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Sea grarsea
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Arnong the various tropical shallow water benthic communities, r*a grass communities
are probably the least studied, althouqh it is generally accepted that Lhey are among the
highest in productivity. Their importance as nursery sreas and as a source of food zupply to
coastal ecosystems via the detritus food web is generally recognized. The distribution and
productivity of the sea grass beds in the Andaman Sea are not yet known. They are generally
found near reef areas and as patches in estuaries from low tide level down [o a depth of
about I metres. Besides their importance as a shallow water habitat for marine organisms and
as primary producers, turtles and dugongs feed directly m sea grasses. Dugongs reared in
captivity at the Phuket Marine Biological CenLer were fed Halophila ov*' a common see
grass species in the area.
Reviewing the ef f ect of pollution m tropical sea grass ecosystem, Zieman (f975)
stated: "of all forms of man-made or induced disturbanees of the estuarine and near-shore
environments, dredging and filling presents lhe greatest potential for damage to the sea grass
beds and has undoubtedly eaused the destruction of more desirable grass bed habitat than any
other form of pollution.r'
Besides direct physical destruction, the secondary effecLs can be far more gerious
depending upon the intensity of dredging. Turbidity reduces light availability to sea grasses in
nearby areas, Fauna associated with grass beds or m the bottom could be smothered by
exeess sedimentation, or suffocated by low oxygen levels due to the high oxygen demand of
the organic matter dredged up from anoxic layers.
Changing sediment composition would change both the flora and fauna of bottom
communiLies, not only of grass beds. These organisms are either food for commercially
important species or young stages of such species. Therefore mining effects m lhe production
of bottom communities need to be estimated.
Conclusion
In conclusion, the effects of tin mining on shallow water ben[hic communities are both
direct and indirect. The direct effect is the physical destruction at the mining site by
dredging. The secondary effect is the disturbance of surrounding areas by suspended
sediment. Tlere is no evidence for other effects from mining besides disLurbance by
sedimentation. Sediment from mining ha6 caused the death of corals in the vicinity and
decreased the population of other benthic macroinvertebrates. Recovery of benthic
macrofaunal populations after mining is possible. At present no information is availabb on
the extent of damage from mining to other benthic communities sr.rch as see grasses or the
detrital bse food web of tnengrove fringe estuarine ecosystems