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State of the Marine Environment Report for Australia: The Marine Environment - Technical Annex: 1

Compiled by Leon P. Zann
Great Barrier Reef Marine Park Authority, Townsville Queensland

Ocean Rescue 2000 Program
Department of the Environment, Sport and Territories, Canberra, 1995

ISBN 0 642 17399 0

Seagrasses

I. R. Poiner & C. Peterken

CSIRO Division of Fisheries
PO Box 120, Cleveland QLD 4163 Australia

Introduction

Seagrasses are rhizomateous marine angiosperms that cover extensive areas in the coastal waters of Australia. Australian seagrasses are characterised by high endemism and high speciation, especially in temperate regions. Australia has the largest number of seagrass species in the world with over 30 species (Walker & Prince 1987) which broadly can be categorised into tropical and temperate groups (Poiner & Roberts 1986). On the basis of species composition and distribution a transition zone occurs at approximately 30S on the east coast and 25S on the west coast (Larkum & den Hartog 1989).

Seagrass communities are of considerable importance in the processes of coastal ecosystems because of their high rates of primary production and their ability to trap sediments and organic nutrients. Their special importance to commercial and recreational fisheries is well documented. Additionally, seagrasses are important in substrate stabilisation, supply and fixation of biogenic calcium carbonate, detrital food chains, nutrient cycling and as substrate for epibiota and as critical habitats for many species (Larkum, McComb & Shepherd 1989).

Commercial and conservation value, and usage

Seagrass communities are important habitats for many commercially important species of fish and crustaceans, providing them shelter from predators and strong currents, and playing a vital role in the food cycle of coastal ecosystems. For example, seagrass communities are the major nursery grounds for juvenile tiger prawns (Penaeus esculentus and P. semisulcatus) and endeavour prawns (Metapenaeus ensis and M. endeavouri) (Poiner, Staples & Kenyon 1987, Staples, Vance & Heales 1985). Post-larvae of both tiger and endeavour prawns settle out from the water column into the shallow inshore seagrass beds and as they grow, they move into and through the deeper water beds. The productivity of the off-shore tiger and endeavour prawn fisheries of Australia are dependent on the presence of near-shore seagrass nursery grounds.

Tiger and endeavour prawn fisheries are very important in Australia. They returned more than $80 million in export earnings in 1993 - approximately 7% of Australia's fisheries export revenue for that year (Australian Fisheries Statistics 1993).

Seagrasses are also critical in the life history of the western rock lobster (Panulirus signus) (Jernakoff 1987) and ornate rock lobster (P. ornatus) (R. Pitcher pers comm) as food and shelter for their peurulus and juvenile stages. The western rock lobster fishery is the most important in Australia returning $217 million in 1993 (Australian Fisheries Statistics 1993). The value of the tropical rock lobster fishery is small in comparison ($4.5 million) but it is extremely important to Torres Strait Islanders.

Seagrass is the principal food resource of the dugong (Dugong dugon). The dugong, the only herbivorous mammal that is strictly marine, is listed as vulnerable to extinction by the International Union for the Conservation of Nature. Seagrass is also a major food resource for the green turtle (Chelonia midas).

Status

Seagrasses of south-eastern Australia

Distribution and extent

South-eastern Australia is defined as the area between the Tweed River in northern New South Wales and Corner Inlet in south-eastern Victoria (Figure 1). Eight species of seagrass are present in this region. Zostera communities are the most common with three species of Zostera growing along this coastline. Zostera capricorni grows from the Tweed River to Mallacoota, where it is replaced by Z. mulleri. Zostera grows across a wide range of habitats and is the dominant genera in terms of occurrence and area. Heterozostera tasmanica occurs as far north as Port Stephens in New South Wales. Three species of Halophila occur in south-eastern Australia although their distribution and status is less well known than is Zostera's. The range of Halophila ovalis and H. decipiens extends the length of New South Wales, while H. australis is present between central New South Wales and all of Victoria. Halophila has a wide habitat range but its growth appears to be very seasonal. It is present in low densities and occurs in small patches. Posidonia australis has a more limited range latitudinally, extending southwards from Wallis Lake, New South Wales. However, it is widely distributed in southern Australia. P. australis occupies fewer habitats than does Zostera or Halophila along the south-east coast, where it avoids hyposaline conditions and unstable sediments, but seems to be more tolerant of wave energy than are the other taxa (West, Larkum & King 1989).

The high energy coastline of south-eastern Australia means that seagrass growth is confined to estuaries and protected embayments. Although a generalised north-to-south distribution pattern can be given, the distribution and dominance of the three seagrass genera (Zostera, Halophila and Posidonia) is dictated by the occurrence and nature of those coastal features which offer a suitable growth habitat for them. The majority of the coastal features where seagrass growth occurs in New South Wales are estuarine. Zostera occurs in 93% of estuaries, Halophila in 66% and P. australis in 43% (West 1983).

Figure 1: Seagrasses of south-eastern Australia

Figure 1

Estuaries have been classified into three basic types according to their entrance characteristics: drowned river valleys, barrier estuaries and coastal lagoons (Roy 1984). Distribution and occurrence of seagrasses varies between type of estuary and is also affected by the age, catchment area and stability of each individual estuary. In general, there is a higher area of coverage by seagrass about 3 km back from the estuary entrance, although Zostera may be found in feeder creeks and cut-off bays further upstream. The species' distribution also depends on substrate type and its stability. All three seagrass genera occur predominantly on marine sand. At Port Hacking, Zostera and Halophila will readily recolonise seaward facing sands subjected to frequent disturbance by heavy seas and storms. Posidonia australis recolonises less readily: for example, at Jervis Bay, seismic testing 20 years ago blasted holes in the P. australis communities. These patches remain bare, even though they are surrounded by P. australis plants (West, Larkum & King 1989).

Coastal lagoons also harbour seagrass and floristic changes occur there over time. These in turn are affected by the frequency with which the lagoon entrance is broached by the sea; eg in Smiths Lake in New South Wales, the coverage by Zostera and Halophila seagrasses is greater during marine conditions than during less saline periods. Hyposaline conditions appear not to suit P. australis which is absent altogether from intermittently open lagoons. There are also several open embayments in New South Wales containing seagrasses. Those at the mouths of large estuaries may undergo periods of high turbidity and low salinities (eg Botany Bay and Batemans Bay). Bays which experience high wave energy are dominated by P. australis (eg Twofold Bay and Batemans Bay) (West, Larkum & King 1989).

Using aerial photographs, West, Thorogood, Walford and Williams (1985) reported that a total of 155 km2 along the New South Wales coast was covered with seagrass. The largest area of seagrass occurred in Wallis Lake (30.785 km2) where there is no Halophila - only Zostera and P. australis; followed by the Clarence River (19.072 km2) of Zostera and Halophila and Lake Macquarie (13.391 km2 ) of Zostera, P. australis andHalophila. There is very little information on eastern Victoria although large areas of seagrass are known to exist at Mallacoota Inlet, Gippsland Lakes and Corner Inlet. It was estimated in 1965 that there were 119 km2 of Posidonia australis growing in Corner Inlet, as well as some Zostera and Heterozostera (Morgan 1986).

Seagrass declines in south-eastern Australia

A number of seagrass communities in New South Wales have been studied in detail due to concerns that an apparent decline in the extent of seagrass beds has taken place. In 1953, the area of seagrasses in Lake Macquarie was estimated at 25.48 km2 as part of an investigation into declining fish stocks. By 1985 the area of seagrass had declined by 11.31 km2 to 14.17 km2. The decline in seagrasses was probably due to increased turbidity in the lake from human activities (King & Hodgson 1986).

Botany Bay is a shallow sand embayment covering 46 km2, with extensive areas of seagrass. Aerial photographs taken between 1942 and 1986 show a loss of 58% of the P. australis communities, an area of about 2.5 km2. Reasons contributing to this loss may include: increased wave climate and erosion due to dredging; major storm events in 1974 and 1975; eutrophication due to sewage input; and grazing by the sea urchin Heliocidaris erythrogramma (Larkum & West 1990). Larkum (1976) suggested that once degeneration in P. australis begins it may be self-perpetuating: there has been no regrowth of P. australis over the 44-year period. Zostera capricorni has replaced much of the P. australis and covers an estimated of 3.09 km2. However, this species has low productivity compared to P. australis. Both Zostera and Posidonia beds in Botany Bay house a diversity of juvenile fish species, and at least five species of commercially important fish utilise Zostera beds alone (Bell & Pollard 1989). The current development of the third runway for Sydney Airport in Botany Bay will probably cause further loss of seagrass from the Bay.

There have been unquantified reports of the decline of Posidonia australis in Corner Inlet (Victoria). No cause could be pinpointed and no man-induced changes are implicated. There is an estimated loss of 50% of Zostera capricorni communities from the estuaries of New South Wales. It is thought that the 60% losses at the Clarence River and the Tweed River over 30 years are due to increased turbidity associated with a general decline in water quality.

Although some of the figures concerning the decline of seagrasses in south-eastern Australia may be exaggerated due to a lack of conformity in survey and mapping methods, there has been an overall decline in the area of seagrasses along the south-east coast. Coastal development and other anthropogenic activities are probably responsible for much of the decline in seagrass area.

Seagrasses of southern Australia

Distribution and extent

This region extends from the Head of the Bight in the west to Bass Strait in the east (Figure 2). Species with warm temperate affinities - in the genera Posidonia and Amphibolis - decline in number from west to east, with a corresponding decrease in ocean temperatures. Encounter Bay is the easterly limit of Amphibolis griffithi; Lacepede Bay of Posidonia sinuosa; Rivoli Bay of P. coriacea and P. denhartogi; and Port MacDonnell of P. angustifolia. The cool temperate species Halophila australis is distributed throughout this region as are seagrasses in the genera Heterozostera and Zostera.

Figure 2: Seagrasses of southern Australia

Figure 2

The distribution of seagrass is a function of the coastal topography and environment. The most extensive seagrass beds are found in Spencer Gulf and Gulf St Vincent in South Australia, with a total of over 5000 km2 of seagrass. Spencer Gulf and Gulf St Vincent offer large expanses of sheltered water for seagrass growth. Both gulfs are dominated by Posidonia species. Posidonia beds in Spencer Gulf extend across 3700 km2 while Gulf St Vincent boasts 1530 km2 of Posidonia with Amphibolis antarctica, A. griffithi and Heterozostera tasmanica more sparsely distributed. Halophila australis is sparse but widespread and Zostera mucronata and Z. mulleri are found intertidally across the gulf (Figure 2) (Shepherd & Robertson 1989).

The clear waters of the Great Australian Bight allow chlorophyllous plants to live at considerable depths. Posidonia angustifolia and P. coriacea are found at depths of 25-30 m at the base of exposed cliffs of the western Eyre Peninsula where they are unaffected by swell. However, seagrass distribution along the exposed coast is patchy and most seagrass is found in the lee of reefs and islands. For example, seagrasses in the lee of Ward Island display a characteristic depth zonation and distribution, with Amphibolis antarctica occupying the shallows and Posidonia coriacea found at 30 m depth. Halophila australis is found on the more unstable sands (Figure 2).

Seagrasses are also distributed along the coastline in coastal lagoons. The larger ones in South Australia are West Lakes (Adelaide), with Zostera mulleri, Heterozostera tasmanica and Halophila australis at different depth zones, and the Coorong, an extensive lagoon system in the south-east of the State supporting Zostera mulleri. The distribution of seagrasses in Tasmania and much of Victoria requires further research. In sheltered waters of Bass Strait, patches of Posidonia australis and Amphibolis antarctica can be found. Tasmania is said to support abundances of Heterozostera and Halophila australis in its more sheltered waters, with large Posidonia australis meadows around its north coast and around Flinders Island (Figure 2) (Shepherd & Robertson 1989).

Seagrass declines in southern Australia

There has been a well documented decline of seagrass on the eastern side of Gulf St Vincent. In the northern section of the gulf, Heterozostera tasmanica dominates the intertidal flats and mixed Posidonia sinuosa and Amphibolis antarctica beds exist subtidally. By 1949, Outer Harbour (Adelaide) had lost over 9 km2 of seagrass after the construction of retaining walls and groynes and subsequent sediment accretion (Sergeev, Clark & Shepherd 1988).

There have also been significant losses of seagrass due to sewage effluent. With the commencement of discharge from the Bolivar outfall in 1967, loss of H. tasmanica around Port Gawler was immediate. By 1976, there were Posidonia losses exhibited by a decline in leaf density, weight and length and an increase in epiphytism. Pre-1978, around the Port Adelaide outfall, 85% of the area was covered by seagrass, comprising 53% Posidonia sinuosa, 2% Amphibolis antarctica and 30% mixed beds. By 1981, the P. sinuosa was reduced by 50% and by 1982, 3.65 km2 had gone. An even more extensive decline of Amphibolis took place, with 15 km2 lost over this period. This suggests that Amphibolis may be particularly sensitive to the effects of effluent. In the southern section of the gulf, where Posidonia angustifolia, P. sinuosa and Amphibolis antarctica dominate, 8 km2 of seagrass was lost around the Glenelg outfall between 1935 and 1987. Overall losses for the gulf amount to more than 60 km2 of seagrass (Sergeev, Clark & Shepherd 1988, Neverauskas 1987).

In Victoria, Port Phillip Bay has large Amphibolis antarctica beds as well as Heterozostera tasmanica and Zostera mulleri beds. Further east, the 680 km2 coastal inlet, Western Port supported extensive seagrass beds when it was first explored in 1899. In the early 1970s, 37% of the area of Western port (250 km2) supported seagrass and macroalgae. H. tasmanica dominated in terms of productivity (50%), with Zostera mulleri and Amphibolis antarctica also present. By 1984, only 72 km2 of seagrass and macroalgae remained, with a 90% loss of Heterozostera tasmanica. Among the surviving seagrass there was a 50% decline in the above-ground biomass, giving an 85% decrease in the standing crop of Western Port. Such a decline appeared to be due to an increase in fine silt coming into the bay from river run-off, and this adhered to the seagrass leaf blades, blocking off light. Initial losses of plants caused further erosion of mud banks and the decline became self-perpetuating (Bulthius 1983).

Seagrasses of Western Australia

The coastline of Western Australia extends from 13S to 35S. Many different habitats and many communities occur over this latitudinal range (Figure 3). The diversity of seagrass species across this region (10 genera and 25 species) is unequalled elsewhere in the world. Tropical species have their southerly limits on this coastline: Thalassia hemprichii and Thalassodendron ciliatum extend to 22S, a latitude which is also the northerly limit of the temperate species Amphibolis antarctica. Cymodocea angustata extends south to Shark Bay; Halodule uninervis and Halophila spinulosa extend south as far as 29S; Syringodium isoetifolium reaches to 32S - 5 further south than its range on the east Australian coast due to the warm Leeuwin Current. Eight Posidonia species are found on the south-western coast of Western Australia. The most widespread species is Posidonia australis with an uninterrupted temperature range from Shark Bay in the north to Lake Macquarie in New South Wales (Kirkman & Walker 1989, Walker 1989).

Figure 3: Seagrasses of Western Australia

Figure 3

Despite the wide latitudinal range of the Western Australian coast, the sea temperature range is very small and the distribution of seagrass species reflects the availability of a suitable environment. In the more southerly latitudes, two Posidonia seagrass complexes are recognised: "ostenfeldii", including P. robertsoniae, P. kirkmanii, P. coriacea and P. Denhartogii and "australis", including P. australis, P. angustifolia and P. sinuosa. Each complex has distinct habitat requirements. Posidonia is very abundant along the south-west coast which is more exposed than coasts further north; and here the members of the "ostenfeldii" complex, with their deep roots and strong leaves, are most successful. Frenchman's Bay near Albany, Princess Royal and Oyster harbours and Geographe Bay have large areas of seagrasses. P. australis and P. sinuosa dominate these protected habitats, with some Amphibolis and other species in small areas. The distribution of species within these bays is a reflection of water movement gradients and depth zonation (Figure 3) (Kirkman & Walker 1989).

Around Perth, there are fringing reefs forming coastal lagoons. Posidonia sinuosa is the dominant species, covering many hundreds of ha in the lagoons. Other "australis" members occur within the lagoons and "ostenfeldii" complexes occupy more turbulent waters. Cockburn Sound is well protected by Garden Island. Here members of the "australis" complex are found together in the shallows with Amphibolis antarctica and Heterozostera tasmanica. The Swan-Canning estuary has 5 km2 of seagrass mainly Halophila ovalis yet with some Zostera mucronata. Rottnest Island has some Halophila ovalis but in these clear waters overlying limestone rock, Amphibolis is the most important genus. In all, 9 species of seagrass are found in the Perth region (Figure 3) (Kirkman & Walker 1989).

Moving north, the sheltered Cliff Head lagoon has a 50 km2 meadow of dense Posidonia. The reefs along this stretch of coast are the centre of the western rock lobster fishery. The seagrass beds are sites of settlement and nocturnal foraging for the juveniles of Panilirus cygnus (western rock lobster) with up to 13% of their diet consisting of seagrass and its associated macrofauna, particularly gastropods (Kirkman & Walker 1989).

Around Dongara further north, Seven Mile Beach is a highly disturbed habitat despite protective reefs. Halophila ovalis, Halodule uninervis, Heterozostera tasmanica and Syringodium isoetifolium form communities in the disturbed sands. There are natural seasonal fluctuations in the amount of cover of up to 80%. However, Amphibolis antarctica and A. griffithii form stable and extensive meadows, which did not fluctuate in area over a 6-year study period (Clarke & Kirkm 1989).

At 26S, Shark Bay, with an area of 13 000 km2 has some of the largest and most diverse seagrass meadows in the world, with seagrass the dominant organism in the bay. The high diversity is thought to result from a regime of intermediate disturbance and occasional cyclonic events. Shark Bay is also the site of an overlap between temperate and tropical seagrass species. The bay has restricted exchange with high evaporation leading to hypersalinity in its eastern parts. Through sediment accretion and slowing down of currents, this huge agglomeration of seagrass has further modified its own environment, with a subsequent build up of barrier banks. Behind these, hypersaline conditions exclude seagrasses, now favouring the growth of stromatolites. At the northern limit of its range, Amphibolis antarctica, is the most abundant species in Shark Bay, with monospecific stands accounting for 3676 km2 or 85% of the total seagrass beds. Mixed A. antarctica and Posidonia australis stands are found in the eastern part of the bay: this is also the northern limit for P. australis. Halodule uninervis is the next most abundant plant. It may form an understorey to A. amphibolis and P. australis or grow on the intertidal flats in sparse mixed stands with Halophila ovalis and H. ovata. These stands are the preferred grazing area for the large number of dugongs that occur in the bay. North-east and east of the bay, monospecific stands of H. uninervis are the site of the dugong summer feeding grounds. Other seagrass species in Shark Bay include 'tropicals' such as Syringodium isoetifolium and Cymodocea angustata (which does not occur further south) (Figure 3) (Walker 1989).

Little is known about the extent of seagrasses north of Shark Bay, Nine genera with 14 species are found here. The coast can be divided into four main habitats. Thalassia hemprichii dominates atoll coral reefs, while large meadows of Enhalus acoroides, Thalassodendron ciliatum and Cymodocea serrulata occur on inshore reefs and banks. Offshore islands and sand cays support large populations of T. ciliatum. On extensive intertidal-subtidal flats, Halodule and Halophila dominate (Walker & Prince 1987).

The extensive and diverse seagrass communities along Western Australia's coastline may be attributed to the general suitability of the coast boasting a variety of habitats, together with a range of tropical and temperate species available for colonisation.

Decline in seagrass in Western Australia

Development of heavy industries and the discharge of industrial waste into Cockburn Sound commenced in the 1950s. By 1969 there were widespread losses of seagrasses and by 1978, 97% of the original 34 km2 of seagrass had been lost (Cambridge & McComb 1984). Examination of the many possible causes showed that increased nutrient loading from effluent had given rise to an explosion in epiphytic growth. As a consequence, there was an overall 63% reduction in light reaching seagrasses in declining meadows. Controls on effluent input have helped to arrest the decline and there is evidence of slow regrowth (Cambridge, et al. 1986, Silberstein, Chiffings & McComb 1986). There have also been losses of seagrass in Princess Royal and Oyster harbours, probably associated with discharges of wastes. There is a 66% loss (over 7 km2) of seagrass from Princess Royal Harbour and a 46% loss (over 8 km2) from Oyster Harbour (Bastyan 1986, Walker & McComb 1990).

Seagrasses of northern Australia

Northern Australia is here defined as the area between Cape Leveque in Western Australia, and Torres Strait (Figure 4). This area encompasses three broad geographical regions: Torres Strait, the Gulf of Carpentaria and the north-western coasts of the Northern Territory and Western Australia.

Figure 4: Seagrasses of northern Australia

Figure 4

Unlike temperate seagrass species, tropical seagrasses tend to occur in mixed-species stands. Thirteen species from seven genera are found across northern Australia. The region has a greater diversity of seagrass species and communities than elsewhere in the Indo-Pacific. Five Halophila species occur in northern Australia: H. ovata, H. ovalis, H. spinulosa and H. decipiens are ubiquitous, while Halophila tricostata is confined to Torres Strait. Halodule uninervis and H. pinifolia range throughout this region. Cymodocea serrulata is also found across the region, but C. rotundata is absent from Western Australia where it is replaced by C. angustata. Other wide-ranging species include Thalassia hemprichii and Enhalus acoroides. Thalassodendron ciliatum is found only in the north-western section, associated with hard substrates and corals (Figure 4) (Poiner, Coles & Walker 1989).

Torres Strait is a shallow (30-50 m deep in the east and 10-15 m in the west) body of water approximately 160 km long (north-south) and 220 km wide (east-west). The area has a complex bathymetry with large numbers of islands, shoals and reefs. Two physiographic regions characterise Torres Strait. The first is the western islands which represent the peaks of the drowned ridge extending from Cape York to Papua New Guinea. Coral reefs fringe many of the islands and are well developed in the shallow water between them. The north-eastern portion of this region is an extensive shallow (less than 10 m) seabed with well developed sand waves. The second physiographic region encompasses the numerous platform reefs, atolls and barrier reefs in the eastern Torres Strait. The dominant feature of this region is a large, north-south oriented platform reef - the Warrior Reef complex that almost bisects Torres Strait. Winds are seasonal, with strong south-east trades in the austral winter (May-October) and north-western monsoons in the austral summer (December-February). Strong tidal currents (more than 1 m per sec.) flow alternatively east and west, but there is no evidence of net current flow through the strait.

Torres Strait supports one of the largest seagrass areas in Australia. A total of 17 500 km2 of seagrass-supporting habitat associated with 295 km of coastline or reef has been identified and mapped. Twelve seagrass species have been recorded and the area supports a greater diversity of seagrass communities than does the rest of northern Australia. As well as the mixed-species reef-flat communities and depth-zoned open coastline communities similar to those found in the Gulf of Carpentaria, two other communities occur: sparsely distributed mixed-species open ocean communities; and subtidal Halophila communities. The open-ocean communities are similar to the diverse mixed species reef-flat communities but occur subtidally (to around 40 m depth) in the extensive shallow waters of the north-western and western Torres Strait with Halophila ovalis, H. spinulosa, Syringodium isoetifolium, Halodule uninervis and Thalassia hemprichii the most common species. Halophila (H. ovalis and H. spinulosa) communities are present subtidally off the large continental islands. The open-ocean communities are unique to the area and extensive. They especially occur in central and western Torres Strait and include very lush deep-water (more than 30 m) seagrass communities (Halophila ovalis and H. spinulosa) in south-western Torres Strait. Very little is known about the role of deep water, open-ocean seagrass communities

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