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

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

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However, corals that become coated with oil when exposed at low tide can be seriously damaged(JOHANNES et al., 1972), while serious effects also appear to result from chronic long-termexposure to even relatively low levels of oil.In general, impacts on corals in the Indian Ocean from oil pollution appear much less frequentthan might be supposed. LOYA (1975, 1976) found an almost complete lack of recolonisation bycorals on reefs at Eilat, chronically polluted by oil. Field and laboratory studies show a decreasedproduction of ova, premature discharge of eggs, damage to larvae, and a reduction in successfulsettlement by larvae when corals are exposed to long-term oil pollution (RINKEVITCH and LOYA,1977, 1979). Parts of the Gulf of Suez (Egypt) have been subject to even more serious chronic oilpollution (WENNINK and NELSON-SMITH, 1979), and ORMOND (1980) has suggested this asone factor responsible for an apparent decline in coral cover on reefs in parts of this area.Eutrophication and SewageSewage discharge can result in severe damage to coral communities. The best studied case isof the killing of corals over most of Kanehoe Bay, Hawaii (BANNER and BAILEY, 1970;MARAGOS, 1972). Coral death has been demonstrated to correlate with nutrient concentrations andturbidity (MARAGOS, 1972), but neither are thought to be directly responsible. The mechanism mayvary with different circumstances in different places; at Aqaba (Red Sea) WALKER and ORMOND(1982) have concluded that sewage discharge killed corals by stimulating the growth of algae, which inturn increase the rate of sedimentation affecting the coral.Other wastes may cause similar effects. In both the Jordanian and Egyptian Red Sea localeutrophication leading to algal growth and coral damage has resulted from the escape of phosphate dustduring loading of ships for export of crushed phosphate rock (FISHELSON, 1973; WALKER andORMOND, 1982).Thermal PollutionDeath of corals resulting from discharge of heated effluents has been observed, for example,in Florida (ROESSLER and ZIEMAN, 1969), Guam (JONES and RANDALL, 1973) and Hawaii(JOKIEL and COLES, 1974). The sensitivity of coral and reef animals to modest temperature increaseshould be especially noted here, since in parts of the Indian Ocean, with higher open water temperatures,coral and reef animals are likely to be living very close to their absolute limit of tolerance.Altered SalinityAlthough some corals live in salinities up to about 50 ppt (KINSMAN, 1964), most corals arekilled by long-term exposure to salinities much above normal (EDMONDSON, 1928). Hypersalineeffluents may be discharged from power plants or desalination plants, or result from restriction ofwater circulation in lagoons or bays. Corals also are affected by a reduction in salinity, and most arekilled in less than a week by exposure to 75% seawater (EDMONDSON, 1928). At various locationsin the Indian Ocean small areas of coral probably have been affected by such changed conditions.SIGNIFICANCEMANGROVE FORESTSThe term mangrove covers a wide variety (60 or so) of species of trees that belong to anumber of essentially unrelated families, but which, often together, alive long the edge of brackish orsea water shores in many parts of the tropics. Here they can form zones of dense forest, up to severalkilometers wide and containing as many as 20 or so different species. Such diverse and expansivecommunities are found only in a few areas; in other areas mangrove stands are much narrower andmay contain only a few species.In recent years it has come to be appreciated, as a result of many investigations in differentparts of the world, that mangroves constitute a highly significant natural habitat and a very valuable17 2

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