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Section 6.5
Benthic Primary Producer Habitat
Benthic (seabed) communities of the region are dominated by marine plants (such as seagrasses, macro
turfing and epiphytic algae) and invertebrates, which acquire a significant proportion of their energy from
symbiotic microalgae that live in coral polyps. Benthic primary producers (BPPs) are subtidal or
intertidal, sequester carbon from surrounding sediment, seawater or air and convert it to organic
compounds through photosynthesis (EPA, 2004).
Section 6.5.1
Benthic Primary Producer Habitat Mapping
The mapping of BPPH was undertaken using auto-classification of Digital Globe Satellite imagery.
Digital Globe Imagery in both panchromatic (black & white) and multispectral (colour) bands for the area
was obtained on a cloud-free day on January 11th, 2006. Each colour band was attributed to a habitat
type, with the habitats ascribed as follows:
•
Bare sand.
•
Seagrass (all species).
•
Macroalgae (all perennial species).
The hard coral colonies found at Gio Batta Patch, Michaelmas Reef and adjacent to the large offshore
Islands (Michaelmas Island and Breaksea Island) were not included in habitat mapping nor BPPH
calculations as their density in any given location was less than 1% cover and these reefs are not
anticipated to be impacted by the dredging or the associated turbidity.
Ground truthing of the management units was comprised of the following investigations:
•
Habitat investigations of the proposed dredge channel, Gio Batta Patch and Michaelmas Reef
during the implementation of the Sampling and Analysis Plan.
•
Historical data from previous habitat mapping of the region.
•
Targeted ground truthing based on the outputs of this habitat mapping exercise that required
validation.
The resulting habitat map is provided in Figure 6.18 and shows the three habitat classifications, the
proposed dredging and reclamation areas, the preferred inner disposal area and the three benthic primary
producer habitat management units.
Section 6.5.2
Land Reclamation Area Habitat
Seagrass in the land reclamation area is dominated by Posidonia australis with a little P. sinuosa,
whereas the seagrass on the south side of the channel is a dense meadow of mixed seagrass dominated by
P. sinuosa and P. australis. Dredging and land reclamation activities will permanently remove 0.01% of
this BPPH in Management Unit 1. The areas lost will be offset as part of the APA commitment to replant
permanent losses of seagrass in Princess Royal Harbour associated with the Albany Port Expansion
Proposal.
A small sub-tidal granite rock pile (approximately 10 m in diameter) lies in the north east corner of the
proposed land reclamation area and would be buried during reclamation.
The water depth is
approximately 3–4 m and the rock reaches to just below the surface at low tide. Presently the rock has a
macroalgal community dominated by Ecklonia radiata with an under-story of red algae and Ulva sp. The
invertebrate community associated with the rock is sparse and is likely a reflection of periodic sand
inundation by resuspended sediment during storm events. On a regional scale, the rock is on the lower
end of the ecological significance when compared to the adjacent rocky shoreline as well as the wider
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King George Sound and offshore Islands. The protection afforded in the small embayment along with the
sandy seabed leads to conditions less conducive to macroalgal and invertebrate communities flourishing.
Anecdotal evidence suggests that fishers use the area as it is protected and occasionally catch King
George Whiting.
Section 6.5.3
Channel Habitat
The proposed channel is predominantly comprised of fine sand (Figure 6.17 a) with no sessile benthic
flora or fauna. The exceptions to this are as follows:
•
Sea pens (Sarcoptilus grandis) that occur on the seabed along the northern batter of Ataturk
entrance between King Point and Vancouver Peninsula; and
•
Sparse clumps of Posidonia coriacea found at varying densities between sites 34 and 45 (Figure
6.17 b).
A map of benthic primary producer habitat (BPPH) in Princess Royal harbour and King George Sound is
provided in Figure 6.18. The presence of seagrass has been used for calculation of BPPH loss in Section
9.2.3. Offshore disposal area habitat is addressed in Section 10.
a) Bare sand showing ripples at 18 m.
Figure 6.17
b) Posidonia coriacea clump.
Benthic Habitat in the Proposed Channel.
Section 6.5.4
Seagrasses
Seagrass meadows are physically and biologically significant in near-shore marine systems, trapping
sediments and providing a refuge from predation for juvenile fish and crustaceans. Seagrass also
performs an important role in stabilising areas of sand on the seafloor, acting to disperse wave energy and
reducing littoral drift and major changes to shorelines. Past investigations of seagrass distribution have
shown that the largest decrease in Princess Royal Harbour occurred between 1962 and 1981 when about
90% of the seagrass meadows in the embayment were lost (Bastyan et al., 1996). Shading, caused by
enhanced algal growth following nutrient enrichment, was concluded to be the major factor in the
depletion of seagrass meadows. Since implementation of management strategies to reduce the nutrient
load of Princess Royal Harbour and harvesting of macroalgae, substantial seagrass regrowth has occurred
both in the deeper basin and in the shallow areas along the southern shore (Bastyan et al., 1996).
Seagrasses are most vulnerable to low light levels which lower the photosynthetic capacity of individual
plants during summer growing months.
Seagrass vegetation in Princess Royal Harbour is dominated by Posidonia australis, P. sinuosa, and
Amphibolus antarctica. Some scattered patches of A. griffithii are also found, but are confined to the
shallower areas of the harbour near South Spit on the southern shore (Smit and Walker, 1999).
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Figure 6.18
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Section 6.5.5
Corals
There are two reef systems (Gio Batta Patch and Michaelmas Reef) adjacent to the proposed channel in
King George Sound (Figure 6.20). Both reefs extend to the west of Michaelmas Island along the 15 m
contour line. The reef systems are partially protected from swell by Flinders Peninsula to the south and
Michaelmas and Breaksea Islands to the east and south-east respectively. However, the reefs still
experience significant wave height from rebounded swell waves and seas generated by strong winds that
are common to the south coast region. The area is fished recreationally as well as frequented by SCUBA
divers; however, the site is prone to swell and is often turbid.
In relation to the Albany Port Expansion Proposal, the proposed channel runs to the south-west of the two
reefs, with the preferred offshore disposal area to the south (Figure 4.2).
Gio Batta Patch
Michaelmas Reef
Figure 6.20
Location of Adjacent Reefs.
Gio Batta Patch
Gio Batta Patch is a heavily dissected limestone reef that rises out of 15 m of water to within
approximately 5 m of the surface. The upper portions of the reef form pinnacles extending from a small
reef top area. The sides of the reef are deeply cut into overhangs and tunnels presumably by the scouring
effect of large seas. The seabed in the vicinity of the reef is a flattened limestone pavement free of fine
sediment and generally devoid of flora and encrusting fauna.
Much of the reef top area is covered by brown algal species such as Ecklonia radiata and Scytothalia
doryocarpa capable of withstanding rough seas (see Figure 6.21a). A few pinnacles extend laterally from
the reef top and are dominated by encrusting invertebrate cover such as bryozoans, ascidians and soft
corals (see Figure 6.21c and d). The more dimly lit areas on the reef walls and overhangs are completely
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covered with encrusting invertebrate life such as sponges, ascidians, soft corals, bryozoans and even small
colonies of black coral (see Figure 6.21e and f).
The reef top and pinnacles are in stark contrast to the adjacent seabed that is heavily scoured limestone
pavement with only a thin film of very coarse sediment in shallow depressions. Some turf algae cover the
limestone but it would appear that the scouring action of heavy seas keeps the seabed cleared of any
significant growth, both floral and faunal.
a) Reef top with brown algae cover.
b) Seabed adjacent to the reef.
c) Pinnacle with bryozoan and ascidian cover. d) Pinnacle with soft coral cover.
e) Encrusting invertebrate cover on wall.
Figure 6.21
f) Encrusting invertebrate cover on overhang.
Gio Batta Patch.
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Michaelmas Reef
Michaelmas Reef (Figure 6.22) is similar to Gio Batta Patch, rising out of 15 m to within 6 m of the
surface; however, it is much larger in area. Michaelmas Reef is much longer than Gio Batta Patch and its
additional width results in protected areas within the centre of the reef and along the northern margin.
a) Reef top with brown algae cover.
b) Seabed adjacent to the reef.
c) Macroalgal covered protected seabed area.
d) Hard coral growing on reef wall.
e) Pinnacle covered in gorgonians and ascidians f) Encrusting invertebrate cover on reef wall.
Figure 6.22
Michaelmas Reef.
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The reef top area, like that of Gio Batta Patch is dominated by large brown algal species adapted to rough
sea conditions (see Figure 6.22a). However, unlike Gio Batta Patch, deeper areas in the middle of the
reef and the seabed along the northern margin are protected from the scouring effect of the large seas and
are covered in macroalgal assemblages including some red algae (see Figure 6.22c). This area of the reef
is by no means calm but it is less prone to scouring and sand blasting during storm events.
The additional protection afforded by the size of Michaelmas reef allows for a greater proliferation of
encrusting faunal species that require slightly calmer conditions. For instance, numerous plate corals
(Coscinaraea marshae) are found on the reef walls (see Figure 6.22d) as well as gorgonian soft corals
(see Figure 6.22e). The seabed around Michaelmas Reef is variable. The exposed side of the reef to the
south is similar to that of Gio Batta Patch with pavement and some very coarse sand; however, on the
northern side of the reef there are limestone rocks laying on top of the pavement and far more sediment
accumulation (see Figure 6.22b).
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