Estuarine, Coastal and Shelf Science (1998) 46, 757–768 Changes in Community Structure and Biomass of Seagrass Communities along Gradients of Siltation in SE Asia J. Terrados a , C. M. Duarte a , M. D. Fortes b , J. Borum c , N. S. R. Agawin a , S. Bach c , U. Thampanya d , L. Kamp-Nielsen c , W. J. Kenworthy e , O. Geertz-Hansen c and J. Vermaat f a Centro de Estudios Avanzados de Blanes, CSIC, Camı ´ de Santa Ba ´rbara s/n, 17300 Blanes, Girona, Spain b M arine Sciences Institute, College of Science, University of The Philippines, Diliman, Quezon City, 1101, The Philippines c Freshwater Biological Laboratory, University of Copenhagen, Helsingørsgade 51, Hillerød, Denmark d Coastal Resources Institute (CORIN), Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand e Beaufort Laboratory, SE Fisheries Science Center, N ational M arine Fisheries Service, N OAA, 101 Pivers Island Road, Beaufort, NC 28516-9722, U.S.A. f International Institute for Infrastructural, Hydraulic and Environmental Engineering, P.O. Box 3015, 2601 DA Delft, The Netherlands Received 11 July 1997 and accepted in revised form 21 October 1997 The patterns of change in species richness and biomass of Southeast Asian seagrass communities along siltation gradients were compared at different sites in The Philippines and Thailand. Seagrass species richness and community leaf biomass declined sharply when the silt and clay content of the sediment exceeded 15%. Syringodium isoetifolium and Cymodocea rotundata were present only in multispecific meadows, while Enhalus acoroides was the only species remaining in heavily silted sediments. The following ranking of species sensitivity to siltation is proposed (from the least to most sensitive): S. isoetifoliumC. rotundataThalassia hemprichiiCymodocea serrulataHalodule uninervis Halophila ovalis Enhalus acoroides . Positive correlations were found between species richness and both community leaf biomass and the leaf biomass of individual seagrass species. The increase in community biomass with increasing species richness was associated with a more even distribution of the leaf biomass among seagrass species. The relationships between percent silt and clay in the sediment and seagrass community leaf biomass and species richness provide useful dose–response relationships which can be used to set allowable or threshold siltation loads in SE Asian coastal waters, and indicate that species loss from seagrass meadows is an early warning of detrimental siltation loads.  1998 Academic Press Limited Keywords: species richness; community biomass; siltation; seagrasses; SE Asia Introduction Fast population and economic growth of countries in Southeast (SE) Asia since World War II have generated increasing pressure on natural resources and ecosystems ( Panayotou, 1993 ). One of the con- sequences of this pressure is deforestation of the region. It is estimated that the surface of forest land in SE Asia has been reduced by 50% between 1850 and 1980, while countries like Thailand or The Philippines have lost more than 70% of their forests ( Liu et al ., 1993 ; Panayotou, 1993 ). Due to the high rates of rainfall, rapid deforestation has resulted in major soil loss, and subsequent siltation of coastal ecosystems. The present sediment yield from SE Asian rivers to the ocean is the largest on earth ( Milliman & Meade, 1983 ) and is driving a wide- spread deterioration of the region’s coastal ecosystems ( Fortes, 1988, 1995 ; Go ´mez, 1988 ), which contain some of the most diverse communities on earth. Seagrasses are prominent components of SE Asian coastal ecosystems ( Fortes, 1995 ), where they sustain high primary production ( Vermaat et al ., 1995 ; Agawin et al ., 1996 ) and provide habitat and food for economically important species ( Fortes, 1995 ). Siltation is associated with a reduction in light avail- ability and an increase of the nutrient load ( Malmer & Grip, 1994 ; Bach, 1997 ; Mitchell et al ., 1997 ), both of which negatively affect seagrass growth ( Giesen et al ., 1990 ; Duarte, 1991 a; Sand-Jensen & Borum, 1991 ; 0272–7714/98/050757+12 $25.00/0/ec970304  1998 Academic Press Limited