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

Along the coasts of the Indian sub-continent there are extensive seagrass beds in southernIndia and in the numerous estuaries and embayments of Sri Lanka. In other areas, however, denseseagrass beds are uncommon or not extensive. This seems to be true of at least some of the coasts ofPakistan and of much of the coasts of western India, probably because the coastal waters are exposedand turbid so that seagrasses can not easily find a foothold or grow well.STATUSBecause their distribution and significance is incompletely known, the extent of damage toseagrass beds within the Indian Ocean can not be assessed accurately. Some impacts, however, havebeen reported. Industrial and agricultural runoff have damaged seagrass beds in Sri Lanka (SALM,1975), and industrial wastes, sewage discharges and overfishing have been reported to have led to thedestruction of a seagrass bed in Mauritius (PROCTER and SALM, 1974). However, damage to andloss of seagrass beds have been most extensive, and most serious in terms of economic consequences,in Saudi Arabia, Kuwait and Bahrain. There large areas of seagrass beds have been lost by extensiveland reclamation and infilling, by dredging to form shipping and boat channels, and by excessivesedimentation resulting from upcurrent infilling or dredging activities.In view of the limited information available on the status of seagrass beds within the IndianOcean, it is perhaps most relevant to summarise (following IUCN, 1984) the range of impacts towhich they, like coral reefs and mangroves, have been found to be susceptible. It should, however,be noted that different species can cope with different stresses to varying degrees, so thatgeneralisations are not easily made, except where the impacts are extremely severe (see ZIEMAN,1975). In general, the most susceptible habitats are the shallow subtidal and intertidal ones, and themost susceptible species are those that lack a significant starch reserve in their rhizomes.Dredging and FillingBecause seagrass beds occur in shallow coastal habitats, they lie in the prime area for coastaldevelopment and are frequently destroyed either by infilling to produce reclaimed land or, alternatively,by dredging for the production of channels for boats, for the exposure of bedrock to provide a moresecure substrate for construction, or for the provision of infilling materials for construction elsewhere.Similar damage also can occur by bottom trawling or where boats or walkers regularly scour seagrasscoveredshallows. Finally, construction associated with dredging and landfill indirectly can influenceseagrass beds by altering water current patterns and producing new water-quality conditions.Infilling causes direct death of seagrass beds through burial, and both infilling and dredgingcan cause deterioration and death of seagrass beds in neighbouring areas because of increased sedlmentloads and increased water turbidity. Heavy sediment loads may suffocate the plants directly, whileincreased turbidity reduces light levels in the water column and on the seagrass, thus reducing orpreventing photosynthesis and growth.Sedimentation also wil smother and kill the flora and fauna associated with the beds. Evenwhere protective curtains or screens are used around sites of dredging and infilling, turbidity may stillbe sufficient to cause damage (ZIEMAN, 1975). These turbidity problems can also affect associatedcoral reefs, leading to their death.EutrophicationEutrophication is of only limited benefit to seagrass communities. Seagrasses can take upextra nutrients, leading to stimulated growth. Enrichment also fosters the development of epiphyticalgae and phytoplankton. However, seagrasses generally require high levels of incident light energyso that increased epiphytic growth and high water turbidity by reducing light levels may actuallydecrease growth of the seagrass.178