A comprehensive water quality monitoring programme has been implemented in the Bay. This programme comprises: the continuous monitoring of water quality at 6 locations using instrumentation suspended from buoys which collects data every 15 minutes and transmit the data to a Harbour Authority computer hourly; the regular monitoring of water quality using mobile equipment deployed from the water quality vessel; and the regular retrieval of water samples for laboratory analysis.

The Cardiff Bay Barrage Act 1993 requires the dissolved oxygen levels in the Bay to be maintained at/or above 5mg/l in all places and at all times. An aeration system has been installed to ensure this standard is maintained. The system works by pumping compressed air through a series of sunken pipes and diffusers, enabling the surface water with a high oxygen level to mix with the deeper water with a low oxygen level.

Other environmental factors such as sunlight, atmospheric pressure, rainfall and wind speed have an influence on water quality within the impoundment. The Harbour Authority has a meteorological station on the barrage that measures and records these parameters at frequent intervals.