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

ecognised, as is common with many marine groups, the highest diversity of seagrasses is found in theIndeWest Pacific where there are seven genera.The principal ecological significance of seagrasses lies in the fact that the seagrass beds theyform constitute an important shallow water habitat. Within the dense array of seagrass blades a largevariety of fish and invertebrates obtain shelter and food, much of which derives directly or indirectlyfrom the seagrasses themselves (DEN HARTOG, 1970; OGDEN, 1980). An important feature of theseagrass plant is its extensive root and rhizome system which enables it to colonise various soft-bottomsubstrates, provides a semi-permanence to the seagrass community, and also enables the plants todraw on mineral nutrients from the seabed.Principal features of the ecology of seagrasses and seagrass communities may be summarisedas follows (IUCN, 1984; see also WOOD et al., 1969; ZIEMAN, 1975):High ProductivityTropical seagrass beds are among the most productive of natural ecosystems. Productivitiesof 0.5-16 g C m-2 day-1 and 500-3,000 g C m-2 yr-l have been recorded (DREW, 1971;PATRIQUIN, 1973; McROY, 1974; BUESA, 1975; ZIEMAN, 1975). A recent review of the topic isprovided by ZIEMAN and WETZEL (1980). Unfortunately few studies have yet been made of theproductivity of the species occurring in the Indian Ocean, although WAHBEH (1980) found anHalophila stipulacea stand at Aqaba (Jordan) to have an estimated productivity of 617 g C m-2 yrl.Direct GrazingTheir cellulose walls make seagrasses difficult for most fish and invertebrates to ingest directlyor to digest (by comparison with algae). Nevertheless many species do feed on seagrasses, andseagrasses are a major source of food for adult Green turtles (CARR, 1952; HIRTH et al., 1973;ROSS, 1979), for juvenile Hawksbill turtles (PHILLIPS and McROY, 1980), and €or the Dugong(LIPKIN, 1975). A summary of other species reported feeding directly on seagrass is given byOGDEN (1980), who estimates that in the Caribbean, for which most the data are available, aboutthirty species of fish and five of sea urchins feed directly on seagrass. In the Red Sea, WAHBEH(1980) found that the sea urchin Tripneustes gratilla, the surgeonfishes Zebrasoma xanthurum andCtenochaetus striatus, and rabbitfishes Siganus spp. were the principal direct consumers of aHalophila stipulacea seagrass bed near Aqaba (Jordan). The urchins accounted for 33.5% and the fishfor 4.9% of the total loss of leaf and rhizome biomass. The bulk, however (61.6%), entered thedecomposer food chain.Production of DetritusIt has generally been concluded that most of the primary productivity of seagrass beds entersthe decomposer food chain (WOOD et al., 1969). The senescent blades break off and decomposeamong the seagrass, on the surrounding seabed, or on an adjacent shore. The resulting detritus andbacteria (see FENCHEL, 1970) are consumed by an abundant meiofauna, by sponges, sessilepolychaetes, many deposit- and suspension-feeding bivalves, a wide variety of crustacea, includingcommercial shrimp species such as Penaeus semisufcatus, and by some fish, including commercialspecies such as Gerres oyena and mullet (Mugilidae) (O'GOWER and WACASEY, 1967; ORTH,1971; BROOK, 1975).Substrate for EpiphytesWell-developed seagrass beds generally support a dense epiphytic community that oftenmakes a major contribution to the primary productivity of seagrass beds (WOOD, 1959; JONES,1968; PENHALE, 1977). Particularly dense epiphytic turfs of sciaphilic algae are found on the stemsand roots of Thalassodendron ciliatum (LIPKIN, 1976; DEN HARTOG, 1970). Many fish andherbivorous molluscs probably graze on the epiphytic algae rather than on the seagrass itself (see, forexample, LIPKIN, 1976; PENHALE, 1977).In addition to their contribution to primary productivity, these epiphytes may play an importantrole in supplying mineral nutrients to the seagrass bed, especially in nutrient-poor waters. Very highrates of nitrogen fixation have been reported for epiphytes on Thafassia testudinum and for bacteria176