Organic Carbon Concentrations in High- and Low-Productivity Areas of the Sulu Sea

The sequestration of anthropogenic carbon dioxide in the form of organic carbon and its eventual deposition in the sediments is an important component of the marine carbon cycle. In the Sulu Sea, Philippines, organic carbon contents in the sediments have been relatively well studied, but the processes that describe the organic carbon distributions in the water column have not been elucidated. Dissolved and particulate organic carbon (DOC, POC) concentrations were measured at several stations in the Sulu Sea during the northeast monsoon of 2007/2008 to understand the dynamics of organic carbon in this unique internal sea. Analyses of primary productivity estimates, beam attenuation coefficient (at 660 nm) profiles, and correlation coefficients among DOC, POC and other parameters (e.g., apparent oxygen utilization) at different layers of the water column indicate that surface primary productivity, upwelling, bottom intensified flows across sills, and ventilation from shallow sills, which may contain semi-labile DOC that is estimated to largely contribute to microbial respiration in the bathypelagic layer, are the major processes that affect the DOC and POC distributions in the Sulu Sea. The variability of these processes should be taken into consideration when assessing the sustainability of internal and marginal seas as carbon sinks.