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Seagrass Beds and
Fauna of St Kilda Harbour
Continued
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5.
Results
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5.1
Seagrass Coverage and Distribution
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The following table presents
data derived from scanned aerial photographs. The layers produced in this
process can be seen in Appendix 1 Study Area and Seagrass Distribution.
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It must be noted
that seagrass does not have a constant standing crop as growth and shrinkage
of the beds occurs due to environmental influence particularly sediment
movement in the harbour area. This can be the result of storm activity,
prolonged winds, severe flooding in the catchments and the dredging of channels
for ferry passage and beach renourishment activity as happens on this study
site. |
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Seagrass
Area in St Kilda Harbour (3 sites)
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Sparse
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Dense
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Total
Seagrass Area sq metres
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1997
= 76,196
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1997
= 62,444
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138,640
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2000
= 23,577
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2000
= 27,937
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51,514
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-
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NET
LOSS
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87,126
sq. metres
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COMMENT
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As mentioned in
Appendix 1 the total amount of seagrass measured is influenced by the quality
of data as much as environmental influences. However the harbour floor is
actively managed to allow the passage of leisure boats from the motor yacht
squadron and ferry services that travel between the St Kilda and Williamstown.
From the dives conducted it was evident that a distinct trench has been
dredged on a regular basis and the abrupt discontinuity of vegetation is
accounted for by this. |
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As this project
is completed intensive work is being undertaken on the edge of our study
area to reclaim enormous amounts of sands for renourishment of beaches 2km
to the north. This will make the inner harbour around Site 3 more vulnerable
to wave action as the sand bar barrier to the north is now removed. The
reliance of seagrass on calmer waters in this area is distinctly shown with
all seagrass beds found in the lee of protective structures away from prevailing
s - sw winds. Any alteration to these will have significant impact on seagrasss
distribution, more so than the removal of the northern sandbar as prevailing
winds are uncommon directly from the north. |
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To more fully understand
the distribution of seagrass in the harbour complex modelling of wave action
and its relationship to protective structures and weather models is required. |
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5.2
Marine Science Flora and Fauna Reports
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Rhodoglossum gigartinoides
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Many plant species
beside seagrass are present in the harbour - pier ecosystem, these are mostly
algal species occuping a variety of habitats from the mud substrate to the
dark recesses of the pier pylons. The algal groups are broken down in to
the Green, Red and Brown algaes to reflect the pigments that each group
contain. These differing pigments have the ability to capture light at different
wavelengths which reflects the depth of light penetration for different
parts of the light spectrum. Each plant has its own unique set of characteristics
to take advantage of the myriad of opportunities that exist in the marine
environment. |
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The animal world
of St Kilda Harbour shows considerable diversity particularly amongst the
worms, shellfish and starfish. There were small schools or individuals of
the commonly recognised fish species such as Cobler, Stingrays, Garfish,
Flathead and Black Bream but the lack of significant cover is not conducive
for the greater diversity of big fish seen on reefs and in deeper waters. |
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The work undertaken
by Guy Werner and Mathew McArthur gives us a professional assessment of
the diversity and distribution of plant and animal life within the harbour.
The following paragraphs are exerts from the second report by Guy Werner
with full reports found in the Appendix 2 flora and Fauna Report. |
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Discussion
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The presence of
dolphins is perhaps a testament to the abundance of prey items within the
breakwater area. |
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The seagrass cover
at the dive sites appeared similar to cover during the autumn survey. Possibly
cover at site one had declined. However, this is difficult to ascertain
due to the qualitative method used and the difficulty of diving exactly
the same spot. |
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The higher diversity
of organisms at site two compared to sites one and three is probably due
to a more varied habitat. While all sites were subtidal and depth did not
vary greatly, sites one and three were flat and fairly featureless except
for the seagrass beds and drift algae. Site two contained some man-made
refuse that formed mini reefs among the seagrass etc. It was also the site
closest to the pier. It appeared that these extra microhabitats offered
a topographically diverse seascape as well as different surfaces for sessile
organisms to grow on. |
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The presence or
absence of species from the collections over time or space must be viewed
critically. There are a variety of reasons a species may or may not be recorded
other than the obvious one (that it was or was not present at the time of
collection). Rare species have a greater probability of being missed by
a survey. Small (e.g. Halicarcinus ovatus) or cryptic (e.g. Platycephalus,
Palaemon, pipefish) species, likewise, are less likely to be seen. Shy
and mobile species (i.e. crabs, fish, sharks and rays) are also likely to
evade detection. Nocturnal species that hide during the day are also unlikely
to be recorded during a diurnal survey such as this. |
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Having said that,
it appears that certain species are widespread within the breakwater (species
noted at all three sites). Two of the Caulerpa species are included
in this list. Their ability, unusual for macro-algae, to grow on soft sediments
is probably the key to their success. Ulva lactuca is an opportunistic,
cosmopolitan species (an introduction?) which grows strongly under enhanced
nutrient levels. Colpomenia peregrina is able to grow epiphytically
on seagrass and this accounts for its widespread distribution within the
breakwater. Pyura stolonifera is a common organism of shallow sediments
around Port Phillip Bay so it is no surprise to record it here. Asterias
and Sabella are well known invaders of Port Phillip. The density
of Asterias within the breakwater appears to have greatly increased
since the autumn survey. No Asterias were found at site one in autumn,
for example. This should be of concern regarding biological diversity within
the breakwater. Asterias is a voracious predator with broad tastes in prey.
Mytilus edulis is a common organism of Port Phillip Bay (The Centre
for Research on Introduced Marine Pests believes it to be an early introduction
to PPB). The beds of it were dense enough once upon a time to support a
dredge industry within the bay. |
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Seasonal or longer-term
community changes may best be detected using the macro-algae and sessile
or easily seen invertebrates. Differences in species observed between this
survey and the last one may indicate which species are seasonal inhabitants.
Dictyota dichotoma, Anthopleura aureoradiata, Chaetomorpha and
Laurencia filiformis were recorded within the breakwater in autumn but
not spring. Ulva lactuca, Caulocystis cephalornithos, Styela clava,
the red jellyfish, Coscinasterias muricata, the cirratulid worm,
the two Epiactis species, Dictyopteris muelleri, Sargassum paradoxum,
Grateloupia filicina, the styelid ascidian, Patiriella calcar
and the sponge were recorded within the breakwater in spring but not autumn.
Some other species that have been recorded from the breakwater are known
to be seasonal annual species. The sporophyte of the Japanese Kelp,
Undaria pinnatifida, grows from approximately the start of winter
to the end of spring. The exotic subspecies Codium fragile tomentosoides
grows throughout the warmer months. Codium fragile found in winter
is likely to be the native subspecies, not the exotic one. |
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Unfortunately, despite
not having observed any eels, we cannot say for certain whether they frequent
the breakwater or not. The author has seen several eels on one dive around
the mouth of St Kilda Marina. Considering the proximity of the marina to
the breakwater (approx. 1 km) we feel it is possible the same eel species
inhabits the breakwater, at least seasonally.Appendix 1 Study Area and Seagrass
Distribution The study area is St Kilda Harbour, Melbourne, Victoria, Australia.
The Sea-grass beds are found in the sheltered waters on the northern side
of the breakwater protected from the prevailing south westerly winds. This
preferred habitat for the sea-grass dictated the extent of the study area.
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Specifically the
site can be sub-divided into sea-grass beds which are found close to shore
on the northern side of the pylon area and 3 main dive sites. |
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| 5.3
Heavy Metal Contamination |
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Site 1 = Mouth of Cowderoy
Outlet (mg/l)
Site 2 = Dive Site 3 Near-shore
seagrass bed (mg/l)
Site 3 = Central Port Phillip
Bay
Site 4 = Corio Bay
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| Sediment Analysis |
Site 1 |
Site 2 |
Site 3 |
Site 4 |
ANZECC Guide
lines |
| Cadmium as
Cd |
<0.1 |
0.1 |
<.05 |
.2 |
2 |
| Zinc
as Zn |
21 |
42 |
<2 |
<2 |
50 |
| Copper
as Cu |
1.9 |
2.3 |
0.6 |
1.1 |
5 |
| Iron
as Fe |
1900 |
4300 |
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| Lead as Pb |
6.6 |
13 |
<0.8 |
<0.8 |
5 |
| Manganese
as Mn |
8.0 |
20 |
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| Chromium as
Cr |
2.5 |
6.5 |
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| Barium as
Ba |
3 |
4 |
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| Nickel as
Ni |
2.1 |
3.3 |
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| Antimony as
Sb |
<0.5 |
<0.5 |
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| Arsenic as
As |
2.4 |
3.7 |
2.8 |
3.2 |
50 |
| Colbalt as
Co |
0.7 |
1.5 |
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| Mecury as
Hg |
0.04 |
0.04 |
.002 |
<.002 |
0.1 |
| Molybdenum
as Mo |
<0.5 |
<0.5 |
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| Selenium as
Se |
<0.5 |
<0.5 |
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Results of the analysis
of St Kilda Harbour sediments |
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Monobutyltin as
Sn = 0.8 ng/g |
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Dibutyltin as Sn
= <0.5 ng/g |
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Tributyltin as Sn = 0.5 ng/g
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Concinasterais muricata
- native seastar
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| Comment |
The results from
the study sediments show a significant level of contamination which would
seem to be contained locally within sediments but the regular disturbance
of the seafloor by dredging and renourishment activities churns the inner
harbour area into a cloud of suspended particle regularly.
With the filter feeding organisms located in such large numbers across the
harbour it is highly likely that heavy metals have entered the food chain.
This harbour system exhibits species diversity at all levels of the food
chain and humans are at the end of this linkage as fishing for small fish
species is popular.
A detailed investigation of the food chain species for heavy metal contamination
would add to our understanding of this diverse harbour ecosystem.
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| From
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Pollution
Technical Annex: 2
State of the Marine Environment Report for Australia
Heavy metals and tributyltin in Australian coastal and estuarine waters
G.E. Batley
CSIRO Centre for Advanced Analytical Chemistry
Menai, NSW 2234
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| Tributyltin
in biota |
The range of data on TBT in Australian aquatic biota has been summarised
in several publications (Batley & Scammell 1991; Maher & Batley
1990; Scammell et al. 1990). The most significant impacts have been on intertidal
oysters but with an apparent lack of impact on subtidal oysters. The reason
for this is postulated to be related to enrichment of TBT in the surface
microlayer. Reductions in populations of scallops in Victoria might also
be attributable to the impact of TBT in the microlayer on larval scallops.
Both problems have now been reversed with the banning of TBT (Batley et
al. 1992). Oyster growth is now normal, with TBT barely detectable, and
larger than ever scallop populations are being reported in Port Phillip
Bay.
The observations in gastropods of imposex (the induction of male reproductive
organs in female animals) caused by TBT has been examined in a number of
sites in NSW (Ahsanullah and Wilson, unpublished results). The impact of
banning on this phenomenon has not been reported. There appears to have
been no significant impacts or accumulation of TBT by other aquatic biota.
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Appendix
1 Study Site and Seagrass Distribution
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The study area is
St Kilda Harbour, Melbourne Australia.
The Sea-grass beds are found in the sheltered waters on the northern side
of the breakwater protected from the prevailing south westerly winds. This
preferred habitat for the sea-grass dictated the extent of the study area.
Specifically the site can be
subdivided into sea-grass beds which are found close to shore on the northern
side of the pylon areas and 3 main dive sites.
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Appendix
2 Flora and Fauna Reports
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Marine scientists
from the Queenscliff Marine Science centre, Victoria, produced the following
reports
1. Autumn season May 2000
2. Spring August 2000.
Members from Earthcare participated in the dives and contributed in the
collection of specimens for identification.
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