Extensive seagrass meadows occur in Gulf St Vincent covering 5,000 km2. They provide a natural habitat and food source for many marine species and stabilise the underlying sediments. Over the last half century, around one third of seagrass along the Adelaide metropolitan coast has been lost and is one of the most visible and significant coastal impacts. While natural events such as storms can cause large-scale loss, the main cause of the initial seagrass loss was poor water quality, particularly increased turbidity and nutrients in stormwater run-off discharged to the Gulf from watercourses and drains.
Infrastructure has been installed that uses physical processes to trap solid waste such as litter and coarse sediment. Sediment basins and gross pollution traps were established in Metropolitan Adelaide, which Green Adelaide now operate and maintain. They own three sediment basins located in the Patawalonga catchment, which is one of the identified sediment sources and they help protect a key area of Adelaide’s coastline. The most productive basin is situated at the confluence of the Brown Hill and Keswick Creeks.
2000-3000 tonnes of sediment per annum have typically been removed from the sediment basins over the last three years. The amount varies according to the rainfall in the preceding season.
According to research, the seagrass decline is slowly being reversed. Removal of sediment from stormwater runoff is not the only solution and other management actions being utilised include seagrass meadow restoration, which provide opportunities to improve blue carbon storage.