Northern Territory

Local First Nations peoples hold much of the biological knowledge of Arnhem Land. Very little marine research has been conducted and consequently there is very little known about the distribution and abundance of marine flora and fauna, including seagrasses along the Arnhem Land coastline^3,4. Much of the information has been based on anecdotal evidence from the 1970s^5,6, surveys from fixed wing aircraft in the 1980s⁷, and survey of intertidal seagrass meadows by helicopter in November 2004⁸.

Seagrass has been reported in the past from Nhulunbuy (Halophila decipiens) and along the north coast of the Tiwi Islands. Ten seagrass species have been reported from the waters surrounding Arnhem Land^7,8: Oceana serrulata, Cymodocea rotundata, Enhalus acoroides, Halodule uninervis, Halophila decipiens, Halophila ovalis, Halophila spinulosa, Syringodium isoetifolium, Thalassia hemprichii and Thalassodendron ciliatum. H. uninervis and H. ovalis are by far the most common on the open sand and mud flats. T. hemprichii and T. ciliatum are often found on reef platforms and around rocky islands. E. acoroides often occurs in sheltered bays.

Goulburn Islands – Castlereagh Bay
Roelofs et al. (2005)⁸ mapped 6,694ha of intertidal seagrass meadows in this region. Approximately half of the meadows mapped were classified as aggregated patches. Intertidal seagrass communities in this region were concentrated around the Goulburn Islands, Maningrida and Milingimbi⁸.

Six seagrass species have been identified from this region: Enhalus acoroides, Halodule uninervis, Halophila decipiens, Halophila ovalis, Thalassia hemprichii and Thalassodendron ciliatum.

The Goulburn Islands seagrasses were typical of tropical reefal seagrass communities and were comprised of Thalassia, Thalassodendron and Enhalus species⁸. The sheltered regions near the Maningrida and Milingimbi communities supported extensive Halophila dominated meadows. Isolated and aggregated seagrass patches were the dominant cover type for this region⁸. The large open bays in the region (Boucuat, Castlereagh, Buckingham and Arnhem Bays) were almost devoid of intertidal seagrass meadows, with only small isolated meadows found in Castlereagh and Boucaut Bays⁸.

Melville Bay
Melville Bay is a large embayment adjacent to Nhulunbuy (pop. 3500), the second largest population in the Northern Territory⁹. Melville Bay is approximately 170 km2 in area, and fringed by mangroves, with several coarse sandy beaches and some rocky shores.

The substrate over most of the Bay is fine mud and silts, typically anoxic, and seagrasses are present but patchy in distribution^10,1 . Seagrasses are largely Halophila spp. although several other species are reported¹¹.

Western Gulf of Carpentaria (Nhulunbuy to Roper River)
Two broad scale surveys have been conducted of the western Gulf of Carpentaria coast. This is a complex coastline with few river inputs, and is consequently less muddy than the southern Gulf. In November 1983⁷ mapped 440km2 of seagrass meadows between Limmen Bight River and Gove. Significant areas of seagrass occured at Groote Eylandt⁷. Roelofs et al.(2005)⁸ mapped approximately 59km2 of intertidal meadows along the mainland coast. The differences between the surveys are possibly a result of different survey methods used and/or because of physical damage by tropical storms and cyclones for which the region is prone.

Eight seagrass species have been identified in this region: Oceana serrulata, Cymodocea rotundata, Enhalus acoroides, Halodule uninervis, Halophila ovalis, Halophila spinulosa, Syringodium isoetifolium, and Thalassia hemprichii^7,8.

Open-coastline communities were the major intertidal meadow types mapped in this region and they were dominated by monospecific stands of H. ovalis and H. uninervis intertidally, and O. serrulata and S. isoetifolium subtidally^7,8. Mixed-species meadows of O. serrulata, T. hemprichii and S. isoetifolium occurred on reef flats, some small sheltered embayments were dominated by E. acoroides, and H. ovalis and H. spinulosa sometimes occurred in river-mouth areas⁷.

In areas surveyed in 2004, seagrass communities were concentrated in the Caledon and Blue Mud Bay regions in the north and Numbulwar in the south⁸. Most meadows with aggregated seagrass patches and they supported abundant dugong and turtle populations, particularly in Caledon Bay and from Limmen Bight to the Sir Edward Pellew Island Group⁸.

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