Variation of partial pressure of CO2 (pCO2) and budgeting of water, salt, nitrogen and phosphorus in the inner Gulf of Thailand were investigated by taking water samples from 22 stations. A series of three sampling cruises were carried out during the wet (August 2009 and September 2010) and the dry (March 2010, 2011) seasons. Values of salinity, temperature, pH and total alkalinity were used to calculate pCO2. Sea surface pCO2 in the dry season (March 2010 and March 2011) varied from 103.7 to 270.3 μatm while those values of the wet season (September 2010) showed wider range from 49.2 to 166.9 μatm. The values of pCO2 were higher (149.8±53.8 μatm) in nearshore and rivermouth stations than those in offshore stations (129.8±33.6 μatm). It can be concluded that the Inner Gulf of Thailand serves as a sink for atmospheric CO2. The net flux of CO2 in the Inner Gulf of Thailand was estimated as an ocean uptake of -22.95×103 tons of carbon per year (-0.06 mol/m2/year).
Water and salt budgets showed seasonal variations. Water and salt were exported from the inner Gulf of Thailand during the wet season at rate of 328 x106 m3/day and 10,115 x106 kg/day, 83% larger than those in the dry season. Results of nutrient budgets for both seasons showed the Gulf acts as a net source for dissolved inorganic phosphorus whereas dissolved inorganic DIN was a net sink. The values of Net Ecosystem Metabolisms (NEM; p-r) and Net Nitrogen Production (nfix-denit) in the inner Gulf of Thailand were negative (-24.3±9.4 mmol C m-2 d-1 and -7.3±0.6 mmol N m-2 d-1) indicating that the Gulf was not only a heterotrophic system as respiration exceeds photosynthesis (photosynthesis-respiration<0) but also a net denitrifying. It can be concluded that inner Gulf of Thailand receives larger amount of organic loads and nitrogen input from terrestrial sources.