The effectiveness of pH in causing mortality of aquatic invasive alien species

Raw water transfers between waterbodies are crucial to water utility companies’ abilities to balance water supply and demand. However, raw water transfers have been highlighted as a high-risk pathway for the spread of invasive alien species (IAS). Implementing biosecurity at the scale of raw water transfers is a considerable undertaking and the impact on water resources and the environment need to be considered when researching methods to biosecure raw water transfers. We explored the effectiveness of pH changes in causing mortality in aquatic IAS. We used two invasive invertebrate species; Dreissena polymorpha, Dikerogammarus villosus and two invasive plant species; Hydrocotyle ranunculoides, Crassula helmsii. pH was adjusted using chemicals already widely utilised within the water treatment process (ferric sulfate-acid and calcium hydroxide-alkali) to produce treatments of acid (pHs 3, 4, 5, 6) and alkali (pHs 9, 10, 11, 12) pH. For D. villosus, complete mortality was only seen following immersion at pH 12 for ≥ 1 hour. For D. polymorpha, 100% mortality was not achieved in any treatment. Mortality was highest in more extreme treatments of pH4 (44%) and pH12 (40%) following immersion for 8 hours. Incomplete mortality of H. ranunculoides was seen for all pH treatments, with the highest mortality (50%) at pH4 and pH 10. Crassula helmsii experienced no mortality following immersion in any of the pH treatments. These results indicate that immersion in water with altered pH is unlikely to be suitable as a biosecurity treatment to slow the spread of aquatic invasive invertebrates and macrophytes in raw water transfers.