4.
Benthic Primary Producer Habitat
4.1.
Seagrass health
4.1.1.
Background
To manage the risk of potential declines in seagrass health caused by a reduction in light
from Project-related sediment plumes, Fremantle Ports undertook a Seagrass Health
Monitoring Program as a component of their Dredging and Spoil Disposal Management Plan
(DSDMP; Fremantle Ports 2010). The DSDMP required seagrass health monitoring to be
undertaken pre-dredging and post-dredging. Additionally, interim seagrass health monitoring
surveys were required if a “Level 4” LAC management trigger based on the light requirements
of seagrass was exceeded, as defined by Section 5.1.1 of the WQMP (Oceanica 2010c)
(further, see Section 3.3.4).
Statistically significant decreases (p<0.05) in mean shoot density between baseline and
interim (or post-dredge) seagrass health surveys at each site were compared against shoot
density triggers for seagrass management (defined within the DSDMP), noting that triggers
were not applicable at reference sites or within zones of predicted seagrass loss.
The health of seagrass stands of the indicator species Posidonia sinuosa, was measured by
determining shoot density at seagrass monitoring sites located adjacent to Project work areas
(Deep Water Channel Area, Offshore Disposal Area, Rous Head Reclamation Area and
Entrance Channel Area; Figure 1-1), on five separate occasions:
1. Baseline survey – baseline seagrass shoot density data were collected at all
monitoring sites in December 2009 (nearshore sites) and January 2010 (offshore
sites), i.e. before commencement of adjacent dredging works, to provide a basis for
comparison with subsequent seagrass health surveys.
Results of the baseline
seagrass survey were provided in the Compliance Assessment Report for Phase 1
works (Oceanica 2010b).
2. Interim I survey – seagrass shoot density was determined at all sites during March
2010 to assess any short-term change in seagrass health at completion of Phase 1
works. It is important to note that this first interim survey was not triggered by LAC
monitoring results of light availability during dredging, but was undertaken to obtain
additional data on any potential impacts. Results of the Interm I seagrass survey were
provided in the Compliance Assessment Report for Phase 1 works (Oceanica 2010b).
3. Interim II survey – seagrass shoot density was determined at sites adjacent to the
Offshore Disposal Area and Rous Head Reclamation Area between 7-12th October 2010
due to exceedance of Level 4 LAC management triggers at:
o two sites adjacent to the Offshore Disposal Area (SH49B and SH50B) between
10-24th September 2010; and,
o one site adjacent to the Rous Head Reclamation Area (SH36A), between
15-19th September 2010 (Oceanica 2010x).
4. Interim III survey – seagrass shoot density was determined at sites adjacent to the
Offshore Disposal Area, Rous Head Reclamation Area and Entrance Channel Area
between 3-12th November 2010 due to exceedance of Level 4 LAC management
triggers at:
o two sites adjacent to the Offshore Disposal Area (SH49B and SH 51) between
12-25th October 2010;
o three sites adjacent to the Rous Head Reclamation Area (SH10, SH11 and
SH36A) between 12th October – 3rd November 2010; and,
o one site adjacent to the Entrance Channel Area (SH18) between 11-17th
October 2010 (Oceanica 2010y).
5. Post-dredge survey – seagrass shoot density was determined at all sites between
7-15th December 2010 to assess any change in seagrass health following the
completion of dredging and spoil disposal operations on the 31 st October 2010
(Oceanica 2011f; Appendix F).
Oceanica: Fremantle Ports: Inner Harbour Deepening –Compliance Assessment Report, Phase 2
59
Interim surveys II and III were undertaken within two weeks of reporting exceedances of
„Level 4‟ LAC management triggers (see Section 3.3.4), and the resultant seagrass health
survey reports were provided to the OEPA (Oceanica 2010x,y). While Table 4-1 briefly
describes the results of all seagrass health surveys, the following summary focuses on results
of the post-dredge seagrass health survey (Oceanica 2011f; Appendix F).
Interim
I
Interim
II
Interim
III
Postdredge
60
17-25 Mar
2010
7-12 Oct
2010
3-12 Nov
2010
7-15 Dec
2010
Trigger
Not triggered (conservatively
undertaken for Proponent
information)
Exceedance of Level 4 LAC
management triggers at:
 two sites adjacent to the
Offshore
Disposal
Area
(SH49B
and
SH50B)
between 10-24 September
2010; and
 one site adjacent to the
Rous Head Reclamation
Area (SH36A), between
15-19 September 2010
Exceedance of Level 4 LAC
management triggers at:
 two sites adjacent to the
Offshore
Disposal
Area
(SH49B
and
SH
51)
between 12-25 October
2010;
 three sites adjacent to the
Rous Head Reclamation
Area (SH10, SH11 and
SH36A)
between
12
October – 3 November
2010; and
 one site adjacent to the
Entrance
Channel Area
(SH18) between 11-17
October 2010.
Completion of dredging and
spoil disposal operations on
the 31st October 2010
Areas monitored
# sites with significantly
lower than baseline mean
seagrass shoot density
Date
# reference sites
monitored
Survey
Interim and post-dredge seagrass health survey dates, triggers, areas monitored
and sites with significantly lower than baseline mean seagrass shoot density
# potential impact sites
monitored
Table 4-1
Deep Water Channel Area
4
4
0
Offshore Disposal Area
4
4
0
Rous Head Reclamation
Area
6
2
1 (SH38A)
Entrance Channel Area
4
2
0
Offshore Disposal Area
4
4
1 (SH34)
Rous Head Reclamation
Area
6
2
1 (SH36A)
Offshore Disposal Area
4
4
0
Rous Head Reclamation
Area
6
2
1 (SM52A)
Entrance Channel Area
4
2
0
Deep Water Channel Area
Offshore Disposal Area
4
4
4
4
0
0
6
2
4
2
Rous Head Reclamation
Area
Entrance Channel Area
2 (SH36A,
SM52A)
1 (SH45)
Oceanica: Fremantle Ports: Inner Harbour Deepening –Compliance Assessment Report, Phase 2
4.1.2.
Results
Deep Water Channel Area
Comparison against seagrass health triggers
During the post-dredge monitoring survey, seagrass health triggers were not exceeded within
the zone of potential influence (ZoI) or zone of potential effect (ZoE) adjacent to the Deep
Water Channel Area (i.e. there were no statistically significant declines in seagrass shoot
density at sites SH2, SH5, SH46A or SH47, compared to the baseline survey; Figure 4-1).
Figure 4-1
Comparison of Posidonia sinuosa mean shoot density recorded during baseline,
interim and post-dredge surveys of the Deep Water Channel Area. * - indicates a
significant difference (increase or decrease) between the post-dredge survey and
the baseline survey for that site. ZoI = zone of influence; ZoE = zone of effect
Oceanica: Fremantle Ports: Inner Harbour Deepening –Compliance Assessment Report, Phase 2
61
Offshore Disposal Area
Comparison against seagrass health triggers
During the post-dredge monitoring survey, seagrass health triggers were not exceeded within
the ZoI adjacent to the Offshore Disposal Area (i.e. there were no statistically significant
declines in seagrass shoot density at sites SH7 or SH51, compared to the baseline survey;
Figure 4-2).
Comparison against reference sites
Due to the absence of baseline data at sites SH49B and SH50B within the ZoE from offshore
disposal activities8, comparisons were made against reference sites at similar or deeper
depths to assess potential impacts. Statistical comparisons determined that post-dredge
mean shoot density at site SH50B was not significantly different to post-dredge mean shoot
density at reference site SH34, and was significantly higher than reference sites SH32 and
SH33 (Figure 4-2). Post-dredge mean shoot density at site SH49B was not significantly
different to post-dredge mean shoot density at reference sites SH32 and SH33, but was
significantly lower than reference site SH34 (Figure 4-2). It is noted that lower seagrass
densities at site SH49B are expected since it is located at a depth 4 m deeper than the
reference sites; unlike site SH50B which is located at a similar depth to reference sites.
Figure 4-2 Comparison of Posidonia sinuosa mean shoot density recorded during baseline, interim
and post-dredge surveys of the Offshore Disposal area. * - indicates a significant
difference (increase or decrease) between the post-dredge survey and the baseline
survey for that site. ** - indicates a significant difference compared to reference sites
during the post-dredge survey. ZoI = zone of influence; ZoE = zone of effect
8
These sites were re-located from the original SH49 and SH50 sites subsequent to Phase 1 works (Oceanica 2010b)
due to an approved re-orientation of the offshore spoil disposal ground, and OEPA request to move the sites to be
within close proximity to the new spoil ground boundary (for further discussion on relocation of the Offshore Disposal
Area, see Oceanica 2010b).
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Oceanica: Fremantle Ports: Inner Harbour Deepening –Compliance Assessment Report, Phase 2
Rous Head Reclamation Area
Comparison against seagrass health triggers
During the post-dredge monitoring survey, seagrass health triggers were not exceeded within
the ZoI, nor at one site within the ZoE, adjacent to the Rous Head Reclamation Area (i.e.
there were no statistically significant declines in seagrass shoot density at sites SH10A,
SH11A or SH12A, compared to the baseline survey; Figure 4-3). However, the post-dredge
mean shoot density of P. sinuosa at the other site within the ZoE (SH36A) was significantly
lower than the baseline shoot density, equating to an exceedance of the Level 2 management
trigger (Figure 4-3).
Results within the predicted zone of loss
Seagrass health triggers were not applicable within the predicted zone of loss (ZoL) as it was
assumed that seagrass within this zone would be lost due to physical disturbance and/or high
rates of sedimentation and turbidity. During the post-dredge survey, the ZoL mean shoot
density of P. sinuosa was not significantly different to baseline density shoots at site SM46A;
however, P. sinuosa could not be located at site SM52 within the ZoL due to loss of seagrass
(Figure 4-3).
Figure 4-3
Comparison of Posidonia sinuosa mean shoot density recorded during the baseline,
interim and post-dredge surveys of the Rous Head Reclamation Area. * - indicates a
significant difference (increase or decrease) between the post-dredge survey and
the baseline survey for that site. ZoI = zone of influence; ZoE = zone of effect; ZoL =
zone of loss
Oceanica: Fremantle Ports: Inner Harbour Deepening –Compliance Assessment Report, Phase 2
63
Entrance Channel Area
Comparison against seagrass health triggers
During the post-dredge monitoring survey, seagrass health triggers were not exceeded within
the ZoI or ZoE adjacent to the Entrance Channel Area (i.e. there were no statistically
significant declines in seagrass shoot density at sites WP1, SH43, SH41 or SH18, compared
to the baseline survey; Figure 4-4).
Reference site results
The mean shoot density at one reference site (SH45) for the Entrance Channel Area was
significantly lower during the post-dredge survey compared to the baseline survey; but there
was no significant difference in shoot density over the same period at the other reference site
(SH44) (Figure 4-4). Seagrass health triggers are not applicable at reference sites.
1600
Entrance Channel Area
Seagrass Shoot Density (n = 14)
1400
Mean Shoot Density (m-2)
1200
1000
800
Baseline
Interim 3
600
Post-Dredge
400
200
0
SH44
SH45*
Reference
Figure 4-4
4.1.3.
WP1*
SH43
ZoI
SH41
SH18
ZoE
Comparison of Posidonia sinuosa mean shoot density recorded during baseline,
interim and post-dredge surveys of the Entrance Channel Area. * - indicates a
significant difference between the post-dredge survey and the baseline survey for
that site. ZoI = zone of influence; ZoE = zone of effect; ZoL = zone of loss
Summary and Conclusions
In summary:
1. The post-dredge monitoring of P. sinuosa mean shoot density demonstrated no
significant declines in seagrass health at most (21 out of 24) sites.
2. While baseline shoot density data were not available within the ZoE adjacent to the
Offshore Disposal Area, post-dredge mean shoot density was either not significantly
different or significantly higher than at offshore reference sites of similar depth.
3. Two sites demonstrated significantly lower mean shoot densities during the postdredge survey than the baseline survey, including one site within the ZoE offshore of
Rous Head and one Entrance Channel reference site; however, the mean shoot density
recorded at these sites during interim surveys has previously fluctuated, and likely
exemplifies natural variability of shoot density within the seagrass meadows rather
than a response to changing light levels.
4. At one site within the predicted ZoL offshore of Rous Head (SM52A), P. sinuosa shoots
could not be located during the Post-Dredge survey due to loss of seagrass. However,
seagrass shoot density at the other monitoring site within the ZoL (SM46A) remained
intact and was not significantly different to that determined during the baseline
survey. As such, not all areas within the ZoL offshore of Rous Head have suffered a
decline in seagrass health let alone a loss.
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Oceanica: Fremantle Ports: Inner Harbour Deepening –Compliance Assessment Report, Phase 2
To further determine the extent of potential impacts from the Project on seagrass habitat, the
total area of seagrass change offshore of Rous Head and broader Project area was estimated
via benthic habitat mapping, in accordance with Ministerial Statement 801 (see Section 4.3).
4.2.
Coral health and cover
The Coral Monitoring Program was designed to monitor for any changes in the coral health at
Hall Bank and to assess whether any potential changes can be attributed to the dredging
program. The details of the monitoring program are outlined in the Dredging and Spoil
Disposal Management Plan (DSDMP; Fremantle Ports 2010).
A pre-dredging survey
established a baseline coral health at Hall Bank; and although water quality monitoring did
not trigger an additional survey, Fremantle Ports decided to undertake an additional „interim‟
survey following Phase 1 of project works (with results discussed in the previous Compliance
Assessment Report; Oceanica 2010b).
Coral health was also assessed following the
completion of major Phase 2 dredging works (SKM 2011; Appendix G), with methods and
results briefly summarised below.
Post-dredge coral health surveys were undertaken at monitoring sites on the northern (HBN)
and western (HBW) sections of Hall Bank on 17 November 2010. At each site, 60 coral
colonies that were considered to be representative of the species composition of the site were
tagged and photographed for analyses during baseline and subsequent surveys, including the
post-dredge survey. The whole-colony images were assessed for partial and total colony
mortality using Coral Point Count with Excel Extensions (CPCe). Areas within the boundary of
the coral colony that were not live coral were categorized using the categories defined in the
CPCe analysis.
The key findings of the post-dredge coral survey (SKM 2011; Appendix G) were as follows:
1. There was very little change in the percentage of coral mortality of the tagged corals
over the survey period. The gross coral mortality at the two monitoring sites (postdredge survey partial mortality minus baseline partial mortality) was below zero
(Figure 4-5). That is, over the duration of the monitoring survey the change in
mortality actually decreased at both sites. From the first baseline survey to the post
dredging survey the percentage coral mortality decreased from 1.53% to 1.23% at
HBN and from 0.55% to 0.38 % at HBW. Since no increase in gross mortality
occurred, the management levels defined in the DSDMP were not exceeded and no
management responses were required.
2. The sampling design had a minimum detectable difference in mortality of 3.59% at
Hall Bank North and 2.49% at Hall Bank West which was sufficient to detect a change
in mortality at the current coral management response levels of 5% and 8% mortality
as outlined in the DSDMP.
3. For the coral cumulative loss surveys, ten random video transects were analysed at
both sites during the baseline and post-dredging surveys. The results found that at
HBN the percentage coral cover decreased from 14.1% to 10.7%; while at HBW the
percentage coral cover increased from 24.0% to 27.9%. To determine if these
changes in coral cover were statistically significant a t-test (p<0.05) was performed
on the results from both sites. The results of the t-test suggest that the changes in
coral cover between baseline and post-dredging surveys at both HBN and HBW were
not statistically significant. In summary, there was no measurable loss of coral
habitat over the duration of the Project.
4. Separate observations indicated that anchoring of recreational vessels on Hall Bank
continues to remain an issue with the potential to cause non-dredging related
mortality to corals.
Oceanica: Fremantle Ports: Inner Harbour Deepening –Compliance Assessment Report, Phase 2
65
Figure 4-5
Average percentage of partial mortality of the coral colonies at Hall Bank northern
(HBN) and western (HBW) monitoring sites before, during and after the dredging
period.
4.3.
BPPH mapping and estimates of spatial change
4.3.1.
Background
A description of the existing marine environment surrounding Fremantle Port, including a
map of proximal benthic primary producer habitats (BPPH; i.e. seagrass, macroalgae and
coral habitats); and an assessment of the direct and indirect losses of BPPH potentially
resulting from the Project were provided as part of the Public Environmental Review (PER)
process (SKM 2009a,b). Direct losses of BPPH may be due to the footprint of dredging or
reclamation works, whereas indirect losses may be due to a reduction in light available to
benthic primary producers as a result of increased total suspended solids (TSS) released into
the water column from dredging activities.
As part of the environmental approval to implement the Project, Ministerial Statement 801
(dated 18 August 2009), detailed the following condition:
5-1
Prior to dredging activities the proponent shall prepare maps showing the:
1.
2.
3.
4.
5.
6.
Gage Roads Benthic Primary Producer Habitat Management Unit;
location and extent of the benthic primary producer habitat types;
boundaries of the direct disturbance area and predicted levels of impact, including
coordinates;
boundaries of the indirect disturbance area and predicted levels of indirect impact,
including coordinates;
the spatially defined offshore spoil disposal ground, including coordinates; and
boundary of the Rous Head reclamation area, including coordinates.
To satisfy the above-stated Ministerial Condition 5-1 a report was produced (Oceanica 2010z)
that provided a baseline map incorporating further groundtruthing to the originally acquired
satellite imagery (June 2007); which showed the benthic primary producer habitats of the
Gage Roads and broader management units, predicted zones of direct and indirect losses, the
offshore spoil disposal ground and the Rous Head reclamation area (Figure 4-6). This report
(Oceanica 2010z) was summarised in, and appended to, the previous Compliance
Assessment Report (Oceanica 2010b), which should be referred to for more detail including
methods and coordinates.
Conditions 5-2, 5-3 and 5-4 of Ministerial Statement 801 further define requirements for
monitoring and reporting BPPH losses realised during and after the dredging campaign, for
assessment against predicted BPPH losses; including a provision that the proponent should
66
Oceanica: Fremantle Ports: Inner Harbour Deepening –Compliance Assessment Report, Phase 2
ensure no losses of coral or macroalgal BPPH habitat and no more than 50 hectares of
seagrass BPPH loss, within the Gage Roads management unit.
Figure 4-6
Baseline benthic primary producer habitat map, showing predicted zones of
potential loss, effect and influence within the Project area.
Oceanica: Fremantle Ports: Inner Harbour Deepening –Compliance Assessment Report, Phase 2
67
4.3.2.
Predictions of BPPH loss
Utilising the baseline benthic habitat map (Figure 4-6) and dredge plume modelling as a basis
for assessment, the areas of BPPH that were predicted to be lost or affected by dredging and
disposal activities within the Gage Roads management unit are provided in Table 4-2 (as
described in the previous Compliance Assessment Report; Oceanica 2010b).
It is noted that revision of the baseline benthic habitat map (Oceanica 2010b) led to higher
loss estimates for seagrass (73 ha) and macroalgae (7 ha), but not coral, within the Gage
Roads management unit than was estimated within the Public Environmental Review
(SKM 2009a,b); and it is reiterated that Ministerial Statement 801 requires < 50 ha loss of
seagrass and no loss of macroalgae to occur from the Project. However, it is also noted that
modelling of the discharge plume from the Rous Head reclamation area did not take into
account any additional management strategies for minimising turbidity, such as silt curtains.
These additional management strategies limited the extent and intensity of the elevated TSS
plume being discharged from the reclamation area, which reduced the likelihood of predicted
impacts to BPPH occurring. Therefore while some losses of BPPH were predicted, realised
losses were expected to be within acceptable limits for cumulative loss of BPPH defined by
Ministerial Statement 801.
4.3.3.
Estimates of spatial change in BPPH
Overview
A post-dredge benthic habitat map (Figure 4-7) of the Gage Roads management unit was
compiled utilising satellite imagery obtained in March 2011 following Project completion (and
methods consistent with those utilised to produce the baseline benthic habitat map; further
see „Methods‟ below). To estimate the spatial change of BPPH (loss or gain) within the Gage
Roads management unit throughout a period including the duration of the Project, a
comparison was made between the BPPH areas defined within the post-dredge habitat map
and the revised baseline habitat map (based on June 2007 imagery; Figure 4-6). Estimates
of cumulative BPPH losses, including historical losses, are discussed in Section 4.3.4 within
the context of the EPA‟s (2004) Guidance Statement for BPPH Protection9.
Methods
The post-dredge habitat mapping process involved searching the Digital Globe image archive
for the best available satellite image for habitat mapping within the first three months
following completion of Project works, which was determined to be World View 2
multispectral imagery captured on 30th March 2011 (sourced through GeoImage)10. The
multispectral image had 8 bands including: Coastal, Yellow, Near Infrared 1, Near Infrared 2,
Panchromatic, Blue, Breen and Red bands.
Supervised image auto-classification was then undertaken to separate vegetated area from
bare sand using ERDAS Image (to classify the subsets of the Red Green Blue bundle image),
performed on more than 12 discrete map sections (split based on water depth and likely
extent of benthic habitats). Classified results were then processed as a shapefile in ArcGIS
10, with reference to the base multispectral image. Isolated patches of vegetated area less
than 15 m2 were merged with the surrounding sand polygon. Similarly, sand polygons with
area less than 15 m2 were merged with the surrounding habitat polygon.
Filtered vegetated polygons were then classified based on the existing baseline habitat layer.
Some polygons also required manual digitising of the features that could not be effectively
captured by the automated classification process. Expert advice from the field professionals
involved in the project was also incorporated.
Unfortunately, an area of approximately 1700 ha in the south west of the Gage Roads
management unit, and overlapping the Offshore Disposal Area, could not be classified due to
the presence of an apparent algal bloom preventing visual penetration to the benthic
9
Note that EPA (2004) was relevant at the time of the environmental impact assessment undertaken for this Project,
but has since been superseded by the EPA‟s (2009) Environmental Assessment Guideline No. 3 – Protection of BPPH
in Western Australia‟s Marine Environment.
10
Following comprehensive search of the archive, this was also the only satellite image available that satisfied both
the requisite timing and spatial coverage.
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Oceanica: Fremantle Ports: Inner Harbour Deepening –Compliance Assessment Report, Phase 2
environment and thus an inability to map habitats. Surface slicks caused by algal blooms
including the cyanobacteria Trichodesmium spp. are a common occurrence within Perth
coastal waters at this time of year.
Habitat categories delineated through this process were the same as identified during
baseline mapping.
The resulting habitat map is presented in Figure 4-7 and discussed below. This refined map
was “clipped‟ to determine the spatial coverage of habitat types within the Gage Roads
management unit.
Results and Discussion
Estimates of the spatial change of BPPH (loss or gain) within the Gage Roads management
unit between the baseline map (June 2007) and post-dredge map (March 2011), including
the duration of the Project, are provided in Table 4-2 and illustrated in Figure 4-8.
An estimated net gain in seagrass habitat of at least 48 ha was determined within the total
area of the Gage Roads management unit subsequent to Project completion (i.e. the
estimated area of seagrass habitat increased from 1534 ha within the baseline map to
1582 ha within the post-dredge map; equivalent to a 3.1% gain in seagrass habitat;
Table 4-2)11.
While up to 73 ha (4.8%) of seagrass habitat within the Gage Roads
management unit was predicted to be either directly or indirectly lost due to Project
dredging, reclamation and disposal activities (as being located within the Zone of Potential
Loss [ZoL] within the baseline benthic habitat map), it was ascertained from the post-dredge
habitat map that at least 49 ha of seagrass habitat remained within the ZoL subsequent to
Project completion; i.e. a loss of seagrass habitat only up to 24 ha within the ZoL (Table 4-2;
Figure 4-7). It is emphasised that potential variation in the quality/penetration of satellite
imagery between baseline and post-dredge mapping events may introduce a (nonquantifiable) level of error in habitat change estimates (e.g. better water penetration of postdredge imagery may lead to the inclusion of habitat areas not originally „seen‟ in poorer
baseline imagery, which may translate to a reported „gain‟ in habitat). If „gains‟ in seagrass
habitat are excluded and only „losses‟ are considered as a highly conservative worst-case
scenario of net loss within the ZoL, then it is estimated that 44 ha of seagrass were „lost‟
between baseline mapping and post-dredge mapping (Table 4-3) (i.e. only the areas within
the ZoL shaded red within Figure 4-8; but this estimate does not include any legitimate gains
in seagrass habitat over the same period)12. Nevertheless, the losses and gains of seagrass
habitat depicted in Figure 4-8, and the finding of 3.1% net gain of seagrass habitat across
the Gage Roads management unit, are consistent and comparable with the level of spatial
change in seagrass habitat locally documented over a similar period in Owen Anchorage
(Oceanica 2007) and offshore of Ocean Reef (Oceanica 2004).
An estimated net gain in macroalgal habitat of 8 ha was determined within the total area of
the Gage Roads management unit subsequent to Project completion (i.e. the estimated area
of macroalgal habitat increased from 117 ha within the baseline map to 125 ha within the
post-dredge map; equivalent to a 6.8% gain in macroalgal habitat; Table 4-2). While up to
7 ha (6.0%) of macroalgal habitat within the Gage Roads management unit was predicted to
be either directly or indirectly lost due to Project activities (as being located within the ZoL
within the revised baseline benthic habitat map), it was ascertained from the post-dredge
habitat map that 6 ha of macroalgal habitat remained within the ZoL subsequent to Project
completion; i.e. a loss of macroalgal habitat of only 1 ha within the ZoL (Table 4-2;
Figure 4-7). If only „losses‟ of macroalgal habitat are considered, then it estimated that
2.5 ha of macroalgae were „lost‟ within the ZoL between baseline mapping and post-dredge
mapping (Table 4-3); but this does not include any legitimate gains in macroalgal habitat
over the same period.
11
The gain of 48 ha is likely an underestimate as it does not include seagrass habitat unable to be mapped within
and surrounding the Offshore Disposal Area (as shown in Figure 4-7 and Figure 4-8).
12
This conservative estimate of seagrass loss within the ZoL also assumes complete loss of all seagrass habitat that
was present within the ZoL associated with the Offshore Disposal Area but could not be mapped for the post-dredge
map (i.e. the light grey shaded area within the ZoL of the Offshore Disposal Area depicted in Figure 4-8).
Oceanica: Fremantle Ports: Inner Harbour Deepening –Compliance Assessment Report, Phase 2
69
The coral community at Hall Bank was not predicted to experience any mortality as a result
of dredging and disposal activities, which was confirmed via post-dredge habitat mapping and
also by direct survey of post-dredge coral cover (Table 4-2; and further see Section 4.2).
Outcome
Ministerial Statement 801 (Condition 5-2) requires that there are no losses of coral or
macroalgal BPPH within the Gage Roads management unit caused by the Project, and that
losses of seagrass BPPH within this management unit caused by the Project do not exceed
50 ha. Based on comparison of the baseline BPPH map to the post-dredge BPPH map of the
Gage Roads management unit at completion of the Project, it was estimated that there was
a:
1. net gain in seagrass habitat of at least 48 ha (equivalent to a 3.1% gain in seagrass
habitat, from 1534 ha to 1582 ha);
2. net gain in macroalgal habitat of 8 ha (equivalent to a 6.8% gain in macroalgal
habitat, from 117 ha to 125 ha); and,
3. no change in the spatial extent of coral habitat (estimated at 5 ha before and after
Project completion).
Table 4-2
Habitat
Category
Coral
Macroalgae
Seagrass
Areas of BPPH within the Gage Roads management unit estimated from pre-dredge
baseline mapping and post-dredge mapping
Total Habitat Area within
Gage Roads Management
Unit
Habitat Area within Zone
of Potential [Direct +
Indirect] Loss
(ha)
(ha)
Habitat Area within Zone
of Potential Effect
Habitat Area within Zone
of Potential Influence
(ha)
(ha)
Baseline
map
Post-dredge
map
Baseline
map
Post-dredge
map
Baseline
map
Post-dredge
map
Baseline
map
Post-dredge
map
5
5
(0%)*
0
(0%)**
0
0
0
5
5
117
125
(+6.8%)*
7
(-6.0%)**
6
34
36
74
82
1534
1582
(+3.1%)*
73
(-4.8%)**
49
144
140
1163
1375
Notes:
* - parentheses indicate percentage of (actual) change in BPPH area, estimated from the difference in area of habitat between the postdredge map and baseline map (as a percentage of that habitat area in the baseline map).
** - parentheses indicate percentage of (predicted) change in BPPH spatial extent, estimated from area of habitat in the Zone of Potential
Loss within the baseline map as a percentage of the total area of that habitat within the baseline map.
Table 4-3
Areas of macroalgal and seagrass habitat change within the Zone of Potential Loss
(ZoL) of the Gage Roads management unit, estimated by comparison of post-dredge
mapping to pre-dredge baseline mapping
Habitat Area within Zone of Potential [Direct + Indirect] Loss
(ha)
Habitat
Category
Macroalgae
Seagrass
Post-dredge map
Net (= Gain +
Unchanged)
Gain
Unchanged
Loss
6
1.6
4.6
2.5
49
19
30
44*
Notes:
* - includes 25 ha of seagrass habitat mapped as loss by comparison of the post-dredge map and baseline map (i.e. the area shaded
red in within the ZoL depicted in Figure 4-8), plus 19 ha of seagrass habitat within the ZoL associated with the Offshore Disposal Area
that were unable to be mapped during post-dredge mapping (and conservatively assumed lost; i.e. the area shaded light grey in
Figure 4-8).
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Oceanica: Fremantle Ports: Inner Harbour Deepening –Compliance Assessment Report, Phase 2
Figure 4-7
Post-dredge benthic primary producer habitat map, showing predicted zones of
potential loss, effect and influence within the Gage Roads management unit.
Oceanica: Fremantle Ports: Inner Harbour Deepening –Compliance Assessment Report, Phase 2
71
Figure 4-8
72
Areas of habitat change between baseline and post-dredge mapping, within the
Gage Roads management unit
Oceanica: Fremantle Ports: Inner Harbour Deepening –Compliance Assessment Report, Phase 2
4.3.4.
Estimates of cumulative spatial change in BPPH
For the purposes of the BPPH assessment undertaken as part of the Project‟s Public
Environmental Review (PER; SKM 2009a,b), the Gage Roads BPPH management unit was
classified as a “Category E” development area within the context of the EPA‟s (2004)
Guidance Statement for BPPH Protection9; which defined an applicable cumulative loss
threshold for BPPH of 10%. Cumulative losses include those caused by the Project plus
historic losses. Assessments to estimate the cumulative spatial changes in BPPH caused by
the Project together with historic losses were undertaken at various stages of
implementation, including: (i) during the Public Environmental Review process (PER;
SKM 2009a,b); (ii) following the update of the baseline BPPH map prior to Project
commencement; and (iii) subsequent to completion of post-dredge BPPH mapping. The
following section discusses the “evolution” of these various estimates of cumulative BPPH
change, as outlined in Table 4-4.
The BPPH assessment undertaken as part of the Public Environmental Review (PER;
SKM 2009a,b) estimated that historic changes in coastline prior to Project commencement
had resulted in approximately 58 ha or 5.2% of total BPPH loss within the Gage Roads
management unit, including 47 ha of seagrass habitat loss and 11 ha of macroalgal habitat
loss (with no historical loss of coral habitat) (Table 4-4). With dredging commencing in
January, the worst-case estimate of BPPH losses due to Project implementation predicted
within the PER was a further 69 ha of seagrass habitat loss (with no further loss of
macroalgal or coral habitat); resulting in a cumulative loss of 11.5% (Table 4-4). While this
estimate of BPPH loss was above the EPA‟s (2004) applicable cumulative loss threshold of
10%, the worst-case nature of the BPPH loss estimate was emphasised, and it was argued
that “the predicted loss is a conservative over-estimation, modelled on an unmanaged outfall
and actual losses will not exceed the cumulative loss threshold” (SKM 2009b).
Subsequent to release of Ministerial Statement 801, the baseline BPPH map for the Gage
Roads management unit was updated, which led to a higher worst-case estimate of 80 ha
total BPPH loss resultant from the Project, including the occurrence of 73 ha of seagrass
habitat and 7 ha of macroalgal habitat (but no coral) within the predicted Zone of Potential
Loss (see Section 4.3.2). However, the updated baseline habitat map also estimated a much
greater extent of seagrass habitat (1534 ha), macroalgal habitat (117 ha) and coral habitat
(5 ha) within the entire Gage Roads management unit than estimated during the PER
assessment; such that the predicted total cumulative BPPH loss of 8.1% (given the same
estimates of historic loss) was proportionally lower than that estimated during the PER
assessment (11.5%), and within the EPA‟s (2004) applicable cumulative loss threshold of
10% (Table 4-4).
Finally, the BPPH assessment based on post-dredge mapping estimated net gains in seagrass
and macroalgal habitat and no change in coral habitat since baseline mapping (as described
in Section 4.3.3), which equated a 3.4% gain of total BPPH within the Gage Roads
management unit (Table 4-4). Given that the updated baseline map estimated a larger area
of total BPPH habitat than within the PER assessment, the absolute historical loss estimate of
total BPPH (58 ha) equated to a lower proportional estimate of historical loss (3.4%)
(Table 4-4). The estimated gains in total BPPH habitat since baseline mapping have thus
offset historical losses, resulting in a cumulative net loss of total BPPH of 0.1%, which is
within the EPA‟s (2004) applicable cumulative loss threshold of 10% (Table 4-4).
Oceanica: Fremantle Ports: Inner Harbour Deepening –Compliance Assessment Report, Phase 2
73
Table 4-4
Areas of BPPH and spatial change within the Gage Roads management unit,
estimated from the Public Environmental Review (SKM 2009a,b), updated baseline
map (Oceanica 2010b) and post-dredge map assessments (this report)
PER Assessment
Habitat
Coral
Unit of measure
Area
Historical
3
Spatial change (ha)
Spatial change (%)
PreProject
Updated Baseline Map Assessment
Post-Project
(predicted)
Historical
3
3
5
0
0%
Cumulative spatial
change (%)
Macroalgae
Area
44
Spatial change (%)
33
1061
Spatial change (%)
Area
Spatial change (ha)
Spatial change (%)
Cumulative spatial
change (%)
5
0
0
0
0*
0%
0%
0%
0%*
0%
0%
110
125
128
117
-11
0
-11
-7
+8*
-25.0%
0%
-8.6%
-5.5%
+6.8%*
-14%
-2.3%
1014
945
1534
1461
1582
-47
-69
-47
-73
+48*
-4.4%
-6.5%
-3.0%
-4.6%
+3.1%*
-7.6%
+0.1%
1656
1576
1712
Cumulative spatial
change (%)
Total BPPH
5
-25%
Spatial change (ha)
1581
-10.9%
1108
Post-Project
5
33
Cumulative spatial
change (%)
Area
Post-Project
(predicted)
0%
Spatial change (ha)
Seagrass
PreProject
Post-dredge
Assessment
1050
981
-58
-69
-58
-80
+56*
-5.2%
-6.2%
-3.4%
-4.7%
+3.4%*
-11.5%
-8.1%
-0.1%
-10%
-10%
-10%
EPA (2004) Category E Loss
Threshold
1714
Note: * - spatial change since Pre-Project (updated) baseline mapping.
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Oceanica: Fremantle Ports: Inner Harbour Deepening –Compliance Assessment Report, Phase 2