The change in land-sea silicate fluxes through the Chao Phraya River

Abstract

The study of the change in land-sea silicate fluxes through the Chao Phraya River focuses on two areas of interest: the seasonal change in silicate and other nutrients in the Chao Phraya River in 1999 and possible historical change in biogenic silica in eight sediment box cores and one 40-year old coral core taken from the Gulf of Thailand. The main objective is to try to evaluate the relationship between silicate fluxes and the historical land use changes in the Central Plain of Thailand. The year-round average of dissolved silicate concentration in the Chao Phraya River in 1999 was 181.58 muM. The silicate concentration in the wet season was higher than silicate concentration in the dry season. In the wet season, larger amount of freshwater transported more dissolved and particulate materials into the river. This amount of materials released more silicate into the river water. Dissolved silicate concentration in the river water decreased toward the river mouth by dilution through the mixing process with low-silicate seawater. In the sediment box cores, we had expected that the biogenic silica concentration accumulated in the upper part of the cores would be lower than in the deeper part because a certain amount of sediment was trapped in major dams upstream. However, the opposite trend was found. The result showed a trend of slight increase in biogenic silica concentration in the upper part of the cores from most stations although the increase was not very distinct. This could be because of the increase in soil erosion downstream in recent years due to land clearance for agricultural practices and other uses was higher than the amount of suspended matter trapped in the dam reservoirs. It is also expected that the fragile siliceous diatom shells would dissolve faster in the tropical seas. The analysis of biogenic silica concentration incoral bands of Porites lutea, showed no clear change in biogenic silica concentration either just before or after the dam construction. However, a very distinct peak was observed during late 1950s to the early 1960s which was the time of the major dam construction.