Vertical phytoplankton distribution and water column stability in Philippine waters

The vertical pattern of phytoplankton abundance is largely determined by the ongoing physical processes that occur in the upper ocean. The occurrence of the Deep Chlorophyll Maximum (DCM) reflects the combination of physical and biological factors that change drastically with depth. This study aimed at determining possible mechanisms governing the vertical distribution of phytoplankton in the Philippines. Vertical fluorescence profiles from oceanographic cruises around Philippine waters in the period of 2008-2012 were calibrated with spectrometrically-determined chlorophyll to obtain corrected chlorophyll profiles. These profiles were initially characterized in terms of shape, DCM concentration and depth of DCM and then related to the stability of the water column using the Brunt-Väisälä frequency. DCM concentration was found to be higher in the Sulu Sea, while DCM depth was generally deeper on the eastern seaboard of the island of Luzon. Initial results show that chlorophyll maximum concentrations do not coincide with maximal vertical stability. This could indicate that other mechanisms such as nutrient input, light availability or turbulence from wind may control the development of the DCM in these tropical waters.