Subtidal low energy over sand

Long description

Subtidal low to very low energy sandy ecosystems including subtidal sandbanks and sand shoals, occurring within bays, rivers, streams and estuaries and open low energy coastlines– including major passage estuaries such as the Great Sandy Strait.

This ecosystem is associated with type 104 but further from the energy source (estuary, beach). These areas are more likely to be areas of deposition and the relevant processes are the northward process of longshore drift due to wave wash and backwash. Previously during times of lower sea level, these areas may have formed part of exposed sand dune systems and have resulted in the morphology formations such as large undersea dunes.

Special values

Low energy sand ecosystems within swales of subtidal dune systems within swales existing between subtidal dune systems may be areas of higher marine productivity, e.g. for biota such as scallops (Tony Courtney DAF and James Daniell JCU pers. Comm).

Sand systems offshore from turtle nesting beaches are inter-nesting habitat for turtles, where they go to incubate eggs before the next laying[1].

These areas are likely to contain unmapped areas of seagrass which are potential habitat for dugong and green turtles.

Diagnostic attributes:

Inundation ‘Subtidal’

Sediment texture ‘Sand’, ‘(slightly gravelly) Sand’, ‘gravelly muddy Sand’, ‘gravelly Sand’, ‘(slightly gravelly) muddy Sand’

Energy magnitude (wave) ‘Low’, ‘Very low’

Qualifiers

Although Naturalness qualifiers for these ecosystems are currently unmapped, these areas often form part of trawl grounds for species such as prawns and scallops.

Distribution

The following relates to distribution of this ecosystem type within the Central Queensland mapping area:

• This is a very extensive ecosystem over much of the mapping area, especially adjacent to the major sandmasses of Cooloola, K'gari, the Burrum Coast, the dune masses between the mouth of the Baffle and 1770, and those off Bustard Head, Middle Island, Hummock Hill Island and Curtis Island. Large undersea dunes are visible offshore from 1770 to Curtis Island, and very large submerged dunes (6-10m in height) occur to the west of Rooney’s Point (K'gari).
• Potentially these areas may include unmapped seagrass – as reflected by a straight line between K'gari and the mainland north of which seagrass is unmapped. Indicative of where seagrass is likely to be can be the movement tracking of seagrass-eating biota such as dugong and green turtles.

Comments

Subtidal sand systems may function very differently depending on whether they are part of a dune system or swale. Research of the FRDC 2018-048 project found areas of high biota diversity in the sandy and gravelly swales associated with subtidal dune systems. The study multi-beam sonar backscatter and field data to identify low relief dune-like Morphology (of compact shelly muddy sand) in depths of 29 to 35 metres, with the scallops mostly in the (low backscatter) swales with the finer sediment. A remote operated vehicle captured sea floor images, and benthic ecosystems are being analysed by Fathom Pacific. Relevant attributes include Terrain morphology i.e. crests and peaks can form part of dunes, channels and depressions may correlate to swales. Planes are flat areas. Further delineation of subtidal dunes and swales may be possible by including the terrain attribute Slope.

References

1. ^ D. Tucker, A, N. Fitzsimmons, N & J. Limpus, C (1995), 'Conservation implications of internesting habitat use by Loggerhead Turtles Caretta caretta in Woongarra Marine Park, Queensland, Australia', Pacific Conservation Biology. [online], vol. 2, no. 2, p. 157. Available at: http://www.publish.csiro.au/?paper=PC960157 [Accessed 13 June 2019].

Last updated: 19 July 2019