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  • Rights: The University of Waikato Te Whare Wānanga o Waikato and Waikato Regional Council
    Published 17 March 2020 Referencing Hub media

    pH is a measure of acidity or alkalinity. pH is short for ‘pondus hydrogenium’ and indicates the concentration of hydrogen in a solution.

    Most natural waterways are in the pH range of 6.5–8. Some streams are naturally acidic (less than 7) or alkaline (greater than 7) depending on the surrounding substrate and bedrock composition. pH also changes over night and day. Biological factors such as photosynthesis and respiration alter the chemical composition of the water. Aquatic life cannot tolerate extreme pH (high or low).

    This Rivers and Us resource is in a downloadable PDF format. Use it as a guide to the scientific methodology for measuring pH. It also contains discussion questions.



    Water has a neutral pH of pH7. So in streams, if pH is too high, that means it can affect aquatic life. So if we add in a chemical contaminant such as ammonia, that can increase the pH and affect fish life, or if a contaminant like an acid gets into the water, that can decrease the pH and make it acidic and affect the ecosystem as well. If there’s large algae blooms, that can affect the pH as well, because algae are plants, and they photosynthesise. So during the day, they take in oxygen and produce carbon dioxide, and that drives the pH down, and then at night when they respire, it drives the pH the other way. So you can have huge pH fluctuations in water bodies, and that can affect fish quite negatively. So it’s very important for waterways to try and remain in neutral pH.


    Dr Eloise Ryan
    Waikato Regional Council
    Footage of algae mats in river, Cawthron Institute


    This video has been developed in partnership with the Waikato Regional Council as part of the Rivers and Us resource.

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