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Great Sandy Strait catchment story

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The Great Sandy Strait is a sand passage estuary with tidal openings to Hervey Bay and Wide Bay. It was listed as a Ramsar site in 1999 and is part of the Great Sandy Marine Park. The Strait is one of the largest, hydrologically diverse and connected passage estuaries in Australia and is enclosed by K’gari, the largest sand island in the world, and the Cooloola sand mass on the mainland. The area is important to Butchulla and Kabi Kabi people.

The catchment story for the Great Sandy Strait is available here (see transcript below). Please note that this catchment story will open as a map journal in a new tab, and you can come back to this map series to view the other catchment stories in the other tabs.

Main image. Network of sand bars and islands, looking north from the Moonboom Islands to Turkey Island and the Mary River, Photo by Gary Cranitch ©Queensland Museum.

Great Sandy Strait – Map Journal Transcript

The Great Sandy Strait is a 70 kilometre long sand passage estuary with tidal openings to Hervey Bay and Wide Bay. It was listed as a Ramsar site in 1999 and provides important habitat for an internationally significant numbers of shorebirds, together with a wide range of other fauna and flora. It is part of the Great Sandy Marine Park and Great Sandy Biosphere Reserve, and includes wetlands listed on the Directory of Important Wetlands of Australia, declared fish habitat areas, and the Hervey Bay—Tin Can Bay Dugong Protection Area. The Strait is one of the largest, hydrologically diverse and connected passage estuaries in Australia and is enclosed by K’gari, the largest sand island in the world, and the Cooloola sand mass on the mainland. The area is important to Butchulla and Kabi Kabi people.

Main image. Network of sand bars and islands, looking north from the Moonboom Islands to Turkey Island and the Mary River, Photo by Gary Cranitch ©Queensland Museum.

Table of contents

  1. Introduction
  2. Benthic depth
  3. Substrate consolidation and sediment texture
  4. Energy source and magnitude
  5. Structural macrobiota—flora
  6. Structural macrobiota—fauna
  7. First Nations peoples
  8. Great Sandy Strait Ramsar site
  9. Water quality
  10. Water flow

Introduction

Tidal inundation is a major characteristic of these ecosystems and this and other biophysical attributes of benthic habitats can be used to classify and map benthic habitats.

Subtidal ecosystems are always submerged due to tidal influence, whereas intertidal ecosystems are found between the high tide and low tide, experiencing fluctuating influences of land and sea.

Intertidal ecosystems are a dynamic complex of plant, animal and micro-organism communities and their non-living environment that interact as a functioning unit, and are exposed at low tides (e.g. mangroves or saltmarsh on muddy substrate).

High values of the systems have been recognised through the presence of Ramsar wetlands, marine park zoning, the Directory of Important Wetlands of Australia (DIWA), declared fish habitat areas (FHAs), dugong protection areas and the Great Sandy Biosphere Reserve, as discussed on the Overview tab of this map series.

Mapping of the benthic ecosystems has been undertaken for Central Queensland (CQ)* including the Great Sandy Strait. A full list of ecosystem types is available on WetlandInfo.**

Main image. Crested terns, Great Sandy Marine Park, Photo by Department of Environment and Science, Queensland Government.

See links at the end of this map journal for further information on the following references.

*WetlandMaps (Department of Environment and Science 2019)

**Intertidal and Subtidal Ecosystem Types of Central Queensland (Department of Environment and Science 2019)

Benthic depth

Benthic depth is the depth of the seafloor using the Australian Height Datum (AHD) and may also be referred to as absolute relief. Benthic depth influences exposure to air, light, temperature, pressure and wind/wave action. It is an indicator of sea floor shape, which in turn influences attributes such as energy and processes of erosion and deposition.

The Great Sandy Strait is mostly shallow (less than 10 metres) however there are several deeper areas.

Shallow areas provide habitat for a range of species, particularly those that photosynthesis (e.g. seagrass and corals). The central Strait is dominated by shallow sand banks and very narrow tidal channels, which influence flow. The mangrove forests and saltmarsh also restrict flow along the landward margin.

The deeper waters in the northern Strait are adjoining the tidal delta and Hervey Bay, in association with the Mary River paleochannel (i.e. the Mary River when sea levels were much lower). In the south, water flows through the broader Wide Bay Harbour and meets channels from Kauri Creek and the Tin Can Inlet. There is a deep hole in the Wide Bay Bar between the Inskip Peninsula and K'gari, which is where sand periodically falls into the sea.*

Video provided by Robbi Bishop-Taylor, Geoscience Australia. Taken from Google Earth Engine - see links at the end of this map journal for further information.

Main image. Shallow water of the Great Sandy Strait, near Dream Island, looking south, Photo by Gary Cranitch. ©Queensland Museum

Substrate consolidation and sediment texture

The Strait is dominated by unconsolidated sediments, mostly sand but also finer muds and coarser gravels, for example those derived from duricrusts. The finer sediments are mobile in high energy areas, but provide habitat for flora and fauna, particularly in lower energy areas.

There are also areas of consolidated substrates, such as the low energy rocky shores of Round, Woody and Little Woody islands and the mainland, and rock bars in some of the coastal streams. Consolidated substrates (reefs) provide habitat for a wide range of species, and are typically more biodiverse than unconsolidated substrates. They provide structural complexity, and support biota including hard and soft corals (e.g. mainly subtidal, either as a veneer on rocky reefs, or as coral reefs), macro-algae (e.g. sargassum seaweed), sponges, molluscs (e.g. oysters, mussels), barnacles, polychaete worms and tubeworms.

There are substrates of intermediate consolidation, such as coffee rock reefs and ledges. Originally coffee rocks were formed by humic acids percolating through sand and cementing sand particles. These structures are periodically covered and uncovered by the tidal re-working of the Cooloola and K'gari sand masses. Coffee rock reefs and ledges provide shelter for fish, and can provide structure for transient to permanent reefal garden (soft corals, sea fans, bryozoans, sponges etc.) depending on the rate of burial and exposure.

Main image. Intertidal gravel, sand and consolidated substrate, Maaroom, Photo by Maria Zann.

Energy source and magnitude

Water column energy plays an important role in the movement of substrates and geomorphological processes (e.g. erosion and accretion). It influences beach formation, estuary configuration (formed through interactions of wave, tide and river energy power), shaping geomorphological features of rocky cliffs, platforms and shores, coral reefs and lagoons.

Energy can also shape the patterns of biota and be correlated with sediment grain size. For example, wave energy from cyclones is capable of tossing boulders across the reef crest and cobble and pebble beaches are usually found in areas of relatively higher wave energy.

Energy source describes how the energy is derived (e.g. from tidal, riverine or wave/wind action), whereas energy magnitude describes the power of water movement. Tidal and riverine (episodic) energy are the main sources/drivers of water movement in the passage.

Tidal movements in the Strait are from the north and south, and it takes much longer for the tidal waters to move around the northern end of K'gari and through Hervey Bay than through the Wide Bay Bar. In the centre of the Strait, south of the Mary entrance, there is a 'dead' energy spot (amphidromic zone) where finer sediments fall out of suspension, and this has provided for the extensive mud islands from Turkey to Moonboom islands and the Sheridan Flats.

Large stretches of the Strait have moderate tidal currents. Currents are relatively high near the Wide Bay Bar and in a small area near south of the Mary River mouth, and relatively low in the central Strait (Turkey Island and Sheridan Flats to Moonboom Islands) and along the shoreline. *

Water current modelling, Photo by eReefs.**

The Mary River has a strong riverine influence over the northern parts of the Strait.

The Mary River is a near-permanent waterway that flows most of the year, except in drought situations. During a flood event, water from the Mary River substantially contributes to the tidal currents and a large flood event can prevent tidal water from entering the river. Freshwater floats on salt water and heavier saline tidal flow comes in under the freshwater surface flow as a salt wedge, with the salt wedge persisting for only a week or two after a flood event.

Most of the Great Sandy Strait has low to very low (or unknown) energy derived from wind/wave. The waters of the Wide Bay Bar have mostly high to very high energy, driven by wind and wave, and this energy extends along the seaward shore of K'gari and Cooloola.

Main image. Jetty on coffee rock, K'gari, Photo by Maria Zann.

*Not currently mapped as part of the intertidal and subtidal mapping for Central Queensland due to limited data.

**eReefs (2019) - see links at the end of this map journal for further information.

Structural macrobiota—flora

Structural macrobiota describes the composition of sessile (attached) habitat-forming species. Their presence increases spatial complexity, altering local environmental conditions that leads to colonisation by a diverse assemblage of organisms. The structuring role of structural macrobiota can include both three dimensional structure, provided by coral or macroalgae, and two-dimensional structure, provided by crust- or mat-forming biota (e.g. turfing algae and encrusting sponges).

Intertidal seagrass meadows, Poona, Photo by Maria Zann.

Structural macrobiota includes flora (plants) and fauna (animals). Flora provides most of the three dimensional structure intertidally, thereby influencing water flow and sediment transport. Subtidally, fauna provides most of the structure and influence on water and sediment.

Intertidal flora of the Great Sandy Strait ranges from:

  • trees and shrubs (casuarinas and some melaleucas), to
  • saltmarsh including grasses, herbs and sedges with bare areas of salt pan (with or without microphytobenthos),
  • mangrove forests dominated by Ceriops spp., Avicennia marina or Rhizophora spp. or mixed communities,
  • seagrass meadows* with strap (e.g. Zostera muelleri and Halophila uninervis) and ovoid (Halophila spp.) growth forms, and
  • macroalgae* with erect (e.g. Asparagopsis spp. and Lobophora spp.), turf and encrusting growth forms.

Leaf form of seagrasses are correlated to ecological patterns of seagrass meadows (e.g. depth, water clarity, sediment size / compaction) and to dugong food preferences.

Most of the mapped flora in the Strait is intertidal, however some seagrass and sparse macroalgae have been mapped subtidally.*

Main image. Mangrove islands and Sheridan Flats along the south-west shore of K'gari, looking south towards Dream and Stewart islands, Photo by Gary Cranitch ©Queensland Museum.

*Seagrass and/or macroalgae has been recorded in this location at least once but this location may not currently support these ecosystems, as both seagrass and macroalgae extent, abundance and diversity typically fluctuate seasonally and between years.

Structural macrobiota—fauna

Faunal structural biota often occur on consolidated substrates, but increasingly soft sediments are recognised as providing habitats for a high diversity of sessile macrofauna. Soft sediment communities often exist in combination where the dominant taxa interchange spatially (e.g. sponge, sea fan, soft coral gardens). Certain taxa usually considered mobile also include sessile forms, e.g. molluscs in the Strait, and siliquilariid (wormshell) reefs in nearby waters.

Structural macrobiota includes fauna (animals) and flora (plants). Fauna provides most of the three dimensional structure subtidally, thereby influencing water flow and sediment transport. Intertidally, flora provides most of the structure and influence on water and sediment.

Structural macrofauna of the Great Sandy Strait includes:

  • intertidal and subtidal hard corals,
  • reefal gardens (subtidal) (e.g. soft corals and/or sponges),
  • molluscs including intertidal oysters and other bivalves and gastropods, and
  • and a range of other invertebrates such as Octocorollians (Gorgonians, sea pens and sea whips), sponges, Ascidians, Crinoids, tube worms and barnacles, often in reefal garden communities.

The Strait also supports a wide range of mobile fauna (i.e. not structural habitat forming) such as worms, sea cucumber, sea stars, prawns, crabs, fishes, sea snakes, marine turtles, rays, sharks, dugongs, dolphins and whales.

Main image. Pelicans, cormorants, terns and other birds, Photo by Tourism and Events Queensland.

First Nations peoples

There is a strong connection to sea country as the Butchulla people are land and sea people. There are water spirits at special places, where Butchulla people won’t go.

Travelling across to K’gari was either by swimming, canoeing and walking. At very low tides the men walked neck-high while towing behind women and children in canoes.

The Great Sandy Strait provided for the people with its plentiful food resources in many ways. Yuloo (Buthoo) were our dolphin brothers who helped by driving fish into the fish traps. The diamond scale mullet was an important fish source, as were turtles and dugongs. Shellfish included oysters, bingus (mud oysters), mangrove whelks and periwinkles. There are important nursery areas for sharks, fish and prawns that provide food sources for people.

Big Woody Island (Tooliewah), Round Island (Weeandin) and Little Woody Island (Nalangoora) were significant places of occupation and water points on the way to K'gari.

Great Sandy Strait Ramsar site

The Convention on Wetlands of International Importance (more commonly referred to as the Ramsar Convention) was adopted in 1971 in the Iranian city of Ramsar. The Convention aims to halt the worldwide loss of wetlands and to conserve remaining wetlands through wise use and management. The Ramsar Convention encourages the designation of sites containing representative, rare or unique wetlands, or wetlands that are important for conserving biological diversity.

Crested terns, Photo by Jenna Tapply, Queensland Government.

Queensland has five Ramsar sites: Currawinya Lakes, Shoalwater and Corio Bays Area, Great Sandy Strait, Moreton Bay and Bowling Green Bay. The Great Sandy Strait was listed as Ramsar site in 1999, and covers approximately 930 square kilometres.

The Great Sandy Strait is a Ramsar site that represents a large near-natural passage system with near intact hydrological connectivity, supports a wide diversity of its wetland types and many different wildlife species. The site is particularly significant for its well-connected hydrology and its extensive near natural mosaic of freshwater, marine and estuarine habitats. At the interface of tropical and sub-tropical influences, it constitutes one of the most expansive, diverse and well-connected coastal wetland complexes along the entire east coast of Australia.

Enclosed by the K’gari (Fraser Island World Heritage Area) and Cooloola sand masses, the Great Sandy Strait’s diverse bathymetry also provides a diversity of hydrological connections. Tidal inflows from the north and south are constrained to rapid flows through deep channels, slowing in broad bays. Channels shallow and are progressively overtaken by sandbanks where the tides dissipate in gentle areas of deposition. Freshwater flows percolate through the groundwater of the sand systems, supplemented by episodic riverine flood flows from the Mary River and smaller estuaries. Floods from the Mary’s large agricultural catchment periodically discharge vast amounts of sediment, nutrients and freshwater over the Strait’s tidal ecosystems and those of Hervey Bay, often with catastrophic consequences for seagrass, dugong and corals.

Mangroves and saltmarshes stretch across large parts of the Ramsar site especially in the centre of the Strait. The mangrove forests are vital to the region’s coastline, particularly during cyclone activity, because they help control coastal erosion and protect the land from strong winds, tidal surges and heavy rainfall.

Seagrass meadows in the intertidal and subtidal areas of the Strait are important for fish, dugong, turtles and other biota. The Great Sandy Strait seagrass meadows support large populations of dugongs and turtles, help absorb nutrients and sediments, stabilise the estuarine banks, provide protection from wave impacts, and store high levels of carbon thereby influencing carbon dioxide levels in the atmosphere and climate change.

Rare wetland types include large examples of patterned fens, freshwater peat wetlands connected to and hydrologically constrained by estuarine tidal flows. In an otherwise sandy environment, consolidated ecosystems provided important refugia for a different suite of biota. Fringing coral reefs surrounding Woody and Round Island in the northern Strait are close to their southern range limits in eastern Australia. Unusual coffee rock ecosystems include rocky shores and subtidal reefal gardens with soft corals and sponges, providing a migratory staging posts for fish.

The Great Sandy Strait Ramsar site provides habitat for numerous wetland-dependent species including waterbirds, marine mammals including several whales and dolphins, marine reptiles including several endangered turtles and the estuarine crocodile, fish and invertebrate species at critical stages of their life cycles.

The Ramsar site supports a rich and abundant birdlife, including seasonally more than 20,000 waterbirds, of which most are migratory shorebirds. It provides vital habitat for a range of species, including some of the largest colonies of fish-feeding birds in eastern Queensland. More than 260 species of bird have been recorded at the site. The site is important for many migratory shorebirds to rest and feed here during their annual migration, including the threatened grey-tailed tattler, far eastern curlew, whimbrel and terek sandpiper.

There are more than 30 migratory species listed under the Conservation of Migratory Species (CMS/Bonn) Convention at the site. These species are also protected under various international agreements such as the Convention for Migratory Species, Japan-Australia Migratory Bird Agreement (JAMBA), China-Australia Migratory Bird Agreement (CAMBA) and/or Republic of Korea-Australia Migratory Bird Agreement (ROKAMBA).

The wetland complex supports passage for fish to access fresh and/or saline waters to meet the critical stages of their life cycles. The site has eight catadromous species including barramundi, two species of mullet (Mugil cephalus and Myxus petardi), longfinned eels, tarpon, jungle perch, Australian bass and bullrouts. These species need free passage to migrate down river to the sea to spawn. The site also has at least 14 amphidromous fish species including bull shark, king threadfin, gudgeons, gobies, fork-tailed catfish, glassfish, gar, milkfish and giant herring. Amphidromous fish move between freshwater and the sea (in both directions), but not for the purpose of breeding.

Main image. Seagrass of the Great Sandy Strait Ramsar site, looking towards Moon Point, Photo by Gary Cranitch ©Queensland Museum.

Water flow

Water flows across the landscape into the waterways of the catchments and the Great Sandy Strait (click for animation)*.

On land, the remaining water either sinks into the ground where it supports a variety of terrestrial and groundwater dependent ecosystems or is used for other purposes. Large volumes of groundwater flow to the Great Sandy Strait through the K'gari and Cooloola sand masses and and porous geologies in the Maryborough basin (Elliott Formation).

The Mary River is the major input of surface water to the Strait. It is a near-permanent waterway that flows at most times except in drought, but large flood events input large volumes of freshwater into the Strait. The Susan River discharges just upstream of the Mary River mouth.*

Pulgul Creek and the mainland catchments are shorter catchments with smaller catchment areas and smaller freshwater inputs. The areas to the south of the Mary River often include wallum streams (tannin-stained 'black water'). Several small creeks flow into the Strait from K'gari which are mostly groundwater-fed, however there is also surface water flow following rain.

Within the Strait itself, tidal waters enter from the north (Hervey Bay) and south (Wide bay). It takes much longer for the tides to go around the northern end of K'gari and through Hervey Bay, and there is a 'dead' energy spot (amphidromic zone) around the mud islands from Turkey to Moonboom islands and the Sheridan Flats.

The Mary River and subcatchments to the north are considered part of the GBR catchments.

Main image. Humpback whale, Photo by Tourism and Events Queensland.

*Mary Basin Draft Water Resource Plan : Environmental Conditions Report (includes Mary River, Burrum River and Beelbi Creek Catchments): Final Report (volume 2) (Department of Natural Resources and Mines, 2004) - see links at the end of this map journal for further information.

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Last updated: 23 July 2020

This page should be cited as:

Department of Environment and Science, Queensland (2020) Great Sandy Strait catchment story – Great Sandy Strait, WetlandInfo website, accessed 24 September 2020. Available at: https://wetlandinfo.des.qld.gov.au/wetlands/ecology/processes-systems/water/catchment-stories/transcript-great-sandy-strait-and-surrounding-catchments/transcript-great-sandy-strait.html

Queensland Government
WetlandInfo   —   Department of Environment and Science