Kimberleys

The Kimberley region of Western Australia extends from the border with the Northern Territory in the north to Sandy Point (Roebuck Bay) in the south. The township of Broome (population 14,000, but grows to 30,000 during the tourist season) is one of the largest population centres in the region. The Kimberley coast displays wide variation with embayments and sounds with wide sandy beaches which give way to mudflats. Mangrove inlets and tidal creeks are interspersed with coastal cliffs. There are numerous offshore islands and much of the coast remains uninhabited. The coast is prone to large tidal variation from <1 to 11m and these strong tidal flows dramatically influence the coastal environment.

Western Australia has the highest diversity of seagrasses in the world, with 25 species represented. These are generally divided into temperate and tropical distributions, with Shark Bay representing the biogeographical overlap. 12 species are represented in the tropics (Thalassia hemprichii, Thalassodendron ciliatum, Enhalus acoroides, Halophila ovalis, Halodule uninervis, Halophila minor, Cymodocea angustata, Syringodium isoetifolium, Cymodocea serrulata, Halophila spinulosa, Halodule pinifolia and Halophila decipiens), one of which is endemic (Cymodocea angustata). Seagrasses are a significantly component of the marine ecosystems and their contribution to the total primary carbon production is critical to regionally important dugong and turtle populations.

Seagrass distribution throughout the region is most likely influenced by shelter, sediment characteristics, water turbidity and tidal exposure. The Kimberley coast region of Western Australia has both arid and wet tropical environments (annual average rainfall <200 mm and >1000 mm respectively). The marine environment is influenced by the warm, south-equatorial current that flows from the east through the Asian and northern Australian region.

Seagrass meadows are mostly found in the sheltered bays along the southern mainland coast of the Kimberley region. Extensive terracing of these expanses of the intertidal zone often result in seagrass high in the intertidal. The majority of the meadows are low - moderate in abundance, and are dominated by Halophila and Halodule species. Seagrasses either occur sparsely in coral reef environments or can attain high biomasses on mudbanks or within high intertidal lagoons, where water is ponded during the falling tide. The environments are otherwise too extreme (tidal movement/turbidity/freshwater runoff in the wet season) for seagrass survival. Subtidal populations of seagrasses are poorly known, but it appears that the northern Kimberley does not have the seagrass richness recorded for the southern Kimberley.

Roebuck Bay

Monitoring: Ongoing, quarterly

Principal watchers:Fiona Bishop, Kirsten Pearce, Danielle Bain, Clare Morton, Grant Morton, Julie Western, Miranda Dibdin, David Trudgen, Wendy Trudgen
Occasional and past watchers: Environs Kimberley, Seagrass-Watch HQ
Location: Town Beach, Demco and Port
Site codes: RO1, RO2, RO3

RO1 position: S17.97671 E122.23855 (heading 160 degrees)
RO2 position: S17.98062 E122.23173 (heading 150 degrees)
RO3 position: S17.99672 E122.21418 (heading 120 degrees)
Best tides: <0.6m (port Broome 62650)
Issues: Urban runoff
Comments: Roebuck Bay is a tropical marine embayment with extensive, highly biologically diverse, intertidal mudflats. The Bay is bounded to the north-west by the township of Broome (population ca. 13,500 in 2001) and extends to Sandy Point in the south. Declared a Ramsar site, it is internationally important for at least 20 species of migratory shorebirds and one of the most important sites for shorebird conservation in the East Asian-Australasian Flyway in Australia and globally. Dugongs (Dugong dugon) and Green turtles (Chelonia mydas) regularly use the bay as a feeding area and as a transit area on migration. The Bay is also a major nursery area for marine fishes and crustaceans, and supports an exceptionally high biomass and diversity of benthic invertebrates (estimated to be between 300 – 500 species), placing it amongst the most diverse mudflats known in the world (de Goeij et al. 2003).

Roebuck Bay has a very large tidal range which exposes around 160 km2 of mudflat, approximately 45% of the total bay area, with tides traveling at up to 20cm/sec mid cycle (Hickey et al. 1998; Piersma et al. 2002). Most of the mudflat area is inundated by each high tide and at times, spring tides and/or cyclones may cause the adjoining coastal flats to become inundated. The tidal system is semi-diurnal with an average tidal amplitude of 5.7m. Tidal range varies from c. 1 m on neap tides to 10.5 m on the highest spring tides. These factors dominate the intertidal ecology.

Extensive seagrass meadows occur in the northern regions of Roebuck Bay, particularly in the Town Beach area, and are dominated by Halophila ovalis and Halodule uninervis (Prince 1986). The most vigorous stands of seagrass grow in areas that are exposed for less than two hours at low tide. Halophila minor occurs sparsely by itself, often in pools which remain in the high intertidal during low tides, or with some H. uninervis. (Prince 1986). Halodule pinifolia has also been reported from northern Roebuck Bay, but mixed with other species (Walker and Prince 1987).

A survey of dugongs in the Kimberley, conducted by the Department of Conservation and Land Management in 1984 (Prince 1986), estimated the population in Roebuck Bay at 50 - 100 individuals. Current population levels are unknown.

Status (Dec10):

seagrass abundance relative to the seagrass guidelines for Queensland (insufficient data to develop local guidelines) indicates that the seagrass meadows at Roebuck Bay are in a FAIR condition (RO1 = good, RO2 = good, RO3 = poor).
seagrass abundance appears significantly higher in 2008 compared to 2007, with the exception of RO1 when the highest abundances reported in December 2007 since sampling established in 2007.
Abundances in 2009 at RO1 and RO2 appear similar or higher than previous years.
Seagrass abundance at RO3 in 2009 significantly lower than 2008, but not significantly different from 2007.
seagrass abundance appears higher in late Dry to early Monsoon (November/December) and lower in late Monsoon to early Dry (April-June).
insufficient sampling events to derive seagrass abundance indicators

all sites dominated by Halodule uninervis with variable composition of Halophila ovalis

Dampier Peninsula

Monitoring: proposed

Principal watchers:Bardi Jawi Land and Sea Rangers (Daniel, Shawn, Mark and Trevor)
Occasional and past watchers: Seagrass-Watch HQ
Location: Chille Creek and One Arm Creek
Site codes: OA1

OA1 position: S16.43804 E123.06846 (heading 30 degrees)
Chille Ck position: S16.51832 E122.86389
Best tides: <4m Karakatta Bay (port 62750)

Issues: none identified
Comments: Dampier Peninsula includes the area of coast north of Broome and includes King Sound. King Sound encompasses the Fitzroy River estuary and is the receiving basin for the Fitzroy River. This region is macro-tidal with low wave energy. There are extensive tidal flats subject to extreme variations in turbidity and tide fluctuations throughout the area. There are also numerous islands in the region. The northern reaches of the sound includes the Buccaneer Archipelago. The region is an important area for dugongs, which have been reported from One Arm Point in the King Sound since 1688 (Adam 2003; Marsh 1991).

The most diverse seagrass meadows in the Kimberley region have been reported on the reef platforms in the One Arm Point – Sunday Island area. Seagrass meadows are located in the shallow waters of the perched fringing coral-reefs. The tidal range is extreme in this location (11m), and during low tides, water is trapped (ponded) on fringing reef flats, while water is several metres below the reef crest on the seaward side. The location with the highest biodiversity of seagrasses was around One Arm Point, where ten species were reported (Thalassia hemprichii, Thalassodendron ciliatum, Enhalus acoroides, Halophila ovalis, Halodule uninervis, Halophila minor, Cymodocea serrulata, Cymodocea angustata, Syringodium isoetifolium and Halodule pinifolia) (Walker and Prince 1987). The majority of the meadows are low - moderate in abundance, and are dominated by Thalassia hemprichii with Halophila ovalis, Halodule uninervis and Enhalus acoroides. Dugongs and turtles are often reported feeding on these meadows.

Located south of Cape Leveque, the Chille Creek seascape is also significantly influenced by the high tidal range (9.6m). Patches of Enhalus acoroides with Halophila ovalis and Halodule uninervis are founds scattered amongst the large tidal pools. Dugongs have been reported to visit these meadows over time.

It is unknown if the seagrasses of the Dampier Peninsula have changed significantly since the 1980’s. In an attempt to provide a better understanding of the status of seagrass meadows and how they change seasonally, Seagrass-Watch monitoring sites are planned to be established in the region by the Kimberley Land Council - Land & Sea Unit in partnership with the Bardi Jawi people.

Status (Apr10):

only 1 site established (no replication due to size of meadow)
site only contains Thalassia hemprichii all sites dominated by Halodule uninervis with variable composition of Halophila ovalis
as there is insufficient data available, it is not possible to give an accurate assessment of the current status.