Toxicity of inorganic aluminium at spring snowmelt—In-stream bioassays with brown trout (Salmo trutta L.)

Although the acid load has decreased throughout Scandinavia, acidic soils still mobilise aluminium (Al) that is harmful to brown trout. We hypothesise that there are thresholds for Al toxicity and that the toxicity can be traced from the water content to gill accumulation and the consequential physiological effects. During snowmelt, yearlings were exposed to a gradient of pH and inorganic monomeric Al (Ali) in humic streams to study the toxic effects and mortality. Gill Al and physiological blood analyses [haemoglobin (Hb), plasma chloride (P-Cl) and glucose (Glu)] were measured. As the water quality deteriorated, Al accumulated on the gills; Hb and Glu increased; P-Cl decreased, and mortality occurred. Moribund fish had significantly increased gill Al and Hb, suggesting that respiratory disturbances contributed to mortality. Decreased P-Cl and plasma availability indicated an ion regulatory disturbance and possibly circulatory collapse. Ali should be less than 20μg/L, and pH higher than 5.0, to sustain healthy brown trout populations. These thresholds can be used to fine-tune lime dose, as both Ali and pH levels have to be balanced to prevent harm in the recovering aquatic biota. Although Al is tightly linked to pH, local variation in Al availability in soil and bedrock affects the Al release and subsequent toxic Ali episodes in some catchment areas. ► Brown trout were exposed in situ to a gradient of pH and Ali in humic streams. ► pH above 5.0 and Ali below 20μg/L sustain healthy brown trout populations. ► Low pH resulted in a tendency to decreased blood chloride levels. ► High Ali increased gill Al and haemoglobin suggesting that respiration was effected. ► The thresholds can be used to support liming strategies in recovering freshwaters.