Chapter 23 Decline and Recovery of Seagrasses
559
loss in the Cockburn Sound West region (Cambridge
and McComb, 1984). Seagrass losses in this region
have also occurred along the deeper waters which
is most likely related to reduction of light associated with the decline in water quality in the Sound
(Cambridge et al., 1986). On the shallow Posidonia
sinuosa meadows north of Sulphur Bay a 3 ha rectangular area of seagrasses was lost between 1981 and
1994 to grazing sea urchins Heliocidaris erythrogramma and Temnopleuris michaelsenii and again in
1991 (Bancroft, personal communication). Loss of
seagrass meadows due to invasions by Temnopleuris
michaelsenii were previously reported near Rockingham in 1972, Careening Bay in 1973, Woodman
Point in 1976 and Warnbro Sound in 1978 (Cambridge et al., 1986). There have also been some increases in seagrass cover in Cockburn Sound: Cambridge (1975) reported crescentic sand blowouts on
Southern Flats, which are now continuous meadows
of P. sinuosa.
In contrast to the main basin of Cockburn Sound,
seagrass meadows on Success and Parmelia Banks
show both measurable gains and losses in coverage.
The gain in coverage between 1965 and 1999 is significant, and demonstrates that colonisation by slowgrowing seagrass species is measurable at a map
scale of 1:10,000 over the mapping period (35 years).
The seagrass assemblages that account for most of
the gains in coverage were Amphibolis griffithii and
Posidonia coriacea. The gains have predominantly
been associated with increases in the size of existing patches at the limit of the mapping resolution
(30 m
2 ) and in infilling, and coalescence of seagrass
patches into larger continuous meadows. Some increases in coverage of Posidonia sinuosa and Posidonia australis on Parmelia Bank were associated with
regrowth of shoots from pre-existing rhizome matte.
In several areas, the gains and losses of seagrass coverage were associated with the spatial displacement
of sand blowouts through beds. The mechanism behind loss and recovery over much of Success and
Parmelia Bank appears to be that the gains and losses
in seagrass coverage are in dynamic equilibrium, and
largely responding to sediment movements. Sediment movement through seagrass beds, in the form
of subaqueous bed forms (sand bars), influences the
survival and growth of seagrasses as they are sensitive to both burial (Marb` a et al., 1994; Marb` a and
Duarte, 1995) and erosion (Patriquin, 1975; Clarke,
1987). As a result, landscape-scale displacement of
seagrasses in the direction of the prevailing currents
is occurring over decades in the Success Bank East
and West and Parmelia Bank West regions. A similar
process was observed in Holdfast Bay, South Australia, where it was estimated that a full turnover of
habitat would occur between sand blowouts and beds
of Posidonia spp. in 140 years (Clarke, 1987; Clarke
and Kirkman, 1989). Clarke (1987) also estimated
maximum movements of blowouts in Holdfast Bay
of 1 m year
−1 during a 32-year period. This rate of
movement is similar to the erosion rates of 1.2 to
1.5 m year
−1 measured from sand blowouts in Thalassia testudinum beds in the West Indies (Patriquin,
1975). Yet it is 10 times less than that recorded
for sand bar movements through beds of Cymodocea nodosa (13.0 ± 0.6 m year
−1 ) in the Mediterranean (Marb` a et al., 1994; Marb` a and Duarte,
1995).
The sand bar migrations ensure that seagrass
species are in a continuous state of colonisation, and
may account for the dominance of Amphibolis griffithii and Posidonia coriacea assemblages on Success Bank. The greatest increases in seagrass coverage in the Success and Parmelia Bank region were
predominantly from the Amphibolis griffithii and
Posidonia coriacea assemblages (Kendrick et al.,
2000), although the published rhizome extension
rates for these species (Clarke and Kirkman, 1989)
cannot fully account for these increases (Kendrick
et al., 1999).
When introducing the case study, we raised the
question of why we lost seagrasses in Cockburn
Sound whereas seagrass area has expanded on neighbouring Parmelia and Success banks. The reason
for loss in the Sound clearly lies in eutrophication (Silberstein et al., 1986), which accompanied
industrial development, possibly exacerbated by a
minor reduction in the already limited exchange
between basin and ocean through causeway construction. Eutrophication combined with the dominance of P. sinuosa, a slow growing meadow forming
seagrass, resulted in extensive loss and little recovery over 30 years. In contrast, although eutrophication occurred on Parmelia Bank (Silberstein et al.,
1996) and to a lesser degree Success Bank, the
mixture of faster growing colonising species combined with slower growing, monospecific meadowforming species, greater hydrodynamic forcing and
therefore higher flushing rates and large scale sand
movement, all suggest the seagrasses on Parmelia
and Success Banks are in dynamic equilibrium
with their environment. This has resulted in greater
559
loss in the Cockburn Sound West region (Cambridge
and McComb, 1984). Seagrass losses in this region
have also occurred along the deeper waters which
is most likely related to reduction of light associated with the decline in water quality in the Sound
(Cambridge et al., 1986). On the shallow Posidonia
sinuosa meadows north of Sulphur Bay a 3 ha rectangular area of seagrasses was lost between 1981 and
1994 to grazing sea urchins Heliocidaris erythrogramma and Temnopleuris michaelsenii and again in
1991 (Bancroft, personal communication). Loss of
seagrass meadows due to invasions by Temnopleuris
michaelsenii were previously reported near Rockingham in 1972, Careening Bay in 1973, Woodman
Point in 1976 and Warnbro Sound in 1978 (Cambridge et al., 1986). There have also been some increases in seagrass cover in Cockburn Sound: Cambridge (1975) reported crescentic sand blowouts on
Southern Flats, which are now continuous meadows
of P. sinuosa.
In contrast to the main basin of Cockburn Sound,
seagrass meadows on Success and Parmelia Banks
show both measurable gains and losses in coverage.
The gain in coverage between 1965 and 1999 is significant, and demonstrates that colonisation by slowgrowing seagrass species is measurable at a map
scale of 1:10,000 over the mapping period (35 years).
The seagrass assemblages that account for most of
the gains in coverage were Amphibolis griffithii and
Posidonia coriacea. The gains have predominantly
been associated with increases in the size of existing patches at the limit of the mapping resolution
(30 m
2 ) and in infilling, and coalescence of seagrass
patches into larger continuous meadows. Some increases in coverage of Posidonia sinuosa and Posidonia australis on Parmelia Bank were associated with
regrowth of shoots from pre-existing rhizome matte.
In several areas, the gains and losses of seagrass coverage were associated with the spatial displacement
of sand blowouts through beds. The mechanism behind loss and recovery over much of Success and
Parmelia Bank appears to be that the gains and losses
in seagrass coverage are in dynamic equilibrium, and
largely responding to sediment movements. Sediment movement through seagrass beds, in the form
of subaqueous bed forms (sand bars), influences the
survival and growth of seagrasses as they are sensitive to both burial (Marb` a et al., 1994; Marb` a and
Duarte, 1995) and erosion (Patriquin, 1975; Clarke,
1987). As a result, landscape-scale displacement of
seagrasses in the direction of the prevailing currents
is occurring over decades in the Success Bank East
and West and Parmelia Bank West regions. A similar
process was observed in Holdfast Bay, South Australia, where it was estimated that a full turnover of
habitat would occur between sand blowouts and beds
of Posidonia spp. in 140 years (Clarke, 1987; Clarke
and Kirkman, 1989). Clarke (1987) also estimated
maximum movements of blowouts in Holdfast Bay
of 1 m year
−1 during a 32-year period. This rate of
movement is similar to the erosion rates of 1.2 to
1.5 m year
−1 measured from sand blowouts in Thalassia testudinum beds in the West Indies (Patriquin,
1975). Yet it is 10 times less than that recorded
for sand bar movements through beds of Cymodocea nodosa (13.0 ± 0.6 m year
−1 ) in the Mediterranean (Marb` a et al., 1994; Marb` a and Duarte,
1995).
The sand bar migrations ensure that seagrass
species are in a continuous state of colonisation, and
may account for the dominance of Amphibolis griffithii and Posidonia coriacea assemblages on Success Bank. The greatest increases in seagrass coverage in the Success and Parmelia Bank region were
predominantly from the Amphibolis griffithii and
Posidonia coriacea assemblages (Kendrick et al.,
2000), although the published rhizome extension
rates for these species (Clarke and Kirkman, 1989)
cannot fully account for these increases (Kendrick
et al., 1999).
When introducing the case study, we raised the
question of why we lost seagrasses in Cockburn
Sound whereas seagrass area has expanded on neighbouring Parmelia and Success banks. The reason
for loss in the Sound clearly lies in eutrophication (Silberstein et al., 1986), which accompanied
industrial development, possibly exacerbated by a
minor reduction in the already limited exchange
between basin and ocean through causeway construction. Eutrophication combined with the dominance of P. sinuosa, a slow growing meadow forming
seagrass, resulted in extensive loss and little recovery over 30 years. In contrast, although eutrophication occurred on Parmelia Bank (Silberstein et al.,
1996) and to a lesser degree Success Bank, the
mixture of faster growing colonising species combined with slower growing, monospecific meadowforming species, greater hydrodynamic forcing and
therefore higher flushing rates and large scale sand
movement, all suggest the seagrasses on Parmelia
and Success Banks are in dynamic equilibrium
with their environment. This has resulted in greater