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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 ...