CHANGES IN COMMUNITY STRUCTURE AND BIOMASS OF SEAGRASS COMMUNITIES ALONG GRADIENTS OF SILTATION IN SE ASIA

The patterns of change in species richness and biomass of Southeast As ian seagrass communities along siltation gradients were compared at di fferent sites in The Philippines and Thailand. Seagrass species richne ss and community leaf biomass declined sharply when the silt and clay content of the sediment exceeded 15%. Syringodium isoetifolium and Cym odocea rotundata were present only in multispecific meadows, while Enh alus acoroides was the only species remaining in heavily silted sedime nts. The following ranking of species sensitivity to siltation is prop osed (from the least to most sensitive): S. isoetifolium-->C. rotundat a-->Thalassia hemprichii-->Cymodocea serulata-->Halodule uninervis-->H alophila 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 biom ass with increasing species richness was associated with a more even d istribution of the leaf biomass among seagrass species. The relationsh ips between percent silt and clay in the sediment and seagrass communi ty leaf biomass and species richness provide useful dose-response rela tionships which can be used to set allowable or threshold siltation lo ads in SE Asian coastal waters, and indicate that species loss from se agrass meadows is an early warning of detrimental siltation loads. (C) 1998 Academic Press Limited.