Geology Reference
In-Depth Information
Promontory, Victoria and halfway down the
coast of Tasmania. Leaves are up to 50 mm long
and the stem height to 1.5 m, while leaves occur
in clusters of 8-10. It mostly grows by itself in
very dense beds and is particularly extensive in
Shark Bay where it forms the dominant cover over
3700 km
2
.
Amphibolis griffi thii
is found on moderately
exposed sand and rock in 0-40 m w.d., from
Champion Bay Western Australia to Victor
Harbor (Fig. 2) again near the centre of the study
area. Leaves are a maximum of 100 mm, twice
the length of
A. antarctica,
and lower in num-
ber (4-5) whereas the maximum stem height is
similar, 1.1 m.
(a)
(b)
Not to scale
Amphibolis
sp.
Posidonia
sp.
SETTING
Fig. 3.
The main attributes of
Posidonia
(right) with
numerous strap-like leaves above the sediment sub-
strate, much like the tropical grass
Thalassia
, compared
to
Amphibolis
(left) that consists of a stem above the
sediment surface from which emerge clusters of leaves.
Oceanography
The shelf of southern Australia is storm-
dominated with high (>2.5 m) modal deep-water
wave heights (Davies, 1980; Short & Hesp, 1982;
Wright
et al
., 1982). Long period (>12 s) swell
waves are common, and wavelengths of 200 m
have been reported. The oceanography is other-
wise complex; shelf water masses, which vary
in character throughout the year are mixtures of
cold Southern Ocean Water, warm Indian Ocean
water, saline waters from the adjacent gulfs, and
water formed in the Great Australian Bight, all
of which fl ow to the southeast. Nearshore water
temperatures are generally less than 20°C, except
in the large gulfs, and become progressively cooler
southeastward.
The seasonal Leeuwin Current (Fig. 2) is a
shallow-water shelf-edge stream of warm
(17-19°C), low-salinity (35.7-35.8‰), low nutri-
ent, tropical surface water that during the winter
fl ows southward along the west coast of the conti-
nent and eastward into the Great Australian Bight
(Cresswell, 1991). This shelf-edge fl ow continues
eastward as the South Australian Current (Fig. 2)
(Ridgeway & Condie, 2004), a fl ow derived from
a warm (17-19°C in winter; 19-22°C in summer)
and saline (35.9-36.4‰) water mass in the cen-
tral and western Great Australian Bight (Rochford,
1986). The generally southeast fl ow of these sur-
face waters is matched by westward geostrophic
fl ow of a cooler intermediate depth, upwelling-
favourable boundary current (Bye, 1972, 1983;
Middleton & Cirano, 2002) called the Flinders
Current. Coast-directed winds result in signifi cant
upwelling (Fig. 2) off Kangaroo Island, the west
seems to tolerate greater extremes in temperature
and salinity compared with related species.
Posidonia angustifolia
grows in moderately
exposed sand between 2 and 35 m w.d. from the
Houtman Abrolhos in Western Australia to Port
Moorowie, South Australia (Fig. 2), in the middle
of this study area. The leaves, although as long as
those of
P. australis
, are thinner, and up to only
6 mm wide. These leaves are easily broken, and
typically coated with dense epiphytes. Rhizomes
lie just below the surface at 5-15 cm. It is often the
dominant species of
Posidonia
in deeper water.
Posidonia sinuosa
lives in moderately exposed
sand and sheltered sand from 0 to 15 m w.d. It
is found from Shark Bay Western Australia to
Kingston, South Australia (Fig. 2) near the eastern
margin of the study area. The leaves are similar to
those of
P. australis
, a maximum of 1.2 m long and
11 mm wide. Rhizomes are fi rmly rooted in sand
and can be removed only with diffi culty.
Amphibolis
The two
Amphibolis
species have woody stems
arising from the rhizome, the stems have a regular
arrangement of leaf scars that are tough and wiry.
The leaves are small relative to the stems. Both
species are restricted to southern Australia.
Amphibolis
antarctica
grows in moderately
exposed sand from 0 to 23 m w.d. and extends
from Carnarvon, Western Australia to Wilson's
