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 Reef conservation and management plans call for Coral reefs, the rain forests of the tropical seas, are monitoring of reef health as well as identifying critical habitats. Worldwide, detailed coral reef maps are considered to be one of the “keystone ecosystems” 2 with reference to global climate change1. As an essentially nonexistent. Spalding et al. has brought ecosystem, coral reefs are sensitive enough to display out a World Atlas of Coral Reefs providing a any kind of changes occurring within the very narrow summary of the geographic distribution and status of range of biophysical parameters of their common world coral reefs. Though the coral reefs have been marine habitats i.e., the shallow, tropical seas of the mapped at global level, the details in terms of geomorphic and ecological categories are not world as well as changes occurring even at small scale 2,3 in the global and local climate. In fact they are available . Worldwide remote sensing studies of coral reefs have focused on mapping the referred to as ‘harbingers’ of climate change. In 4,5 addition to being high in biological productivity they geomorphological classes . Mapping of ecological are also sources of huge deposits of calcium components of coral reefs have not been attempted using satellite data6. Indian coral reefs have been carbonate. Coral reefs are also sites of CO2 sink. They also ensure coastal protection, are centers of scientific mapped at habitat level using Indian Remote Sensing (IRS) satellite data7. research, provide natural raw materials for deriving pharmacological products especially the life saving Looking at the paucity of information on the coral drugs. Coral reefs have an important ecological reefs extent, its various habitats, ecological and function by serving as habitat, nursery, and feeding geomorphological zones worldwide, spatial inventory areas for many marine species. They represent crucial of coral reefs for the Central Indian Ocean (CIO) at sources of income and resources for the coastal eco-geomorphic level has been carried out using the population through their role in fishing and tourism. data from RESOURCESAT-1 (IRS-P6) satellite. Despite their persistence and importance over Central Indian Ocean covers the countries of India, geological time, coral reefs appear to be one of the Sri Lanka, Bangladesh, Maldives and British Indian most vulnerable marine ecosystems. Ocean Territory. Reefs in Pakistan could not be BAHUGUNA et al.: SPATIAL INVENTORY & ECOLOGICAL STATUS OF CORAL REEFS 685 delineated from the current satellite data and therefore from the reef-scape. For selecting the low tide image, have not been included in this study. Main objectives we have referred to the predicted tide data of major of the study were: i) To evolve a classification system and minor ports for Indian reefs given by the for mapping coral reefs at geomorphological level Maritime Board. For other regions we have carefully with broad ecological zones, ii) Generate coral reef browsed and selected the data based on the exposure habitat maps at eco-geomorphic level using of reefs. Data of October to April months were Resourcesat-1 data, iii) To infer the ecological status selected to take care of the seasonal vegetation like of the CIO reefs. algae on the reef. Algal growth peaks from October to March in most of these areas. Materials and Methods Study Area Methodology Coral reefs in the CIO occur within the Using the classification system for mapping Indian 7,8 reefs as a base along with intensive literature geographical limits of 23 North to 8 South latitudes 4,5,9,10,11 and 69 East to 94 East longitudes. In the CIO survey we have evolved a common, region, the major reef formations are present in the comprehensive classification system (Table 1) to map Indian sub-continent and the Maldives. As an the CIO coral reefs. Ecological components are oceanographic ecotone this region epitomizes the bio- limited to assemblages of plants (algae and seagrass) geographical transition observed over the entire and the inhabited substrata. In order to map the live Indian Ocean from its west to the east. CIO serves as corals very high resolution data 1m or better is a ‘critical link’ between the robust, endemic reefs required, therefore this study is limited to mapping found in the enclosed seas of the Middle East geomorphological zones having the probability of live (Arabian Gulf and the Red Sea) marking the north coral occurrence (like outer reef crest, shallow pools, western continental margin of the Indian Ocean and etc.). After frequent ground checks using GPS, areas the Southeast Asian reefs: the region of highest reef having live corals along with other categories like biodiversity in the world. To the west, the reefs from algae or shallow pools have been labelled in maps as south India (Lakshadweep) to the British Indian algae with live corals or shallow pools having live Ocean Territory or the Chagos Archipelago include corals etc. An interpretation key was generated which certain elements of more typically Indian Ocean helped in identifying and mapping different reef species, as well as small number of species which features based on optical characteristics along with locational-attributes. characterize the Western Indian Ocean. According to Spalding et al.2, maximum reef diversity in the Methodology to map and monitor the Indian coral Central Indian Ocean is restricted to a “narrow path” reefs at eco-morphological level evolved earlier8,12,13,14 –centred to the southern Maldives and Chagos has been used for mapping the CIO reefs. Standard Archipelago. digital as well as visual analysis procedures such as pre-processing of images (image corrections and Data used image enhancements), georeferencing, subset CIO reefs have been mapped using data from generation, delineation of reef boundary and high tide RESOURCESAT-1 (IRS-P6) satellite launched on line, visual interpretation of satellite data for reef October 17, 2003. This satellite has three sensors: an classification, etc., were employed to generate the Advanced Wide Field Sensor (AWiFS) having spatial coral reef map using ERDAS Imagine software. resolution of 56 m and two Linear Imaging and Self Geographic projection and Modified Everest datum Scanning Sensors LISS-III and LISS-IV having 23 m have been chosen for the geo-referencing of satellite and 5.8 m spatial resolutions respectively. images. Due care has been taken to ensure geo- Primarily, LISS-IV data pertaining to 2004 to 2007 referencing with Root Mean Square Error (RMSE) to has been used in the present study. However, in case be less than one pixel. Regions having coral reefs of non-availability of cloud-free and low tide (subsets) were extracted from the full image to get exposure of coral reefs in LISS-IV data, LISS-III and maximum details of the reef region. Digital imagery AWiFS data have been used. Data acquired at low displayed as colour composite image by combining tides and clear sky conditions were carefully selected green, red and near-infra red band (or the standard as high tides modify and reduce the signature coming False Colour Composite Combination) was processed 686 INDIAN J. MAR. SCI., VOL. 42, NO. 6, OCTOBER 2013 Table 1 Classification System for Mapping the CIO Reefs Reef Type Geomorphological Geomorphological Ecological Zone Zone Sub-zone Fringing/Platform/Patch/ Reef Front/Windward Reef Edge* Pinnacle Reef Front* 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 Algae (dense), Seagrass* (After-Tsunami) 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.