Recovery of Posidonia meadows in Oyster Harbour, southwestern Australia

This study examines the first stages of recovery of seagrasses (Posidoniaceae) after extensive losses during the 1980s were documented in Oyster Harbour, a marine inlet near Albany on the south coast of Western Australia. Two mechanisms of recovery were evident for the dominant seagrass species Posidonia australis Hook. f. and Posidonia sinuosa Cambridge and Kuo: horizontal spread by underground stems (rhizomes) at the edges of existing stands and gaps within meadows, and establishment of seedlings in bare areas previously vegetated by seagrass meadows. Rhizome extension rates, estimated from the expanding edges of seagrass stands in Oyster Harbour, ranged from 8-26 cm yr−1 for P. australis at 11 sites and 8-15 cm yr−1 for P. sinuosa at 3 sites. Successful long-term seedling recruitment was recorded in many areas, with seedlings older than one year present and developing into patches on areas of bare sand previously vegetated by seagrass. Long-term measurements of rhizome extension rates for P. australis were made at one site in Oyster Harbour (Eastern Shore) over 2.5-6 yrs on more than 250 transplants in 4 plots and 22 naturally established patches. Rhizome growth rates for transplants averaged 15-18 cm yr−1 over 4 yrs. The average rate of leaf formation was 10 leaves yr−1 over 2.5 yrs for 5 harvested transplants. Radial growth of 22 established patches of P. australis averaged approx. 20 cm yr−1 over 6 yrs. However, the measurements along different radii showed great variation, with some patches expanding by more than 2 m in one direction over 6 yrs, indicating that a few rhizomes were growing at more than 30 cm yr−1. The seagrass loss in Oyster Harbour was the result of nutrient and sediment influx from an extensive rural catchment in the 1980's. Water quality has improved during the 1990s and this study demonstrates that recovery is occurring as a result of expansion of existing stands and seedling establishment. There has also been good survival and growth of P. australis transplants, and at the current rates being measured, individual transplants 1 m apart will grow together to form a meadow in less than 5 yrs, providing an alternative to natural recovery at a limited scale.