Dynamics of dissolved organic carbon in the northwestern Indian Ocean

Abstract

We report here the non-conservative behaviour of DOC in the northwestern Indian Ocean by studying this parameter together with other carbon and nitrogen components. This contrasts with earlier reports of conservative behaviour. Concentrations of DOC, 3-4 times higher than those reported earlier, were found to decrease northward from the equator. Total carbon dioxide (TCO₂) increases in proportion of the oxygen utilized, thus revealing the dominant biological role in the carbon turnover. The CO₂ added through dissolution of biogenic debris is found to decrease southward, in general. Decomposition of organic material contributes at least 64% to the CO₂ addition that increases southward, the rest being from dissolution of skeletal material. Evidence is provided for the utilization of oxygen and nitrate for DOC oxidative decomposition. Accumulation of DOC without its complete oxidation to CO₂ could be the main reason for the TCO₂ decrease in southern Arabian Sea. Relationships of DOC with nitrification and denitrification processes show that the microbial population plays a major role in regulating the DOC contents in the seawater of this region. Consumption/decomposition by denitrifying bacteria and other micro-organisms responsible for nitrogen cycling in the sea are found to be intimately related to the DOC dynamics and are responsible for decreased DOC concentrations in the north. DOC accumulation in the southern Arabian Sea seems to facilitate bacterio-particulate aggregate formation and consequent nitrification, which results in excess nitrate. Application of a one-dimensional advection-diffusion model to the present data set provides evidence for the non-conservative nature of DOC in the Arabian Sea.