CO2 and acidification effects on larval frog immune function, growth, and survival.

Variation in environmental chemistry affects animal health and demographics. For instance, acidification of aquatic ecosystems (e.g., due to acid rain) can have consequences for organismal fitness. Elevated atmospheric concentrations of CO₂, which reacts with water to produce carbonic acid, may affect aquatic animal physiology in multiple ways, including via reductions in pH. Such impacts have been assessed to a large extent in marine systems. Similar effects are also likely occurring in freshwater ecosystems but are relatively understudied. The goal of this project was to determine how changes in pH and CO₂ concentrations influence traits and survival of animals in freshwater systems, using larval American Bullfrogs (Lithobates catesbeianus (also known as Rana catesbeiana) [Shaw, 1802]) as a model. We hypothesized that elevated CO₂ concentrations and reduced pH negatively affect larval frog immune function, growth, and survival, which we tested in a laboratory aquarium experiment. Over the 27-d experiment, tadpoles were exposed to 4 treatments: 1) near-neutral pH and no gas addition (mean ± SE pH = 7.17 ± 0.05), 2) CO₂ gas sparged into aquariums (pH = 5.32 ± 0.04), 3) acidified water without gas addition (pH = 6.29 ± 0.08), and 4) ambient air sparged into aquariums (control for effects of gas addition) at near-neutral pH (pH = 7.46 ± 0.03). We measured tadpole innate immune response, growth, and survival. Results from the experiment partially supported our hypothesis, with lower immune function in tadpoles exposed to acidified water or elevated CO₂ compared with the control treatments and lower final mass and survival under elevated CO₂. These findings motivate additional research on pH and CO₂ effects on freshwater animals and suggest that increased environmental acidity and elevated CO₂ can lead to negative impacts, which could contribute to reductions in wildlife health and population declines in light of ongoing environmental changes. [ABSTRACT FROM AUTHOR]