Threats to seagrasses
Most coastal resources tend to be degraded including seagrass areas, which can be affected by many activities and coastal developments like mangrove forest and coral reefs. The degradation of seagrass is directly and indirectly caused by natural factors and anthropogenic factors. Natural factors such as elevated seawater temperature generate effects on seagrass’ life cycle; the long period exposure to air may cause dryness and heat on seagrass; temperature and sunlight also affects on flowering and seeding of seagrass, while the seagrass may be eaten by grazers. Seagrass would be deteriorated if there are many grazers (Fong, 1999). Additionally, natural disasters such as cyclone may cause degradation to seagrass, for examples, the cyclone that hit Hervey bay in Australia destroyed all seagrass beds in that area (Preen and Marsh, 1995), another one was Typhoon Linda that hit the South of Vietnam causing extensive damages on seagrass beds at Ko Konda and the seagrass beds in that area were later changed in the next period of time (Hoa, 2001). In Thailand, the tsunami affected on the seagrass beds in the Andaman Sea causing about 4 km2 of destroyed seagrass areas. Besides, there are many human activities which generate impacts on seagrasses including coastal development which produce sediments and coastal wastewater such as marine mining, road construction, factories, coastal communities and shrimp farming. Seagrasses and marine animals can be destroyed by destructive fishery such as small trawl nets, push nets and large beach seines. Unfortunately, the tsunami that occurred on December 26, 2004 caused enormous effects on about 2,500 rai of seagrass beds, especially the seagrass areas at Ban Tung Nang Dam and the North of Ko Prathong of Phang Nga Province. In some area, it was found that large amount of sediments were dispersed and transported by waves hitting and covering seagrasses; their leaves were turned to brown color and died within 2-3 weeks later. It took about 2-3 months for its recovery.