Stress respiration and changes in CO2 concentrations affect the risk for H2S toxicity – Insights from model calculations

Carbon dioxide (CO 2 ), alkalinity, and pH are closely linked, but little information exists on how CO 2 fluctuations influence hydrogen sulfide (H 2 S) toxicity in aquaculture systems. As fish respire, CO 2 accumulates and lowers pH, which increases the proportion of total sulfide present as toxic H 2 S. During periods of elevated activity or stress, CO 2 production rises further, potentially worsening this effect. We calculated how respiration driven CO 2 increases can affect pH and the sulfide balance under typical post smolt RAS conditions (12 °C, 12 ppt, 25-200 mg/L alkalinity as CaCO 3 ). Our results show that relying solely on inlet measurements can underestimate H 2 S risks. At high alkalinity and low CO 2 , only 5-10% of total sulfide appears as H 2 S at the inlet. If CO 2 increases from 5 mg/L at the inlet to 15 mg/L in the tank, the resulting pH drop could increase H 2 S up to 2.4 fold. During stress, CO 2 could reach 30 mg/L, leading to a further ~50% increase in H 2 S in the tank. Although higher alkalinity generally reduces overall H 2 S toxicity, it also makes systems more sensitive to rapid shifts in the sulfide equilibrium when CO 2 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 H 2 S mortality events. Risk assessments for H 2 S must therefore consider the effects of alkalinity, stress and CO 2 on sulfide balances within a system. • Increases in CO 2 raise the risk of H 2 S toxicity in aquaculture systems. • Stress respiration exacerbates H 2 S toxicity, creating a potential feedback loop. • High alkalinity protects from H 2 S but also masks risks for toxicity. • Monitoring H 2 S before the fish tanks is not sufficient to assess risks in RAS. • Risk assessment must consider total sulfide and expected maximal changes in pH.