Abstract
Status of Mangroves in Muthupet (Southeast Coast of India): A Remote Sensing ApproachT. T. Ajith Kumar Lecturer Centre of Advanced Study in Marine Biology Annamalai University Parangipettai-608502 Tamil nadu tt_ajith@yahoo.co.in T. Thangaradju and L. Kannan Centre of Advanced Study in Marine Biology, Annamalai University Parangipettai - 608502, Tamil Nadu Thiruvalluvar University, Vellore, Tamil Nadu Muthupet mangroves form a unique ecosystem, covering an area of about 6800 ha, along the southeast coast of India. Background information on the areal extent and status of this mangrove environment is scanty and hence its protection becomes difficult. However in the recent times, synoptic and repetitive coverage provided by the satellites have opened up immense possibilities in terms of monitoring and management of coastal resources including the mangroves. So, a study has been carried out in the Muthupet mangroves using satellite imageries validated by ground checks. The data collected have helped detect changes in the areal extent and assess the present status of Muthupet mangroves. This in turn would help develop a multidisciplinary management plan to conserve and manage this ecosystem for posterity and sustainable utilization of its resources. Introduction Coastal wetlands serve as essential habitats for a variety of flora and fauna, energy storage units, water purifier, sediment traps, storm barriers and shore stabilizers. They also serve as nurseries and feeding grounds for plethora of fish, crustaceans, birds and act as repositories for nutrients and organic matters. Based on the vegetational attributes, these wetlands can be divided into three major habitats viz. mangrove swamps, sea grass meadows and coral reefs. It is bathed between sea water during high tides and fresh water during low tides. Of these, mangrove ecosystem is more fragile but yet highly productive. It is unique with characteristic floral-faunal assemblages, though with a relatively low degree of species diversity compared to other tropical forests. Mangroves are of great ecological, economic and social significance. Further, mangroves occurring along estuaries, backwaters and the deltas function as the most important links between the land and sea. Such mangrove forests are estimated to cover an area of about 17 million hectares world over. The total area of mangroves in India is estimated to be 6,740 sq. km. In Tamil Nadu, mangrove coverage is about 150 sq. km. at Pitchavaram and Muthupet (Ajith kumar, 1998).  Plate 1. A view of Muthupet lagoon bordered with mangroves Mangroves of Muthupet Muthupet is situated 400 km south of Chennai and lies close to Point Calimere on the Southeast coast of Penisular India( 100 25’ N ; 790 39’ E ). It is at the southern end of the Cauveri delta covering an area of approximately 6800 ha. of which only 4% is occupied by well-grown mangroves. Various tributaries of the river Cauveri flow through Muthupet and adjacent villages. At the tail end, they form a lagoon before meeting the Palk Strait. The northern and western borders of the lagoon are occupied by a sand spit which is devoid of mangrove vegetation (Plates 1 & 2). This mangrove forest was declared as reserve forest in February 1937 by the Government of Tamil Nadu (Shanmugasundaram, 1985).  Plate 2. Gate (water) way to the Muthupet mangrove and cottages for reposing The Muthupet mangrove ecosystem embraces a heterogeneous mixture of mangrove elements of plants and animals. Among the six principal mangrove species, Avicennia marina, is most common and abundant followed by Exoecaria agallocha, Aegiceros corniculatum, Acanthus ilicifolius, Suaeda maritima and S monoica in that order. Five species of seaweeds viz. Chaetomorpha sp., Enteromorpha sp., Gracilaria sp., Hypnea sp., Ulva sp., and two species of seagrasses namely Halodule sp. and Halophila sp. occur in the mangrove water channels. 76 species of phytoplankton, 90 species of zooplankton, 113 species of insects, 3 species of amphibians 7 species of reptiles and 13 species of mammals have also been reported from this area (Kalidasan, 1991; Oswin, 1998). Because of its multifarious uses and values, extent of Muthupet mangroves has been reduced over a period of time due to direct and indirect natural and man made pressures. In addition, few brackish water aquaculture farms and salt pans have been developed in and around this area. However, ground truth observations on the water quality of the Muthupet mangrove biotope have revealed seasonal variations in the physical-chemical and biological characteristics, depicting the typical mangrove environmental features (Ajithkumar, 1998). Materials and methods The status of Muthupet mangroves and their areal extent has been studied using satellite data. Geocoded Landsat TM (1986) and IRS LISS II (1996) imageries (Plates 3 & 4) were used for visual interpretation and the mangrove wetland maps were prepared based on image-interpretation key elements. A topographic map in the scale of 1:50,000 (sheet No. 58N 7 and 58N 11) obtained from the Survey of India was used to incorporate various manifestations in the image. From the interpreted maps, the areas of mangroves were calculated using a digital planimeter. The interpretation key elements (tone, texture, pattern and association) for the coastal wetlands were followed as suggested by Nayak (1993) and Krishnamoorthy (1995). Accuracy of mangrove mapping, using images was tested by comparing with ground truth data. The mapping accuracy was estimated based on sample basin, assuming a binominal distribution for the probability of success/failures of sample tests as discussed by Nayak (1991) and Nayak (1992).  Plate 3. Landsat TM imagery of Muthupet mangrove environment (1986) Results and discussion Available records (prior to 1912) reveal that the plant species of Muthupet mangroves were exploited throughout the year for sales as fiber as well as for fuel wood by the Chatram department. Cattle grazing and felling were the the main reasons for high degree of degradation of Muthupet mangroves. Present analysis of satellite data (Land sat TM -1986 & IRS LISS II 1996) shows that the mangrove cover in Muthupet is around 1290 ha. in 1986 & 1270 in 1996. Thus, the loss of mangrove area is about 20 ha. between 1986 and 1996 and so there is an imperative need for conserving the existing area of mangroves for posterity and sustainable utilization of their resources. As in the present study, Institute of Remote sensing, Anna University, Chennai (1991) prepared the wetland morphology maps for Cauveri delta using aerial photographs of Landsat TM of 1986 and SPOT FCC of 1989. The results indicated that about 40% of the mangrove area has been reduced in general along the Tamil Nadu coast. Hence, it is quite obvious that the mangrove cover along the Tamil Nadu coast in general, and Muthupet in particular, have been reduced from year after year. However, it should be noted that during the present study, it was found that there is only a small loss of mangroves (20 ha.) from 1986 to 1996 which could be attributed to the protective and conservation measures undertaken here. The present study suggests that the application of remote sensing techniques using recent high resolution satellite data should be made periodically. Such data adequately supported by ground truth will be of much use in watching the health of the mangrove forests continuously and at close quarters, for their proper management.  Plate 4. IRS 1B LISS II FCC of Muthupet mangrove environment (1996) Acknowledgements Authors thank the Director, C.A.S. in Marine Biology and authorities of Annamalai University for facilities and encouragements. References
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