Seagrass loss

Seagrasses are very important sediment stabilisers and trappers. But a third of seagrass meadows along the Adelaide metropolitan coast have died since 1950.

Most of the seagrass has been lost within 2km of shore. Four km offshore, seagrasses appear to be relatively healthy. Extensive offshore loss occurred around the Port Adelaide and Glenelg wastewater treatment plant sludge outfalls, but these are no longer in use.

Poor water quality resulting from stormwater run-off and effluent disposal has most likely been the initial cause of seagrass loss. Once there are gaps in the seagrass meadows, the sand below the meadow edge can be eroded by waves. This is thought to have increased the rate of seagrass loss and made it difficult for plants to recolonise the seafloor, even though water quality has improved.

Finer-grained sand that was once trapped by seagrass meadows has been released and washed ashore. Because the sand is fine, it accumulates in the sandbars and washes north to Largs Bay. Although in the short term this sand has added to protection of the coast, it is unsuitable for replenishing Adelaide's beaches in the longer term. This is because it tends to remain in the underwater part of the beach and is moved too quickly by waves.

As a result of the loss of sand from the seabed, the level of the seabed has steadily become up to one metre deeper and the wave energy reaching our beaches has increased. This causes a larger quantity of sand to drift north along the coast.

More information 

Refer to our reference material page.