Ovoid seagrass in deep water

Long description

Deep (greater than 10 metres and potentially to 35m) subtidal seagrass meadows dominated by seagrasses with an ovoid growth, mostly Halophila ovalis but also Halophila capricorni, Halophila decipiens (fern-like, see type 50) and Halophila minor. These species have high tolerance to low light in deep water but growth is intermittent, recurrence in deeper water, and patchy spatially and temporally. They are able to grow in lower light conditions due to their lower biomass and associated lower respiratory and photosynthetic demands[20].

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.

Special values

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[11][18][8].

The fisheries value of seagrass habitat as nursery grounds for juvenile commercial fish and prawn species in Queensland is well documented[15][19]. 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[6][13].

Seagrass meadows, particularly those containing H. uninervis and H. ovalis, provide food for dugong[18] and green sea turtles[9]. Dugongs feed mostly on the roots of mostly H. uninervis whereas green turtles tend to graze the leaves of mostly H. ovalis. Halodule uninervis[18] and H. ovalis[14] 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[14][1][9].

Deep water seagrasses provide feeding habitat for dugong and potentially a refuge if nearshore seagrasses are impacted by runoff etc. In Western Australia, deep water meadows provide  dugong habitat with evidence of long deep dives to feed on rhizomes with high nutrient content. Long, deep dives may be more costly in time and energy than the shorter dives characteristic of feeding in shallow water. Potentially deep water seagrasses provide a refugia for dugong feeding when shallow nearshore seagrasses are impacted by flood runoff[18].

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)[14].

Diagnostic attributes

Inundation 'Subtidal'

Structural macrobiota 'Ovoid seagrass'

Benthic depth 'Deep (10-30m)'

Note that seagrasses can grow to 35m depth in clear waters (McKenzie pers. Comm.)

Qualifiers

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 and deepwater seagrasses are highly intermittent (subject to water clarity). The extent and biomass of seagrass meadows along the Queensland east coast are typically maximal in late spring and summer, and minimal over winter[2][5][17].

Distribution

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).

Halophila spp. are early colonisers 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.

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
• Baffle Creek intertidal and subtidal seagrasses mapped in 2007 were unable to be allocated to a leaf growth form[12].
• Very large areas of this ecosystem mapped in Central Queensland. 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[16][5]
• Seagrass meadows, particularly those containing H. uninervis and H. ovalis, provide food for dugong[18] and green sea turtles[9]. Both of these species were observed in Port Curtis during recent seagrass surveys[16]. Dugongs travelled along the deeper sea floor feeding on seagrass during long-distance movements from Burrum Heads to Breaksea Spit in winter, only coming to the surface to feed[18]
• 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[10] and may either be absent or not represent current seagrass extent
• Thalassia hemprichii does not occur in Central Queensland.

Comments

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[11][3]. 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[4][11]. Deep meadows are generally less susceptible to temperature than intertidal and shallow seagrasses[7].

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 Trace elements (the presence of nutrients and other potential contaminants)).

Additional Information

Seagrass-Watch

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, Science and Innovation

Seagrasses - Australian Institute of Marine Science

A Vulnerability Assessment for the Great Barrier Reef (Seagrass) - Great Barrier Reef Marine Park Authority

Seagrass Restoration Network

References

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2. ^ Bruinsma, C & Danaher, K (2001), Queensland Coastal Wetland Resources: Round Hill Head to Tin Can Inlet.. [online], vol. QI99081, Department of Primary Industries, Queensland Government., Brisbane. Available at: http://era.daf.qld.gov.au/id/eprint/3545/.
3. ^ Campbell, SJ & McKenzie, LJ (2004), 'Flood related loss and recovery of intertidal seagrass meadows in southern Queensland, Australia', Estuarine, Coastal and Shelf Science, vol. 60, no. 3, pp. 477-490, Elsevier.
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8. ^ Kilminster, K, McMahon, K, Waycott, M, Kendrick, GA, Scanes, P, McKenzie, L, O'Brien, KR, Lyons, M, Ferguson, A & Maxwell, P (2005), 'Unravelling complexity in seagrass systems for management: Australia as a microcosm', Science of the Total Environment, vol. 534, pp. 97-109, Elsevier.
9. ^ a b c Kuiper-Linley, M, Johnson, CR & Lanyon, JM (2007), 'Effects of simulated green turtle regrazing on seagrass abundance, growth and nutritional status in Moreton Bay, south-east Queensland, Australia', Marine and Freshwater Research. [online], vol. 58, no. 5, p. 492. Available at: http://www.publish.csiro.au/?paper=MF06241 [Accessed 5 April 2019].
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16. ^ a b Rasheed, MA, Thomas, R, Roelofs, AJ, Neil, KM & Kerville, SP (2003), Port Curtis and Rodds Bay Seagrass andBenthic Macro-Invertebrate Community Baseline Survey.
17. ^ Sheaves, M (2005), 'Nature and consequences of biological connectivity in mangrove systems', Marine Ecology Progress Series. [online], vol. 302, pp. 293-305. Available at: http://www.int-res.com/abstracts/meps/v302/p293-305/ [Accessed 15 March 2019].
18. ^ a b c d e f Sheppard, JK, Preen, AR, Marsh, H, Lawler, IR, Whiting, SD & Jones, RE (2006), 'Movement heterogeneity of dugongs, Dugong dugon(Müller), over large spatial scales', Journal of experimental marine biology and ecology, vol. 334, no. 1, pp. 64-83, Elsevier.
19. ^ Watson, R, Coles, R & Lee Long, W (1993), 'Simulation estimates of annual yield and landed value for commercial penaeid prawns from a tropical seagrass habitat, Northern Queensland, Australia', Marine and Freshwater Research. [online], vol. 44, no. 1, p. 211. Available at: http://www.publish.csiro.au/?paper=MF9930211 [Accessed 8 April 2019].
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Last updated: 22 July 2019