Spatial inventory and ecological status of coral reefs of ...nopr.niscair.res.in/bitstream/123456789/24796/1/IJMS 42(6) 684-696.pdf · Indian Journal of Geo-Marine Sciences Vol. 42(6),

Indian Journal of Geo-Marine Sciences Vol. 42(6), October 2013, pp. 684-696

Spatial inventory and ecological status of coral reefs of the Central Indian Ocean using Resourcesat-1

Bahuguna A., Ray Chaudhury N., Bhattji N., Ajai & Navalgund R. R. Space Applications Centre (ISRO), Ahmedabad – 380015, India

[Email: [email protected]]

Received 8 February 2012; revised 14 September 2012

Spatial inventory of coral reef habitat at eco-morphological level has been carried out for the Central Indian Ocean (CIO) using Resourcesat-1 satellite data. CIO covers India, Sri Lanka, Bangladesh, Maldives and the British Indian Ocean Territory. A comprehensive classification system has been evolved and standardized to map the coral reefs of CIO. Coral reefs of the CIO occupy 18,252.13 sq km area. Maldivian and Indian reefs cover 79.63% (14533.60 sq km) and 13.06% (2383.87 sq km) of CIO reef area respectively. Coral reefs of British Indian Ocean Territory enjoy pristine ecological status among the CIO reefs. In India, Lakshadweep followed by Andaman and Nicobar islands have certain reefs which may be termed as pristine with the rest being in vulnerable condition. Gulf of Kachchh reefs are in degraded condition. Sri Lankan reefs are also in vulnerable condition. Banladesh has a single reef which is in degrading condition. This inventory is the first of its kind providing baseline data not only for management and conservation but also forms an important input for climate change studies.

[Keywords: Coral reefs, Central Indian Ocean, Resourcesat -1, Habitat Mapping]

Introduction

Coral reefs, the rain forests of the tropical seas, are considered to be one of the “keystone ecosystems” with reference to global climate change1. As an ecosystem, coral reefs are sensitive enough to display any kind of changes occurring within the very narrow range of biophysical parameters of their common marine habitats i.e., the shallow, tropical seas of the world as well as changes occurring even at small scale in the global and local climate. In fact they are referred to as ‘harbingers’ of climate change. In addition to being high in biological productivity they are also sources of huge deposits of calcium carbonate. Coral reefs are also sites of CO2 sink. They also ensure coastal protection, are centers of scientific research, provide natural raw materials for deriving pharmacological products especially the life saving drugs. Coral reefs have an important ecological function by serving as habitat, nursery, and feeding areas for many marine species. They represent crucial sources of income and resources for the coastal population through their role in fishing and tourism. Despite their persistence and importance over geological time, coral reefs appear to be one of the most vulnerable marine ecosystems.

Reef conservation and management plans call for monitoring of reef health as well as identifying critical habitats. Worldwide, detailed coral reef maps are essentially nonexistent. Spalding et al.

2 has brought out a World Atlas of Coral Reefs providing a summary of the geographic distribution and status of world coral reefs. Though the coral reefs have been mapped at global level, the details in terms of geomorphic and ecological categories are not available2,3. Worldwide remote sensing studies of coral reefs have focused on mapping the geomorphological classes4,5. Mapping of ecological components of coral reefs have not been attempted using satellite data6. Indian coral reefs have been mapped at habitat level using Indian Remote Sensing (IRS) satellite data7.

Looking at the paucity of information on the coral reefs extent, its various habitats, ecological and geomorphological zones worldwide, spatial inventory of coral reefs for the Central Indian Ocean (CIO) at eco-geomorphic level has been carried out using the data from RESOURCESAT-1 (IRS-P6) satellite. Central Indian Ocean covers the countries of India, Sri Lanka, Bangladesh, Maldives and British Indian Ocean Territory. Reefs in Pakistan could not be

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delineated from the current satellite data and therefore have not been included in this study. Main objectives of the study were: i) To evolve a classification system for mapping coral reefs at geomorphological level with broad ecological zones, ii) Generate coral reef habitat maps at eco-geomorphic level using Resourcesat-1 data, iii) To infer the ecological status of the CIO reefs.

Materials and Methods

Study Area

Coral reefs in the CIO occur within the geographical limits of 23 North to 8 South latitudes and 69 East to 94 East longitudes. In the CIO region, the major reef formations are present in the Indian sub-continent and the Maldives. As an oceanographic ecotone this region epitomizes the bio-geographical transition observed over the entire Indian Ocean from its west to the east. CIO serves as a ‘critical link’ between the robust, endemic reefs found in the enclosed seas of the Middle East (Arabian Gulf and the Red Sea) marking the north western continental margin of the Indian Ocean and the Southeast Asian reefs: the region of highest reef biodiversity in the world. To the west, the reefs from south India (Lakshadweep) to the British Indian Ocean Territory or the Chagos Archipelago include certain elements of more typically Indian Ocean species, as well as small number of species which characterize the Western Indian Ocean. According to Spalding et al.2, maximum reef diversity in the Central Indian Ocean is restricted to a “narrow path” –centred to the southern Maldives and Chagos Archipelago.

Data used

CIO reefs have been mapped using data from RESOURCESAT-1 (IRS-P6) satellite launched on October 17, 2003. This satellite has three sensors: an Advanced Wide Field Sensor (AWiFS) having spatial resolution of 56 m and two Linear Imaging and Self Scanning Sensors LISS-III and LISS-IV having 23 m and 5.8 m spatial resolutions respectively.

Primarily, LISS-IV data pertaining to 2004 to 2007 has been used in the present study. However, in case of non-availability of cloud-free and low tide exposure of coral reefs in LISS-IV data, LISS-III and AWiFS data have been used. Data acquired at low tides and clear sky conditions were carefully selected as high tides modify and reduce the signature coming

from the reef-scape. For selecting the low tide image, we have referred to the predicted tide data of major and minor ports for Indian reefs given by the Maritime Board. For other regions we have carefully browsed and selected the data based on the exposure of reefs. Data of October to April months were selected to take care of the seasonal vegetation like algae on the reef. Algal growth peaks from October to March in most of these areas.

Methodology

Using the classification system for mapping Indian reefs as a base7,8 along with intensive literature survey4,5,9,10,11 we have evolved a common, comprehensive classification system (Table 1) to map the CIO coral reefs. Ecological components are limited to assemblages of plants (algae and seagrass) and the inhabited substrata. In order to map the live corals very high resolution data 1m or better is required, therefore this study is limited to mapping geomorphological zones having the probability of live coral occurrence (like outer reef crest, shallow pools, etc.). After frequent ground checks using GPS, areas having live corals along with other categories like algae or shallow pools have been labelled in maps as algae with live corals or shallow pools having live corals etc. An interpretation key was generated which helped in identifying and mapping different reef features based on optical characteristics along with locational-attributes.

Methodology to map and monitor the Indian coral reefs at eco-morphological level evolved earlier8,12,13,14

has been used for mapping the CIO reefs. Standard digital as well as visual analysis procedures such as pre-processing of images (image corrections and image enhancements), georeferencing, subset generation, delineation of reef boundary and high tide line, visual interpretation of satellite data for reef classification, etc., were employed to generate the coral reef map using ERDAS Imagine software.

Geographic projection and Modified Everest datum have been chosen for the geo-referencing of satellite images. Due care has been taken to ensure geo-referencing with Root Mean Square Error (RMSE) to be less than one pixel. Regions having coral reefs (subsets) were extracted from the full image to get maximum details of the reef region. Digital imagery displayed as colour composite image by combining green, red and near-infra red band (or the standard False Colour Composite Combination) was processed

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Table 1 Classification System for Mapping the CIO Reefs

Reef Type Geomorphological Geomorphological Ecological Zone Zone Sub-zone Fringing/Platform/Patch/ Pinnacle

Reef Front/Windward Reef Front*

Reef Edge*

Reef Slope* Reef Crest* Algal Ridge/Algal Pavement* Macro and Micro Algae* Moat* Live Corals, Algae* Reef Flat* Outer Reef Flat* Coral Zone/

Mixed Coral Zone, Algae* Inner Reef Flat* Macroalgae, Seagrass,

less corals* Reef Flat (Degraded)* Macroalgae* Sanded Reef Flat* Macroalgae, Seagrass* Muddy Reef Flat* Macroalgae dense* Sandy Muddy Reef* Algae, Seagrass* Boulder Zone* Algae* Rubble Zone* Algae* Shallow Tidal Pools* Algae (matty)* Exposed Reef Flat

(After-Tsunami) Algae (dense), Seagrass*

Eroded Reef Area (After Tsunami)

Backwash/Detritus Deposit (After-Tsunami)

Sand Deposition (After-Tsunami)

Sand Patches* Sand Cay* Sandy Ridge* Algae* Rocks* Coralline Shelf* Barrier/Ribbon Reef Atoll/Faros Atoll Outer Rim Lagoon Shallow sandy lagoon

(Depth 1-5 meters)

Lagoon of Intermediate Depth (Depth 5-20 meters)

Deep Lagoon (Depth > 20 meters)

Lagoonal Reef Slope Lagoonal Reef Patches Coral Knoll Lagoonlet Submerged Reef* Submerged Reef

(After-tsunami)

Coral Head* Live Coral* Beach* Dune Vegetation* Island* Mudflat Sub-tidal Mudflat Inter-tidal Mudflat High-tidal mudflat

Mangroves Jetty High Tide Line

*:The Geomorphological Zones/Sub-zones & Ecological Zones marked with asterisk are common to all Types of reefs.


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and enhanced digitally. Image enhancement, was solely done to improve the appearance of the imagery to assist in visual interpretation and analysis. As the coral reefs often occur in submerged conditions, care was taken to enhance the coral reefs by softening the land pixels (contrast and brightness stretching) from the image. Visual interpretation of intra-reef geomorphic features, ecological and litho-substrates was done to classify the reefs using the interpretation key evolved during this study. Same were labelled and a map was composed for each subset. Unsupervised Classification based on Iterative Self-Organising Data Analysis (ISODATA) clustering algorithm available in ERDAS IMAGINE image processing software has been used for pixel level area estimation. Digital classification accuracies from ISODATA algorithm on Indian reefs have been between 77-92% at 90% confidence level8. Maximum visible reef edge seaward on the image is discerned as the reef boundary. Since satellite data of low-tide exposure was preferred, hence it became easier to discern the upper limit of tidal reach and accordingly the High Tide Line was delineated. Major reef typology and intra-reef geomorphic features were delineated with reference to the elements of location, association, size and shape whereas the ecological and litho-substrates were discerned with reference to their tone and texture and also according to their spatial occurrence. Main typology of reef is decided with reference to its association and distance to other land features. It is important to mention here extensive ground truthing/reef surveys of Indian reefs was carried out, however the reefs of Sri Lanka, Bangladesh, Maldives and British Indian Ocean Territory were not surveyed. The signatures and knowledge of Indian reefs were extrapolated to these reefs for visual interpretation, e.g., the coral reefs of Lakshadweep are similar to Maldivian reefs, therefore, the signatures of Lakshadweep reefs were extended to reefs of Maldives. The Gulf of Mannar and the fringing reefs of Sri Lanka are similar. Reefs of British Indian Ocean Territory have similarity with the Lakshadweep. We have also taken help from experts in the field and the available literature2,10,15 for interpreting the unsurveyed reefs.

Results & Discussion

Indian Reefs

In the Indian coast, major coral reef regions are Gulf of Kachchh (Gujarat), Malvan in Maharashtra, Lakshadweep, Palk Bay and Gulf of Mannar (Tamil Nadu) and the reefs of the Andaman and Nicobar Group of Islands. Resourcesat-1 satellite images depicting the coral reefs for the four major reef regions of India are shown in Fig. 1. Field photographs of common corals found in these regions are given in Fig. 2. The total area under coral reefs mapped for the Indian coast is 2383.87 sq km. There is a unique linear ribbon reef connecting India and Sri Lanka. The details for the above regions are discussed below: Gulf of Kachchh reefs, occupying 352.5 sq km area, are northernmost of the Indian reefs, mostly consisting of dead coral boulders and pebbles. They comprise fringing, platform and patch type of reefs along with some coral pinnacles. Well defined reef slope and reef front mainly occurs in the northern and western regions. Algal/Reef ridge follows the reef crest in the north and is the most distinct feature of the island reefs like Bural Chank (Fig. 1A), Kalubhar, Paga, etc. Ridge is markedly absent on the eastern or the leeward side of the reefs. On certain reefs a clear distinction between outer and inner reef flat is observed from the satellite data. Outer reef flat here is narrow and the percentage of live corals is more. On the outer reef flats of Bural Chank, Paga, Ajad and Kalubhar, live corals are found to co-habit with dense algae (mapped as Algae Dense with Live Corals) or sometimes even under dense cover of algae. Inner reef flat has lesser percentage of live corals and has more of dead coral boulders, shallow pools, algae (calcareous as well as fleshy algae; Sargassum is found in abundance here) and sparse seagrass. Recently, increase in macro algae has been observed, particularly of the fleshy algae like Ulva. Muddy reef flat or thick mud depositions (thickness of mud varies from few centimeters to more than a meter) are observed on almost all the reefs in the leeward or the eastern side. An encrusting alga belonging to Vaucheria family (carpet or matty algae) grows profusely on this muddy zone. Area of the sanded reef flat varies within the inner reef flat, sometimes adjoining the linear beach. Seasonal cover of vegetation is observed during October-April months, mainly consisting of algae and species of Ipomoea.


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Periodic monitoring using multitemporal satellite data has given insight on the ecological status of these reefs. Sand deposition is one of the major cause of reef degradation. At many places, the degradation of coral reefs due to bleaching has also been observed

(for e.g., the Pirotan reef which lies in the core Marine National Park, Jamnagar, Gulf of Kachchh). Temporal satellite images of Pirotan Island for a period of 20 years (1986-2006) shown in Fig. 3 reveals successive degradation of Pirotan reef from a coral dominated

Fig. 1 Resourcesat-1 LISS images showing coral reef habitats of A) Bural Chank Reef, Gulf of Kachchh; B) Bangaram Island,

Lakshadweep; C) Krusadai Reef Complex, Gulf of Mannar and D) Interview Island, Andaman.

Fig. 2 Field photographs of different types of corals in India a) Bural Chank Reef, Gulf of Kachchh b) Bangaram Island, Lakshadweep c)

Krusadai Reef Complex, Gulf of Mannar and d) Interview Island, Andaman.


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habitat to a macroalgae dominated habitat. It is due to mass coral bleaching following anomalous rise in sea surface temperatures; effect of sedimentation (both sand as well as mud) as well as anthropogenic pressures. Pirotan reef covers an area of 12.90 sq. km. A comparison between 1996 and 2002 IRS data revealed that mud on Pirotan reef rose from 3.7 sq. km to 7.7 sq. km while algae cover on muddy reef flat increased from 6.3 sq. km to 10.9 sq. km. Deposition of both silt and sand effectively hinders the occupancy of live corals on reef flat. Fringing reefs of Dhani bet have significantly degraded with more mud depositions. Areas having live corals are almost non-existent here. It can also be inferred that the fringing type of reefs have degraded the most due to more anthropogenic pressure. Malvan coast in Maharashtra forms a part of the Western Ghats where the Sahyadri mountain ranges gradually meet the Arabian Sea. A chain of submerged and exposed rocky islands extend southwards comprising several islands at the southern tip and the Sindhudurg fort at the northern tip. Sindhudurg is a low fortified island on the coastal reef, which is joined to the mainland by a dying reef. During low tide, the exposed area of the bay has large rock pools with rocks partially exposed and the corals are generally seen attached to the rocks. Corals occur here in spite of the region being highly turbid. Malvan

reef is a very small reef having an area of 0.28 sq km. Coral reef is well defined in the eastern part of the Malvan Island. Inspite of the corals remaining submerged even at very low tides, it was possible to delineate this zone using LISS IV data. This reef lacks proper reef zones. Windward reef front, reef crest and outer reef flat are not present here. Reef flat is broader on the west and northwest. Submerged corals are present in the northeast. Seagrass is observed to be linearly distributed along the sandy beach in the east. Lakshadweep archipelago consists of a chain of well developed coral reefs, lying about 200-470 km off the Kerala coast. Southernmost island of Minicoy is separated from rest of the islands by a 180 km wide stretch of sea. Coral formations built up on the Laccadive-Chagos submarine ridge rising steeply from a depth of about 1500 m to 4000 m form an interrupted chain covering a distance of over 2000 km along with the Maldives and the Chagos Archipelago3. Coral reefs of Lakshadweep, covering 933.7 sq km area are atoll type except for Pitti (Submerged Bank), and Androth (Platform reef). The growth of reefs is widest on the western side presumably because the growth is facilitated by the continuous supply of the nutrients on the western side. Along with well developed atolls, there are submerged coral growth and coral heads. Reef front is mapped in the western, southwestern and northern

Fig. 3 IRS LISS sequel of Pirotan Reef, Gulf of Kachchh, India


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reef regions of Lakshadweep reefs. Outer reef flat has a prominent boulder zone or the algal ridge on the western seaward side. Porites and Favia are commonly found here. Sandy Ridge and Rubble Zone occur on certain reefs on the seaward side after the reef front. Reef flat of Lakshadweep is an aligned coral zone comprising dense coral growth. Live corals like Acropora and Pocillopora dominate in this zone along with sporadic occurrence of Favites and Heliopora. Sand, coral rubbles and shingles and algae are also present. Seagrass occupies the shallow lagoonal area landwards along the beach on many reefs. Shallow western lagoon of the Kadmat inhabits extensive seagrass beds apart from seaweeds and corals. All the atolls have centrally or westerly located lagoons with clear waters. Shallow lagoon on certain reefs (Baliyapaniyam, Cheriyapaniyam, Bitra, Perumal Par, Suheli Par, Minicoy, Chetlat, Bangaram and Agatti) harbour lagoonal patch reefs comprising massive coral colonies of mainly Porites, Acropora and Pocillopora with sporadic presence of Acropora. These patch reefs are found at depths of 1.5-5 m. The most conspicuous feature of the coral reefs of Bangaram (Fig. 1B), Bitra, Cheriyapaniyam, Baliyapaniyam and Minicoy is the presence of a number of coral knolls in the lagoonal region. On an average, knolls posses about 70-80% live coral cover. There are more than 100 such knolls in Bangaram, Cheriyapaniyam and Bitra. Minicoy has 11 and Baliyapaniyam has 12 coral knolls. Extensive Coralline shelf is present near Agatti-Bangaram, Pitti and Kalpeni atolls and the platform reef of Androth.

Among all the Lakshadweep islands, the coralline shelf joining the two reefs, Bangaram and Aggati, is most unique. From the north of Aggati, the shelf extends northwards and joins the western portion of the Bangaram reef. The shelf is wider on the western region and is linear and parallel in the rest of the regions and could be differentiated clearly using satellite data. The shelf as such has not been earlier charted. Bathymetric charts show the contour of 30-40 m (area which has been marked by coralline shelf in the present study). In the north of Lakshadweep islands, two atolls, namely, Cheriyapaniyam and Baliyapaniyam which were not charted earlier in the Survey of India topographical maps or Naval Hydrographic bathymetric charts have been mapped using satellite data. Sandy beach/storm beaches (12 sq km) characteristically make up the coast of all the Lakshadweep islands, comprising coralline sand, dead coral boulders and sandstone.

Some of the Lakshadweep reefs (Bitra, Perumal Par, Baliyapaniyam, Cheriyapaniyam) are in near pristine ecological status. Lagoons have clear waters harbouring rich coral patches along with other fauna like Green turtles, Sting rays, Sea Cucumber, and a plethora of fish. The reefs are rich in diversity. However, reefs like Kadmat, Kavaratti, Agatti, and Minicoy are in vulnerable conditions. These reefs have seagrass cover that has significantly increased within a span of 5 years. The increase is mainly attributed to eutrophication due to human pressure. Bangaram islands too are being surrounded by seagrass (an activity observed only in last 5 years). Sand deposition has also increased here. This reef otherwise harbours rich corals in the lagoonal and reef areas and also has coral knolls having dominant cover of live corals. In view of this, the islands need to be managed effectively to stall degradation and improve their ecological status. Gulf of Mannar has a number of offshore platform reefs, patch reefs and coral pinnacles, lying along the 140 km stretch from south of the Pamban Pass to north-east of Tuticorin. The islands lie at an average distance of 8 km from the mainland.

A linear Ribbon reef situated between 79º27′- 79º42′ E longitudes and 09º04′-09º09′ N latitudes extends from south-eastern tip of the Rameswaram Island going up to the northwest of Sri Lanka. This is a spectacular landform connecting the two countries and is known as Adam’s Bridge16. Spalding et al.2 mentioned Adam’s Bridge as a string of reefs stretching Sri Lanka. Distinctly visible reef crest lies between the back reef and fore reef zones. There are a total of 25 sand cays along the reef out of which 9 fall in the Indian side and the remaining are part of Sri Lanka. Adam’s Bridge reefs are now being increasingly subjected to heavy sedimentation round the year. Sedimentation has resulted in excessive sand deposition and degradation of the reefs. Percentage of live corals is negligible.

The fringing reef in the Palk Bay lies in an east-west direction along the mainland from the Pamban channel at the Pamban end of the bridge to Rameswaram Island. The reef flat is relatively broad from Pamban channel to southern end near Ramnad and narrow from Pamban to south of Rameswaram. Reef flat is followed by sandy beach on the landward side. Live coral percentage is less here, as these reefs are also subjected to excessive sedimentation particularly in the monsoon.


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Gulf of Mannar has a number of offshore platform, patch reefs, coral pinnacles and atoll. Reefs having a total area of 75.93 sq km, are present from south of the Pamban Pass to Tuticorin. Windward Reef Front covers the southern area from the seaward edge that slopes into deeper water to the landward edge of the shelf platform. In north the signature of the windward reef front was not distinct due to excessive turbidity of water. It is conspicuously absent in the northern region. The ecological components of Reef Flat include live corals, algae and seagrass. Live corals are represented by Acropora, Montipora, Pocillopora,

Porites, Favia, Favites, Goniastrea, Echinopora,

Leptastrea, Platygyra, Galaxea, Goniopora, etc. Micro-atolls were observed on this zone (in the field) especially in the northern region. Major portion of the reef flat is occupied by algal growth comprising mainly Turbinaria sp. Other species are Padina, Halimeda, Sargassum and Gracilaria. Central part of the reef [mainly Krusadai Reef Complex (KRC) (Fig. 1C) and Manouli Reef Complex (MRC)] makes a shallow lagoonal environment covering an area of about 1.2 sq. km. Bottom of the lagoon is mainly sandy. The sea grass ecosystem dominates the MRC with 87% cover. It is represented by Cymodocea

rotundata, Cymodocea serrulata, Halodule,

Halophila ovalis, Halophila ovata, Enhalus

acoroides, Thalassia hemprichii, etc.

Presence of macroalgae, seagrass in major portions of the reef and high level of sedimentation is indicative of degrading ecological condition of the reef. Tsunami of 26th December, 2004 has also rendered damages to the northeastern reefs of the Gulf 17. Coral blocks were found to be uprooted during a ground survey carried out after the tsunami. Reefs near Tuticorin are also in a degrading condition due to thermal ash deposits. Gulf of Mannar reefs are also subjected to illegal quarrying.

Andaman and Nicobar group of islands are separated by the 160 km wide Ten Degree Channel. The western side of the Andaman Islands has coral reefs of fringing type running intermittently for about 350 kilometers. Earlier study7 using satellite data revealed that reefs are in good condition with broad reef flat in the west. Eastern side has narrow fringing reefs in certain places. The east coast is a sheltered type with creeks and monsoonal river estuaries and bays. As the Andaman and Nicobar reefs are closer to Indonesia and Southeast Asia, the reef diversity in this region is therefore closer and similar to the Indonesian

and the Southeast Asian centre of biodiversity than to Indian reefs. Coral species diversity of these reefs is the maximum (219 species) among all Indian reefs. Windward reef front comprising live corals surrounds the entire reef on the seaward side followed by non-aligned coral zone shoreward. Live corals of various species (mixed coral zone) including Porites,

Goniopora, Pocillopora, Acropora (Fig. 2D), Symphyllia Fungia, Echinopora and Goniastrea sp occupy the zone. The reef flat comprises sandy substrate with intermittent rocks, live massive coral colonies (20 cm to > 1 m in diameter) of Porites, Acropora, Symphyllia, Lobophyllia, Goniopora,

Heliopora and Montipora (covering 50-70% area) and algae. Patchy algae are dominated by Sargassum,

Padina, Halimeda and Caulerpa. Turbinaria increases in percentage as we move seaward. Turbinaria is observed to dominate the outer reefs where the wave energy is high whereas, Sargassum, Halimeda and Padina dominate the inner reef flat portions. Cymodocea and Halophyla seagrass were also found on the reef flat. Shallow pools on the sanded reef flat have corals like Porites, Acropora and Pocillopora. Jolly Boys is the only reef, which has large sanded reef flat making up almost 70-80 % of the reef system. Rock exposures occur within the reef system north of the Belle reef, Tarmugli reef, etc. The Andaman group occupies reef area of 968.44 sq km while the Nicobar Islands cover an area of 53.02 sq km of coral reefs.

The coral reefs of the Andaman and Nicobar islands were in near pristine condition prior to tsunami of 26th December, 200417. When giant tsunami waves smashed onto shores, the coral in nearby shallow areas were destroyed, crushed and shrouded in debris. Massive backwash returned to sea carrying a deadly cargo of detritus material and deposited on coral reef. Erosion due to breakage has resulted in loss of 54% of Andaman reefs (covering 227 sq km area). The subsidence (for e.g., in the north and middle Andaman the subsidence has resulted in increase of water level to the extent of 1.2 m) along with the high intensity waves have caused physical damage to these reefs. Earthquake has caused about 15 – 60 cm wide and 80 – 100 m long cracks in the outer reef flat and the reef slope (ground observation). Earthquakes have also resulted in elevation of areas wherein the reefs have become exposed. Significant erosion is observed in the easterly-located Havelock group of reefs (comprising 5 reef complexes). A total


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of 7.5 sq km reef area of Havelock Island has undergone erosion. Just after tsunami 2.1 sq km area eroded as observed in the AWiFS data of 16th February 2005. The erosion reportedly increased to 7.5 sq km area as mapped using IRS LISS III data in 3rd August 2005 data. Gradual erosion is indicative of continuing subsidence due to recurring earthquakes. About 22% of the total area occupied by the reef has experienced damage due to backwash (covering 93.9 sq km area). Sentinel reef located in the western part has undergone damage entirely due to the backwash. However, significant improvement was noticed after 11 months. Backwash like sand and detritus has washed off significantly. About 41% of reef area was restored (i.e. the backwash got washed off), sand depositions came back to their original extent, and detritus reduced to 3.7 sq km. Similarly sand that covered 800 m in Havelock just after tsunami was observed to have washed off subsequently resulting in an increase in the sand free shallow reef flat area. Elevation of the reef flat leading to exposure and dying of corals at a large scale occurred particularly in the west of the North Andaman where the reef flat now stands exposed most of the time and comprises sand, algae and intermittent shallow pools. In the Nicobar group of reefs, erosion was more prominent. A total of 69% of reefs have totally broken off and eroded. Coral reefs of the entire Little Nicobar Island (earlier covering an extent of 9.8 sq km) have been eroded. Trinkat and Camorta Reefs have suffered maximum damage. These two islands shared a common reef area (they were joined by the coral reef growth) and had a total reef area of 60 sq km prior to tsunami. About 50% of its area has eroded totally and 49% area has been severely damaged due to backwash. A large extent of Great Nicobar reef (covering 98 sq km and making up 83.8% area) has totally eroded.

The reef area of Indian coral reef regions under the present study has been compared with the previous inventory on Indian coral reefs7 using satellite data of 1987 to 1991 (Table 2). Coral Reefs of Sri Lanka

Sri Lanka has almost continuous and well developed fringing reefs in the north and north-west and patchy reef growth in the east. Reef covers 475.70 sq km area. Significant reef systems of Sri Lanka are Karaitivu, Jaffna Lagoon, Eulaitivu, Andalaitivu, Delft Island, Palitivu and Punkudutivu, Nainativu and Bar Reef, the latter happens to be a Marine Sanctuary. These small island reefs survive in a comparatively high turbid condition. Mud depositions on the reef flat have been noticed on the fringing reef near Nainativu, Jaffna lagoon (Fig. 4A) in the north-west. Punkudutivu in the north-west has a significantly large fringing reef system. After tsunami, sand deposition appears to have increased on the northern reef. Reef Ponnaveli has been exposed significantly after the tsunami and earthquake of 26th December 2004. South western reefs of Sri Lankan coast have more of the narrow fringing reefs of Bentota, Hikkaduwa Marine Nature Reserve near Galle, Weligama, Polhena Reef, and Kalametiya Kalapuwa Sanctuary. These reefs have characteristically long narrow reef flats, sometimes sanded. In certain cases submerged reefs have also been detected from the LISS-III images for this region. Reef near Kalpitiya has a shallow lagoon which shows signs of eutrophication in certain places as well as high sediment concentration. South-eastern coast of Sri Lanka has some offshore prominences of reefs and submerged coral banks. In the eastern coast fringing reefs occur in Vandeloos Bay, Vellaiyadimadu and surrounding area, near Verukal, Seruwila Allai Sanctuary, Kunittivu and surroundings, Great Sober Island Sanctuary. Intra-reef zonation pattern could not be inferred from the satellite data in the Sri Lankan reef system probably because of narrow fringing reefs. Prominent reef features appearing for these reefs are mostly the long narrow reef flat, reef front/reef slope and the adjacent beaches.

Coral reefs of Sri Lanka are in vulnerable condition. West coast has coral reefs that are narrow and fringing and the east coast has sparse distribution of coral reefs. Coral reefs in the north-west of Sri Lanka are in good ecological condition e.g., Delft Island, Palitivu & Punkudutivu, Mannar Island.

Table 2 Comparison of coral reef area of India over two decades

Reef Region 1987-91 2004-07 Difference in area (in sq km)

Gulf of Kachchh 460.20 352.50 107.70

Malvan Not mapped 00.28 00.28

Lakshadweep 816.10 933.70 117.60

Gulf of Mannar 94.30 75.93 18.37

Andaman & Nicobar 959.30 1021.46 62.16

Total 2329.90 2383.87 53.97


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Among the western and south-western coast reefs, the Hikkaduwa Marine Nature Reserve and Bar Reef Marine Sanctuary have coral reefs that are in good ecological condition. However all these reefs have been damaged to some extent due to tsunami. Major damage has occurred due to the deposition of sand/debris on the reefs. Coral Reef of Bangladesh

In Bangladesh, the only coral reef is around the St. Martin’s Island (locally known as Jinjiradwip or Narikel Jingira) covering 4.0 sq km area. Reef occurs in the extreme south, off the mainland coast of Bangladesh in the north eastern Bay of Bengal. Reef development in this region has occurred in an unfavourable condition as there is continuous freshwater discharge of mighty rivers like the Ganges, Brahmaputra and Meghna. AWiFS image shows an exposure of a fringing reef around the island (Fig. 4B). The eastern side of the island shows the reef in continuity whereas to the western side, the reef abruptly ends in the central portion. In the northwestern and northern side of the island there is a wide reef front area with a narrow reef flat. The flat however widens onto the east. Existence of boulders is visible in the northwestern reef front area. Higher spatial resolution satellite data obtained during low

tide exposure is required to obtain in-depth information on this reef.

Coral Reefs of Maldives

Located south of the Lakshadweep islands of India and running for about 800 kilometers north to south in the CIO, the Maldives form a definite sequence of large oceanic atolls. The Maldivian reefs are unique among CIO reefs. These are the only faros atoll reefs. They cover an area of 14533.60 sq km. Each reef system forms a chain of individual atolls around a deep central lagoon and each individual atoll has its own lagoon (thus called Faros atoll). These atolls rise steeply from the base of the submerged Chagos-Laccadive ridge and are aligned in two parallel chains. Faros are more common in the northern Maldives, but absent at Fua Mulaku atoll in the south. Fua Mulaku is the only patch reef in Maldives having a small lagoonlet18. Thiladhunmathi atoll in the north is the largest faros atoll occupying 2408.70 sq. km area. Makunudu atoll is a uniform atoll with no faros atolls. In its central deep lagoon there are a number of patch reefs as well as coral knolls. North Male atoll has more than hundred faros and mini-atolls in the lagoon. Gaa Faru is a small, elliptical, atoll situated to the north-west of North Male atoll. Gaa Faru atoll has coral zone lining the entire inner edge of the

Fig. 4 Resourcesat-1 images showing A) Jaffna Lagoon, Sri Lanka; B) St. Martin’s Island, Bangladesh; C) Felidu Atoll, Maldives and

D) Blenheim reef, BIOT.


694

lagoonward reef. South Male atoll has eastern faros atolls with sandy shallow lagoons and sanded reef flats while the west is devoid of sand depositions. Ari atoll consists of ring shaped faros reefs that form its outer edge and has numerous mini atolls inside its lagoon. This atoll has only four inhabited small islands in the south making the region comparatively free of anthropogenic pressure. There are no visible signs of eutrophication and algal growth here. Toddu atoll in the north east of Ari atoll has a well defined inner reef flat which shows aligned coral zone. Felidu atoll (Fig. 4C) is another beautiful atoll. It is a boot-shaped atoll with the horizontal arm having shallow sandy lagoonal floor and sanded reef flat while the vertical arm is devoid of any sand. Huvadhoo atoll in the south has number of coral knolls both in its central deep lagoon as well in the lagoons of mini-atolls. Addu, the southernmost atoll has well defined reef slope and reef crest on the outer side. Reef flat is narrow. To the east after the sandy region, a well defined lagoon slope is observed with coral zone that goes down comparatively gently to the lagoon floor. Seagrass lines the sandy region in the east. Lagoon has reef patches and is covered with growing corals. Few Atolls like Addu have visible impacts of anthropogenic pressure. The western arm of the atoll has more signs of anthropogenic pressure. Southern region has extensive areas showing eutrophication and algae cover. More than any other nation outside the Western Pacific, the Maldives is dependent on coral reefs for maintenance of land area, food, export earnings and foreign currency from revenues.

British Indian Ocean Territory (BIOT) coral reefs

Coral reefs of British Indian Ocean Territory (BIOT) serve to be the link between Maldivian atolls

and the reefs of Western Indian Ocean. BIOT forms the southernmost extension of the Chagos-Laccadive Ridge. These reefs and the islands are highly isolated – the nearest reef structures are those of Maldives, some 500 kilometers to the north. Reefs of BIOT can be classified into atolls, faros atolls and submerged coral banks covering 854.96 sq km area. Reef structures although resemble the Maldivian atolls but among all the reefs of BIOT, only Peros Banhos qualify to become a faros atoll as per its definition.

Due to the unavailability of cloud free data of RESOURCESAT-1 satellite, major portions of Colvocoresses reef, Great Chagos Bank, Cauvin Bank, Egmont reef, Pitt Bank, Ganges Bank, Centurion Bank and Wight Bank could not be mapped. Speakers Bank is a large submerged coral bank. About 50 to 65 m wide reef edge is visible in the satellite data. Certain reef patches in the lagoonal area are also visible. Another submerged bank that could be detected from LISS III data is the Victory Bank. Due to cloud cover, the western portion of the Bank could not be detected. The Blenheim reef (Fig. 4D) appears to be in good condition. Numbers of small, mostly symmetrical patch reefs with live corals are present in its lagoon. Reef flat is narrow but it has an extensive reef slope plunging deep down into the lagoon. The reef crest however, is fostering the foundation of a juvenile beach. Saloman atoll, lying to the east of Blenheim has a deep lagoon with a wide reef flat and a comparatively narrow lagoonal reef slope. Reef crest is narrow and filled with sand deposition. This reef has five major islands with permanent vegetation and five smaller islands are in the stage of their territorial expansion on the reef flat itself. Lagoonal reef slope is characteristically wide at certain places. Reef of Peros Banhos practically resembles the faros atoll structure of other Maldivian Islands. Circumfering a deep lagoon the main structure of faros atoll of Peros Banhos stands to the east of Saloman Island. This reef however, bears certain signs of concern for the overall reef health as algal growth on reef flat is observed. Atoll of Diego Garcia marks the end of linear extension of the axis line of Maldivian Atolls. This reef housed the southernmost British base in the Indian Ocean. Even on the narrow reef flat of Diego Garcia a strip of island with permanent vegetation runs continuously. Lagoonal reef slope shows two distinct slope profiles: the sandy shallow slope adjacent to the island and the relatively deeper slope which plunges down to the

Table 3 Countrywise reef area in CIO

Sr. No. Country Reef Area (in sq. km)

1 India 2383.87

2 Sri Lanka 475.70

3 Bangladesh 04.00

4 Maldives 14533.60

5 BIOT 854.96

(mapped so far)

Total 18252.13


695

lagoon floor. Coral zones can be easily identified along with coral knolls present in the lagoon from the Resourcesat-1 image. Probably as a result of their isolation, these reefs are the most pristine and best protected ones in the Central Indian Ocean. There are submerged banks which harbour coral reefs and its flora and fauna in good diversity and health. Country wise reef area of CIO region mapped under the present study is given in Table 3.

Conclusion

Baseline mapping of the Central Indian Ocean reefs covering India, Sri Lanka, Bangladesh, Maldives and BIOT at ecological as well as geomorphic level has been carried out for the first time using satellite data. A comprehensive classification system has been evolved to map the coral reefs of the world at eco-morphological level. Mapping at ecological and geomorphic level has given significant insight on the ecological condition of these reefs, highlighting regions which have undergone degradation, or vulnerable to degradation and the regions which can be called pristine or in good ecological conditions. Among the Indian reefs, Gulf of Kachchh and Gulf of Mannar reefs are in degrading condition. Lakshadweep and Andaman and Nicobar reefs are in vulnerable condition. In Bangladesh the only reef is found in St. Martin’s Island (fringing type). Maldives, the country with largest atolls of the world is closely followed by the British Indian Ocean Territory (BIOT) in terms of similar kind of reefs. The coral reefs including the lagoonal system occupy 18,252.13 sq km area in the Central Indian Ocean region. Indian reefs cover 2383.87 sq km area. Highest reef area excluding the lagoons is occupied by the Indian reefs (1734.9 sq km) followed by the Maldives reefs (1654 sq km). In CIO, reefs of Maldives and BIOT are found to be in pristine condition compared to the reefs of India, Sri Lanka and Bangladesh. Acknowledgements

Authors acknowledge Dr. Shailesh Nayak, Secretary, Ministry of Earth Sciences, Govt. of India and Dr. M.V.M. Wafar, Coral Reef Scientist, National Institute of Oceanography, Goa for their guidance and help in evolving the classification schemes for mapping coral reefs of the Central Indian Ocean. Dr. A.K. Kumaraguru and Dr. N. Marimuthu, Madurai

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