AN ASSESSMENT OF NUTRIENT BIOGEOCHEMICAL CYCLES IN THE INNER GULF OF THAILAND

Abstract

The nutrient dynamics of the Inner Gulf of Thailand were studied using a simple steady-state budget model, according to the Land–Ocean Interaction in the Coastal Zone modelling guidelines. Two sampling campaigns were carried out during the wet (October 2011) and the dry (February 2012) seasons. For each season, budgets for water and salinity, dissolved inorganic phosphate (DIP) and nitrogen (DIN) were determined. The study indicates that the Inner Gulf exports water at the rate of 133-562 MCM.day -1 in the form of residual flux (V R ) at the inner-outer Gulf interface. This results in export of salt to the inner-outer Gulf interface at the rate of 4.3x10 3 –17x10 3 ton.d -1 . Exchange between the Inner Gulf water and the adjoining seawater replaces this salt loss. Mass balance calculations indicate that the Inner Gulf is a net source for DIP and DIN to the adjacent outer Gulf water at the rate of 4.0x10 4 -3.4x10 6 molP.d -1 and 4.1x10 6 - 20.5x10 6 molN.d -1 , during the dry and wet season respectively. The high DIP and DIN exports during the wet season probably reflect the inputs coming from the agricultural, domestic and industrial wastes during the severe flooding over Thailand in 2011. Stoichiometric analysis yields the values of net ecosystem metabolisms (NEM; p-r) and net nitrogen production (nfix-denit) in the Inner Gulf at the rate of 4.4x10 3 tons C.d -1 and 0.7x10 3 tons N.d -1 , indicating that the Inner Gulf is an autotrophic (sink of nutrients) and net nitrogen fixing (nfix-denit >0) ecosystem during the wet season. However in the dry season, the Inner Gulf remains to be autotrophic (p-r = 47 tonsC.d -1 ) but shifted to be a net denitrifying ecosystem (nfix-denit = -88 tonsN.d -1 ). Results from the nitrogen and phosphorus biogeochemical cycling revealed the importance of river discharges in the transport and transformation of these substances within the Inner Gulf of Thailand.