Addition of NaOH, limestone slurry and finegrained limestone to acidified lake water and the effects on smolts of atlantic salmon (Salmo salar L.)

A neutralization experiment comparing NaOH, limestone slurry and finegrained limestone was performed using smolts of Atlantic salmon as testfish. Smolts were raised on chronically acid Lake Liervatn (pH = 4.9–5.4, conductivity = 55 μ S cm⁻¹, Ca = 1.3 mg l⁻¹, labile Al = 40 μg l⁻¹). As a result testfish were sublethally stressed prior to the experiment, as indicated by low levels of plasma chloride. During the experiment, smolts were held in keepnets in the middle of large plastic enclosures without sediment contact. Rapid changes in pH and Al-speciation were recorded after addition of the neutralizing agents. No mortality of fish occurred during the 3 days exposure. Plasma chloride levels in fish exposed to limestone slurry, limestone and the lowest concentration of NaOH (pH = 5.9) did not differ significantly from levels in fish from the reference group. Fish exposed to the highest concentration of NaOH (pH > 7.45), however, experienced a significant decrease in plasma chloride levels. Increased sublethal stress in treatments with NaOH was presumably caused by the presence of aluminate ions [Al(OH)₄⁻] at high pH and by low concentrations of Ca. The importance of maintaining pH below 7 when using bases with monovalent cations is emphasized. Adding inorganic aluminium to the lake water induced loss of plasma chloride within 48 h at 70 μg labile Al l⁻¹ at pH 5.1 and 1.2 mgCa l⁻¹.