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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

    Pest plants invade aquatic ecosystems and compete with native plants for the same resources. Pest plants can range in size from microscopic algae to large willow trees.

    Algae naturally occur in our lakes and rivers. They become a pest when nutrients from urban wastewater, fertilisers or animal wastes enter the waterways and stimulate excess growth – algal blooms.

    Dr Eloise Ryan talks about ‘good’ and ‘bad’ aquatic plants.

    The following resources provide information about pest plants:




    There are good aquatic plants and bad aquatic plants. So our good native aquatic plants provide food and shelter for aquatic invertebrates and fish and water birds, and they also provide refuge for things like our freshwater crayfish and our little native fish, such as the common bully. So when we lose these native aquatic plants, sometimes we can get really bad introduced species.

    Pest plants are things like alligator weed or Egeria. And the problem with these pest plants is that they grow very fast and rapidly and choke our streams and waterways, and they displace our native plants. They take up the complete stream. For example, native aquatic plants – there might just be a few of them and they don’t cause you any bother – but pest plants completely grow very fast and take up the whole water column and slow the water movement in the waterways, and when these pest plants die, they decay and use up the oxygen in the stream as well, and it’s another source of nutrients into our streams.

    Also they can be a nuisance to boaties and swimmers, where your boats might get wrapped up in weed or you’re trying to swim and your legs get caught in weed, which isn’t very good.

    Dr Eloise Ryan
    Waikato Regional Council
    Slender knotweed still, tangawhenua; Blunt pondweed still, Marley Ford; common duckweed still, Kate McCombs; and Egeria still, ryvesie. All sourced from iNaturalist NZ and released under CC-BY-NC 4.0


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

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