Seagrasses of Southwest Australia: Estuaries
Estuaries are transition zones where rivers meet the ocean, creating an environment with large seasonal fl uctuations in temperature, salinity, and light. These difficult growing conditions provide some unique challenges for seagrasses. In southwest Australia, estuaries are usually closed by a sand bar at the mouth, cutting them off from the ebb and fl ow of the tide for long periods. Winter rains flow down-river into the estuaries, raising the water level until it breaks through the sand bar. Seawater then starts to fl ow in and out with the tide until movement of sand along the beach by waves will once again close the sand bar, usually in late summer. Because of these seasonal changes, only a few types of seagrass such as Ruppia, Halophila, and Zostera grow in these estuaries. However, they are very important as they provide shelter and food for many species of fish, crabs, shellfish, and prawns.