The effects of naturally occurring high and low molecular weight inorganic and organic species on the yolk-sack larvae of Atlantic salmon ( Salmo salar L.) exposed to acidic aluminium-rich lake water

Yolk-sac larvae of Atlantic salmon ( Salmo salar L.) were exposed to water from two acidic, aluminium rich lakes, Nepptjern and Lomstjern. The average pH of the lakes during the experiments were 4.85 and 5.24. The concentrations of total monomeric aluminium (Al a) were high in both lakes, i.e. 640 ± 22 and 451 ± 16 μg Al 1 −1, but the total amount of organic carbon (TOC) differed significantly (1.89 ± 0.04 and 11.6 ± 0.1 mg Cl −1). The larvae were also exposed to cation exchanged and hollow-fiber ultrafiltered water from both lakes. Based on the hollow-fiber ultrafiltration, the toxic aluminium species present were as the inorganic, low molecular weight fraction (mol. wt. < 3 000 Da). Within this fraction, the highest mortality rates [LT 50=24.6±0.5 h ( n = 6)] were observed in the lake water having the highest amounts of the hydrolysis products, i.e. Al 3+, Al(OH) 2+ and Al(OH) 2 +. The aluminium-fluorides, AlOHF +, AlF 2+, and AlF 2 + were also found to be toxic, but less toxic [LT 50=49.6±1.1 h ( n=4)] than the aluminium hydrolysis products. Removal of high molecular weight species including inorganic and organic Al species, did not affect the toxicity or the mortality rates. No acute toxicity was observed when the larvae were exposed to the cation exchanged eluates. Thus, the organic aluminium-complexes seemed to have no acute toxic effect, at least at the concentration levels tested.