). In all the above studies the salinity was found fluctuating widely which was mainly due to the

influence of rainfall and influx of the fresh water into the study areas.

The hydrogen ion concentration (pH) of water may influence many biological and chemical characteristics of marine waters (30 Saad, 1978). In general hydrogen ion concentrations fluctuated in all the stations during the study period and the temperature as suggested by several authors (Zingde et al 1985; Tiwari and vijayalakshmi 1993; 28 Soundarapandian et al., 2009; Sushanth and Rajashekhar, 2012; Medudhula et al., 2012). Similar trend in pH was reported by Seenivasan (1998) from the Vellar estuarine system, Palanichamy and Rajendran (2000) from Palk Bay, Prabu et al., (2008) from Pichavaram mangroves, Damotharan et al., (2010) from Point Calimere coastal water. Ramalingam Manikannan et al., 2011 from Great Vedaranyam Swamp of the Point Calimere Wildlife Sanctuary, South-east coast of India.

Dissolved oxygen showed a wide range of variations throughout the study period at all stations. In general, the maximum values recorded during the monsoon month and minimum during the summer month. This is attributed to the variations in freshwater inflow and tidal ingress (Eswari and Ramani Bai 2002). This was accompalished by lowering of salinity and air and surface water temperature values. This is in conformity with the earlier works that high concentrations of dissolved oxygen in low saline water coupled with low temperature during monsoon. It was noted that in all the stations, the standing crop of diatoms were more during summer months (when dissolved oxygen concentrations was low due to cessation of freshwater flow) as reported by Jeyachandran (1989) in Pitchavram mangroves.

Int.J.Curr.Microbiol.App.Sci (2014) 3(7) 31-39

34

Table.I List of isolated of Diatoms

S.No Name of the Diatoms S.No Name of the Diatoms 1 Actinocyclus ehrenbergii var.ralfsii (W.Smith) 42 Navicula granulate (Bailey) 2 Achnanthes hauckiana Grun. 43 N. lyra Ehr. 3 Amphora coffeaeformis (Ag.)Kutz 44 N. monilifera Cleve 4 A. marina (W.Sm.)V.H 45 N. rhynchocephala Kützing 5 A. proteus Grag 46 N.rostellata Kützing 6 A. ovalis (Kutz) (Kutz) 47 N. salinarum Grunow 7 Bacteriastrum furcatum Shadbolt 48 Navicula sp 8 Bellerochea malleus (Brightwell) Van Heurck 49 Nitzschia closterium (Ehrenberg)W-Smith 9 Biddulphia mobiliensis Bailey 50 N.flexa Schumann

10 B. retiformis Mann 51 N.granulate Grun 11 Caloneis permagna (Bailey) Cleve 52 N. insignis Greg 12 Chaetoceros lorenzianus Grun 53 N. obtuse var scalpelliformis (Grun) 13 Cocconeis costata Greg 54 N. punctate (Wm.Smith) Grun 14 Coscinodiscus centralis Her 55 N. punctate var.coarctata (Grun) Hust 15 C. kutzingii A.Schmidt 56 Nitzschia spp 16 C. normanii Gregory in Greville 57 Pinnularia viridis (Nitzsch) Ehrenberg 17 C. rothii Ehr Grun 58 Pinnulariosigma reana Desikachary,

etal(= Navicula reana (Castr) De Toni) 18 C. radiatus Her 59 Planktoniella sol (wallich) Schutt 19 Cyclotella striata (Kutz) Grun 60 Pleurosigma aestuarii (De breb.)W.Smith 20 C. stylorum Btoo 61 P. normanii Ralfs in Pritchard 21 Diplomenora cocconeiforma A & S Plaze 62 Rhabdonema mirificoom w.smith 22 Diploneis bombus (Ehr.) Her 63 Rhopalodia gibberula Kutz 23 D. crabro Her 64 Rhizosolenia alata Brightwell 24 D. interrupta (Kutz) Cleve 65 Skeletonema costatum (Greville) Cleve 25 D. subovalis Cleve 66 Surirella minuta Brébisson in Kützing 26 D. weissflogii (A.Schmidt) Cleve 67 Synedra formosa Hantzsch 27 Frustulia rhomboids (Ehr). Var. saxonica DeToni 68 S. fulgens (Greville) W.Smith 28 Grammatophora marina (Lyngb.) Kutz 69 Thalassionema nitzschioides Grunow 29 Guinardia flaccida (Castracane) Peragallo 70 Thalassiothrix frauenfeldii Grunow 30 Gyrosigma attenuatum (Kutz.) Rabh 71 Thalassiothrix longissima Cleve 31 G.balticum (Ehr).Cleve 72 Thalassiosira decipiens (Grunow) 32 G. distortum var. parkeri (M.B.Harrison) Cleve 73 Thalassiosira lineata Jouse (=

coscinodiscus lineatus van Heurck) 33 G. scalproides (Rabh.) Cleve. 74 Triceratium dubium Brightwell 34 Lauderia annulata Cleve 75 Triceratium favus Ehrenberg 35 Licmophora abbreviate Agardh 76 Trachyneis aspera (Ehr) Cleve 36 Mastogloia apiculata W.Smith 77 Tropidoneis Lepidoptera (Gregory) Cleve 37 M. smithii Thwaites ex.W.Sm 78 Tropidoneis vitrea (W.Smith) Cleve 38 Melosira nummuloides Agardh 39 M. sulcata (Ehr) Kutz. = (Paralia sulcata (.Ehr.)

Cleve) 40 Navicula arabica Grun 41 N. clementis var. linearis Brander ex Hustedt

Int.J.Curr.Microbiol.App.Sci (2014) 3(7) 31-39

35

Table.II Diatom population Density (cell numbers/ ml) during 2010-2011

Stations Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May

I 5652 5952 5831 5968 4240 3632 4096 5760 6010 10448 13600 1569

6

II 5888 6032 6011 6192 4368 2367 3984 4320 5090 10896 13680 1540

8

III 4560 4816 4713 5408 3856 3104 4320 5440 6100 11088 13552 1590

4

IV 5664 5904 5814 6048 3536 3328 4656 5040 5900 11248 14560 1579

2

Table.III Distribution of Diatoms in genera during June 2010 to May 2011 Stations S.No Name of the Diatoms

I II III IV 1. Actinocyclus 1 - 1 1 2. Achnanthes - - - 1 3. Amphora 2 - 3 2 4. Amphiprora 1 - - 1 5. Asterionellopsis 1 - 1 - 6. Bacteriastrum 1 1 - 1 7. Bellerochea - 1 1 - 8. Biddulphia 2 - 2 2 9. Caloneis 1 - - - 10. Chaetoceros 1 - 1 1 11. Cocconeis - 1 1 - 12. Coscinodiscus 3 2 2 - 13. Cyclotella 1 - 1 2 14. Diplomenora 1 - 1 - 15. Diploneis 2 - 4 2 16. Fragilaria - - - 1 17. Frustulia - - 1 1 18 Grammatophoa - - 1 - 19. Guinardia 1 - - 1 20. Gyrosigma 3 - 2 1 21. Lauderia 1 - - 1 22. Licmophora 1 - 1 1 23. Mastogloia 1 2 - 1 24. Melosira 1 - 1 1 25. Navicula 7 4 5 5 26. Nitzschia 3 - 3 2 27. Pinnularia 1 - - - 28. Pinnulariosiga 1 - - - 29. Planktoniella 1 - - 1 30. Pleurosigma 1 1 1 1 31. Rhabdonema - - 1 - 32. Rhopalodia - - - 1 33. Rhizosolenia - 1 - 1 34. Skeletonema - 1 - 1 35. Surirella - 1 1 - 36. Synedra 1 - - 2 37. Thalassionema 1 - 1 - 38. Thalassiothrix 1 - 1 - 39. Thalassiosira - - - - 40. Triceratium 1 - 1 - 41. Trachyneis - - - 1 42. Tropidoneis 2 - 2 2

Total 45 15 40 38

Int.J.Curr.Microbiol.App.Sci (2014) 3(7) 31-39

36

Fig.1 Variations in Diatom population density recorded during June 2010 to May 2011 at Station I

Fig.2 Variations in Diatom population density recorded during June 2010 to May 2011 at Station II

Fig.3 Variations in Diatom population density recorded during June 2010 to May2011 at Station III

Fig.4 Variations in Diatom population density recorded during June 2010 to May 2011 at Station IV

Int.J.Curr.Microbiol.App.Sci (2014) 3(7) 31-39

37

Higher dissolved oxygen concentration observed during monsoon season may be due to the cumulative effect of higher wind velocity joined with heavy rainfall and the resultant freshwater mixing (Das et al., 1997; Vijayakumar et al., 2000; Prabu et al., 2008; Damotharan et al., 2010).

Nutrients concentrations showed distinct seasonal variations. High reactive silicate present at the bottom sediments might go into upper surface layers when the bottom region is agitated by wind action during the monsoonal floods. Low values of silicate recorded during the summer may be due to the abundant planktonic diatoms for their biological activity (Gouda and Panigrahy, 1992). In addition to planktonic diatom uptake, some related processes like absorption and co-precipitation of soluble silicon might also govern the distribution of dissolved silicate in the marine environment (Sivakumar, 1982; Gouda and Panigrahy, 1992 and Sushanth and Rajashekhar, 2012).

Inorganic phosphate concentration ranged from 1.44 to 7.8µmL-1. Minimum concentration was observed during the month of May 2011 at station IV and maximum (7.8µmL -1) during the month of October 2010 at station IV. Similar observations have been made by Senthilkumar et al., (2002); Ramalingam Manikannan et al., (2011).

Organic phosphate concentration value varied between 1.9 and 14.83µm L -1. The lowest value was recorded during the month of April and May 2011 at station II and peak value was recorded in the month of July 2010 at station IV. The same results was obtained by various authors Gowda and Panigrahy, (1992); Sushanth

and Rajashekhar, (2012); Medudhula et al., (2012).

Nitrate concentration ranged from 2.0 to 9.86 µm L -1.The minimum value recorded was 2.0 µm L-1 during the month of Sep 2010 at stations IV. The maximum value recorded was 9.86µm L -1 during the month of October 2010 at station I. The same was recorded by Mathevan (1994) from Cuddalore Uppanar estuary, Satpathy (1996) from coastal waters of Kalpakkam. Das et al., (1997); Govindasamy et al., 2000; Medudhula et al., (2012); Sushanth and Rajashekhar, 2012.

Diatom Population Density

Diatom population density ranged between 2367 and 16696 cells/ml. Minimum population density was observed during the month of November 2010 (Late monsoon) and maximum was recorded in the month of May 2011 in all four stations (Table- II & Fig.-1-4). Minimum population was reported from station II (2367 cells/ml). Planktonic diatoms totalling 78 species belonging to 42 genera were collected from all the stations in the study area (Table- I & III).

In station I 45 species belonging to 29 genera, at station II 15 species 10 genera, at station III 40 species 25 genera at station IV ,38 species belonging 27 genera (Table- III). Similar observations were made by many workers (Nagasathya and Thajuddin, 2008; Ya-hui,Liang et al., 2011; Godrijan et al., 2012; Olakunbi Kunrunmi, 2012).

References

Boyer, C.S., 1926. Proc Acad. Natl.Sci.phila., 78 (78): 229-583.

Damotharan, P., Perumal, N.V., Perumal, P., 2010. Seasonal variation of

Int.J.Curr.Microbiol.App.Sci (2014) 3(7) 31-39

38

physico-chemical characteristics of Point Calimere coastal waters (South east coast of India). Middle-East. J. Sci. Res., 6(4): 333-339.

Das, J., Das, S.N., Sahoo, R.K., 1997. Semidiurnal variation of some physicochemical parameters in the Mahanadi estuary, east coast of India. Indian J. Mar. Sci., 26: 323-326.

Desikachary, T.V., 1986. Madras science foundation. Madras, pp.77.

Desikachary, T.V., 1988. Madras science foundation, Madras.pp. 67

Eswari, Y.N.K. and Ramani Bai, R.J., 2002. Mar. Biol. Assoc., India, 44(1-2): 205-211.

Godrijan, J., Mari , D., Ime ek, M., Janekovi , I., Schweikert, M. and Pfannkuchen, M., 2012. Diversity, occurrence, and habitats of the diatom genus Bacteriastrum (Bacillariophyta) in the northern Adriatic Sea, with the description of B. jadranum sp. nov. 55(4): 415 426.

Gouda,R. and Panigrahy, R.C., 1992. Seasonal distribution and behavior of silicate in the Rushikulya estuary, East Coast of India. J.Mar.Sci., 21:111-115.

Govindasamy, C., Kannan, L., Azariah, J., 2000. Seasonal variation in physico-chemical properties and primary production in the coastal water biotopes of Coromandel coast, India. J. Environ. Biol., 21: 1-7.

Hendey, N.I., Fishery inves. Ser,1964, IV, London, pp.317.

Jeyachandran, S., 1989. Ph.D, Thesis Annamalai University,

Kaliyaperumal, C., 1992. Ph.D. Thesis. Annamalai University, India, pp, 215.

Lim, D., Douglas, M. and Smol, J., 2001. Diatoms and their relationship to environmental variables from lakes and ponds on Bathurst Island, Nunavut, Canadian High Arctic.

Hydrobiologia, 450: 215 230. Mathevan P.M, 1994. Ph.D Thesis,

Annamalai University, pp.159. Medudhula,T., Samatha, C. and Chintha,

S., 2012. Analysis of water quality using physico-chemical parameters in lower manair reservoir of Karimnagar district, Andhra Pradesh. In. J. of Environmental Sci., 3(1): 12.

Nagasathya, A. and Thajuddin, N., 2008. Diatom Diversity in Hypersaline Environment. J. of Fisheries and Aquatic Science, 3: 328-333.

Olakunbi Kunrunmi, 2012 Effects of Physico-chemical Parameters on Phytoplankton of a Tidal Creek, Lagos, Nigeria. J. of Environment and Ecology, 3(1):2157-6092.

Palanichamy, S., Rajendran, A., 2000. Heavy metal concentration in seawater and sediments of Gulf of Mannar and Palk Bay, southeast coast of India. Indian J. Mar. Sci., 29: 116-119.

Palpand, C., 2011. Inter. J. Biodi. and Conser, 3(4): 120-130.

Prabu, V.A., Rajkumar, M. and Perumal, P., 2008. Seasonal variations in physico-chemical characteristics of Pichavaram mangroves, southeast coast of India. J. Environ. Biol., 29(6): 945-950.

Ramalingam, M., Subramanian, A. and Abdul Hameed, M.H., 2011 Seasonal variations of physico-chemical properties of the Great Vedaranyam Swamp, Point Calimere Wildlife Sanctuary, South-east coast of India. Afr. J. Environ. Sci. Technol. 5(9).

Saad, M.A.H .,1978.Seasonal variation of some physico-chemical conditions of Shatt al Arabestuary, Iran.Estuar.Coast.Mar.Sci., 6: 503-513.

Satpathy, K.K., 1996.Seasonal distribution of nutrients in the Coastal waters of

Int.J.Curr.Microbiol.App.Sci (2014) 3(7) 31-39

39

Kalpakkam, East coast of India. Indian J.Mar.Sci., 25:221-224.

Seenivasan, R., 1998. Spectral reflectance properties of the Vellar estuarine environment, southeast coast of India, M.Phil. Thesis, Annamalai University. India.

Senthilkumar, S., Santhanam, P.and Perumal, P., 2002. Diversity of phytoplankton in Vellar estuary, southeast coast of India. In: The 5th

Indian Fisheries Forum Proceedings. Jena SJK & M Mohan Joseph (Eds.). Published by AFSIB, Mangalore and AeA, Bhubanewar, India.

Sivakumar, V., 1982. Ph.D.Thesis, Annamalai University, India, pp:78.

Soundarapandian, P., Premkumar, T., Dinakaran, G.K., 2009. Studies on the physico-chemical characteristic and nutrients in the Uppanar estuary of Cuddalore, South east coast of India. Curr. Res. J. Biol. Sci., 1(3): 102-105.

Stevenson, R.J., and Pan, Y., 1999, Diatoms as indicators of coastal paleo- environments and relative sea-level change, in Stoermer, E.F., and Smol, J.P., eds., The diatoms: Applications for the environmental and earth sciences: New York, Cambridge University Press, p. 277-418.

Stoemer, E.F. and Smol J.P.,1990. The diatom application for the environmental and Earth science, Cambridge Univ, Press, United Kingdom. pp. 469.

Subrahmanyan, R., 1946. Indian Acad. Sci., 24(13): 85-197.

Sulochana, B., Muniyandi, K., 2005. Hydrographic parameters off Gulf of Mannar and Palk Bay during an year of abnormal rainfall. J. Mar. Biol. Assoc. India, 47(2): 198- 200.

Sushanth, V.R., Rajashekhar, M., 2012 Seasonal variation in diatoms in

response to physico-chemical characteristics of coastal waters of Uttara Kannada district, West Coast of India. In.ternational J. of Environmental Sci., 2 (3):1543- 1552.

Tiwari and Vijayalakshmi.,1993.Zooplankton composition in Dharamter Creek adjoining Bombay harbour. Indian. J. Mar .Sci., 22:63-69.

Vijayakumar, S.K., Rajesh, K.M., Mendon, R. and Hariharan, V., 2000. Seasonal distribution and behaviour of nutrients with reference to tidal rhythm in the Mulki estuary, southwest coast of India. J. Mar. Biol. Assoc. India, 42: 21-31.

Ya-hui, L., Jun-rong, C., Chang-ping, L.I., Xue-song, P., Xing-yue, L., 2011. Studies on Biodiversity and Ecological Importance of Marine Diatoms GAO. Journal of Xiamen University (Natural Science).

Zingde, 1985., Physico-chemical investigation in Auranaga river estuary(Gujarat). Mahasagar-Bull. Natl.Ins.Oceanogr., 18:307-321