Join Date: Feb 2012
Location: Houston, TX
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Has to do with nutrients cycling from the ocean bottom to the surface. Tropical waters are warm, and warm water floats on top of the cooler bottom water, inhibiting upflow. Consequently, there is less life, nutrients, silts, etc, being circulated, which stunts the overall ability of tropical waters to sustain life. This results in water with less in it and it is therefore more transparent.
This is also why artic waters are so dense with life. With a small temperature gradient between surface and bottom, nutrients from the bottom are constantly be lifted to the surface.