Tsunami Impacts on Biodiversity of Seagrass Communities in the Andaman Sea, Thailand: (1) Seagrass Abundance and Diversity

Evaluation of the impact of a catastrophic disturbance on biodiversity is often difficult due to a lack of sufficient quantitative data on biological communities prior to the disturbance. Since 2001, we have been monitoring the abundance and biomass of seagrass and its associated animal community along the Andaman Sea coast of Thailand, an area that was hit by a tsunami originating in the Indian Ocean on Dec. 26, 2004. To quantitatively evaluate the effects of the disturbance on the biodiversity of the seagrass community we carried out a comparative analysis of seagrass species diversity and abundance based on pre-/post-disturbance data at several seagrass beds at Kuraburi, northern Phang-nga (most-affected region) and Trang (less-affected region). Species diversity, coverage and biomass of seagrass declined greatly after the tsunami at one seagrass bed (Thung Nang Dam) in Kuraburi. The effect of the tsunami was less obvious at two other beds a few kilometers away from Thung Nang Dam, and at the seagrass beds in Haad Chao Mai National Park in Trang (located ca. 200km south of Kuraburi), where temporal change before and after the tsunami was the least obvious. A steady decline in biomass was observed in the three seagrass beds that had not been severely affected by the tsunami, possibly associated with other types of disturbance such as river discharge. Analyses revealed that the effect of the tsunami on seagrass ecosystems was highly variable even on small spatial scales, and that other factors causing disturbance to the seagrass beds are important factors of the observed temporal variation.