Within the lighted portion of the water column in Philippine waters there are two distinct layers easily distinguishable by their thermal structure. Superimposed upon the discontinuity layer or the main thermocline is the mixed or isothermal layer of uniformly high temperature. Owing to phytoplankton activity, the isothermal layer is generally deficient in the chemical nutrients known to regulate and limit plant growth. The thermocline below militates against the vertical movements of the richer waters beneath it, as a consequence of which there is no active replenishment of the impoverished waters within the isothermal layer.
While it is highly desirable to study the thermal structure of both layers in order to acquire a more thorough understanding of the mechanisms controlling nutrient exchange, the BT (bathythermograph) tows did not penetrate deep enough to reach the lower boundary of the main thermocline. The maximum BT penetration never exceeded 150 meters. Below this depth there were still evident significant temperature gradients as shown by the vertical temperature distribution curves based on the hydrographic data.
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