Alum dosing effects on fish and aquatic invertebrates: Utuhina Stream 2024

Abstract

To reduce external phosphorus loading to Lake Rotorua, the Bay of Plenty Regional Council has been granted resource consent to dose the Utuhina Stream with alum (aluminium sulphate). Aluminium binds phosphorus, preventing its uptake by phytoplankton and thereby inhibiting growth. Aluminium forms monomeric species outside of circum-neutral pH (pH 68), and these species can disrupt osmoregulation and respiration of aquatic organisms. The alum dose rate to the Utuhina Stream is dependent on discharge, with a maximum application rate of 1 mg Al L-1. This report presents the results of an on-going assessment of the fish and aquatic macroinvertebrate communities of the Utuhina Stream for 2024. Macroinvertebrates, fish and kōura (freshwater crayfish; Paranephrops planifrons) were sampled from one control (site 1) and two treatment (sites 2 & 3) reaches of the Utuhina Stream. Common bully (Gobiomorphus cotidianus) were the dominant species of the fish community, with juvenile trout and kōaro (Galaxias brevipinnis) also captured. Kōura were present at all sites but variable in abundance. There was a notable decline in the semiquantitative macroinvertebrate community (for soft-bottomed streams; MCI-sb) score at Sites 1 and 3 in 2024 compared to long-term site averages, with Site 1 declining into the ‘poor’ category. Interannual variation in MCI-sb scores is typical for these reaches of the Utuhina Stream and have been attributed to flood-related disturbances to stream bank morphology and in-stream vegetative cover (Ling 2021). Tissue aluminium concentrations were determined from common bully (flesh, gill, liver) and kōura (flesh, gill, hepatopancreas) by inductively coupled plasma mass spectrometry (ICP-MS). There was some evidence of aluminium bioaccumulation in the liver tissues of common bully at Site 2, but overall concentrations were similar to long-term averages and there were no notable differences for the same tissues between sites. Kōura tissue aluminium concentrations for 2024 were similar to long-term averages across all tissues and sites. A small number of individuals had elevated gill concentrations at Site 3 compared to long-term average concentrations, but this is unlikely to result in harmful physiological impacts. The data presented in this report further supports the conclusion by Tempero and Ling (2024) that alum dosing does not appear to have a notable effect on the fish and kōura community and that site and interannual differences appear to be due to hydrological and habitat variability.