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

Abstract

Alum (aluminium sulphate, Al₂(SO₄)₃) dosing is used to reduce eutrophication of lakes by sequestering phosphorus and preventing its uptake by phytoplankton for growth. When applied to water at circum-neutral pH, alum forms aluminium hydroxide (Al(OH)₃), a precipitate that is benign at environmentally relevant concentrations and moderate concentrations. However, at low and high pH, dissolved ionic aluminium species evolve which are known to be toxic to a range of aquatic organisms. In the Rotorua lakes, alum dosing occurs under circum-neutral pH (6–8) conditions, however, transiently elevated pH (10) conditions may occur due to algal uptake of carbon dioxide (CO₂) for photosynthesis. Under elevated pH, particulate Al(OH)₃ solubilises to form aluminate (Al(OH)₄⁻), which can interfere with osmoregulation and respiration. Similarly, at circumneutral pH, solid Al(OH)₃ predominates and is generally benign, but in high concentrations can irritate and damage gill tissue causing inflammation and excess mucus production which impairs oxygen uptake, CO₂ elimination, and ultimately causing death. The effects of aluminium under intermittent elevated pH on fish gill function, respiration rates, and metabolism have not yet been investigated.

This study investigated the impacts of aluminium on rainbow trout (Oncorhynchus mykiss), common bully (Gobiomorphus cotidianus), and kōura (Paranephrops planifrons) osmoregulation and respiration under diurnal pH cycles, through two experiments. The first being a dose exposure study whereby rainbow trout and kōura were exposed to alum derived aluminium at 2 mg L⁻¹ under diel pH cycling (pH 7–10) over 10 days. Measured parameters included haematocrit, haemoglobin, mean cell haemoglobin concentration (MCHC), and blood plasma osmolarity in rainbow trout and haemolymph osmolarity in kōura. A significant increase (Student’s t-test, P <0.05) in the mean cell haemoglobin concentration (MCHC) in the rainbow trout control group indicated erythrocyte swelling, suggesting a generalised stress induced response. However, other haematological variables such as plasma and haemolymph osmolarity, haematocrit and haemoglobin concentrations were not significantly different (Student’s t-test, P >0.05) between control and treatment groups. Histological examination of kōura and rainbow trout gill tissue was also conducted, with abnormalities observed within both the control and treatment groups for each species. However no significant differences (Kolomogorov-Smirnov, P >0.05) were observed between groups. It was concluded that organisms can physiologically compensate for aluminium induced stress and disturbance at the gills.

A second experiment utilised intermittent flow respirometry to determine mass specific metabolic oxygen consumption rates (MO₂) of rainbow trout, common bully, and kōura exposed to 2 mg Al L⁻¹ and diel pH fluctuations (pH 7–10) over 60 hours. It was expected that precipitation of dissolved aluminium onto the gill surface during pH transitions and/or binding of dissolved aluminium species would inhibit the respiratory gas exchange resulting in increased gill ventilation from hypoxic and hypercapnic conditions. There was a significant negative correlation (Pearson’s, r² = 0.34, P <0.0001) between pH and MO₂ possibly indicating kōura are sensitive to the experimental conditions. However, there were no consistent trends in differences between control and treatment groups for any of the tested species. Data interpretation was hindered by the necessity to exclude samples from the rainbow trout and common bully control groups due to aberrations in the oxygen sensor data. However, mean MO₂ was within expected ranges for rainbow trout and kōura, indicating that significant respiratory impacts were not occurring within the period of exposure. Although definitive conclusions could not be made regarding changes to MO₂ in response to aluminium and diel pH cycling, it is unlikely that acute impacts would result from these conditions in the natural environment.

From this research it is concluded that the combined impacts of aluminium and diel pH cycling are unlikely to cause acute osmoregulatory or respiratory impairment or cause death in kōura, common bully, or rainbow trout.