1 Environmental Information System, Centre for Advanced study in Marine Biology, Parangipettai - 608 502, Tamilnadu, India
Abstract
Comparing old data with present data is the only way to analyse the ecosystem and assess changes caused by nature and anthrophogeic influences. Coral reef constitute a sensitive and fragile ecosystem little researched in India. The coral reef of Palk bay has been thoroughly studied by Gopindhapillai (1969), Thomas (1969), Appukuttan (1969), Mallik (1983), Silas and Fernando (1985), Umamaheswara Rao (1989), Marichamy (1983). This paper aims to analyse the present status of the coral reef of Palk Bay by comparing it with previous studies. The conclusion is that the Palk Bay coral reef ecosystem is in good health but certain areas need attention.
Distribution of Palk Bay Reef
The reef in Palk Bay runs parallel to land (east to west direction) between longitudes 79° 17' E and 79° 8'E at the latitude 9° 17'N. The Bay is a very shallow flat basin and the depth never exceeds 15 metres. The average depth is 9 meters. The coral reef in Palk Bay starts from Munakad as a wall-like formation 1-2 m broad and run east upto Tonithurai a distance of nearly 5.5 km. Here the reef width is more than 300 metres. East of Pamban pass, the reef again starts near Thangachimadam and ends near Agnitheertham (Rameswaram) (Mahadevan and Nagapan Nair 1969). The Palk strait between India and Ceylon is about 75 km wide, with a water depth of 9-13 mt, except where local coral reef rises above sea level.
Physical Parameters of The Reef
The south-west and north-east monsoons are both active in this area but the south west monsoon contributes very little to the total annual rainfall. Rain is moderate to heavy during the period October to January. The mean annual rainfall varies from 820 mm to 1650 mm. The monthly average temperature of the waters of Palk Bay ranges between 24.6°C and 29.1°C (minimum during December and the maximum during May). The tidal elevation is around 1 m. Palk Bay is generally calm, except during the onset of the north-east monsoon when turbulent condition prevail (C.S. Pillai, 1969). The salinity of the water decreases gradually along an axis in the south-west direction, running from the strait. There is a point of high saline water in the south-west corner of the bay. This may be because of the incursion of Gulf of Mannar water through the Pamban pass.
Map 1 - Map showing the Palk Bay coral reef
The density of the water also decreases along an axis in the south-western direction from the strait. The strength of the north-east wind at Nagapattinam is about 8 to 10 knots (North of Palk Bay) and at Pamban the wind strength is only 2 to 4 knots. The temperature, salinity, density and dissolved oxygen of the surface waters of the Palk Bay indicate that the Bay of Bengal waters entering the Palk strait have a major influence on the hydrographic condition of Palk Bay. The Gulf waters influence the hydrological parameters only to a minor extent (Murty and Udayavarma, 1964). Silt settlement has a remarkable effect on the distribution and diversity of coral reef associated plants and animals. This appears to have a greater influence in the inshore regions of Palk Bay especially during the north-east monsoon.
The inshore waters of Palk Bay become muddy during the monsoon because of the presence of suspended sand and silt stirred up from the sandy shore by wave action. Cyclonic winds with high velocity capable of generating mechanical damage to corals of this area are common during the monsoon season. A severe cyclone in 1964 with a wind speed of 100 k. devastated most of the corals (Pillai, 1971).
The bottom sediments of Mandapam consist of silt and clay, clayey silt and sand, fine to medium sand, coarse sand, and coarse sand with gravel. Distribution of various size classes indicates that the offshore sediment in this area is usually unimodal with the primary mode around 1.5 to 2 (medium sand). A secondary mode is sometimes present around 3.5. Beach samples have a prominent mode around 2.25, 1.75, 2.75 and 3.25, suggesting the polymodal nature of the sediment (Mallik, 1983).
Review of the Coral Reef Ecosystem of Palk Bay
Gopinadha Pillai (1969) classified the reefs of Palk Bay into five zones - shore, lagoon, shoreward slope, reef crest and seaward slope. The shore of the reef is mostly sandy with dead pieces of corals, except at the extreme eastern and near the Pamban bridge where one can see traces of sandstone's. The vegetation on the shore comprises Cocos nucifera, Borassus flabellifera, Casurina equisetifolia, Azadirdicata indica and few other thorny shrubs.
The width of the lagoon varies from 200 to 600 meters at different places with a depth of 1 to 2 metres. The bottom is sandy with molluscan shells and pieces of disintegrating corals. Living corals are practically absent in the lagoon, probably due to the absence of any hard substratum on which coral planulae can settle. Sponges such as Hercina fusca, Dysidea fragilis, Spirastrella inconstans and Calispongia diffusa are fairly common at the bottom. The vegetation is composed of Cymodocea sp, Ulva reticulata, Turbinaria sp, Padina sp, Halimeda sp and Amphiora sp are common Holothuria scabra, Holothuria arta and Pentaceraster australis are common inhabitants of the sandy lagoon floor (Pillai 1969).
Corals distributed along the shoreward slope are encrusting and massive types with comparatively large polyps, such as Favia pallida, Favus, Favites virens, Goniastrea pectinata, G retiformis, Platygyna lamellina, Hydrophora sp., Cyphastrea sp., Leptastrea sp., symphillia sp. and Goniopora sp. Living colonies of Ponies sp are rare or small in size. Galaxea fascicularis, Turbinaria peltala, Pavona varians are the rarest species. This zone of the reef supports a good many reef dwellers like encrusting sponges, bryozoans and calcareous algae. Among the fleshy corals Lobophylum sp and Sarcophylum sp are represented.
The reef crest is often completely exposed at low tides. Corals are very rare at the reef crest, probably because of the influence of exposure to sun light. However Heptastrea transversa and Goniopora duofaciata are occasionally seen under the rocks.
The coral growth of the reef along the seaward side slope is comparatively richer than on the shoreward side. Majority of corals are ramose genera viz., Pocillopora sp, Acropora sp and Montipora sp. The vegetation comprises of Turbinaria sp, Sargassum sp, Padina sp, Caulerpa sp and rarely Cymodocea sp. Halimeda sp and a few other encrusting calcareous algae are commonly seen.
A total of 61 species of algae has been collected. They are distributed among the three major groups - green algae (14 genera and 28 species), brown algae (8 genera and 13 species), red algae (17 genera and 20 species). The frequency occurrence of different species in the quardrat samples show that Halimeda opuntia is the dominant algal member of the reef. Species of Caulerpa and Sargassum are the other most common plants found in the reef. The physical conditions such as the nature of the substratum and water level above the substratum, influence the distribution of flora in the coral reef area (Umamaheswara Rao, 1989). Boring sponges are the major group among the marine organisms causing considerable destruction to the reef system. The bores made by the sponges weaken the entire reef, making it more susceptible to the wear and tear caused by the waves. There are altogether 20 known species of boring sponges from the Gulf of Mannar and Palk Bay, falling into nine genera. The most conspicuous genus is Cliona, both in number of species and in distribution (Thomas, 1969). Among the coral boring organisms, bivalve molluscs cause considerable destruction to coral reefs. They act as biological agents in the erosion of hard coral stones. In Palk bay and Gulf of Mannar, only 17 boring bivalve species have been recorded from this area (under 10 genera of six families) (Appukuttan, 1969). Asir Ramesh (1996) recorded a total of 73 species of molluscs associated with corals in Palk Bay viz., 46 species of gastropods belonging to 17 families, and 27 species of bivalves belonging to 13 families.
The dried sea horse (Hippocampus kunda) is in great demand in south-east Asian countries, especially in Singapore and China - not only for extraction of soup which is a delicacy but also for its medicinal values. Along the Ramnad coast, the dried sea horse is used as a medicine to arrest whooping cough in children. The dried sea horse is finely powdered and the roasted. This powder is mixed with honey and administered as a engulfing medicine. In some places the powder is mixed with coconut oil and pasted on the cut wounds. It is also used for curing asthma (Marichamy et al., 1993).
Dugongs are long living animals with a low reproductive rate. They have a long gestation period and a large gap between each off spring. Around 25 dugongs were caught accidentally in this region during 1960. In Palk Bay Karangadu, Nambuthaalai, Morepanai and Mullimunai are minor fishing villages. Valivalai (drift net) shore seins and Thirukkaivalai are used to capture dugong in the shallow regions. Explosives (Country bombs and dynamites) are used for capturing the dugong in Thiruppalaikudi and Devipatnam (Ramnad District). During the 1960's the fisherman of Palk Bay region bitterly complained about the disappearance of large beds of algae owing to the cyclone in 1964, and turtles and dugongs almost disappeared in this area. Fishermen, now report that the algal beds have sprung up once again (Silas and Fernando, 1985).
Present Status of Palk Bay
Nearshore areas of Palk bay are polluted because of increased coastal urban development. Sewage outlets are increasing the suspended load, turbidity, nutrient etc. The coral reefs are under stress wherever processing industries let out their sewage. The indiscriminate cutting of near shore forest, leads to coastal soil erosion with huge quantities of nutrients that aggravate the physical stress on the coral reef.
The Palk Bay lagoon has a width of around 230 m. from the shore. The lagoon contains a large number of boulders, occupied by various species of scleractinian corals. Table reef are also found in the lagoons. These newly found boulders and table reef are formed by a process of wind drift. The green algae population is greater in areas close to the sewage outlets of processing industries than in healthy reef systems. Perna virdis, a rare component of the coral reef ecosystem, is densely distributed in Palk Bay. Six scleractinian coral species are recorded from the lagoon of Vellaperukkumanthai reef whereas Gopinadha Pillai has identified two species (porities somaliensis and favia pallida) from the lagoon (Table 1). Fishermen suggest that the sponge population and soft coral population have decreased over the past two decades. Our investigations also confirm an increase in the boring sponge species and a decrease in the macrosponge species.
The shoreward slope of the reef has a width of 70 m in the area between 230 m and 300 m from the shore. The coral population has been increasing remarkably in distribution and diversity along the shoreward slope. The 1969 record of Gopinadha Pillai shows 11 species in this area, however, present investigation shows 20 coral species with a density of 50 colonies/10m2. Padina sp and Halimeda sp are most common algae present in this zone. The sponge population is comparatively higher than in the lagoon. The coral species Platygyra lamellina, Hydnophora sp, Galaxea fascicularis and Turbinaria pelata recorded by Pillai (1969) are no longer present in Palk Bay (Table 1).
The reef crest area are exposed to sunlight during low tide for a very long period (maximum 7 hrs). Over exposure to sunlight leads to animal mortality and radiation infection. Thereby the animal population in this area is lower than in the other zones of the reef. Slopes of the reef crest area are densely populated by algal groups.
Gopinadha Pillai recorded all the ramose corals in the seaward slope of the reef. However, our present investigation shows that ramose corals are also distributed along the shoreward slope and lagoon. The present study indicates that 10 scleractinian species are present in the seaward slope, whereas the previous record (Gopinadha Pillai 1969) shows only 6 species (Table-l).
Fishermen, report the presence of the dugong and turtles are in Palk Bay. Awareness of the ecological and genetic significance of the coral reef and legislation's brought about by the Marine Park Authority have contributed to reduce the interest of fishermen in dugong and turtle fishing. Mollusc fishers report a reduction in mollusc capture over the decade.
Asir Ramesh (1966) expressed the opinion that Gulf of Mannar corals are healthier than the reef's of Palk Bay. The sick status of Palk Bay is because of the discharge of the processing industries and fish landings. Present data reveals that coral reef's of the Palk Bay are undergoing rejuvenation.
References
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