Ovoid seagrass in shallow water
Shallow subtidal seagrass meadows dominated by seagrasses with an ovoid growth, mostly Halophila ovalis.
Disclaimer: Ecosystem type descriptions are based on biophysical attributes identified in Central Queensland through expert advice and supported by scientific literature. Not all ecosystem types are mapped based on current inventory, and many of the ecosystems described here may also occur in other parts of Queensland.
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Shallow (less than 10 metres) subtidal seagrass meadows dominated by 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:
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. Dugongs feed mostly on the roots of mostly H. uninervis whereas 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.
Halophila spp. is an early coloniser and often found in areas subject to disturbance, such as the receiving waters of runoff from built up areas along the Queensland coastline. 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.
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).
Structural macrobiota 'Ovoid seagrass'
Benthic depth 'Shallow (<10m)'
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 (SeagrassWatch).
Most tropical and sub-tropical species are found in water less than 10 metres deep. 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 including 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:
Other relevant attributes include Water clarity, Sediment texture, and Trace elements, together with Energy magnitude, Energy source (wave) and Freshwater volume however deeper seagrass tend to be less influenced by energy and freshwater except in a severe storm. 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. Shallow subtidal meadows are generally less susceptible to air and surface temperature than intertidal seagrasses, but more susceptible than deeper seagrasses.
Mapping represents locations where 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
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
Last updated: 22 July 2019
This page should be cited as:
Department of Environment and Science, Queensland (2019) Ovoid seagrass in shallow water, WetlandInfo website, accessed 1 February 2023. Available at: https://wetlandinfo.des.qld.gov.au/wetlands/ecology/aquatic-ecosystems-natural/estuarine-marine/descriptions/45/