Seedling development and patch formation of the seagrass Posidonia australis in a southeast Australian estuary

Abstract

Seedlings of the seagrass Posidonia australis are rarely found and little is known about their development. St. Georges Basin, an estuary in southeast Australia, harbours relatively large numbers of P. australis seedlings and patches. Seedlings of varying age were collected from four sites within the estuary. Mature plagiotropic shoots (horizontal rhizomes) on existing beds were tagged to measure vegetative growth and provide a plastochrone interval with which to age the seedlings. The abundance of P. australis seedlings ranged from 0.001 to 0.014 seedlings m −2. Based on the plastochrone intervals (plagiotropic: 33±0.8 days; orthotropic: 48.0±1.4 days) estimated seedling age ranged from 0.7 to 8.2 years. Both the numbers of shoots and leaf areas were related to seedling age, with increases particularly evident between 2 and 5 years. Of 57 seedlings either collected or observed in situ, only 6 had produced a plagiotropic shoot. Rhizome growth for the seedlings ranged from 5.1±0.5 to 17.1±1.3 cm per year, which was slower than the mean rhizome growth of mature plants (21.4±1.0 cm per year). A development time-line was constructed for P. australis for the first 5 years of seedling growth based on the data collected. On average, production of a second shoot occurred at the age of 2.7±0.3 years, and thereafter approximately one shoot was produced per year. Plagiotropic shoots (horizontal rhizomes) did not begin to develop until 4.3±0.7 years.