Thresholds for Survival of Brown Trout during the Spring Flood Acid Pulse in Streams High in Dissolved Organic Carbon

Abstract

The survival of brown trout Salmo trutta embryos and first-year juveniles was studied using in situ bioassays during the snowmelt-driven spring flood in 12 streams in northern Sweden. Unlike in most previous studies on the impact of acidity on brown trout, the streams in this study were high in dissolved organic carbon (DOC), and as a result organic acids play a primary role in controlling pH. During the spring flood period DOC concentrations increased strongly in most streams and, in combination with dilution of acid-neutralizing capacity, resulted in a decrease in pH and an increase in total and inorganic monomeric aluminum. High mortality of brown trout juveniles occurred during the spring flood and was best correlated to the high H⁺ concentration. No toxic effect could be directly attributed to measured inorganic aluminum concentrations. An empirical model to predict juvenile brown trout mortality in DOC-rich streams was developed, and based on these predictions a critical chemical threshold of pH 4.8-5.4 is proposed for first-year juveniles. There was high embryo and yolk sac fry survival during the spring flood, even at sites with pH as low as 4.0, suggesting that the pH threshold in DOC-rich waters is lower than the previously established threshold for low-DOC systems. We discuss the complex role that DOC has in humic-rich surface waters, where it both causes a pH decrease and protects against aluminum toxicity in fish. The results suggest that first-year juveniles are likely to be the stage most vulnerable to the effects of episodic pH depression associated with the snowmelt period in northern boreal systems. This results from asynchrony in the seasonality of the spring flood acid pulse and the seasonality of trout embryo development, which is slow in cold northern waters.