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Intertidal seagrass meadows dominated by the seagrasses with an ovoid growth, mostly Halophila ovalis but also: Halophila capricorni, Halophila decipiens and Halophila minor. These meadows can also support other growth forms, including Halophila spinulosa, Halophila tricostata, Zostera muelleri subsp. capricorni*, Cymodocea rotundata, Cymodocea serrulata, Enhalus acoroides, Halodule pinifolia, Halodule uninervis, Thalassia hemprichii, Thalassodendron ciliatum and Syringodium isoetifolium.
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 (mostly H. uninervis and H. ovalis), and many fisheries species (e.g. prawns and fishes)
- fisheries habitat (e.g. food, refuge and reproduction)
- coastal protection, erosion control and sediment capture
- tourism, recreation, education and research.
The fisheries value of seagrass habitat as nursery grounds for juvenile commercial fish and prawn species in Queensland is well documented. Sea cucumbers may also be collected from seagrass meadows 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.
Seagrass meadows, particularly those containing H. uninervis and H. ovalis, provide food for dugong and green sea turtles. Green turtles tend to graze the leaves of mostly H. ovalis. Halodule uninervis and H. ovalis are reported to be the most nutritious seagrasses due to high nitrogen and starch content. Grazing of H. uninervis and H. ovalis has been shown to increase production of a nitrogen-rich standing crop. Preferential grazing of H. ovalis can prevent the expansion of the often dominant Zostera muelleri and increase the abundance of the H. ovalis (i.e. cultivation grazing).
Species from the Halophila genus are early coloniser and often found in areas subject to disturbance, such as the receiving waters of runoff from built up areas. Colonising species typically have fast shoot turnover, short lifespan, fast sexual maturation and development of a (dormant) seed bank. They have low physiological resistance to disturbance but a rapid ability to recover.
Inundation 'Intertidal – Lower low', 'Intertidal – Mid low', 'Intertidal – Upper low', 'Intertidal – Low undifferentiated', 'Intertidal – Lower medium', 'Intertidal – Upper-medium', 'Intertidal – Medium undifferentiated', 'Intertidal – High', 'Intertidal – Undifferentiated', 'Intertidal – High undifferentiated', although usually occurring below mean sea level.
Structural macrobiota 'Seagrass – ovoid'
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.
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
- 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
- Halophila ovalis provides food for green sea turtles, and was observed in Port Curtis during recent seagrass surveys
- Intertidal meadows of ovoid seagrass in the Central Queensland mapped area are typically dominated by H. ovalis with some H. decipiens
- 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 may or may not represent current seagrass extent
- Seagrasses were mapped in Baffle Creek but their growth form has not been identified
- 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 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 negatively impact seagrass meadows.
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
Case study: Hervey Bay seagrass and dugong - Queensland Government
Saltmarshes, seagrasses and algae - Department of Agriculture and Fisheries
Seagrass-Watch: guides and manuals
Seagrasses in Queensland (pamphlet)
Seagrass - Queensland Government
Seagrasses - Australian Institute of Marine Science (AIMS)
A Vulnerability Assessment for the Great Barrier Reef - Great Barrier Reef Marine Park Authority
Seagrass Restoration Network
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Last updated: 22 July 2019
This page should be cited as:
Department of Environment and Science, Queensland (2019) Intertidal ovoid seagrass, WetlandInfo website, accessed 1 July 2022. Available at: https://wetlandinfo.des.qld.gov.au/wetlands/ecology/aquatic-ecosystems-natural/estuarine-marine/descriptions/11/