The long term goal is to understand the interplay of physical and chemical factors in controlling biological production in coastal tropical waters. 

In this framework marginal seas play a critical role in coupling terrestrial processes to the global ocean and their productivity and role in the global carbon cycle is generally controlled by the availability of inorganic nutrients.  The South China Sea (SCS) is an excellent site to test hypotheses and develop understanding of nutrient limitation and both primary (nutrient loading) and secondary (N2-fixation) influences of riverine input on plankton production.  The juxtaposition of the Mekong outflow and upwelling in the SCS allows exploration of their impact on the abundance, diversity, and activity of plankton through field surveys, shipboard experiments, and modeling.  The project team will carry out nutrient and metal amendment experiments and measure N2- and CO2-fixation to evaluate their response to upwelling, mesoscale dynamics and riverine input.  This project has broad relevance since little is currently known about the interplay between river plumes, advection, and upwelling in providing nutrients to tropical oceans.
The Mekong is the 9th largest river in the world and the larges draining into the SCS.  Both the Mekong basin and the SCS are small enough that anthropogenic changes in land and water use can propagate through the river system and have an impact on oceanic properties on short time scales.  Current dam construction, increased fertilizer use, increased sewage loading, and mangrove destruction all are likely to affect the riverine nutrient flux into the SCS over the next few years. This project will make an important contribution to our understanding of nutrient dynamics and ecosystem response to rapidly changing riverine forcings in this important marginal sea. 

Figures above: MODIS-Aqua image(s) generated by the automated Ocean-Color Data Processing System (ODPS). They show the evolution of Chlorophyll-a concentrations from July 19 (left), to August 20 (center) and August 23 (right), 2007. Note how fast the bloom associated with the plume changes its shape. Monthly climatologies and a movie of the evolution through the years 1997-2005 can be found at http://www.ldeo.columbia.edu/~ajit/scs/page_01.htm

This project will allow us to:
1.   Measure the fluxes and explore the mechanisms of transport of N, P, and Fe from the Mekong into the SCS. 
2.   Quantify the impact of riverine inputs and upwelling on nutrient limitation and N2-fixation in the water column of the SCS, identify the major phytopankton groups involved, and evaluate the controls on N2 fixation and the degree of spatial and temporal niche differentiation among diazotroph assemblages.
3.   Explore the impact of land use changes and climate processes including wind and radiative forcing in altering nutrient dynamics in the SCS.

This project will also create new research linkages among US and Vietnamese ocean scientists, including opportunities for American students to take part in an international project in an important but relatively unstudied marginal sea.