Water Flow in Seagrass Ecosystems - Hydrodynamic environment of seagrass meadows

Brief description

The hydrodynamic environment within the seagrass canopy of Amphibolis griffithii, Posidonia australis, Posidonia sinuosa and mixed Posidonia beds was studied, and compared with a bare sand site (See thumbnail). Electronic current meters developed by the author (Chapter 2 of thesis) was used to measure water velocity profiles.
In addition, the hydrodynamic regimes of sheltered and exposed Posidonia sinuosa sites were studied, as well as sediment size fraction distribution within a dense Posidonia sinuosa meadow. These studies were undertaken in Shoalwater Bay located south of Perth, Western Australia.

Lineage

Maintenance and Update Frequency: notPlanned

Statement: - Profiles through different meadows -

Posidonia australis, Posidonia sinuosa, Amphibolis griffithii, and a 50:50 mixed patch of the two Posidonia species were chosen for the study. A series of Posidonia patches were chosen in Shoalwater Bay and the Amphibolis griffithii meadow was located in northern Warnbro Sound near Penguin Island, all south of Perth, Western Australia (see thumbnail).

Previous studies suggest that a homogeneous seagrass meadow will produce maximum reduction of flow within approximately 1 m of the meadow edge. Patches of at least 5 x 5 m were therefore selected, to avoid any possible edge effects. Measurements of the water velocity profiles within the selected patches were carried out using the electronic current meters developed as part of the thesis (Chapter 2). Field deployment was carried out as described in Chapter 2 of thesis.

Statement: - Study of exposed and sheltered meadows -

Two field sites were randomly established at a distance of approximately 500 m from each other, within a continuous Posidonia sinuosa meadow located in Shoalwater Bay, south of Perth. A sheltered site was located inshore of Bird Island where it was sheltered by a series of limestone reefs from the open ocean swells. An exposed site was located approximately 500 m further offshore, near Bird Island and was not sheltered by the offshore reef system. While these sites were selected on an arbitrary basis, comprehensive measurements of water velocity profiles, sediment grain size analysis and seagrass standing crop were made, to ensure all major potential variables were assessed and included in the analysis. Sediment grain size analysis described in another section of the metadata, while standing crop measurements methodology is given in (Water Flow in Seagrass Ecosystems - Water Motion and Productivity; ID - a9f1e890-43bf-11dc-972b-00188b4c0af8

Statement: - Sediment grain size analysis -

Sediment sample sets, obtained simultaneously with the biomass samples from the inshore and offshore sites were collected approximately every three months over 14 months from December 1991 to February 1993. On each sampling occasion a 10 m transect was randomly established at each of the inshore and offshore sites. The transect was oriented parallel to the shoreline to avoid confounding the inshore/offshore variable. Ten biomass and sediment samples were collected immediately following biomass sampling, from the cleared seagrass quadrats (0.25 m2). Ten samples were taken from each site (inshore and offshore) at each sampling date, corresponding to the number of biomass samples collected. Samples were collected in December 1991, February, April, July, October and December 1992, and February 1993.

Samples were collected by sawing a toothed perspex tube (length 200 mm, diameter 60 mm) into the sediment after the above-ground biomass was collected. Sediment was collected from the surface to a depth of approximately 10 cm. A plastic lid was screwed on to the top of the core tube, and the core extracted from the seabed. The sediment sample was released into a plastic bag for storage by removing the plastic lid. The collected material was frozen until processed.

Laboratory processing of sediment samples consisted of drying in an oven at a temperature of 100 degrees Celsius fro at least 48 hours. Obvious large pieces of plant material and shells were removed from the samples. Each total sediment sample was weighed (dry weight) and then graded into a standard size fraction series, using a standard set of test sieves (2.0, 1.0, 0.50, 0.25, 0.125, 0.063 mm; Endecotts Ltd.). Each fraction was weighed enabling a size fraction distribution to be calculated.