Carbon budget in the eastern and central Arabian Sea: an Indian JGOFS synthesis

Abstract

The carbon budget for the eastern and central Arabian Sea was constructed using results from the Modular Ocean Model and biogeochemical data collected largely under the Indian Joint Global Ocean Flux Study programme. The study region (east of 64°E and between 11° and 21°N) was divided into two vertical boxes; a surface box of the top 100 m that largely undergoes exchanges with atmosphere and exhibits relatively strong seasonal variability and a subsurface box between 100 and 1000 m. Water transport rates in surface layers were maximal (up to 83 × 10¹² m³) in the southwest monsoon season. Sinking from surface driven by convection (25 × 10¹² m³) largely supports lateral outflows of water in subsurface layers in the northeast monsoon. Surface waters are renewed 10 times faster (τ = 0.8 years) than intermediate waters (τ = 8 year). A net supply of 25 Tg C yr^-1 is estimated to the upper 1000-m water column of the study area by the physical pump. Photosynthetic activity (234 Tg C yr^-1) does not seem to support total carbon demands (1203 Tg C yr^-1) by bacteria and microzooplankton and mesozooplankton in the surface layers. Carbon demand rate requires organic carbon nearly double that of the total living biomass production rate (644 Tg yr^-1) suggesting that most of the demand might be met from internal cycling involving zooplankton grazing/excretion activities. Sinking flux (69 Tg C yr^-1) from surface accounts for about 30% of the total photosynthetic production rate indicating intense remineralization of organic matter in the surface layers of the Arabian Sea. Grazing and excretion of carbon by the microzooplankton and mesozooplankton appear to easily sustain perennial supersaturation of carbon dioxide in surface waters of the Arabian Sea and emission of 32 Tg C yr^-1 to the atmosphere.