Seagrasses are very important sediment stabilisers and trappers, but a third of seagrass meadows along the Adelaide metropolitan coast have been lost. Poor water quality resulting from drain discharge, stormwater run-off and effluent disposal has most likely been the initial cause of seagrass loss.
When there is a large scale loss of seagrass meadows, the finer-grained sand that was once trapped by the seagrass is released and washed ashore. The mobile sand accumulates in the sandbars but because it’s so fine, it is moved along quickly by littoral drift and provides little long-term protection.
As a result of the loss of sand from the seabed, the level of the seabed has become up to one metre deeper in places. This has resulted in an increase in the wave energy that reaches our beaches and has contributed to the movement of large quantities of sand along the coast.
The monitoring program
In 1987, a long-term seagrass monitoring program was introduced. Brass rods were installed at particular seabed locations and are surveyed annually to identify any changes in the seabed level, including seagrass species composition and condition. Photographs are also taken at each rod location and are added to the extensive photographic library that the Department maintains.
Monitoring has shown that local deepening of the seabed continues for a number of years after the seagrass has been lost and may progress until either a hard clay or calcrete layer is exposed or the seabed depth reaches the limit of the wave action.
In recent times, however, seagrass regeneration has been noted at various locations along the Adelaide metropolitan coast. These sites are re-visited regularly to monitor the ongoing success of regeneration.
The rod program is complemented by seagrass profiles at some locations. The data collected during profile surveys provides a larger survey area, contributing to a better understanding of the processes occurring at a particular location.