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.

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Southwest Australia has warm temperate water with a mixture of tropical influences from the Leeuwin Current and cool southern waters. This mixing of tropical and temperate water results in diverse seagrass communities occurring in a wide variety of coastal habitats. Almost half of the world's ~60 seagrass species can be found along this 1,500 km of coast. Seagrasses are important to the marine environment as they stabilise sediments and trap nutrients, helping to maintain water quality.

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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.

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On the south coast, a diverse range of seagrasses grow in habitats protected from the full force of waves by islands and headlands. South coast seagrasses grow on silica sand to a depth of about 30 m and on carbonate sand beyond 30 m depth. Many seagrasses grow in these sheltered zones, including most of the species found on the west coast. Beyond the shelter of the headlands, waves roll in from the open ocean and seagrasses are subjected to very high wave action.

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Along some 600 km of the west coast, a line of offshore islands and submerged reefs provides protection from the full force of ocean swell and zones of sand deposition. The longitudinal reef and islands remain from former coastlines drowned by rising sea levels - the most recent rise was about 6,000 - 12,000 years ago.

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Effective management of estuarine systems that contain submerged aquatic macrophytes, requires knowledge of whether macrophyte populations are stable, increasing or decreasing in terms of aerial coverage and abundance. This study established three transects within Wilson Inlet and monitored them five times during 1996 for percent cover of Ruppia megacarpa and maximum depth limits of these meadows.

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Full description … Thirty 3 m diameter plots were established in Warnbro Sound, Western Australia in early 1991, within an Amphibolis griffithii meadow, and the shoot densities manipulated to 0, 25, 50, 75 and 100% (control) by pruning. These experimental manipulations were monitored and maintained on a monthly basis until March 1993. The effects of these density changes on sediments were investigated.

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Experimental manipulation of canopy density in a temperate seagrass (Amphibolis griffithii (Black) den Hartog) meadow: effects on sediments

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The phenotypic plasticity and performance of two growth-forms of Caulerpa racemosa (Forsskal) J. Agardh, one found in estuarine conditions, the other growing in a coastal reef environment was studied in laboratory culture. There was no change in the morphology of either growth-form when cultured through a range of salinity (10—40%o) and light (20—140 μEm-2s-1) conditions.

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