Understanding the distribution of seabed habitats such as seagrass beds, bare sands, rocky reefs, and rhodolith beds is important for successful coastal environmental management. However, studying the dynamic structure and composition of the seabed can also provide powerful insights into the processes that determine the distribution and diversity of habitats. In Esperance Bay, research is being conducted into the relationships between habitat diversity, sediment transport, wave energy, and the 3D structure of the seabed. Gaining an understanding of these physical processes provides information about the stability of habitats, and also assists the prediction of habitat distributions in areas where detailed mapping has not yet been conducted.
The rugged southern coast of Western Australia near Esperance has a unique inner shelf environment, with a limestone seabed punctuated by numerous granitic outcrops. This archipelago was first charted by Matthew Flinders in 1802, and comprises over 100 dome-shaped islands, and more than 1500 small islets and exposed shoals. A recent ecological study by the University of Western Australia has revealed that this environment contains a diverse array of subtropical and temperate flora and fauna. This is partly due to the influence of the eastward flowing Leeuwin Current, which warms the usually cool sea to over 20ºC in summer. This coast is exposed to the most extreme wave energy of the entire Australian coastline, and the seabed is subjected to powerful wave abrasion down to almost 100 m during storms. On the inner shelf of the Recherche Archipelago, the average seafloor depth is about 40 m, and the majority of the islands occur in waters less than 60 m deep.
The Recherche Archipelago is a challenging environment in which to assess habitats, due to frequent extreme weather conditions. Recently, surveys were conducted to investigate the effects of sediment transport, erosion, and accumulation on the distribution of seabed habitats. In the Recherche Archipelago, seabed grab sampling, core sampling, underwater video, sub-bottom profiling, and swath acoustic mapping (or multibeam sonar) has revealed the structure and composition of the seabed in unprecedented detail. In addition, acoustic backscatter images were produced by the swath mapper, providing textural information about the seabed which helps to delineate habitats such as rhodoliths.
For more information about the acoustic habitat survey methods, visit the Coastal CRC's coastal water habitat mapping webpage.
Figure 1. High resolution bathymetry (Reson 8125) of the seabed around the Woody Island group in the Recherche Archipelago, showing areas of sediment buildup to the north-east of islands; A) acoustic backscatter delineation of a rhodolith bed; B) limestone structures on the seabed; and C) seabed sediment dunes. Backscatter imagery processed by Curtin University Centre for Marine Science and Technology.
Close to the shore, the seabed comprises well-rounded quartz sand, which is often colonised by extensive beds of seagrass. The nearshore zone is dominated by an east to west long-shore sediment transport system. In protected bays, seagrasses encourage this sediment to build up over time.
Further offshore, the seabed is quite different. The majority of the substrate comprises a hard and mostly flat limestone surface, which is generally swept clean of sediment by the strong currents generated by heavy wave action during storms. This provides an ideal substrate for a diverse range of attaching marine organisms such as sponges, bryozoans (lace corals), ascidians (sea squirts), corals, and marine algae. Wide but low sediment dunes were discovered in various locations. These dunes are probably mobilised during storms, and may periodically bury limestone reef habitats as they move across the middle shelf in a shoreward direction.
The numerous steep sided granite islands have a strong influence on sedimentary and biological processes in the Recherche Archipelago. The islands provide shelter from the dominant south-westerly swell, and therefore provide habitat for a diversity of animals and plants that are adapted to different levels of wave exposure. Many of the attaching and free living organisms (such as molluscs, foraminifera, calcareous algae, and bryozoans) produce skeletons of calcium carbonate, which over time create large quantities of coarse sediment. This material accumulates as areas of mostly bare sand on the sheltered sides of the rocky islands, producing yet another type of seabed habitat. Core samples indicate that sediment accumulation is generally slow, however rapid pulses of accumulation also occur, possibly related to large storm events. These events may bury biota such as seagrass, which subsequently take a long time to re-establish themselves.
Figure 2. Conceptual model of the distribution of sediments and habitats on the Recherche Archipelago inner shelf, showing the directions of sediment transport and areas of seabed erosion.
Extensive rhodolith beds occur in partially sheltered environments. Rhodoliths are free-living red algal encrustations that range in size from a few millimetres to 5 cm in diameter. Individual rhodoliths are slow growing, occur in various shapes or growth forms, and due to their complex shapes, provide habitats for numerous types of small marine organisms. Core samples from rhodolith beds show that they produce large quantities of calcium carbonate sediment, and also provide the only sheltered environment in which fine muddy sediment can accumulate. Contaminants produced by human activities are often strongly associated with muddy sediments, and therefore may build up in these areas of seabed.