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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 West, Ayesha Moss, Carla Eisemberg, Chris Nutt, Connie Grohmann, Dianne Bennett, Kylie Weatherall, Gary Lienert, Heather Beswick, Jon Hall, Juanet West, Julia Rau, Kandy Curran, Kevin Smith, Neil Hamaguchi, Kent Dequito, Liz Kent, Malcolm Lindsay, Michelle Teoh, Pat Lowe , Rose Barker, Stacey Newton , Tessa Mossop

Occasional and past watchers:Adrian Boyle, Alessandro Nicoletti , Alex Watson, Alistrair Sherwin, Ana Dalitz, Anthony Aris, Anthony Richardson, Bel Catchside, Ben Ansell, Brendan Smith, Caterina Carson, Cath Ralston, Catherine Bishop, Catherine Cochrane, Catriona Webster, Chris Howe Ping, Chris Sampi, Christine Elsasser, Clare Morton, Claudia Curran, Corey Thorne, Craig Hamaguchi, Curtis Robinson, Dane Freeman, Danielle Bain, Darren Stevens, David Trudgen, Deanne Bird , Dominic Yanawana, Dwayne George, Elizabeth Malone, Emily Burke, Emma Ellis, Erica Shedley, Fiona Galloway, Frankie O'Conner, Franky O'Connor, Gay Marsden, Georgia Wheeler, George Bishop, Gerard Bennett, Grant Morton, Hannah Beadle, Hannah Curran, Howard Pedersen, Jackie Cullen, Jacqueline McKenzie, Jaime Jackett, Jane Lawton, Janine Bedros, Janine Furtado, Jason Fong, Jason Robertson, Jason Simmons, Jeanette Hunt, Jeanie Govan, Jenna Cowie , Jenny Costigan, Jessica Bangu, Jessica Koleck, Jill Duncan, Jo Jones , Joey Munro, John Curran, Josh Coates, Jeremy Cussan, John Graff , John Hopiga, John Sawyer , Joy Pyrchl, Judith Howell , Julia Noakes, Julie Western, Kate Golson, Karen Gorman, Kenneth Callaghan, Kevin George, Kirsten Pearce, Lauren Hutton, Lauren Johnson, Leong Teoh, Linda Danne, Lindsay Sawyer, Louise Beames , Louise Mullin, Luke Halling, Luke Mischker, Luke Puertollano, Mandy Soynonoff, Mark Warren, Martin Pritchard, Maurice Connor, Melissa Williams , Michelle Haage, Michelle Smith, Miranda Curran, Miranda Dibdin, Mizuyo Ichinohe, Nancy VozoffJo Jones, Naomi Findlay, Nigel Jackett, Pam Jennings, Pat Foley, Patrick Kitcchner, Peter Danne, Phillip Matsumoto, Philippa Girgiw, Querida Hutchinson, Rachael Leamy, Rebeka Wilson, S. Fong, Sam Tonkin, Sarah McMillan, Seth Lovell, Seth Roberts, Sharon Ferguson, Stephen Russell, Steve Reynolds, Tameeka Dodd , Taro Bin Amat, Teressa Coutts, Theo Dann, Thomas Aaron, Thomas King, Tim Crawford, Tim Malone, Todd Quartermaine, Tomoki Mishina, Troy Sinclair, Trudy Classens, Vanessa McGuinnes, Vanessa Rippon, Virginia Westwood, Viv de Boer, Volker Mischker, Wade Freeman, Wendy Trudgen, William Miller, Zenaida Cayaon, Seagrass-Watch HQ

Location: Town Beach, Demco and Port

Site codes: RO1, RO2, RO3, RO4

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 (November 2018):

  • seagrass abundance (% cover) declined from 2017 to 2018, but remained in a good state (grade A) in 2018
  • seagrass abundance follows a unimodal pattern of growth annually, with higher abundances in late-dry to early-monsoon (October-December) and lower in late-monsoon to dry (April-July) seasons of each year
  • seagrass abundance appears to be primarily driven by environmental factors which modify the interactive effect of sea water temperature (over preceding 2 weeks) and light availability. A dominate negative influence on available light appears to be runoff from seasonal rainfall in the preceding 3 months.
  • although seagrass abundance fluctuated between years, there were no detectable long-term trends at any site
  • all sites dominated by Halodule uninervis with variable composition of Halophila ovalis
  • macroalgae fluctuates both within and between years, but remains low
  • epiphyte abundance is similar across sites and generally greater in late-dry/early monsoon of each year
  • a persistent seed bank of the foundation seagrass exists at all sites, with higher densities in the wet season (December to May)
  • RO3 may be more vulnerable to large scale disturbances as its ability to resist is lower due to lower seagrass abundance

A pilot report card was developed in 2017 by Seagrass-Watch HQ using two indicators of seagrass condition, based on their significance to seagrass resilience: abundance, which represents the state of the seagrass to resist stressors; and seed banks, which represents the capacity of the seagrass to recover from loss/disturbance. The two indicators are given a health score, which is then converted to an annual grade (good, fair, poor and very poor) for easier communication (for more details please see the detailed science report, McKenzie et al. 2017). The abundance indicator relies on a baseline of 3–5 years of minimal disturbance, whereas the seedbank indicator is compared to a baseline of the long-term mean (2012–2016). These two indicators are then combined to give an overall condition grade.

The report card represents the annual relative health of seagrass in Roebuck Bay, and shows that seagrass condition has fluctuated over the 12 years of monitoring. Seagrass condition was in a fair state when monitoring was established in 2007, but improved to 'good' (A grade) the following year and remained 'good' until 2014. In 2015 when there was lower abundance and fewer seeds, seagrass condition declined to 'fair', but in 2016 seagrass recovered back to 'good' health (A grade) with increased abundance and seed banks. Although seagrass abundance declined during 2018, the condition remained good. Good seagrass abundance coupled with a large persistent seed banks suggests the seagrass in northern Roebuck Bay remain resilient, with a high ability to resist or recover from future disturbances.

Seagrass condition report card for Roebuck Bay: the first figure shows the annual trend in the two indicators, abundance and seedbank; the second figure shows the annual index (combined indicator scores). Grade A / Green = 'good' seagrass health, to Grade D / Red = 'very poor'. For more detail of report card scoring see McKenzie et al. 2017.

Seagrass abundance is seasonal, being most abundant in late-dry to early monsoon season (October-December) and lower in late-monsoon to dry (April-July). All sites have fluctuated between years, with some consistent changes amongst sites, especially between Town Beach and Demco. The Port site is generally lower abundance of seagrass and seasonality was less pronounced. The changes in abundance over the years appears to be driven primarily by environmental factors that alter seawater temperature and light availability. Seagrass abundance is reduced by high rainfall in the preceding 3 months; it decreased during the wetter-than-average 2011–2014, and increased during the drier 2015–2016. This appears a consequence of increased run-off causing turbid/less clear water and therefore less light for seagrass growth.

Two species of seagrass (Halodule uninervis and Halophila ovalis) were confirmed from Roebuck Bay, with a further two species (Halodule pinifolia and Halophila minor) under taxonomic review. The opportunistic foundational species Halodule uninervis dominates the meadows in both abundance and canopy height. The colonising species Halophila ovalis fluctuated seasonally in composition, declining in the wet season (monsoon and late-monsoon).

The abundance of macro-algae fluctuated both within and between years but has remained low at all sites over the monitoring period. Macroalgae generally consists of Halimeda species (e.g. Halimeda opuntia). Although macroalgal abundance has declined at RO3 since monitoring was established, no long-term trend is apparent at the other sites.

Epiphyte abundance is similar across all sites, being significantly higher in 2007–2009 and 2013–2014, and generally greater in late-dry/early monsoon of each year, most likely owing to increasing daylight paired with moderate temperatures.

Seagrass are flowering plants which produce fruits and seeds. Some seagrass seeds persist in seabed sediments for several years, forming a bank from which meadows can recover when plants are lost or disturbed. A persistent seed bank of the foundation seagrass Halodule uninervis exists throughout the meadows of northern Roebuck Bay. Although highly variable, densities appear higher in the wet season (December to May) after the main seagrass growth period (September to December). Analysis suggests that meadows may require only a moderate seed bank for recovery capacity, and that greater sized seed banks provide greater probability of recovery (i.e. greater probability of viable seeds) but not necessarily greater abundance.


 

 

 

Dampier Peninsula

 

Monitoring: suspended

Occasional and past watchers: Todd Quartermaine, Trevor Sampi, Chris Sampi, Nathan Sampi, Kevin George, Dwayne George, Phillip McCarthy, Terry McCarthy, Mark Shadforth (Bardi Jawi Land and Sea Rangers) and 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 (Mar13):

  • only 1 site established (no replication due to size of meadow)
  • site only contains Thalassia hemprichii
  • abundances have declined since 2012 and are currently in a poor state (the lowest since monitoring was established)

 

 

 

 
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