139736eo.pdf (20MB) - Japan Oceanographic Data Center

NATURAL CAUSES OF REEF DEGRADATIONA coral reef is subject to a variety of natural environmental stresses primarily associated withwater movement, geological dynamics and biological interactions. Destructive wave action and strongcurrents experienced during monsoons, cyclones and freak storms can cause severe mechanicaldamage to coral reefs. PILLAI (1971a), reporting on the effect of a severe cyclone that swept throughsoutheastern India and eastern Sri Lanka in December 1964, stated that “... the ti’ranching corals,especially Acropora spp. were the worst hit and very many large colonies of A. corymbosa and A.surculosa were found uprooted and washed ashore. Several corals, both ramose and massive, werelater found dead on the reefs. The alcyonarians were reduced greatly in numbers at several places. Onthe whole there was a reduction of corals in most of the reefs”. Currents also can bring about changesin water quality such as temperature, salinity, turbidity and nutrients which affect coral development.Exposure to anomalously low tides can expose reefs to excessive heat, freshwater inundation ordestructive wave action.Various geological phenomena also are known to cause reef damage. Sedimentation causessevere reef damage by inhibiting coral growth through reduced incident light available to thezooxanthellae within coral polyps, by interfering with the planktonic food supply, or by smotheringthe coral polyps. PILLAI (1971a) reported the mortality of a large number of coral colonies in theinshore waters of Palk Bay due to silt and suspended sand stirred up by monsoon winds. Large-scaledestruction of corals due to silt stirred up during heavy winds also has been reported in Mandapam inthe Gulf of Mannar. Other geological phenomena such as landslides, earthquakes, volcanic activity,and tectonic uplift and subsidence also can cause coral reef damage.Among the biological interactions, competition constitutes a major limitation to the growth ofreef-building organisms. Boring organisms, such as barnacles, molluscs, sipunculids, polychaetes,sponges and algae as well as grazing organisms such as parrot fishes (Scaridae), are also known tocause some degree of damage to coral reefs. Predation is another factor of great importance, the mostwell-known predator of coral being the ‘Crown of Thorns’ starfish (Acanthaster planci ) which hasbeen responsible for the complete destruction of several reef areas along the east coast of Sri Lanka(DE BRUIN, 1972).HUMAN ACTIVITIES CAUSING REEF DEGRADATIONHuman activities are of greater importance in several places than natural causes in causingcoral reef degradation and have been responsible for extensive damage to many coral reef areas inSouth Asia. In 1980, the Coral Reef Working Group - 1 of the International Union for Conservationof Nature and Natural Resources (IUCN) recognized 24 human-related activities that could causedamage to coral reefs. Of these, the activities considered relevant to South Asia are listed in Table 1 ona three point scale to indicate the extent of the problem.CORAL REEFS OF THE CENTRAL INDIAN OCEANSEWELL (1932, 1935), STODDART (1971, 1972), PILLAI (1972), DE SILVA (1983)SCHEER (1972,1984), MAHADEVAN and NAYAR (1972), MERGNER and SCHEER (1974), andPILLAf and SCHEER (1976) have summarized the information available on corals and coral reefs ofthe Indian Ocean, with special emphasis on the Central Indian Ocean. Perhaps with the exception ofSEWELL, most of the early workers “ ...were less concerned with the structure of reef communitiesand their physiographic expression than with the taxonomy and biogeography of the reef biota”(STODDART, 1971). However, many recent workers have made in-roads to bridge this gap in ourknowledge of coral reefs (STODDART, 1971, 1972; MERGNER and SCHEER, 1974; DAVIES etal., 1971; and SCHEER, 1971).155