Dissolved Carbon Dioxide

Dissolved carbon dioxide (CO2) is the portion of respired and bacterially produced carbon dioxide that dissolves in the water column, expressed in mg/L. It accumulates whenever production of CO2 (fish respiration, microbial degradation of organic matter) exceeds removal by photosynthesis and air-water gas exchange; in poorly buffered or heavily stocked systems, free CO2 can climb from typical background levels below 5 mg/L to 15-25 mg/L or more. Elevated CO2 depresses blood pH through acidosis and reduces the oxygen-carrying capacity of haemoglobin (the Root effect), so fish in high-CO2 water show reduced appetite and growth even when dissolved oxygen appears adequate. Management relies on vigorous aeration and degassing towers, especially in recirculating aquaculture systems where CO2 can be stripped less efficiently than in flow-through units. CO2 levels rise predictably with biomass and feeding, so routine measurement alongside dissolved oxygen and pH gives early warning of under-ventilation; values above 10 mg/L warrant corrective degassing, and above 20 mg/L are considered harmful for most salmonids and many marine species.