556
D. I. Walker, G. A. Kendrick, and A. J. McComb
sand blowouts was size specific with patches
<25 m
2 being colonised over a 50 year period, but
larger patches remained the same size. The scaling
of mapping studies of these slower growing species
needs to capture these very slow expansion rates.
Missing from landscape scale mapping studies
are the appropriate temporal and spatial scales for
observing recovery in seagrass meadows from
known recruitment and growth rates of the specific
seagrass species being studied.
There is a paucity of good rhizome growth data
for many seagrass species. Rhizome production is
difficult to study directly, and reconstruction methods have limitations. We have found that rhizome
extension rates, branching rates, and mortality rates
are not readily available for many seagrass species.
Yet these growth measurements are important for interpreting expansion observed in mapping exercises
and managing for recovery of seagrass meadows.
In summary, these mapping exercises have shown
large-scale (up to 58%) losses due to environmental
perturbations, but there have been gains of up to 21%
shown in areas where recruitment has occurred or
following removal of perturbation. We conclude that
human impacts will tend to remove seagrass faster
than it can recover, but recovery can and does occur.
IV. Case Study 1: Why did We Lose
Seagrasses in Cockburn Sound Whereas
Seagrass Area has Expanded on
Neighbouring Parmelia and
Success Banks?
A. Geographical and Historical Setting
Cockburn Sound and Success and Parmelia Banks
are relatively protected marine waters to the south of
Fremantle, Western Australia (Fig. 1). The sediment
is unconsolidated carbonate sand and has formed
largely from the onshore transport of sands over the
past 7,500 years. Cockburn Sound is a sheltered marine embayment located to the south of Parmelia
Bank. It is 16 km long by 9 km wide, and consists
of a deep central basin (17–22 m deep) surrounded
by shallow sandy platforms ranging in width from
50 m to 3 km, that support seagrasses (Fig. 1).
Parmelia Bank is the northern boundary of Cockburn
Sound and is approximately 4 km south of Success
Bank. Success Bank extends in an east-west direction offshore just south of the port of Fremantle.
Water depths where seagrasses are found are generally <10 m.
Prior to 1954, Cockburn Sound was used mainly
for recreational purposes and commercial fishing,
but its relatively protected deep waters also constituted an excellent anchorage, hence its choice as an
outer harbour for the Perth-Fremantle area. Industrial development commenced with the establishment of an oil refinery in 1955, and the next 25
years saw the addition of iron, steel, alumina, and
nickel refining or processing plants, chemical and
fertiliser production plants, and a bulk grain terminal. With the development of the industrial complex, wharves and groynes were built and channels
dredged for shipping access. A wastewater treatment
plant was commissioned in 1966, which discharged
south west of Woodman Point at the northern end of
the Sound, while at the southern end of the Sound
a rock fill causeway was built in 1971–1973 connecting Garden Island with the mainland. Industrial
expansion resulted in deterioration of the Cockburn
Sound environment, leading to extensive loss of seagrass. The Fremantle Port Authority (FPA) channel
that enables shipping access into Cockburn Sound
bisects Success and Parmelia Banks. This channel was originally dredged prior to the 1950s and
maintenance dredging has been conducted occasionally since then. Shell sand dredging also occurs on
Parmelia and Success Banks, and is a major threat
to seagrass meadows in the region.
B. Seagrass Species
In 1999 in Cockburn Sound, seagrass beds were continuous over hectares and predominantly Posidonia
sinuosa (and Posidonia angustifolia) with small areas of Posidonia australis. On Success and Parmelia
Banks seagrass coverage was generally more spatially patchy and consisted of more equally represented assemblages than in Cockburn Sound. The
seagrass species Posidonia sinuosa, Posidonia australis, Posidonia coriacea, Amphibolis griffithii, and
Amphibolis antarctica occur as single- and mixedspecies assemblages.
C. Loss and Recovery of Seagrasses
The decline in area of seagrass cover on shallow
shelves that border Cockburn Sound was dramatic
and the largest loss was concentrated over a 5 year
D. I. Walker, G. A. Kendrick, and A. J. McComb
sand blowouts was size specific with patches
<25 m
2 being colonised over a 50 year period, but
larger patches remained the same size. The scaling
of mapping studies of these slower growing species
needs to capture these very slow expansion rates.
Missing from landscape scale mapping studies
are the appropriate temporal and spatial scales for
observing recovery in seagrass meadows from
known recruitment and growth rates of the specific
seagrass species being studied.
There is a paucity of good rhizome growth data
for many seagrass species. Rhizome production is
difficult to study directly, and reconstruction methods have limitations. We have found that rhizome
extension rates, branching rates, and mortality rates
are not readily available for many seagrass species.
Yet these growth measurements are important for interpreting expansion observed in mapping exercises
and managing for recovery of seagrass meadows.
In summary, these mapping exercises have shown
large-scale (up to 58%) losses due to environmental
perturbations, but there have been gains of up to 21%
shown in areas where recruitment has occurred or
following removal of perturbation. We conclude that
human impacts will tend to remove seagrass faster
than it can recover, but recovery can and does occur.
IV. Case Study 1: Why did We Lose
Seagrasses in Cockburn Sound Whereas
Seagrass Area has Expanded on
Neighbouring Parmelia and
Success Banks?
A. Geographical and Historical Setting
Cockburn Sound and Success and Parmelia Banks
are relatively protected marine waters to the south of
Fremantle, Western Australia (Fig. 1). The sediment
is unconsolidated carbonate sand and has formed
largely from the onshore transport of sands over the
past 7,500 years. Cockburn Sound is a sheltered marine embayment located to the south of Parmelia
Bank. It is 16 km long by 9 km wide, and consists
of a deep central basin (17–22 m deep) surrounded
by shallow sandy platforms ranging in width from
50 m to 3 km, that support seagrasses (Fig. 1).
Parmelia Bank is the northern boundary of Cockburn
Sound and is approximately 4 km south of Success
Bank. Success Bank extends in an east-west direction offshore just south of the port of Fremantle.
Water depths where seagrasses are found are generally <10 m.
Prior to 1954, Cockburn Sound was used mainly
for recreational purposes and commercial fishing,
but its relatively protected deep waters also constituted an excellent anchorage, hence its choice as an
outer harbour for the Perth-Fremantle area. Industrial development commenced with the establishment of an oil refinery in 1955, and the next 25
years saw the addition of iron, steel, alumina, and
nickel refining or processing plants, chemical and
fertiliser production plants, and a bulk grain terminal. With the development of the industrial complex, wharves and groynes were built and channels
dredged for shipping access. A wastewater treatment
plant was commissioned in 1966, which discharged
south west of Woodman Point at the northern end of
the Sound, while at the southern end of the Sound
a rock fill causeway was built in 1971–1973 connecting Garden Island with the mainland. Industrial
expansion resulted in deterioration of the Cockburn
Sound environment, leading to extensive loss of seagrass. The Fremantle Port Authority (FPA) channel
that enables shipping access into Cockburn Sound
bisects Success and Parmelia Banks. This channel was originally dredged prior to the 1950s and
maintenance dredging has been conducted occasionally since then. Shell sand dredging also occurs on
Parmelia and Success Banks, and is a major threat
to seagrass meadows in the region.
B. Seagrass Species
In 1999 in Cockburn Sound, seagrass beds were continuous over hectares and predominantly Posidonia
sinuosa (and Posidonia angustifolia) with small areas of Posidonia australis. On Success and Parmelia
Banks seagrass coverage was generally more spatially patchy and consisted of more equally represented assemblages than in Cockburn Sound. The
seagrass species Posidonia sinuosa, Posidonia australis, Posidonia coriacea, Amphibolis griffithii, and
Amphibolis antarctica occur as single- and mixedspecies assemblages.
C. Loss and Recovery of Seagrasses
The decline in area of seagrass cover on shallow
shelves that border Cockburn Sound was dramatic
and the largest loss was concentrated over a 5 year