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Posidonia Australis Complex Seagrass Meadows Draft Description 1 POSIDONIA AUSTRALIS COMPLEX SEAGRASS MEADOWS DRAFT 2 DESCRIPTION 3 4 Draft Description 5 The ecological community consists of the assemblage of plants, animals and micro-organisms 6 associated with seagrass meadows dominated by species from the Posidonia australis 7 complex and occurs mostly in temperate regions of Australia. The ecological community is the 8 climax community of a successional process that occurs over decades to centuries (Kendrick et 9 al, 2008). 10 11 Name of the ecological community 12 The ecological community was nominated as Posidonia australis seagrass beds or Posidonia 13 dominated seagrass meadows. The Threatened Species Scientific Committee recommended 14 that the ecological community be assessed as Posidonia seagrass meadows and it was placed 15 on the 2011 Finalised Priority Assessment List (FPAL) by the Minister for the Environment, 16 with an assessment period ending in December 2013. 17 Further consideration of the definition of this marine ecological community, including an 18 expert workshop, resulted in a focus on the species of the Posidonia australis complex. The 19 P. australis complex is one of two distinct groups of Australian Posidonia species; the other 20 being the P. ostenfeldii complex1. The two groups can be separated on the basis of their leaf 21 and rhizome characteristics but also their ecological characteristics. The three species of the 22 P. australis complex (P. australis; P. angustifolia; and P. sinuosa) are the meadow forming 23 species of the Posidonia genus (Cambridge and Kuo, 1979). It is therefore recommended that 24 the name of the ecological community be Posidonia australis complex seagrass meadows 25 (hereafter referred to as ‘the ecological community’). 26 27 Location and physical environment 28 The ecological community occurs in temperate/subtropical waters from Shark Bay (25°S) on 29 the west coast, across southern Australia to Wallis Lake (32°S) on the east coast, around Bass 30 Strait islands and along the north coast of Tasmania. Latitude 25°S on the west coast and 30°S 31 on the east coast of Australia is considered to form a boundary between temperate and 32 tropical seagrass species (Larkum, 1976). Posidonia australis complex seagrass meadows 33 occurring around islands within the geographic range are also included within the ecological 34 community. 35 36 The ecological community typically occurs in subtidal2 waters where salinities are marine (30– ◦ 37 50 /◦◦), at depths less than 20 m on sand and silty mud substrate (Cambridge and Kuo, 1979; 38 Carruthers et al., 2007). Posidonia species of the ecological community have been recorded at 39 depths greater than 20 m. For example, off the west coast, P. angustifolia has been recorded 40 at a depth of 45 m (Carruthers et al., 2007). However, at depths greater than 20 m, the 41 meadow structure and function typical of the ecological community is diminished. These 42 occurrences are therefore excluded from the definition of the ecological community. The 43 upslope limit of the ecological community is imposed by the requirement for sufficient 44 immersion in seawater and tolerable disturbance by water movement (Duarte, 2002). In some 1 In contrast to Posidonia australis, species of the P. ostenfeldii complex are clump forming and are found in more exposed, high energy environments (Kuo and Cambridge, 1984). 2 Subtidal zones are submerged most of the time, exposed briefly during extreme low tides around full and new moon events. 1 cases, ponding and retention of water at the shallow edge of meadows enables the ecological 2 community to exist below tide level (Kirkman, pers. comm., 2013). For example, in Corner 3 Inlet, Victoria, large expanses of the ecological community are exposed at most low tides 4 (Kirkman, 2009). 5 6 The ecological community typically occurs in sheltered habitats, with the nature and degree of 7 protection from water movement varying across its range. For example, 8 On the south eastern coast of Australia, the high wave energy typical of the coastline 9 largely restricts the ecological community to estuaries (e.g. Port Hacking, Lake 10 Macquarie, New South Wales) and open embayments (e.g. Jervis Bay, New South 11 Wales and Corner Inlet, Victoria). Some small isolated patches occur near protected 12 rocky platforms and in the lee of offshore islands (e.g. in the lee of Broughton Island, 13 Bushrangers Bay at Bass Point and in waters at Bittangabee Bay) (West et al., 1989). 14 Along the southern Australian coast, the ecological community occurs in sheltered 15 habitats affording protection from strong, prevailing south west oceanic swells, such as 16 gulfs (e.g. Spencer Gulf and Gulf St Vincent, South Australia), embayments (e.g. 17 Princess Royal Harbour, Western Australia and estuaries (e.g. Oyster Harbour, 18 Western Australia) (Shepherd and Robertson, 1989). On exposed coastlines, the 19 ecological community occurs where reefs or islands give local protection from ocean 20 swell (Carruthers et al., 2007) (e.g. Waterloo Bay, South Australia; Thompsons Bay, 21 Rottnest Island) or at depths which sufficiently reduce wave energy (e.g. at the base of 22 exposed cliffs of western Eyre Peninsula) (Shepherd and Robertson, 1989). 23 On the west coast of Australia the ecological community mostly occurs inshore of a 24 broken line of limestone reefs and barrier islands (Kirkman, 1985; Carruthers, 2007) 25 which give protection from the full force of oceanic swell. The breaks in the reef 26 system can be up to many kilometres in length and this is reflected in the occurrence 27 of the ecological community (Kirkman, 1985). The ecological community reaches its 28 northern limit on the west coast in the semi-enclosed basin of Shark Bay (Walker, 29 1989). 30 31 Vegetation 32 The ecological community occurs as continuous to patchy monospecific and multispecies 33 seagrass meadows dominated by species from the Posidonia australis complex - 34 P. angustifolia, P. australis and P. sinuosa. 35 36 The Posidonia australis complex species are slow growing, have persistent rhizomes and are 37 typically the meadow-forming species of the Posidonia genus (Cambridge and Kuo, 1979). 38 There is differentiation in the distribution and habitat preferences of the Posidonia australis 39 complex species that can affect the relative dominance in a meadow of one or other of the 40 species (refer Table 1). For example, P. australis is the only Posidonia species occurring on the 41 east coast. Where P. australis and P. sinuosa co-occur, P. australis typically dominates 42 P. sinuosa in very shallow and more disturbed areas as the shorter and wider leaves of 43 P. australis are resistant to mechanical damage caused by water movement . Conversely, 44 P. sinuosa typically dominates P. australis in slightly deeper and more sheltered water 45 (Kirkman and Walker, 1989; Shepherd and Robertson, 1989). In deeper water and in deeper 46 sand, P. angustifolia is often the dominant species of Posidonia (Shepherd and Roberston, 47 1989). 2 1 Table 1 Distribution, habitat preference and meadow structure of Posidonia australis complex 2 species. 3 Sources: Cambridge and Kuo, 1979; Edgar, 2012. 4 Posidonia Distribution Environment Depth (m) Meadow species structure P. australis Shark Bay, WA Sheltered sand and 1-15 Dense around the southern mud environments monospecific and southeastern meadows, coastlines to Wallis although Lake, NSW and along frequently the northern coast of found with Tasmania and Bass other Posidonia Strait Islands species P. angustifolia Houtman Abrolhos, Moderately exposed 2-35 Dense WA to Encounter Bay, sand environments monospecific or South Australia and mixed species along northern coast meadows of Tasmania P. sinuosa Shark Bay, WA to Moderately exposed 1-15 Dense Kingston, SA and sheltered sands monospecific meadows; usually found with P. australis in sheltered embayments or with Amphibolis species in less enclosed waters 5 6 7 Both continuous and patchy meadows of the ecological community often contain mosaics of 8 bare sand or other seagrass species that occupy edges, blowouts3 and small areas of meadows 9 (Kirkman and Kuo, 1990). In some cases, sparse meadows of the ecological community may 10 have an understorey of smaller seagrass species. Seagrass species that may be found growing 11 with the ecological community include: Amphibolis antarctica, A. griffithii, Cymodocea 12 angustata, Halophila uninervis, H. ovalis, Heterozostera tasmanica and Syringodium 13 isoetifolium. 14 15 Also associated with the ecological community is a diverse assemblage of benthic flora, in the 16 form of micro and macro epiphytes and non-epiphytic algae and a complex layer of 17 periphyton4 on the seagrass leaves (Klumpp, et al., 1989; Carruthers, 2007). 18 3 Blowout – an area in a seagrass meadow denuded of seagrass through natural or anthropogenic disturbance. Blowouts are typically areas of instability in sediment movement, shape and size and seagrass species composition (Kirkman, 1985). 4 Periphyton comprising of diatoms, macroalgae, encrusting algae, bacteria, fungi and sedimented debris (Klumpp, et al., 1989) 3 1 Fauna 2 The ecological community is highly productive and provides habitat, shelter and food for a 3 diverse array of fauna permanently and temporarily inhabiting the ecological community. 4 Posidonia australis complex seagrasses are not usually a direct food source for grazers. 5 However, a few fish, notably monacanthids (Wressnig and Booth, 2008) graze the leaves. 6 Mostly, the seagrass provides a stable
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