Sediment Properties along Gradients of Siltation in South-east Asia

Marine sediment properties were quantified along SE Asian siltation gradients and across different habitat types to assess the extent of terrestrial sediment influence and derive indicators of of terrestrial sediment input. Three sites in the Andaman Sea (Ranong, Phuket and Trang; Thailand), one site in the Gulf of Thailand (Pak Phanang; Thailand) and three sites in the South China Sea (Bolinao at the Luzon Island, Puerto Galera at the Mindoro Island and El Nido at the Palawan Island, all in the Philippines) were studied. The following main habitat types were covered: mangroves, seagrass beds, coral reefs, mud flats, river mouths and deep channels. Particularly in Bolinao and El Nido, distinct fronts in siltation were identified at about 7 km from the source. Mud (<63 μm), water content, organic matter, total nitrogen, total phosphorus and iron co-varied along the first axis of a principal components analysis, which correlated negatively with the pattern in medium to coarse sand (>250 μm), total and inorganic carbon as well as calcium. Interstitial ammonia and phosphate varied with the fine sand fraction along the second axis. Two-way analysis of variance showed that water depth, distance from silt source, site and habitat type all contributed to the variance, but site explained most. Linear regressions showed positive correlations between silt and water content, organic matter, total nitrogen, total phosphorus and iron, but negative correlations with calcium suggesting iron and calcium as markers for terrigeneous and marine origin, respectively. The composition of the sediment particulate matter groups the habitat types in two clusters: (1) silty types as river mouths, shallow mud bottoms, deep channels and mangrove stands with high contents of organic matter and nutrients versus (2) non-silty types as seagrass beds and coral reefs with low contents of nutrients and organic matter. Median settling velocities of the silt loads varied from 0•6 m d−1to 27 m d−1. Experimentally determined susceptibility to resuspension identified a critical water content of 50%.