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Subtidal seagrass meadows dominated by wide (broad) strap growth forms, particularly Zostera muelleri subsp. capricorni*. These meadows can also include other seagrasses with wide strap growth forms such as Cymodocea rotundata, Cymodocea serrulata, Enhalus acoroides, Halodule uninervis and Thalassia hemprichii, together with other growth forms such Halophila spp., Syringodium isoetifolium and Thalassodendron ciliatum. These meadows can also include the H. uninervis wide growth form where data is available.
Seagrasses are not a taxonomically unified group, but rather an ecological group that arose through convergent evolution and includes several different families. They are all flowering plants that live underwater and need light to photosynthesise. They also produce seeds. They grow on muds, sands and fine gravels which may be mobile. Meadows may include other structural macrobiota such as encrusting algae, erect macrophyte algae, bryozoans, sponges and molluscs (e.g. bivalves, cockles, whelks, razor clam beds), together with mobile invertebrate fauna such as sea cucumbers, crabs (e.g. commercial sand crabs and other portunids) and polychaete worms.
*Revision of Zostera capricorni has resulted in classification to subspecies. In Queensland, Zostera capricorni has been revised to Zostera muelleri subsp. capricorni.
Seagrasses provide a wide range of services, including:
- primary production, carbon fixation and nutrient removal
- support numerous herbivore- and detritivore-based food webs, including food for dugongs and green turtles, and many fisheries species (e.g. prawns and fishes) and other structural macrobiota (e.g. shellfish reefs of razor clams)
- fisheries habitat (e.g. food, refuge and reproduction)
- coastal protection, erosion control and sediment capture and stabilisation
- tourism, recreation, education and research.
The fisheries value of seagrass habitat as nursery grounds for juvenile commercial fish, prawn, crab and crayfish species in Queensland is well documented. Sea cucumbers may also be collected for commercial aquaculture. Not only do seagrass provide habitat for fish, but the proximity of seagrass meadows to other ecosystems (mangroves, coral reefs) increases their abundance in these ecosystems.
Structural macrobiota 'Seagrass - strap broad (wide)'
Seagrass ecosystems vary in Period and Trend (seasonally and from year to year). The species composition, extent and biomass of seagrass meadows can vary seasonally and between years. The extent and biomass of seagrass meadows along the Queensland east coast are typically maximal in late spring and summer, and minimal over winter.
Seagrass meadows grow throughout the world’s coastal waters, with large areas along Queensland’s coastline. Approximately 58 species of seagrass have been recorded across the globe with about 30 recorded in Australian waters and at least 15 in Queensland.
Most tropical and sub-tropical species are found in water less than 10 metres deep and wide strap seagrasses are typically found in shallow water and the intertidal area (coastal areas and within estuaries). Of the 13 species identified in north-eastern Queensland, all occurred in water depths less than 6 metres below mean sea level (MSL) and only four occurred in water more than 20 metres below MSL. Three general depth zones of seagrass species composition for tropical waters have been observed a shallow zone less than 6 metres deep with high species diversity, likely to include all species found in a region; a zone between 6 and 11 metres where the most commonly found seagrasses were the pioneering Halodule and Halophila species; and a zone deeper than 11 metres where only species of the genus Halophila were commonly found.
Seagrasses form dynamic communities of mixed species. Enhalus acoroides, T. hemprichii and T. ciliatum are considered persistent whereas Cymodocea spp. and S. isoetifolium are considered opportunistic, Z. muelleri and H. uninervis are considered colonising to opportunistic. Wide strap seagrasses are typically more enduring than early colonising species of Halophila spp., with Zostera spp. forming both transitory and enduring meadows. Transitory meadows increase and decrease in extent and biomass seasonally, and can re-establish following complete loss through sexual reproduction. Enduring meadows may fluctuate but generally remain to some extent through seasons and years.
The following relates to distribution of this ecosystem type within the Central Queensland mapping area:
- Seagrass mapping represents the maximum known extent of that ecosystem since 2000 and details concerning period and trend of each meadow are documented within the structural macrobiota attribute dataset.
- Subtidal meadows of wide strap seagrass occur in the nearshore area and are typically dominated by Z. muelleri but some areas are dominated by C. serrulata. Extensive seagrass meadows in Port Curtis are the only described large area of seagrass between Hervey Bay and Shoalwater Bay, and are therefore very important regionally and regularly mapped and monitored. Hervey Bay seagrass meadows are highly significant for their dugong and turtle populations. They experience flood impacts from the Mary River but the Booral seagrass meadows recover more rapidly. Estuarine seagrass (potentially Zostera spp.) is also present in Baffle Creek both intertidally and shallow subtidally.
- Spatial Inventory of seagrass in Hervey Bay, its tributaries/estuaries and the Great Sandy Strait was last updated in 2003 and seagrass meadows may have changed in extent since it was last mapped. Some estuarine seagrasses (excluding Baffle Creek) were mapped only once at a very broad scale 1984-88 and may either be absent or not represent current seagrass extent.
- Thalassia hemprichii does not occur in Central Queensland.
Other relevant attributes include Water clarity, Energy magnitude and Energy source (wave) together with Sediment texture, Freshwater volume and Trace elements. Seagrasses need light to be able to photosynthesise and turbid water inhibits light penetration, thus the Benthic depth that light can penetrate is a major control. Severe storms (cyclones), and/or high rainfall, river discharge and the associated low Water clarity and high concentrations of nutrients and other potential contaminants, and sediment deposition leads to seagrass loss (see SeagrassWatch annual reports for inshore seagrass monitoring in the Great Barrier Reef Marine Park. Trace elements (nutrients N, P), herbicides and other contaminants are known to affect seagrass health and other Structural macrobiota. This includes epiphytic algae and macroalgae which are indicators of high nutrients.
Water temperature is also relevant. Marine heatwaves can also negatively impact seagrass meadows. Zostera spp. are particularly susceptible to temperature stress. The optimal temperature for primary production of Zostera is 24 degrees Celsius, with deficient primary production (i.e. energy loss) above 35 degrees Celsius, compared to 40 degrees Celsius for deficient primary production in H. uninervis and C. serrulata. Deeper subtidal meadows are generally less susceptible to air and surface temperature than shallow subtidal and intertidal seagrasses.
Mapping represents locations that seagrass has been recorded at some point in time, and therefore locations where seagrass may grow now or in the future providing environmental conditions are suitable (e.g. wave action, Water clarity, sediment stability, temperature and the presence of nutrients and other potential contaminants).
Seagrass - Queensland Government
Seagrass (Case study: Hervey Bay seagrass and dugong) - Queensland Government
Saltmarshes, seagrasses and algae - Department of Agriculture and Fisheries
Seagrasses in Queensland (pamphlet)
Seagrass - Department of Environment and Science
Seagrasses - Australian Institute of Marine Science
A Vulnerability Assessment for the Great Barrier Reef (Seagrass) - Great Barrier Reef Marine Park Authority
Seagrass Restoration Network
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Last updated: 11 July 2019
This page should be cited as:
Department of Environment and Science, Queensland (2019) Subtidal wide strap seagrass, WetlandInfo website, accessed 13 April 2023. Available at: https://wetlandinfo.des.qld.gov.au/wetlands/ecology/aquatic-ecosystems-natural/estuarine-marine/descriptions/48/