Toxicological effects of aluminium in relation to diel pH changes on fish and kōura

• Main Authors: Fensham, Emily, Ling, Nicholas, Tempero, Grant Wayne
• Format: Report
• Language: English
• Published: 2022
• Subjects: Carbonic acid
• Online Access: https://hdl.handle.net/10289/15078; https://doi.org/10.15663/ERI.Report.160

Since 2006, alum (aluminium sulphate; Al₂(SO₄)₃) has been applied to the Utuhina and Puarenga Streams at a targeted dose rate of 1 mg Al L⁻¹ to control phosphorus loading to Lake Rotorua. Alum dosing is widely used for water quality restoration and, under circumneutral pH (6-8), is considered to have no significant toxicological impacts at low to moderate (0.1–2 mg Al L⁻¹) concentrations. At circumneutral pH, alum forms aluminium hydroxide (AlOH₃), a white insoluble precipitate which adsorbs phosphorus, reducing its availability for phytoplankton growth. However, at high or low pH, monomeric and hydroxy aluminium species occur in varying proportions with respect to acidic (i.e., Al³⁺, AlOH²⁺, Al(OH)²⁺) and alkaline (i.e., Al(OH)₄⁻) conditions with increasing toxicity to aquatic organisms. Under typical conditions, the pH of Lake Rotorua is near pH 7, but due to its limited buffering capacity may reach pH 10 during intensive algal blooms. These diel increases in pH occur due to the photosynthetic uptake of CO₂ during the day, thereby increasing the environmental hydroxide concentration and raising the pH of the lake. At night, respiration releases CO₂, driving down the pH due to the formation of carbonic acid. Diel pH cycling has the potential to solubilise alum-derived aluminium, resulting in toxicological impacts on aquatic biota. Previous research has primarily focused on the toxicological impacts of aluminium under acidic or, more recently, alkaline conditions, however, potential impacts during transient exposure to alkaline pH have not been reported. The University of Waikato was contracted to investigate the effects of aluminium at 2 mg L⁻¹ in association with diel pH cycling on rainbow trout (Oncorhynchus mykiss), common bully (Gobiomorphus cotidianus) and kōura (Paranephrops planifrons) osmoregulation and respiration. Potential osmoregulatory effects were investigated by exposure of rainbow trout and kōura to aluminium at 2 mg L⁻¹ under diel pH cycling (pH 7–10) over 10 days. No significant differences ...