No, the adjustment is done by the red valve. It regulates the amount of water exiting the skimmer. If you close it, the water level inside the skimmer will go up and if you open, the water level inside the skimmer will go down.
In terms of regulating the flow of water from the pump, it cannot be done as the pump only runs in one speed. Regulating the amount of air intake is not recommended as it will decrease the efficiency of the skimmer.