Great Sandy Strait

Seagrass meadows provide a major marine habitat in the Great Sandy Strait. The meadows form part of significant Ramsar wetlands sites, are within the proposed Great Sandy Marine Park (Northern Section), and provide critical nursery habitat for regional prawn and finfish fisheries.

Seagrass distribution was first mapped in the Great Sandy Strait in July/December 1973¹. Seagrass was found south of the co-tidal line, which occurs at Moonboom Islands and within Tin Can Inlet. No seagrass was found north of Moonboom Islands, including Urangan. Aerial photographs and ground truthing at 25 locations, were used to map an area of seagrass covering >4,800 hectares (~5,232 hectares¹). There were six species of seagrass within the study area, although the total extent of the subtidal Halophila spinulosa meadows could not be estimated.

Lennon and Luck², estimated that the Great Sandy Strait had approximately 12,300 hectares of seagrass covering extensive intertidal and subtidal areas. This estimate is based on remote sensing analysis and may have overestimated the intertidal (confused with algae) and underestimated the subtidal (high turbidity) seagrass habitat.

In October-November 1992 an aerial photographic survey of the Strait was conducted and significant decreases were reported in Tin Can Inlet³. Increases in seagrass distribution however, were reported in the northern section of the Strait, between River Heads and Urangan, and Blackfellow's Point and Moon Point. Seagrass community changes were also reported, especially in the dense monspecific Cymodoea serrulata meadow off Kauri Creek, which changed to sparse C. serrulata subtidally and Z. muelleri ssp. capricorni intertidally.

In 1994, a broad scale survey of the Great Sandy Strait seagrass meadows was conducted (mainly by air) which reported an increase in distribution of meadows south of Urangan to River Heads compared with 1992 ⁴. In June 1994, long-term monitoring transects were established throughout the Great Sandy Strait. Resurveys were conducted in March 1995, November 1996, February 1998, September 1998 and February 1999. Large decreases in seagrass distribution were recorded in 1996 and recovery to February 1999 remained low⁵.

In December 1998 a detailed dive survey of the Great Sandy Strait was conducted which estimated 5,554 ±1,446 ha of seagrass habitat 6. Seven species of seagrass were present in the Great Sandy Strait (Zostera muelleri ssp. capricorni, Halodule uninervis, Halophila ovalis, Halophila decipiens, Halophila spinulosa, Cymodocea serrulata and Syringodium isoetifolium). Most of the meadows throughout the Great Sandy Strait were intertidal on large mud- and sand-banks, and were predominantly in the northern and central sections.

Subtidal meadows contributed to only 5% (256 ±105 ha) of the total seagrass distribution of the Great Sandy Strait. Subtidal meadows were mostly in the northern and southern sections of the Strait in narrow bands along the edge of intertidal banks, or extending across the large subtidal banks. Subtidal meadows were dominated by Halophila species (H. spinulosa, H decipiens, H. ovalis) or Z. muelleri ssp. capricorni. Algae were often mixed within the subtidal meadows with cover ranging between 5 and 40%.

Flooding of the Mary River and other tributaries in the Sandy Strait in February 1999 caused the complete loss of seagrass meadows in the northern Great Sandy Strait and loss of some other regions in the central and southern Sandy Strait region⁶.

In February 2002 the total area of seagrass throughout the Great Sandy Strait had recovered to 7007 ±1945 hectares^7,⁷. This was greater than the pre-flood survey conducted in December 1998.

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REGIONAL REPORT CARD (November 2017)

Seagrass meadows throughout the Great Sandy Strait support significant fisheries, turtle and dugong populations
Seagrass meadows in the Great Sandy Strait region were classified as fair in abundance in 2017.
Seagrass abundance in the Great Sandy Strait has fluctuated since 1999 (fair to good to poor and back to fair), but there is no apparent long-term trend.
Species composition has remained stable across the sites.
Macroalgae abundance is generally low, but seasonally increases in the middle of each year at most sites. Episodic algal blooms occurred in mid 2002.
Epiphyte blooms regularly occur at most sites in the mid-latter part of the year, with a dramatic decline in the summer months. Epiphyte abundance has continued to increase (not significantly) at some sites on the western shores of the Great Sandy Strait.
Sediment grain size has remained relatively stable of the monitoring period, with only a few sites becoming either more muddy or more sandy.
Seagrass-Watch data provides understanding of seasonal trends and effects of climatic patterns on seagrass meadows
Management controls depend on public and government support.