STRESS RESPIRATION AND CHANGES IN CO2 CONCENTRATIONS AFFECT THE RISK FOR H2S TOXICITY – INSIGHTS FROM MODEL CALCULATIONS
Abstract
Carbon dioxide (CO2), alkalinity, and pH are closely linked, but little information exists on how CO2 fluctuations influence hydrogen sulfide (H2S) toxicity in aquaculture systems. As fish respire, CO2 accumulates and lowers pH, which increases the proportion of total sulfide present as toxic H2S. During periods of elevated activity or stress, CO2 production rises further, potentially worsening this effect. We calculated how respiration driven CO2 increases can affect pH and the sulfide balance under typical post smolt RAS conditions (12 °C, 12 ppt, 25–200 mg/L alkalinity as CaCO3). Our results show that relying solely on inlet measurements can underestimate H2S risks. At high alkalinity and low CO2, only 5–10% of total sulfide appears as H2S at the inlet. If CO2 increases from 5 mg/L at the inlet to 15 mg/L in the tank, the resulting pH drop could increase H2S up to 2.4-fold. During stress, CO2 could reach 30 mg/L, leading to a further ∼50% increase in H2S in the tank. Although higher alkalinity generally reduces overall H2S toxicity, it also makes systems more sensitive to rapid shifts in the sulfide equilibrium when CO2 rises. Our results clearly suggest that stress and its secondary effects on pH and chemical equilibria may be the missing link to explain the sudden onset and severity of H2S mortality events. Risk assessments for H2S must therefore consider the effects of alkalinity, stress and CO2 on sulfide balances within a system.
Highlights
(1) Increases in CO2 raise the risk of H2S toxicity in aquaculture systems. (2) Stress respiration exacerbates H2S toxicity, creating a potential feedback loop. (3) High alkalinity protects from H2S but also masks risks for toxicity. (4) Monitoring H2S before the fish tanks is not sufficient to assess risks in RAS. (5) Risk assessment must consider total sulfide and expected maximal changes in pH.
Figure 1: H2S as fraction of total sulfides calculated for different alkalinities, at tank inlet, at baseline respiration and after stress respiration has been triggered (5, 15 and 30 mg/L CO2, respectively). The horizontal dashed line indicates the critical limit of 40 µg S/L which could trigger a stress response in a tank stocked with Atlantic salmon (Ciani et al., 2023). The solid part of the bars represents the fraction of total sulfides present as H2S, total bar height indicated total sulfides (H2S and HS-). (T = 12°C, salinity = 12 ppt).
References
Meriac, A., Holan, A. B., Kolarevic, J., & Thorarensen, H. T. (2026). Stress respiration and changes in CO2 concentrations affect the risk for H2S toxicity – Insights from model calculations. Aquacultural Engineering, 114, 102736. https://doi.org/10.1016/j.aquaeng.2026.102736
Ciani, E., Kvæstad, B., Stormoen, M., Mayer, I., Gupta, S., Ribičić, D., & Netzer, R. (2023). Early warning through video monitoring: Dissolved hydrogen sulphide (H2S) affects Atlantic salmon swimming behavior in recirculating aquaculture systems. Aquaculture, 740201. https://doi.org/10.1016/J.AQUACULTURE.2023.740201