Salinity Tolerance Thresholds and Acclimation Potential in the Invasive Macrophyte Crassula helmsii

The salt gradient is a major driver of the composition of aquatic plant communities. The ionic composition of salt and exposure regime can influence the response of macrophytes, either positively or negatively. In European freshwaters, the New Zealand pigmyweed (Crassula helmsii) is a highly invasive macrophyte that should exhibit a small tolerance to salt, raising interest in salt-based control strategies. Two ex-situ experiments were conducted to evaluate the effects of salinity on C. helmsii. The first experiment assessed survival and morphological responses to four salinity levels (0, 15, 30 and 60 g/L) using either road or sea salts. The second experiment compared gradual versus direct exposure to elevated salinity (20 and 30 g/L). Salinity exposure reduced apical growth and biomass. No significant differences were detected between the effects of road salt and sea salt. Median effective concentration (EC50) was approximately 33 g/L. Moderate salinity stimulated branching and root production, whereas higher salinity (> 15 g/L) inhibited growth and increased mortality. Gradual exposure did not improve survival significantly compared with direct exposure. C. helmsii continued to grow at salinity levels up to 15 g/L, exceeding previously reported tolerance thresholds for the tested species. These findings indicate substantial tolerance to short-term stress and delayed mortality under progressive salinisation may suggest potential acclimation ability. Eradication of C. helmsii would require prolonged exposure to salinity levels approaching seawater concentration that should negatively affect other freshwater organisms and ecosystem functioning. Species salinity tolerance and potential acclimation suggest an increased invasion risk of C. helmsii for temporarily salinised freshwater ecosystems.