Over the winter months in the southwest the weather conditions produce quite a different environment for the local flora and fauna. The calm, clear waters of summer are replaced with rough, turbid seas and the beaches undergo a rapid change. Pounding waves on the shore create a steeper incline of the beach as the sand is mobilised by the waves. As low pressure weather systems with cold fronts cross over the coast, the large associated swells bring up from the sea an abundance of seagrass wrack.
Seagrass wrack serves an important part of the coastal ecosystem. It is mainly the dead seagrass that has been cleaned out of the meadows by repetitive wave action which ends up on the shore, although in severe weather, healthy seagrass can also be uprooted. The most common species observed in wrack inside Geographe Bay are wireweed (Amphibolis griffithi) and strapweed (Posidonia australis). Huge mounds of seagrass wrack are evident on the shores of Geographe Bay from June until September which provides numerous benefits to the coastal ecosystem.
Firstly it can help to counteract coastal erosion, protecting the sands of the beaches by providing a fibrous cushion on which the waves can impact. As the waves impact the wrack further, it is compacted into the sand and essentially holds piles of sand together, preventing it from washing away.
Secondly as the seagrass wrack decomposes it provides a nutrient source which benefits life on the shore. Crustaceans, including amphipods, isopods and crabs feed on the decaying leaves and flying coastal insects are often seen around a wrack mound. Nesting birds such as seagulls and terns will also use dead seagrass leaves to build and line their nests.
Often the common fishes of seagrass habitats are also caught up in the wrack and include leatherjackets and boxfish and these may be found stranded on the beach. Occasionally yellow-bellied sea snakes are also observed and, as always on beaches, care should be taken as these reptiles may still be alive after being stranded.
By – Sophie Teede, Marine Scientist at the Jetty