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

associated with the roots (e.g. GOERING and PARKER, 1972). However, other workers havereported much lower levels of nitrogen fixation in seagrasses (e.g. McROY et al., 1973; ZIEMAN,1975).Sulphate ReductionZIEMAN (1975) reported that seagrasses may also be important in the sulphate cycle. Thereare no reports of this application by seagrasses in the Indian Ocean. However, the roots of seagrassesoften are found in reducing conditions.Reduction in Current Velocity and Sediment BindingDense stands of seagrass often reduce open water current velocity almost to near zero close tothe substrate. This allows increased sedimentation and the development of an infauna and inflora thatwould not survive otherwise. The seagrass roots and rhizomes further bind the sediment that hassettled. Of the species of seagrass reported from the Indian Ocean, only H. ovalis and H. ovata are notconsidered to be significant sediment binders (DEN HARTOG, 1970).Refuge for Commercially Important SpeciesA variety of commercially important species of fish, mollusc and crustacea use seagrass bedsas a refuge and frequently as a source of food as well. Pearl oysters are closely associated withseagrass beds in the Arabian Gulf (BASSON et al., 1977), and seagrass beds are also important areasfor the juveniles of penaeid shrimps, including the commercially important P. semisulcarus (PRICE,1982). There is evidence to suggest that the collapse of the commercial shrimp fishery around Kuwait(D.A. JONES, personal communication) is closely associated with the extensive destruction of theseagrass beds during industrial development in that area.Given their significance to the various fisheries, seagrass beds are considered to be a resourceof considerable economic importance. The approximate economic value of the fishery generated by aseagrass dominated bay in Saudi Arabia (Tarut Bay) has been estimated by BASSON et al., (1977). Ifseagrass in the bay were incorporated into food chains, the estimated value of the fish yield would be$8 million. If instead, grass were converted at the same efficiency (1% overall) into shrimps, thecalculated value would be nearly $12 million. On the other hand, if the seagrass were grazed directlyby Green turtles, at an efficiency of lo%, the turtle yield would be $46 million. The value of thesenatural resources is based on 1977 prices, and the calculations are very approximate and largelytheoretical. Nevertheless, they deomonstrate the considerable value of local biological resourceswhich, if managed correctly, are renewable.OCCURRENCEIn general (see UNEP, 1985) knowledge of the occurrence and extent of seagrass beds in theIndian Ocean is very incomplete, save in two or three countries, notably Indonesia and Saudi Arabia,where recent surveys specifically have examined the question. In the eastern Indian Ocean (Indonesia)13 species, of which Thalassia hemprichii is the most widespread, have been recorded (SOEGIARTOand POLUNIN, 1982). In the western Indian Ocean (East Africa) nine species have been recorded(ALEEM, 1984) and in the Red Sea (Saudi Arabia) ten species (IUCN, 1984).Fewer species are recorded from any one of the island groups of the western Indian Ocean; sixhave been recorded in the Seychelles (TAYLOR, 1968), and six in Mauritius, although some of thesespecies are abundant around parts of many of the higher granitic islands (i.e., Seychelles, theMascarenes, the Andamans and the Nicobar Islands).Around the periphery of the region, under cooler or more saline conditions, the number ofspecies may be reduced. Only three species, Halophila stipulacea, H. ovalis and Halodule uninervis,occur within the Arabian Gulf, but there, as indicated above, they nevertheless form an extensive andeconomically significant habitat.177