Water chemistry changes in the gill micro-environment of rainbow trout: experimental observations and theory

Soft water of low buffer capacity was drawn from near the branchial surface of rainbow trout (Salmo gairdneri) at 15°C, using opercular catheters, to determine pH changes in water passing over the gills. Latex masks allowed measurement of ventilation volume, and concentrations of carbon dioxide, oxygen, ammonia, and titratable base in expired water were compared to concentrations in inspired water. Water passing over the gills was more basic than inspired water if the inspired water was pH 4-6 (maximum increase: +0.7 pH units near pH 5). Expired water was more acidic than inspired water if the inspired water was pH 6-10 (maximum decrease: -1.7 pH units near pH 9). Ventilation volume (∼0.37 l·kg^-1·min^-1) and oxygen consumption (∼1.7 mmol·kg^-1·h^-1) were constant in the pH range 4.6-10.1, but both increased by 1.6-2.4× near pH 4. Carbon dioxide transfer near the gills was about 100 μM, ammonia transfer about 15 μM, and titratable base added at the gills was about 30 μM. A theoretical model using CO₂, titratable base, and ammonia added at the gills, the titration characteristics of the defined soft water medium, and aquatic equilibria for CO₂ and ammonia, adequately explained the experimentally observed changes in pH near trout gills. Our observations and predictive model indicate that any gill contaminant whose toxicity varies with pH may be more or less toxic at the gills than predicted from bulk water chemistry alone.