The bubbles rise after coming out of suspension as a gas. Prior to this change in state they contribute to the overall mass. As the bubbles form the volume increases proportionately and this reduces the mass density but as weighed in a vacuum the weight would remain constant. Only after reaching and leaving the surface will the mass of the remaining fluid be reduced by the mass of the gas particles.
rojash, I�ve often wondered how they derived the term flatulence?
Brrrrrruuuummmpph . . . Hey! What do ya know? I feel much lighter thank you!