will a dehumidifier...?

will a dehumidifier absorb any of the water in an ebb and flow resivoir?

No, not directly. It will pull hum out of the air. Until the ebb evaps it will stay water.

Like tSunami13 said. It will just absorb the water that is evaporating from the set up. But the dryer the air is the quicker it will evaporate.