Initial assessment of salinity tolerance in the invasive aquatic weed Frogbit (Hydrocharis laevigata)

Frogbit (Hydrocharis laevigata) is an aquatic invasive species with a high capacity to disperse, which can lead to rapid colonisation and potential domination of the littoral zone of affected freshwater systems. Little is known about the tolerance of this species to saline waters, or whether it has the potential to impact estuaries or tidal sections of rivers. On receipt of a communication that Frogbit had been observed in an estuary in south-eastern Australia, we conducted a glasshouse experiment with the aim of making an initial assessment of the salinity tolerance of Frogbit. Mature and juvenile Frogbit ramets were placed in open 1-L opaque, cylindrical containers with water and six sodium chloride concentrations (control (0 g/L), 1, 2, 5, 10 and 20 g/L) replicated 12 times each. Ramets were assessed as alive or dead at seven, 14 and 21 days. Bayesian logistic regression was used to model Frogbit responses to salinity. At 21 days, all except two ramets remained alive in the control, 1 g/L and 2 g/L NaCl treatments. At 7 days, the probability of mature and juvenile survival began to decline sharply at 5 g/L and was approximately zero at 15 g/L on average. The probability of survival reduced further with time at these intermediate concentrations. All ramets were dead in the 20 g/L solution at day 21. Our results show that while Frogbit may tolerate slightly elevated salinity for short periods, it is unlikely to persist in parts of estuaries where seawater prevails. However, further research is urgently required to understand the extent to which Frogbit can occupy tidal waterways with regular influxes of fresh water on receding tides.