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

Temperature and SalinityThe effects of alterations in temperature and salinity, such as might arise from coastalconstruction work or effluent discharge, depend upon the species concerned. For example,Syringodium is reported to die at temperatures below 20°C (ZIEMAN, 1975), whilst Halophila ovaliscan tolerate seawater temperatures lower than 10°C. Conversely, Halophila stipulacea is very salinityadaptablewhilst Cymodocea serrulata is very sensitive to a decrease in salinity (DEN HARTOG,1970).OvergrazingAs already discussed, although many herbivores do not eat seagrass, a variety of species,particularly of fish and sea urchin, feed directly on the plant, and where these grazers are present invery large numbers, overgrazing and destruction of the grass bed can occur. Such overgrazing byreef-associated fish frequently results in a bare halo around patch reefs in the Caribbean (RANDALL,1965; OGDEN, 1972), and in some localities epidemic overgrazing by sea urchins is reported (CAMPet al., 1973). In the,Red Sea recently there has been loss of seagrass at Aqaba (Jordan) throughovergrazing by large numbers of the urchins Diadema setosum and Tripneustes gratilla (BENAYAHUand LOYA, 1977; MASTALLER, 1979; WAHBEH, 1980), and such population outbreaks ofechinoids may occur as a result of the loss of predators taken by man (LOWRY and PEARSE, 1973;BREEN and MANN, 1976). Thus intensification of some fisheries can indirectly result in the loss ofseagrass beds.Oil PollutionAs seagrasses are generally subtidal, they are less susceptible to damage from oil thanorganisms in the intertidal zone. However, in some regions large areas of reef flat, sometimescontaining seagrasses, are exposed at low tides and these seagrasses probably would be killed bydirect contact with oil. Also surface floating rafts of oil, if present for long enough, can affectsignificantly seagrasses by reducing light penetration, by increasing water temperature throughabsorption of light energy, and by reducing oxygen exchange at the air/oil/water interface.In addition to direct effects on the seagrass, more serious and possibly longer-term damagemay occur through interaction between the oil and the sediments in which the seagrass is growing.Much of the oil and tar from oil spills eventually sinks to the seabed, and here oil and sediment canagglomerate into more buoyant lumps and pellets which, in relatively shallow water, can be removedby wave and current action. DIAZ-PIFFERER (1962) recorded the loss of 3,000 m3 of sand from aPuerto Scan beach in less than a week due to this effect. Where oil settles over seagrass beds, loss ofsediment can lead to uprooting of the grass and damage to or destruction of the bed.In this connection it should be noted that a number of oil spill dispersants cause floating oil tosink so that seagrass beds that might otherwise be safe from oil could be directly contacted by it, orimpacted by destabilisation of sediments as just described.MetalsSeagrasses are reported to have a very high uptake of metals (e.g. PARKER et al., 1963),despite the fact that the roots are often in reducing conditions that limit the lability of metal ions. Notonly can excess metals be concentrated in the seagrass but they become available for furtheraccumulation in the food chain. This has been of particular concern in some tropical regions wkrehigh concentrations of metals have been released into the environment from desalination plants (seeCHESHER, 1975).DiseaseIn the 1930's a large-scale and so far permanent loss of beds of the temperate seagrass Zosteramarina occurred on both sides of the Atlantic (see STEVENS, 1936) that was attributed to a fungaldisease (TUTIN, 1934). So far as is known, disease has not caused extensive damage to seagrassbeds in the tropics. However, susceptibility to disease is frequently a response to increased179