Seagrasses of south-west Australia: A conceptual synthesis of the world's most diverse and extensive seagrass meadows
South-west Australia contains extensive seagrass meadows along 2,500 km of coastline from the shallow subtidal to 50+ m water depths, and in many of the 51 bar-built estuaries along the coast. There are geomorphological differences between the south and west coasts that result in different patterns of swell exposure influencing the processes that structure seagrass habitats. In this paper, 'sheltered', 'exposed' and 'estuarine' seagrass habitat types are defined for south-west Australia to synthesize processes influencing seagrass communities. Sheltered habitats in south-west Australia are characterized by high light, low to moderate water motion and sporadic disturbance from storms, making them ideal habitats for a diversity of seagrass assemblages. Exposed seagrass habitats are characterized by the presence of strong and consistent ocean swells (3-8 m), predominantly from the south or south-west and seagrasses exhibit a suite of adaptive traits to survive the effects of exposure to ocean swell and associated sand movement. These include morphological features such as heavy fiber reinforcement to strengthen the aboveground stems or leaves, deep vertical rhizomes and life history traits such as rapid growth and high seed set. Within estuarine habitats highly dynamic seagrass communities are the result of fluctuating annual cycles in temperature, light and salinity. Compared to global seagrass meadows, coastal south-west Australian seagrass habitats experience high light, low nutrients and high water movement. Despite these differences, similarities with other regions do exist and here we place the habitats of south-west Australia into a global context using comparative data. The wide array of morphology and life history traits displayed among seagrass species of south-west Australia are presented in a conceptual model including habitat type, physical stressors and seagrass responses. The combination of adaptations to the habitats and processes that define them make south-west Australia a region where there is an unusually high number of co-occurring seagrass species, the highest in the world for a temperate environment (19 species), and approaching the species diversity of many tropical environments. Linking aspects of seagrass habitat, physical aspects of the environment and seagrass life history provides a context for applying knowledge gained from seagrasses in south-west Australia to other coastal ecosystems throughout the world.