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用遥感和GIS技术管理印度马纳尔湾的珊瑚礁
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Abstract
The coral reefs characterise an ecosystem of high biological diversity, having the greatest number of species of any marine ecosystem. Livelihood of many millions of people is dependent on this unique ecosystem as a considerable proportion of their food and earning is from the productivity of coral reef. Coral reefs are considered as one of the most important critical resources for various ecological, environmental and socio-economic reasons. Coral reefs act as a barrier against wave action along coastal areas thus preventing coastal erosion. In addition, coral reefs protect mangroves and seagrass beds in certain areas, which are the breeding and nursing grounds of various economically important fauna. Coral reefs are also important breeding, spawing, nesting, and feeding areas for many economically important varieties of fishes and other marine organisms. As a result of the increasing human population along the coastal area, anthropogenic impacts on the coastal zone have become severe over the past few decades. Coral ecosystem also face many threats, of which some are of natural origin like storms and waves particularly tropical storms and cyclones that cause major intermittent damage to reefs. The majority of damage to coral reefs around the world has been through direct anthropogenic stress. In Gulf of Mannar, the coral reefs have been used as a source of calcium carbonate and building blocks. In general, coral reefs in Gulf of Mannar can be categorised as 揹egrading? and hence, monitoring and management of these valuable marine resources are of prime importance. In this contest Survey of India Topographic Sheet (1969), Naval Hydrographic Chart (1975) IRS LISS-II (1988), IRS LISS-III (1998) satellite data and ARC-INFO and ARC-VIEW GIS software were used for coastal geomorphology, seafloor, shoreline, coastal land use/land cover and coral reef mapping from Gulf of Mannar. The current status of coral reef, changes and important coastal problems (coastal geomorphological and land use/land cover changes) for coral reef degradation of Gulf of Mannar are identified.
1. Introduction
Coral reef system as also the ecosystem of the tropical rain forest, are the most matured marine ecosystems of our planet. They play an important role in global biochemical processes and in the reproduction of food resources in the tropical regions. Coral reefs act as a barrier against wave action along coastal areas thus preventing coastal erosion. In addition, coral reefs protect mangroves and seagrass beds in certain areas, which are the breeding and nursing grounds of various economically important fauna. Coral reefs are also important breeding, spawing, nesting, and feeding areas for many economically important varieties of fishes and other marine organisms. The people living along the coast obtain a considerable proportion of their food and earnings from the productivity of coral reefs. Coral reef ecosystems are very sensitive to external impacts both natural and manmade, which violate their homeostasis (Sorokin 1992). The majority of damage to coral reefs around the world has been through direct anthropogenic stress (Grigg and Dollar 1990). According to Bryant et.al (1998), 57% of the world抯 coral reefs are potentially threated by human activity such as coastal development, destructive fishing, over exploitation, marine pollution, runoff from deforestation and toxic discharge from industrial and agricultural chemicals. As global pressures on coral reefs and related ecosystems grow with developing economies and increasing coastal populations, the need for careful monitoring , planning and management become essential (Knight et.al 1997).
Gulf of Mannar, extends from Tuticorin to Rameswaram Island in the SW-NE direction, lies between 78?5?& 79?0?E longitudes and 8?7?& 9?5?N latitudes, to a length of about 140 km. There are 21low islands (Figure 1) situated at an average distance of about 8km from the coast and running almost parallel to the coastline. These islands are broadly grouped into Tuticforin, Vembar, Keelakkarai and Mandapam groups. These areas are endowed with a combination of ecosystems including mangroves, seagrass, seaweeds and corals reefs. Different types of reef formations have also been observed along the periphery of the islands viz. fringing reef, patch reef and coral pinnacles.
Figure 1 Location map of the study area (Gulf of Mannar)
In South India the coral reefs have been used as a source of calcium carbonate, building blocks and rubble for construction of roads (Mahadevan and Nayar 1972). Blasting and dredging activities result in high sedimentation on the coral reef of Gulf of Mannar there by leading to its degradation. The coral reefs of Tuticorin group of Islands in Gulf of Mannar have been damaged due to the discharge of effluents from petrochemical and other industries along the coast, and fly ash discharges from thermal plants (Ramanujam and Mukesh 1998). In general, coral reefs in Gulf of Mannar can be categorised as 揹egrading? and hence, monitoring and management of these valuable marine resources are of prime importance.
To study coral reefs area effectively and to monitor changes over timw, accurate, rapid and cost-effective mapping techniques are required. Satellite remote sensing is widely used as a tool in many parts of the world for the management of resources and activities within the continental shelf containing reefs. Preliminary studies carried out in India by Space Application Center (SAC) Ahmedabad, have proved the importance of remote sensing data in mapping and monitoring of the coral reef (Muley et al. 1986; Nayak et al. 1987). Anjali Bahuguna and Nayak (1998) have mapped the coral reefs of Gulf of Kachchh, Lakshadweep, Palk Bay, Gulf of Mannar and Andaman and Nicobar Islands using IRS and SPOT satellite data. Mumby et al (1998) used digital airborne sensor of Landsat MSS, Landsat TM, SPOT XS, and SPOT PAN and merged Landsat TM/SPOT PAN for mapping the coral reef in Turks and Caicos Islands they also studied coral reefs in situ by visual technique.
2. Objectivs of the Study
The objectives of the present study are to
Assess the current status of coral reefs in Gulf of Mannar based on remote sensing and GIS approach
Assess the changes that have occurred in the spatial distribution of coral reefs during the period 1988-1998 using multidate remote sensing data
Study the coastal geomorphology and its influence on the coral reefs
Study the influence of landuse/landcover changes on the coral reefs
Suggest suitable management measures for sustainable management and preservation of coral reefs in Gulf of Mannar region
3. Materials and Methods
Images of IRS LISS-II (April 1988) and IRS LISS-III (May 1998), Survey of India Toposheets (1969) and Naval Hydrographic Charts (NHO) 1975 were been used in this study.
To cover the objectives of this study, six types of approaches have been attempted are listed below:
Analyse and interpretation of optical remote sensing data (IRS LISS-III data) for coastal geomorphology
Analyse and interpretation of optical remote sensing data (IRS LISS-III data) and Survey of India Topographic sheets (1969) for shoreline change mapping.
Interpolation of Bathymetry using Naval Hydrographic charts and bathymetry survey
Analyse and interpretation of multidate optical remote sensing data (IRS LISS-II and LISS-III data) for coastal land use/land cover change deduction
Socio-economic studies
Analyse and interpretation of multidate optical remote sensing data (IRS LISS-II and LISS-III data) for coral reef change deduction.
Coastal Landform mapping
Geocoded IRS1D LISS ?III imagery on 1:50,000 scale were visually interpreted based on image characteristics, and identified and mapped various coastal landform categories along the coast of Gulf of Mannar. In the present study, an image interpretation key indicating tone/colour, size and pattern developed by Space Applications Center, Ahmedabad SAC (1991), has been adopted in this study. The basic information like transport network, tanks, rivers etc are transferred from SOI toposheet. After identification and delineation, an accuracy test was made for 118 sample points on SOI toposheet. The study area map at 1:50, 000 scale was divided into grids and intersecting points of each grid within the study area were taken as sample points for validation of classified satellite data in ground truth checking. Over the ground, out of the 118 sample points, 105 points were found to be correctly interpreted giving an accuracy of about 90 per cent. The georeferenced coastal land form map was digitized, edited labeled and projected using ARC-INFO GIS. Finally a coastal landform map was generated using intercept operation of ARC/VIEW. The area statistics of coastal landforms in the map were generated.
Shoreline Change
Geocoded IRS LISS III data of May 1998 and SOI topographic sheets of 1969 were used to prepare shoreline maps on 1:50,000 scale. Multi-date shoreline maps of 1969 and1998 were digitized and projected using polygonal using ARC-INFO GIS and were overlaid using tic coordinates of the study area. Overlaid maps were edited and labeled. Finally a temoporal shoreline change map was generated using intercept option of ARC-VIEW and and identified erosion and accretion areas along the coasts of island and offshore islands in Gulf of Mannar wit sufficient ground truth verification.
Bathymetry Mapping
Bathymetry map of study area on 1: 50,000 scale was prepared manually using 1975 Naval Hydrographic Chart. The prepared bathymetry map was digitized into ARC-INFO and a digital bathymetry map was prepared. The prepared bathymetry map was updated during fieldwork, which was carried out during April 1999. The depth of the sea bottom was measured (within 10m depth) using ecosounder around Mandapam and Tuticorin groups of islands and some coastal regions near Tuticorin and Mandapam coastal area. The depth values are recorded at a particular location with reference to chart datum (1975). The measured depths values were tide corrected with respect to time and then converted with respect chart datum (0.2m accuracy). Measured tide table from the Tuticorin port was used for final data conversion to chart datum.
Land use/Land cover Mapping
Geocoded multi-date IRS (1A LISS ?II of June 1988 and 1D LISS ?III of May 1998) imageries on 1:50,000 scale were visually interpreted based on image characteristics, and various land use / land cover categories were identified and mapped around Gulf of Mannar. The basic information like transport network, tanks, rivers etc are transferred from Survey of India toposheet. After identification and delineation, an accuracy test was made for 150 sample points on SOI toposheet. The study area map at 1:50, 000 scale was divided into grids and intersecting points of each grid within the study area were taken as sample points for validation of classified satellite data in ground truth checking. Over the ground, out of the 150 sample points, 139 points were found to be correctly interpreted giving an accuracy of about 92 per cent. The georeferenced land use maps of 1988 and 1998 were digitized in ARC/INFO and were overlaid using TIC coordinates of the study area. Digitized maps were edited and labeled. Finally a temporal land use/ land cover change map was generated using intercept operation of ARC/INFO.
Socio-Economic Studies
The fifth approach is by collection of socioeconomic data. The result of this approach is expected to provide information on (i) various economic activities of coastal communities in Gulf of Mannar and (ii) identification of coral reef degradation due to economic activities.
Coral reef Mapping
IRS LISS-II and IRS LISS-III imageries on 1:50,000 scale were visually interpreted based on image characteristics, various coral reef were identified and mapped categories in Gulf of Mannar. In the present study, the classification system developed by Space Application Center for the national coral reef mapping project (Anjali Bahuguna and Nayak, 1994) has been adopted. After identification and delineation, an accuracy test was made for sample points on Survey of India Topographic sheets (SOI). The study area map on 1:50,000 scale was divided into grids and intersecting points of each grid within the study area were taken as sample points for validation of classified satellite data in ground truth checking. Over the ground, out of the 112 sample points, 95 points were found to be correctly interpreted giving an accuracy of about 84.82 percent. The geo-referenced coral reef maps were digitized, edited, assigned corresponding labels and projected using ARC-INFO. Finally a coral reef map was generated using intercept statistics of coral reef classes in the map.
4. Result and Discussion
Coastal Land form
The coastal plain between Dhanuskodi and Tuticorin has various geomorphic units with different types of configurations (Figure 2). The geomorphic units, interpreted from remotely sensed data and checked subsequently through fieldwork, have been categorized into four genetic classes ?marine, fluvio-marine, fluvial, aeolian and biogenic landforms.
Figure 2 Coastal geomorphology map of Gulf of Mannar derived from IRS LISS-III satellite data
Marine Landforms In the coastal zone varies marine landform features such as beaches, spit, beach ridges, swales and backwater zone, mudflat, offshore islands, coral reefs, wave cut platform, sea cliff, sea cave, water logged land and strand lines have been identified.
Beach
Beaches are extensively developed along the entire coast of Gulf of Mannar except at some places. The shore between Tuticorin to south Sippikulam ( 2.04 km? , Vaippar River and Gundar River (2.56 km?, Gundar River and Palar River,(2.64 km?, Palar River and Kottakkarai River (2.189 km?,Kottakkarai River and Marakkayarpattanam (2.18 km?southern coastal parts of the Rameswaram Island (2.91 km?and the western part of the Rameswaram Island from Pamban to Peikkarumbu are observed as a important beach areas in Gulf of Mannar coast. All along the shore the beach is observed to be gently sloping and marked with altered crusts and troughs that are formed due to wave action.
Spit
Among the various depositional landform features encountered, the formation of spit is a significant feature of recent age. South of Tuticorin coastal area two spit formations have been observed. It appears to have been built by the sediments brought by long shore current during southwest monsoon. As the Gulf of Mannar is on the lee of the northeast monsoon, there is no long shore drift from the northeast that might be the cause for the inward curving of this spit (Ahmad 1972). The southwestern shore of Rameswaram has a tongue shaped spit. SOI toposheet of the year 1969 does not shown any spit but recent IRS LISS-III imagery (1998) clearly shows the spit. Hence it may be assumed that these spits are recently formed. It can be explained that the Rameswaram spit may have been the result of littoral current from Palk Bay to Gulf of Mannar during northeast monsoon period.
Beach ridges
Beach ridges are moderately undulating terrain features of marine depositional type, formed during Pleistocene to Recent age, in the plains of Gulf of Mannar coast. In the Gulf of Mannar coast areas between Mandapam and East of Vaippar River are covered by well-developed beach ridges. There are twelve beach ridges observed in the region. Almost all beach ridges in the study area are parallel to each other, and cover an area of 155.49 km?and trend from east to west and northeast to southwest direction. On the basis of the nature and dispositions of beach ridges, it can be grouped into (i) Beach ridges south of Vaigai River, (ii) Beach ridges between Kotangudi River and Palar River, (iii) Beach ridges between Palar River and Gundar River system, (iv) Beach ridges between Gundar River and Vaippar River and (v) Beach ridges south of Vaippar River.
Swales and backwater zone
Swales and backwater zones are seen between coastal plains of Mandapam and Kottakkarai River; they are branched and arranged in series of linear patterns. They are situated almost parallel to the present coastline. Prominent backwater zones have been observed in the coastal plains between Valinockkam and Vaippar River, Mandapam and Southeast of Tiryppullani near Tinaikkulam. These are divided in to two parts by beach ridges. The coastal areas between Mandapam and Tinaikkulam, Valinokkam and Krishnapuram and North of Terku Mukkaiyur and Tukukankulam consist of prominent and wide backwater zones. These three backwater zones are connected by small, linear and narrow swales to the sea by means of few creeks, which supply water from sea to backwater channels during high tide. The basin bed is composed of silt and mud. The adjacent low lying area, as a part of swale zone is used at present for salt production.
Mud flat
Mud flats are wide expense of deposit of clay, silt, ooze, etc (Davies 1972). The mudflats are observed near Vaippar River mouth, around Valinokkam backwater lagoon, Kallar River mouth and Gundar River mouth. The area covered by mudflat has been estimated to be 14.50 km?
Off shore islands and coral reefs
A chain of 21 low islands has been observed along the offshore region of Gulf of Mannar(Table 3). It extends from south of Rameswaram to Tuticorin. All islands are made up of a calcareous framework of dead reef and sand. They have a low and narrow sandy coast and some of them have rocky coast. Around all offshore islands, well-developed coral reefs have been noticed. Geomorphologically, coral reefs in this area are of fringing type, though some patchy corals are also observed in between Appa Island and Pilliyarmuni Island, and in some areas like Bharathi nagar coast and southeast coast of Kariya Shuli Island.
Wave cut platform
Table 3. Areal Distribution of Offshore Islands and Types of Reefs and their Areal Extent
Sl. No Islands Island Area(km? Coral Reef
Reef type Area(km?
1 Van Island 0.245 Fringing type 1.090
2 Koswari Island 0.241 Fringing type 1.474
3 Kariya Shuli Island 0.166 Fringing type 0.702
4 Uppu Tanni Island 0.377 Fringing type 0.644
5 Shalli Island 0.126 Fringing type 0.249
6 Nalla Tanni Island 1.248 Fringing type 1.250
7 Anaipar Island 0.229 Fringing type 0.888
8 Palliyarmunai Island 0.134 Fringing type 0.687
9 Appa Island 0.410 Fringing type 3.612
10 Talairi and Valai Island 1.072 Fringing type 9.268
11 Muli Island 0.170 Fringing type 1.208
12 Musal Island 1.836 Fringing type 27.73
13 Manalli Island 0.353 Fringing type
14 Manalli Putti Island 0.037 Fringing type
15 Pumurichan Island 0.187 Fringing type 8.320
16 Kovi Island 0.482 Fringing type
17 Kursadi Island 0.740 Fringing type
18 Shingle Island 0.191 Fringing type 1.023
19 Near Bharathi nagar --- Patchy type 0.131
20 Between Appa Island and Pilliyarmunai Island --- Patchy type 3.981
21 Southeast of Kariya Shali Island --- Patchy type 0.969
Wavecut platforms are common in the coast of Mandapam, Ramaswami Madam, Pudumatam, Valinokkam etc. At Pudumadam coast, hard and tough sandstone platform occupies the intertidal zone. South of Valinokkam coast very extensive wavecut platform has been observed and erosional features are widely seen.
Sea cliff and sea cave
Along the coast of Gulf of Mannar cliffs have been observed in Mandapam, Rameswaram, Pudumatam and Appa Island coastal areas. Generally the sea cliff and caves are made up of calcareous sandstone and located at the high water level. Due to intensive action of waves on cliffs, at some places, sea caves are formed. Such caves have been observed near Mandapam coastal area and Southwestern and Southern coastal areas of Appa Island (Figure 3). At some places, these features have been destroyed due to slumping of upper cliff materials.
Figure 3 Sea caves and cliffs at Appa island coast
Waterlogged land
Water logged land is the area where the water is at or near the surface and has been stagnant for most part of the year. Such lands usually occupy low-lying areas, topographically. In the study area around the northern part of Rameswaram Island some waterlogged lands have been observed. In the areas like Pillaikulam, Surantidal and Mangaud, this type of features has been observed. The total water logged land in the study area has been calculated to be 5.96 km?
Strandlines
In the Gulf of Mannar coast from Tiruppullani to Mandapam, eight series of strandlines in curvilinear form have been observed. The general trend of the strandline is in the east to west direction. In the south of Rameswaram area also, curvilinear strandlines have been observed.
Fluvio-Marine Landforms
Deltaic plain
Deltaic plains are predominantly controlled by fluvial processes. In the study area deltaic plain is considered to be of Pleistocene to Recent age (Loveson 1993). The areas around the river courses of Vaippar, Gundar, Palar and Kottakkarai Rivers, had vast deltas, but at present they are found to be inactive. The deltaic plains are marked by flat and vast areas, having vegetation cover. Number of tanks has been noticed on the deltaic plains. The total area has been estimated to be about 221.69 km?
Fluvial Landforms
Flood plain
Well-established rivers usually have their floors covered with alluvium, in which the normal flow channel is covered. The surface of low relief on the alluvium from the banks of the low-water channel to the base of the valley walls is called the flood plain of a river. Flood plain and their major morphologic subdivisions are primarily deposited landforms.
Floodplains have been very clearly observed along the riverbanks of Kallar, Vaippar and Gundar Rivers in study region. These flood plains are inactive, which is covered with thick vegetation. The total estimated area of flood plain in study region is 24.43 km?
Natural levee
Land forms in deltaic regions include natural levees bordering river channel and backed by lowering of the swamp or flooded depression in the lower parts of the river valleys (Bird 1984).The over bank deposits are located more or less parallel to the riverbank. Generally, the levees are mainly seen adjoining the meandering course of the rivers. A natural levee has been observed in the Gundar River basin. The width of these levees encountered ranges from 2 to 3 km. and covers an area of 1.10 km?
Flood basin
Flood basin occupying the shallow reaches of the flood plain consists of mostly gray brown sandy and silty clay and clay. Kottakkarai River surface has a flood basin with an area of 26.83 km?
Aeolian Landforms
Sand dune complex
Almost entire coastal plains in the study area are covered by sand dune (Figure 4). The area in between swale system and shoreline is marked by dune complex. The area covered by dune complex has been estimated to be about 469.10 km? However, extensive spread is observed around Rameswaram Island, Mandapam, Mangudi, Bharathinagar, Valinokkam, Thunamariyur, Terku Nerippaiyur and Tuticorin areas. In other places the areal spread is less. Numereous dunes have been observed the places like Tuticorin, Taruvaikkulam, Sippikkulam, Terku Narippaiyur, Bharathinager near Keellakkarai, Valinokkam, Mandapam and Rameswaram are situated on the dune complex. Aeolian process is dominating in this zone and its intensity can be seen in the migrating dunes in Tuticorin itself. Thruvaikkulam and the other above-mentioned areas also exhibit features similar to those in Tuticorin area.
Figure 4 Sand dunes at south of Rameswaram Island
Teri dune complex
In the coastal plains between south of Vaippar River and Tuticorin near Maravanmadam seven patches of teri dune complexes have been observed with a thick cover of vegetation. In the area north of Panaiyur, oval shaped teri dunes with sparse vegetation have been observed. The areal extent of this dune has been calculated to be 6.27 km? Another two teri dunes have been observed near Kumarapuram. They cover an area of 2.67 km? Near Pandiyapuram, rounded dune complex has been observed with thick vegetation. The area of this dune has been estimated to be 4.27 km? Near Milavittam small rounded patches of teri dune complex covering an area of 2 km?has been observed. Two other teri dune complexes have been observed near Maravanmatam area with thick vegetation. They cover an area of 3.08 km? All teri dune complexes in this area are trending in the northeast to southeast direction.
Biogenic Landform
Back swamp
Back swamps occur in marshy areas along the coast; they particularly occur at the edge of the tropical or sub-tropical seas, in bays lagoons and estuarine regions (Gerlech 1973). Small back swamp areas have been observed in the areas near the mouth of Korampallam odai around Tuticorin coast and west of Rameswaram Island. They cover a total area of 1.87 km? These swamps are covered by mangrove vegetation.
Coastal Erosion and Accretion
The shoreline change map derived from Survey of India Topographic sheet (1969) and IRS LISS-III (1998) imageries are shown in figure 5 and 6. In the coastline between Dhanuskodi and Tuticorin, erosion and accretion areas have been observed. The areas of erosion and accretion have been estimated as 4.34 km?and 23.49 km?respectively over a period of 30 years (1969 to1998). The average widths of accretion and erosion have been calculated from previous shoreline (1969) and present shoreline (1998) as 143.11m and 106.50 respectively during the past 30 years. The rates of accretion and erosion have been approximately calculated to be 4.7m/year and 3.5m/year respectively along the study area.
Figure 5 Shoreline change map between Tution and Vaippar River (1969-1998)
On the basis of the distribution of coastal erosion and accretion, the study area has been classified into 1) Shoreline between Tuticorin and Vaippar River, 2) Shoreline between Vaippar River and Gundar River, 3) Shoreline between Gundar River and Palar River, 4) Shoreline between Pallar River and Kottangudi River, 5) Shoreline between Kottangudi River and Thoniturai and 6) Shoreline of Rameswaram Island.
Island Erosion and Accretion
There are 21 islands between Tuticorin and Rameswaram. These islands are sedimentary landforms. Fringing reef along the windward side of the islands protects the islands from direct wave action. Morphology of sandy islands is very dynamic. The morphological variations of islands occur due to natural and anthropogenic agents. The natural agents include erosion, accretion, wave, current, sea level variation etc. Anthropogenic impacts are construction of breakwaters, discharging of effluents, mining of coral reef etc.
The comparison of 1998 map with 1969 map of islands of Gulf of Mannar shows changes in their shape, size and location and these have been caused by erosion and accretion. The total areas of erosion and accretion were calculated as 4.16 km?and 3.31km?respectively during 30-yearsperiod (1969to1998).
Island erosion and accretion are caused mainly by the action of wave and wave-induced current and longshore current along the shores of islands. In the study region the waves are in the northeast and southwest direction and wind direction is similar to that of wave direction. According to Chandrasekar (1996), coral reefs are destroyed by siltation, logging and illegal mining in Tuticorin group of islands. During the last 50 to 60 years, mining of stony corals from the reef area, especially from, Tuticorin group of islands, for building, industrial and chemical purposes have destabilised the formation of Tuticorin group of islands (Ramanujam et al 1995). Hence the waves hit directly on south, southeast and southwest shores of these islands, causing erosion. These eroded sediments are then transported by wave-induced currents and deposited at the leeward sides of these islands. By such repeated processes, the windward sides of the island get reduced and leeward sides of the islands are accreded. Hence size, shape and location of these islands have changed. All islands in Tuticorin group have been migrating towards mainland (Figure 5). According to Ramanujam et al (1995) landward migration and deterioration of islands in Tuticorin region are caused by sea level variation and mining of reef material.
In Keelakkarai and Mandapam groups (Figure 6) of islands, generally the islands are seen to move towards south (seaward side). The island erosion has been mostly identified along the northern side of the islands (landward side). Evidences of submerged trees and sharp edged coasts are found along the northern shores of these islands. This is because the long shore current and tidal current flow towards south, along the northern shore of islands, then erode the coast, this eroded materials are transported and deposited on seaward side of the coast. While high velocity waves are moving towards northern shores of islands with the littoral sediments and coming across the coral reefs, these sediments will be dropped on the coral reefs, wave speed reduces and turn into a wave-induced current. By repeated action of such processes the area between Islands and reef edge get shallow and reefs have submerged. Evidence of submerged reefs is identified along south east of Kursadai Island.
Figure 6 Shorline map of Ramswaram island (1969-1998)
Bathymetry Mapping
Any changes in sea floor may be the result of sea-level variation or to a change in the elevation of land surface. Changes in absolute water-surface levels are worldwide due to the interconnectivity of the oceans and are termed eustatic changes. Changes in the absolute level of the land are localized. They may be due to tectonic adjustments or due to adjustments caused by their distribution of weight on the land surface. As and when sedimentation or ice build-up occurs, such changes are known as isostatic. A rise in the sea level or down warping of land would involve the opposite movements of sea and land. Synonymous with positive and negative changes are the forms of sea-level transgression and regression, although in many cases these terms also refer to the horizontal movement of the shoreline associated with vertical changes of sea level.
Recent depth contour map of 1999 (Figure 7) has been compared with bathymetry map of 1975 (Figure 8a&b); it reflects that the seafloor level decreased along the coastal and around the islands in the study area. It may be due to emerging of land or lowering of sea level (due to tectonism) and sediment deposit. In very few places particularly at river mouths and in island areas, the sea floor level has increased, which may be due to erosion caused by anthropogenic activities.
Figure 7 Bathymetry map of Gulf of Mannar (1975)
Figure 8a Bathymetry map of Tuticorin coastal region (1999)
Figure 8b Bathymetry map of Tuticorin coastal region (1999)
The average depth reduction of seafloor along the coast of the study area has been estimated as 0.51m over a period of 24 years. The average decrease and increase of depth around the islands in the study area have been calculated as 0.56m and 0.38m respectively. Assuming that the rate of change of depth of sea floor is uniform over a year, the rate of decrease of depth is estimated as 0.021m/year along the coast and 0.023 m/year around the island, and also the rate of increase of depth as 0.015 m/year around the island. The annual sediment deposit on Gulf of Mannar sea floor is 0.001m/year (Basanta Kumar Jena 1997), so it will become 0.024m for a period of 24 years. As found from the present study, the decrease of depth for the period of 24 years (1975 to 1999) is 0.51m. Out of this 0.51 m of decrease of depth, sedimentation will account for about 0.024m. The remaining 0.486 m reduction in depth may be due to emerging of land or lowering of sea level (by tectonic activities). From this, the rate of emerging of land or lowering of sea level can be estimated as 0.02m/year.
Land use/land cover mapping
Geocoded IRS LISS-II (April 1988) and IRS LISS-III (May 1998) imageries on 1:50,000 scale were used for visual interpretation to prepare the land use/land cover map. In the present study, the classification system developed by National Remote Sensing Agency for the national land use/land cover mapping (Gautham and Narayan, 1982) has been adopted. The land use/land cover maps derived from IRS LISS-II and IRS LISS-III imageries are shown in the Figure.9 and 10. The areal distribution of various land use/land cover classes for the years 1988 and 1998 and their changes are shown in Table 4.
Figure 9 Coastal landuse/landcover map of Gulf of Mannar coastal area derived from IRS LISS-II satellite data (1988)
Figure 10 Coastal landuse/landcover map of Gulf of Mannar coastal area derived from IRS LISS-III satellite data (1998)
Table 4. Areal Distribution of Land use/Land cover and its Changes observed during the period from 1988 to 1998
Sl.No. Land Use/ Land Cover Classes Area(km? 1988 Area (km?1998 Changes observed from 1988 to 1998
1 Settlement 22.070 34.930 +12.860
2 Crop land 539.860 417.660 -122.200
3 Fallow land 50.170 52.560 +2.390
4 Agricultural plantation 185.560 192.570 +7.010
5 Natural Forest 3.860 0.062 -3.800
6 Forest plantation 115.290 145.020 +29.730
7 Mangroves 0.651 1.510 +0.860
8 Island vegetation 3.810 2.280 -1.530
9 Salt affected area 19.910 6.800 -13.110
10 Water logged area 3.620 7.070 +3.450
11 Marsh vegetation ---- 0.375 +0.375
12 Scrub land 243.480 294.020 +50.540
13 Rocky coast ---- 2.540 + 2.540
14 Mud flat 2.860 4.700 + 1.840
15 Sandy area 39.280 36.800 -2.480
16 Tanks 70.840 74.850 + 4.010
17 Salt pan 27.730 48.930 +21.200
18 Aquaculture ponds ---- 3.170 +3.170
19 Flood affected area ---- 0.800 +0.800
Land use/land cover changes
Multi temporal satellite data used in the present study enabled observing the land use and land cover changes in the study area from 1988 to 1998. Over the past 10 years, areas of some land use classes have increased, areas of some classes have decreased and some categories have changed in to another category in the study area. These changes are taken place due to the increase in population in towns and villages along the coast and various kinds of economic activities.
The major land use/land cover changes has occurred in the following classes: (1) cropland area has reduced from 539.86 km? to 417.66 km? (2) fallow land increased has from 50.17 km?to 52.56 km? (3) agricultural plantation has increased from 185.56 km?to 192.57 km? (4) forest plantation has increased from 115.29 km?to 145.02 km? (5) scrublands has increase from 243.48 km?to 294.02 km? (6) sandy area has reduced to 36.80 km?and (7) tanks has increased from 70.84 km? to 74.85 km?
Socio-Economic study
Fishing has been the sole occupation of several thousands of fishermen families living along the coast of Gulf of Mannar. They have been in close proximity with the sea, so that, their life-style, culture, community and social life have centered around the sea. The main livelihood activities of the fishermen residing in the villages adjoining the study area are (i) fishing, (ii) seaweed collection, (iii) chanks collection, (iv) coral mining, (v) fire wood collection and (vi) agriculture.
Coral reef mapping
Geocoded IRS LISS-II (April 1988) and IRS LISS-III (May 1998) imageries on 1:50,000 scale were used for visual interpretation to prepare coral reef maps. In the present study, the classification system developed by Space Application Center for the national coral reef mapping (Anjali Bahuguna and Nayak, 1994b) has been adopted. The coral reef maps derived from IRS LISS-II and IRS LISS-III imageries are shown in Figure.11 and 12. The areal distribution of coral reefs, reef vegetation, degraded coral reef, etc for the years 1988 and 1998 are shown in Table 5.
Figure 11 Coral reef map of Gulf of Manner derived from IRS LISS-III (1988) satellite data
Figure 12 Coral reef map of Gulf of Mannar derived from IRS LISS-III (1998) satellite data
Table 5. Areal distribution of coral reefs and its changes observed during the period from 1988 to 1998
Category Area (km?1988 Area (km?1998 Changes1988-1998
Reef area 73.70 48.18 -25.52
Reef vegetation 12.31 10.15 -2.16
Degraded reef (Coral mining) - 2.68 +2.68
Degradation of coral reefs
The degradation of coral reefs in the Gulf of Mannar is quit severe due to the human stress (anthropogenic) and also by natural agents (Mahadevan and Nagappan Nayar 1972; Pillai 1975; Venkataramanujam et al 1981; UNEP 1985; Wafar 1986; Anon 1987; UNEP/IUCN 1988; Venkataramanujam and Santhanam 1989; Chandrasekaran 1996; Dhandapani 1997; Ramanujam and Mukesh 1998). Arjan Rajasuriya et al (1999) is of the opinion that the coral mining for lime, sand mining, pollution, sedimentation, fisheries, population pressure, commercial shell collection and industrial development has led to the increase in coral reef degradation in India.
The total coral reef area in Gulf of Mannar based on the present study (1998) is about 61.01km? of which reef area covers 48.18 km? reef vegetation covers 10.15 km?and degraded coral occupies 2.68 km?
The analysis of multi-date satellite data indicates that nearly 25.52km?area of coral reef was lost over a period of ten years (1988-1998). Through remote sensing the demarcation of dead coral is possible within 2 feet beyond not possible. During the time of ground truth it was identified that nearly 67.2% of the corals were dead corals, 13% of coral reef are directly removed by coral mining and the remaining 19% were live corals. During the time of ground truth, the following activities were observed to be the major causes in degradation of coral reefs in the study area (1) anthropogenic Stresses (Human Activities) and (b) natural Stresses
Anthropogenic stresses (Human activities)
Increase in human population and economic activities in the study area have increased the pressure on the adjacent reefs. The major causes of coral reef degradation are:
Over fishing and destructive fishing practices
Sea weed collection
Commercial shell collection
Coral mining
Poor land use practices
Coastal urban development
Harbour and dredging activities
Industrial development and pollution
Fishery is the primary economic activity of the people of the coast of Gulf of Mannar. There are about 50,000 fishermen engaged in fishing activity. The marine fish catch from Gulf of Mannar during 1992-1996 was between 0.55 and 1.02 lakh tons, with an average of 78, 511 tons. The pelagic fishes formed 54.38%, demersal fishes 34.85%, orcstation 5.69% and mollusks 5.08%. Nearly 20% of total fishes in Tamilnadu are being caught from Gulf of Mannar, of which 44,600 tons of pelagic fishes and 35,200 tons of demersal fishes. The fishermen collect these holothurians by diving in shallow reef areas. Much of the fishing is carried out on / and near the coral reef area. In many areas of Gulf of Mannar, the majority of the reefs have been destroyed by fishing due to modern fishing techniques like mechanised launches with trawling net and gill net and also by destructive fishing practices such as dynamite fishing near Tuticorin area. These destructive methods cause direct damage to the corals and indirectly affect their bgrowth by increasing turbidity and suspended sediment in the coastal waters and reducing the clarity of seawater and increasing sediment loads on reefs.
Seaweeds form one of the most important marine living resources in Gulf of Mannar. Because of their commercial value, the seaweeds are harvested by fishermen for agar industry. More than 1000 fishermen and women are engaged in seaweed collection in the study area and nearly 5000-7000 tons (dry weight) seaweeds are harvested annualy. The continuous harvesting of seaweeds results in the decrease in areal extent of seaweed area, which was estimated to be about 2.16 km?area over ten years (1988-1998). The decreasing of seaweeds may lead to coastal erosion and removal of the coral reefs.
Commercial shell collection is another human activity which causes coral reef degradation to some extent. Shell collection is one of the important economic activities of the fishermen living in the study area. Nearly 770 fishermen are engaged in this occupation. This activity increases the sedimentation in coastal water leading to the death of coral.
Coral mining activity has caused extensive degradation of reefs, coastal erosion and sedimentation in a number of countries all over the world. In the study area coral mining is the major human activity, which causes coral reef degradation. Coral reefs are used on a large scale as raw material by the lime industries. In Tuticorin group of islands this activity is very active and the corals have been used for building, industrial and chemical purposes. The rapid development of lime based industries in and around Tuticorin has accelerated the coral mining. Nearly 250 skilled divers and about 50 boats are engaged in the coral reef mining. The use of explosives for reef mining, has produced very serious problems to the coastal and marine ecosystems like coastal and island erosion and sedimentation, coral reef degradation and reduction of marine faunal population. Pillai (1973) estimated the annual exploitation of reefs in the study area to be about 90,000m? Venketaramanujam et al (1981) reported that about 15,000 tons of the corals are removed from Tuticorin group of islands annually. Mahadevan and Nayar (1972) estimated that 25,000 tons of corals are exploited annually in Gulf of Mannar. The analyses of multi-date satellite data indicate that nearly 2.68 km?of coral reef area has been mined over a period of ten years from 1988 to 1998 and nearly 0.267 km?of shoreline of Tuticorin group of islands have been eroded over a period of thirty years from 1969 to 1998. The net result of removal of corals is that the depth has increased to about 1m. This has also been confirmed by the bathymetry map pertaining to Tuticorin group of islands.
Another cause for the damage of coral reefs is poor landuse practices, such as agricultural activities, changing land use practices and deforestation etc, which increases land derived sediment flowing onto coral reef. In the present study it was observed that nearly 470.22 km?of land behind the coast of Gulf of Mannar has been under agriculture land, of this nearly 417.66 km?area fall under the category of cultivated land. Cultivation in this area is poor and intensive method of cultivation is generally practiced during rainy season. The poor agricultural practices increase the agriculture waste like pesticides and fertilizers, which are dumped into the coastal water through surface runoff during rainy season and through rivers. This encourages rapid algal growth, which chokes coral polyps and cut off the supply of light and oxygen. This problem is common in Tuticorin group of islands, Vembar group of islands and Keelakkarai group of islands.
Another major cause for the coral reef degradation is changing land use practice. The analysis of multidate satellite data indicates that nearly 2.45 km?of cropland has been converted in to salt pan (1.86 km? and aquaculture pond (0.59 km?; 2.74 km?of agriculture plantation has been converted into cropland (1.77 km? and salt pan (0.97 km?; 10.74 km?of forest plantation has been converted into agriculture plantation (6.19 km? and forest blank (4.54 km?; Nearly 68.78 km?of scrubland has been converted into settlement (7.10 km?, saltpan (2.59 km?, aquaculture ponds (0.18 km?, forest plantation (4.31 km?, agriculture plantation (12.10 km?, fallow land (7.60 km? and cropland (34.90 km? over a period of ten years (1988-1998) along the coast of Gulf of Mannar. Another important human activity encountered in Gulf of Mannar is destruction of natural forest and island vegetation. Through remote sensing it has been clearly identified that nearly 3.80 km?of natural forest has been destroyed for fuel wood and cultivation by human beings along the coastal plains of the study area, and nearly 1.53 km?of island vegetation has been cut down for fuel wood over a period of ten years (1988-1998). These changing landuse practices, destruction of natural forest and island vegetation accelarate the soil, coastal and island erosion and increases the input of inland-suspended soil into coastal waters. The increase in suspended soil and other suspended materials in coastal waters smothers the reefs and causes their degradation.
Increasing urban population along the coastal area has lead to pollution due to sewage discharge into the coastal waters. Discharge of sewage is responsible also for the loss of coral reefs in large quantities. Along the coast of Gulf of Mannar, five coastal towns are located and due to rapid growth of population, the urban areas have increased in area from 22.07 km?to 34.92 km?over a period of ten years (1988-1998). As a result of this rapid urbanization there is an increase in the amount of discharge of sewage waste into coastal waters, which has caused the death of corals in Gulf of Mannar.
Dredging and other harbour related activities like anchoring and ship grounding have increased sedimentation in the coastal waters and caused the degradation of coral reefs (Brock et al 1966). In the study area the New Tuticorin harbour has been constructed at the upstream side of the Tuticorin group of island. This harbour has a breakwater, which has changed the current flow pattern and sediment movement along the Tuticorin coast and islands. The periodic dredging operations at the entrance of the harbour and anchoring of ships have increased the amount of silt plume, which inturn has increased water turbidity, and lowered the light intensity, leading to coral death. This problem has been extensively observed in Tuticorin group of islands.
Industrial development and their waste discharge into coastal water affect the coral reefs. The coast of Gulf of Mannar is experiencing an accelerated growth in the rate of industrialization, since the New Tuticorin port became operational. The industrial development has led to marine pollution and coral reef degradation. The dumping of fly ash slurry into Karapad Bay by the Thermal power station resulted not only in filling up of an extensive portion of the bay, but also letting out of ash directly in to the sea causing extensive damage to the coral reef. Dharangadhare Chemical Works Ltd, Plastic Resins and Chemical Ltd and Tuticorin Alkali Chemical Ltd are the major chemical industries in Tuticorin area, which discharge their effluents into open sea. These effluents contain mercury, sodium carbonate, ammonium chloride and sodium bicarbonate etc. which are harmful to the coral reefs and the environment.
Natural stress
atural?problems are those that are not caused by man, but occur naturally over the long history. Natural problems such as storms, waves, sea level variation, fresh water runoff, volcanic activity etc cause the degradation of coral reefs. Various authors have studied coral degradation due to natural activities in the Gulf of Mannar (Foot 1888; Stoddart and Pillai 1973; Pillai 1975). Through remote sensing and extensive ground truth it has been identified that natural activities such as monsoons, waves, currents, tides and sea level fall have caused the coral reef degradation in Gulf of Mannar.
During the northeast monsoon period highly turbid and low saline water from Palk Bay is transported to Gulf of Mannar by long shore current along Pamban channel. This increases the silt in Gulf of Mannar waters and cause the death of coral reef. According to Pillai (1971) several corals die yearly during northeast monsoon period. In addition to this, during the low tide period, the tidal current moves towards south and transport some amount of sediment to Gulf of Mannar. Towards the seaward side of the island, the high velocity waves hit against the reef edge (wave breaker) and a fall in the velocity of waves cause the wave induced current forms which moves towards the island and drop the littoral sediment on reef leading to coral degradation. The prolonged and repetition of such action, result in the area between island and reef edge to became shallow. The multi-date bathymetry map analysis indicates that nearly 0.24m depth has decreased over a period of 24 years on the seaward side of Pumurichan, Kovi and Kursadi Island. This process has also caused the coral degradation. This problem is observed mainly in the Mandapam group of islands.
Many authors report that the sea level in Gulf of Mannar is on a regression phase (Foot 1988; Sewell 1935; Ahmad 1972; Stoodart and Pillai 1972 and Loveson and Rajamanicam 1990). The sea level fall causes rising of coral reef. The raised reefs are the reliable indicator of sea level fall. Raised reefs have been observed in many places of Gulf of Mannar. The multi-date shoreline map and bathymetry map indicate that sea level has fallen by 0.36m over a period of 24 years (1975-1999) and the shoreline has migrated 143.11m towards the seaward side over a period of 30 years (1969-1998).
Sedimentation is a major factor controlling the distribution of reef organisms and overall reef development (Macintyre 1988). The reduced level of light due to suspended sediment in the water column can reduce coral growth (Hubbard et al 1986) and has an impact on natural zonation patterns (Morelock et al 1983). Excessive sedimentation can also discourage the settlement of coral larvae. Most of the studies on the effects of turbidity on corals have concentrated on anthropogenic-increased sedimentation and turbidity. According to present study, nearly 67.2% of the coral reefs in Gulf of Mannar is not in living condition due to sedimentation and turbidity caused by anthropogenic and natural activities. The anthropogenic activities like destructive fishing methods, seaweed collection, commercial shell collection, coral mining, intensive agriculture, changing land use practices, deforestation and industrial waste input etc. and natural activities like monsoon, wave action, ocean current and tides were identified as the agents that increase the sedimentation and turbidity in coastal waters of Gulf of Mannar.
5. Recommendations
Based on the present study some of the followings are the recommendation for manage the coral reefs in Gulf of Mannar
It is necessary to create awareness among the coastal communities in the study area, in order to protect and conserve the coral reefs through effective involvement of educational institutions and NGOs.
Stringent measures need to be under taken with immediate effect to ban coral mining and to take into task those involved in or those who encourage the exploitation of corals for any purpose. Patrolling the coast to check coral mining should be carried out.
Law should be enacted to regulate and stop trawl boat operation in the zone earmarked for non-mechanized boat. The Department of Forest and the Department of Fisheries should take steps to stop anchoring of vessels on coral reefs, pair trawling and dynamite fishing.
Indiscriminate picking of budding seaweeds needs to be banned.
Commercial shell collection should be controlled and closely monitored.
Marine Resources Management Centres should be established to improve the skills of fishermen communities in areas other than coral mining, which in turn will lead to efficient management of coral reefs.
Initiatives to train the coastal fishermen in mechanized boat operation, shell collection, seaweed collection and conservation of coral reefs need to be taken up so that they could find alternate sources of livelihood.
Deforestation along the coast and islands of Gulf of Mannar should be banned. The Forest Department should take up afforestation along the coast and islands of Gulf of Mannar to protect soil erosion.
Discharging of untreated sewage and urban wastes into the coastal waters should be totally banned.
Dumping of any kind of material that would affect the coral reef ecosystem should be banned. |
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发表于 2006-12-19 18:22:31
发表于 2006-12-24 22:31:25
