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

Coating of Aerial Roots by Fine Sediment/ErosionThis can occur though natural sediments released near mangrove stands by dredging orlandfill activities, through excessive organic matter input or through the introduction of chemicals intomangrove waters. In St. Croix (U.S. Virgin Islands) release of N aOH (used in bauxite extraction)into mangrove waters caused precipitation of hydroxides of various metals, including magnesium,onto aerial roots, and thus the death of mangrove plants (ODUM and JOHANNES, 1975).Oil PollutionThe serious effect of oil pollution on mangrove stands has been demonstrated on a number ofoccasions. For example, SPOONER (1970) observed that an oil spill in Tarut Bay, Saudi Arabia,defoliated mangroves although the mangrove plants survived. And within the Red Sea major oil spillsalready have occurred on the Egyptian Coast, severely impacting several stands of Avicennia marina,particularly on Abu Minqar Island, near Hurghada (BARRATT, 1982).Oil pollution damages mangroves by an immediate smothering effect on aerial roots, andunder circumstances.of heavy oiling, mangrove plants can be killed in 48-72 hours (IUCN, 1983a).In addition, oiling can cause chronic poisoning as a result of the uptake of volatile petrochemicalfractions through aerial root and leaf lenticels. More detailed discussion of the effects of oil onmangrove can be found in BAKER (1982).Fresh Water FloodingAs a result of inland construction works or coastal engineering interfering with land drainage,mangroves can be flooded with fresh water and subsequently die (see ODUM and JOHANNES,1975). Thus the Sundarbans mangrove ecosystem has been severely affected by the Farakka Barrage,which has diverted waters of the Ganges; as a result floods, soil toxification, land instability and pestinfestations have increased, while fertility, water quality, forests and fisheries have declined (IUCN,19 8 3 a).Herbicide DamageMangroves are unusually susceptible to herbicide damage because of the critical role of thefoliage in salt secretion. Moreover it appears that herbicidal damage can prevent re-establishment ofany new mangrove community for at least six years and possibly for much longer, This type ofdamage could become increasingly significant in the region because of the rapidly increasing amountsof pesticides being used by many of the countries bordering the Indian Ocean (WHITMORE, inUNEP, 1985). In addition, the potential for such damage also should be taken into account whenconsidering the use of potentially toxic oil dispersants in clearing up oil spills in mangrove areas,Thus it is appropriate to note here that decimation of mangrove resources has stimulatedinternational concern and activity. In 1978 SCOR, in collaboration with UNESCO's division ofMarine Sciences, established a working group on Mangrove Ecology, and in 1980 IUCN established aworking group on Mangrove Ecosystems. UNESCO continues to be active in promoting activitiesrelated to mangrove research and management, including mangrove mapping by remote sensing, aUNEP/UNESCO regional project on sedimentology in mangrove areas, and the development of anetwork of mangrove managers and scientists in the Indian Ocean.SIGNIFICANCESEAGRASS BEDSSeagrasses are true flowering plants which propagate both vegetatively and by the dispersal ofseeds. They occur in soft-bottom, shallow-water marine environments and are usually permanentlysubmerged, although they occasionally are found intertidally. About 50 species in twelve genera are175