The Bohol Sea is a relatively unstudied marginal sea located in the southern part of the Philippines. Hydrographic data from the Philippine Straits Dynamics Experiment (PhilEx) cruises reveal a complex three-dimensional circulation composed of two overturning cells that may be referred to as “double-estuarine type.” This type of overturning circulation promotes upwelling of cold, nutrient-rich waters within the Bohol Sea associated with entrainment and eddy processes that promote phytoplankton blooms. Evidence from ocean color imagery supports entrainment in the eastern basin and eddy formation in the southwestern basin. However, PhilEx researchers found that the cyclonic Iligan Bay Eddy in the southwestern basin did not conform to the paradigm of cyclonic eddy upwelling. Although upwelling was evident through doming isotherms within the water column, the surface projection of this signal was suppressed by the presence of a thick barrier layer, particularly during cruises in December 2007 and January 2008, a known La Niña period. Long-term trends in chlorophyll data followed trends in rainfall and the ENSO 3.4 index, with elevated (reduced) chlorophyll during dry El Niño years (wet La Niña years). By promoting stability of surface layer stratification and preventing vertical transport of nutrients, the barrier layer is thus a mechanism by which the El Niño-Southern Oscillation influences phytoplankton biomass in the Bohol Sea.

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