Long-term variations of nutrients in the Upper Gulf of Thailand

Abstract

Long-term variations of nutrients in the Upper Gulf of Thailand was conducted using the secondary data gathered during 1987-1994. The results indicated that concentrations of nutrients in the Upper Gulf had not changed over the period of study but showed seasonal variations. High concentrations were found near the coast especially in the eastern part of the Gulf and also in the central pat of the Gulf when the evaporation was extremely high. In dry season, nitrate and phosphate varied within the range of 0.2-10 and 0.1-2 umol l-1, respectively. During wet season, large amount of freshwater input into the Upper Gulf, and higher concentrations were observed. Concentrations of nitrate and phosphate were 0.5-13 and 0.05-3 umol l-1, respectively. Nutrient budgets in the Upper Gulf were established using secondary data from the Fishery Department collected in 1987, 1990 and 1994 which were the most completed and covered the whole area. Two approaches were used. The first approach was a single longitudinal one dimension model which was conducted by dividing the study area into 2 parts: coastal and offshore area (with a boundary at 20 km from the bay head). Longitudinal mixing coefficients of coastal and offshore area was 2x105-3x106 and 2x107 cm2s-1, respectively. The mixing rate of coastal area was lower than of the offshore area. The coastal area showed net productions of total phosphorus and nitrate plus nitrite through the year, while for some months, net destructions of phosphate and nitrate were found. Some nutrients in the coastal area were exported to the offshore area where most of total phosphorus in this area was removed as sediment, but nitrogen showed net production in some periods. It was also found that there were some exchanges of phosphorus and nitrogen, between offshore area and the Lower Gulf of Thailand. The second approach was a box model in which the study area were divided into six boxes in order to determine the internal transport of water and nutrients of the Upper Gulf. The result of water and salt budget indicated that water transport in the Upper Gulf varied seasonally. The calculated result of nutrient budget was indicated that the large amount of exchange fluxes of nutrients, both phosphorus and nitrogen (nitrate plus nitrite) was mostly occurred in the vinicity of mouth especially in the eastern part of the Gulf. The major sources of nutrients in the Upper Gulf of Thailand came from the four major rivers drained into the area and regeneration within the system. There were very small variation of nutrients overtime. Some excess nutrients were removed to sediment and some were exported to the Lower Gulf in some periods.