An interesting little observation.
After adding CO2 to the meads yesterday, the airlock was filled with fresh water and attached to the carboys. The water levels in the two chambers were equal.
Within a short time the meads began to generate a small vacuum shifting the levels of the airlock. This must be an uptake of CO2 in the meads. I thought the meads would be saturated by CO2, but apparently not. This will also push pH down a bit.