Acidification increases sensitivity to hypoxia in important forage fishes

Hypoxia (low dissolved oxygen [DO]) and CO₂-induced acidification are important aquatic stressors that are exacerbated by anthropogenic nutrient inputs and are expected to increase in severity with increasing atmospheric CO₂ and higher global temperatures. Understanding how species respond to changes in DO and pH is critical to predicting how climate change will affect estuarine ecosystems, including the extreme shallow margins of these systems, where factors such as respiration, photosynthesis, and tides create daily fluctuations of DO and pH, and strong correlations between the 2 stressors. To determine how acidification affects the sensitivity to hypoxia of 2 important forage fishes, the silversides Menidia menidia and M. beryllina, we recorded opercula ventilation rates, aquatic surface respiration (ASR, where fish breathe in the oxygenated surface layer during hypoxic events), and mortality as we lowered either DO or both DO and pH simultaneously. Fish subjected to low DO and low pH in the laboratory performed ASR and died at higher DO concentrations than fish subjected only to hypoxia. Additionally, fish beat their opercula slower, which may have contributed to the differences in ASR and mortality that we saw. These results indicate acidification can increase mortality under hypoxia not only directly but also indirectly by increasing vulnerability to predation during increased use of ASR.