The effect of salinity on the germination of Ruppia tuberosa and Ruppia megacarpa and implications for the Coorong: A coastal lagoon of southern Australia

• Salinity > 120 g/L is a factor causing the decline in the distribution of Ruppia.
• The germination of Ruppia tuberosa seeds occurred at salinities up to 90 g/L.
• The germination of Ruppia tuberosa turions occurred at salinities up to120 g/L.
• The germination of Ruppia megacarpa seeds occurred at salinities up to 30 g/L.
• Salinity needs to be maintained <90 g/L in the southern end of the Coorong.

This study examines the influence of salinity on the germination of Ruppia tuberosa J. Davis and Tomlinson (seeds and turions) and seeds of Ruppia megacarpa R. Mason. This information is used to demonstrate how freshwater riverine inputs maybe used to restore the probability of germination of Ruppia sp. in a major RAMSAR site, a coastal lagoon in South Australia (The Coorong) where salinity increases have been implicated as the major cause of the decline of Ruppia sp. a major food source for local and migratory water birds. Increased salinity led to a decrease in germination rates for R. tuberosa and R. megacarpa. The salinity thresholds for germination were 85–90 g/L for R. tuberosa seeds, 120 g/L for R. tuberosa turions and 30 g/L for R. megacarpa seeds. An increase in salinity also led to an increase in mean time to germination of 15 days for R. tuberosa seeds (at 85 g/L), 24 days for R. tuberosa turions (at 120 g/L) and 10 days for R. megacarpa seeds (at 30 g/L). Seeds of R. tuberosa that failed to germinate at elevated salinities germinated on transfer to low salinities at an enhanced rate compared with the initial rates. Turions of R. tuberosa did not respond in a similar manner because of ion toxicity after exposure to high salinities. This study suggests that salinity affects imbibition, inducing ion toxicities on seeds and turions, and consequently influences the germination process. Using recent elevated salinity levels of 40 to over 120 g/L in the Coorong, modelling suggests that seed germination of R. tuberosa is likely to be restricted to the less saline northern half of the Coorong, but R. megacarpa germination is unlikely to occur. The modelling also suggests that riverine inputs that reduce salinity levels would restore R. tuberosa and R. megacarpa communities in the water-bodies where they were once abundant, but have declined in response to human induced salinisation.

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