Parks Victoria Technical Paper Series No. 86
Mud Islands Seagrass and Coastline
Mapping 2011–12
David Ball
Department of Primary Industries
January 2013
Parks Victoria Technical Series No. 86
Mud Islands Mapping 2011-12
EXECUTIVE SUMMARY
Mud Islands supports the largest colonies of breeding waterbirds and seabirds in Port Phillip
Bay and is a Ramsar site. It also forms part of the Port Phillip Heads Marine National Park.
Mud Islands is a dynamic coastal system and has undergone large historical changes in its
position, size and orientation due to erosion and deposition of sand from shoals in the
surrounding Great Sand.
The primary objective of this project was to monitor seagrass habitat and coastline changes
at Mud Islands - Port Phillip Heads Marine National Park from aerial photography during
2011 and 2012.
Automated image processing techniques combined with manual editing were applied to map
the seagrass cover and coastline from the existing digital aerial photography captured on 7
April 2011 and 23 January 2012. Previous observations at Mud Islands showed that the
seagrass is predominantly Zostera nigricaulis with smaller areas of the intertidal Zostera
muelleri. The aquatic macrophyte Lepilaena marina is also present growing amongst the
intertidal seagrass. Small beds of Caulerpa spp. are intermixed with the seagrass beds on
the eastern coast.
While the majority of seagrass remained unchanged between 2011 and 2012, seagrass
losses were observed on the outer edges of beds along the western shore. This seagrass
loss was probably due to sand deposition in the shallow regions on the western shores.
Conversely, seagrass cover increased in the deeper regions seaward of the main seagrass
beds on the northern and eastern shores in areas which appear to be subject to less wave
energy and lower rates of sand movement.
Other seagrass losses were observed within the central lagoon area, although this area has
not been previously ground-truthed and some of the vegetation loss may be due to changes
in macroalgae cover.
The major coastline change affected the southern entrance to the central lagoon where a
sand spit which had formed across the entrance since before 2008 was breached between
April 2011 and January 2012, most likely due to storm waves hitting the southern shore. A
new southern entrance approximately 15 m wide is now located where the sand spit was
breached. The other main coastline change observed was a reduction in the beach width on
the southwestern shore of the western island.
It is recommended that Parks Victoria continue to monitor coastline and seagrass changes at
Mud Islands through aerial imagery interpretation, with a particular focus on the status of the
two entrances to the central lagoon and the effect of sand deposition on seagrass beds along
the western shores.
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Parks Victoria Technical Series No. 86
Mud Islands Mapping 2011-12
CONTENTS
FOREWORD [OPTIONAL] .............................ERROR! BOOKMARK NOT DEFINED.
EXECUTIVE SUMMARY............................................................................................ II
CONTENTS...............................................................................................................
III
INDEX OF FIGURES AND TABLES......................................................................... IV
SECTION 1 INTRODUCTION ................................................................................... 1
1.1 Objectives......................................................................................................................... 1
SECTION 2 METHODS ............................................................................................. 2
Study Site ..............................................................................................................................................2
Aerial Photography ................................................................................................................................2
Mapping .................................................................................................................................................3
Ground-truthing .....................................................................................................................................3
SECTION 3 RESULTS .............................................................................................. 4
Seagrass and Coastline Mapping ........................................................................................... 4
Seagrass Changes ................................................................................................................................4
Coastline Changes ................................................................................................................................6
Historical Changes ................................................................................................................................9
Mapping Accuracy .............................................................................................................................. 11
SECTION 4 DISCUSSION ...................................................................................... 12
ACKNOWLEDGMENTS ........................................................................................... 13
REFERENCES ......................................................................................................... 14
III
Parks Victoria Technical Series No. 86
Mud Islands Mapping 2011-12
INDEX OF FIGURES AND TABLES
FIGURES
Figure 1. Mud Islands study area ........................................................................................... 2
Figure 2. Mud Islands aerial photography 7 April 2011 (left) and 23 January 2012 (right)....... 4
Figure 3. Mud Islands habitat mapping 7 April 2011............................................................... 5
Figure 4. Mud Islands habitat mapping 23 January 2012 ....................................................... 5
Figure 5. Seagrass cover and beach changes at Mud Islands between 2011 and 2012 ......... 6
Figure 6. Sand deposition in the shallow region on the northwest shore of Mud Islands
between 1990 and 2012 ........................................................................................ 7
Figure 7. Coastline changes at the southern entrance to Mud Islands’ central lagoon between
1990 and 2012....................................................................................................... 8
Figure 8. Mud Islands coastal topography and seagrass changes 1939 to 2012 (1939 to
2009 data from Ball et al. (2009)) ......................................................................... 10
TABLES
Table 1. Seagrass and coastal topography area changes at Mud Islands .............................. 9
4
SECTION 1 INTRODUCTION
Mud Islands supports the largest colonies of breeding waterbirds and seabirds in Port Phillip
Bay and is a Ramsar site. It is also forms part of the Port Phillip Heads Marine National Park
and is listed on the Register of the National Estate. Mud Islands is a dynamic coastal system
and has undergone large historical changes in its position, size and orientation due to
erosion and deposition of sand from shoals in the surrounding Great Sand. Mud Islands is
currently characterised by two adjacent islands enclosing a shallow central lagoon with two
narrow entrances at its northern and southern ends. The ongoing changes in coastal
topography at Mud Islands, and particularly the deposition of sand on its western shores,
have affected seagrass habitat at the site.
1.1 Objectives
The project objectives were to:
•
Monitor annual seagrass habitat and coastline changes at Port Phillip Heads Marine
National Park – Mud Islands from aerial photography.
•
Detect changes in the status of the Mud Islands’ coastline and entrances to the central
lagoon which may impact shorebird habitats.
•
Provide information on coastal dynamics at Mud Islands and their potential impact on
maintenance of natural values.
SECTION 2 METHODS
Shallow seagrass habitat and the coastline at Mud Islands were mapped from aerial
photography from 2011 and 2012.
Study Site
The study area was the extent of the Mud Islands - Port Phillip Heads Marine National Park
(Figure 1).
Figure 1. Mud Islands study area.
Aerial Photography
Aerial image interpretation is an effective method for mapping the distribution of shallow
marine habitats where there is clear seawater and the imagery has been collected under
optimum conditions which include (Finkbeiner et al. 2001):
•
Clear sunny conditions, minimal cloud and/or haze.
•
Optimum sun angle of 20–30°to avoid solar flaring on the sea surface.
•
Low wind to avoid surface ripples and waves.
•
Low swell on the day of collection, and on the preceding day to minimise suspended
sediment in the water column.
Low-tide, preferably within 1 hour of low-tide.
•
Suitable aerial photography covering the study area flown on 7 April 2011 and 23 January
2012 was available from the Department of Primary Industries and Department of
Sustainability and Environment respectively. The digital aerial photography had three visible
colour bands (red, green, blue) and also included an infrared band. The pixel resolution of
the digital photography was 30 cm in 2011 and 35 cm in 2012.
Mapping
Automated image processing techniques combined with manual editing were applied to map
the seagrass cover and coastline from the digital aerial photography.
Light attenuation through the water column affects the reflectance values of seabed habitats
at different depths visible in aerial photography. At Mud Islands these difference in habitats
visible in the aerial photography can be observed between shallow depths (up to
approximately 2 m) and deeper depths (>2 m). This can cause habitats visible in deeper
water to appear similar to different habitats in shallow water (e.g. shallow sparse seagrass
and deep bare sediment). To minimise this effect, the aerial photography was split into
separate mapping regions representing shallow/intertidal areas, deep areas and the central
lagoons. The area covering the sand beaches was also split into a separate mapping region
and the sand dunes/salt marsh areas were masked in the image so that they were not
included in the habitat classification.
Unsupervised classification is a method to transform a digital image into a thematic map. An
unsupervised classification defines those groups of pixels with a similar spectral signature
and which are assumed to represent the same type of habitat (Anon. 1998). An unsupervised
classification was applied to each aerial photograph mapping region using all four bands (i.e.
red, green, blue, infrared). The output from this process was a single band image with 5–15
pixel values representing the number of groups or categories defined by the unsupervised
classification.
Following each unsupervised classification, a low pass filter was applied to smooth out any
image noise. The classified image was then overlayed on the aerial imagery to allow a visual
comparison. The pixel values within the classified image were then assigned to a thematic
classification based on the dominant habitat type as follows:
• Sand beach.
•
Bare intertidal/subtidal sediment (no visible seagrass).
•
•
•
Sparse seagrass (more sediment than seagrass visible within a seagrass bed).
Medium-dense seagrass (more seagrass than sediment visible in a seagrass bed).
Undefined (seabed not clearly visible in image).
The final phase of the mapping was to convert the classified raster images to vector layers
with a smoothing algorithm in ArcGIS. The polygon layers from the different photography
mapping regions were merged into a single layer for each year in ArcGIS. The Eliminate
function in ArcGIS was then used to remove all polygons with an area <2 m2 which was
assessed to be the smallest habitat feature which could be consistently differentiated from
the aerial photographs.
The unsupervised classification mapping approach succeeded in accurately differentiating
most of the seagrass habitat boundaries. Factors such as ripples on the sea-surface,
turbidity and/or depth influences prevented the unsupervised classification method from
accurately delineating habitat boundaries at some places within the image. These anomalies
and errors from the automated mapping were fixed by manually correcting the habitat
boundaries in ArcGIS.
Ground-truthing
No aerial photography ground-truthing was undertaken as part of the current study. However,
the northern and eastern seagrass beds at Mud Islands were ground-truthed annually with
underwater video between 2008 and 2011 for the Channel Deepening Baywide Seagrass
Monitoring Program (Hirst et al. 2009a, b, 2010, 2011).
SECTION 3 RESULTS
Seagrass and Coastline Mapping
The seagrass and coastline were mapped from aerial photography captured on 7 April 2011
and 23 January 2012 (Figure 2). The 2011 and 2012 mapping are presented in Figure 3 and
Figure 4 respectively. Previous video and SCUBA observations showed that the seagrass at
Mud Islands is predominantly Zostera nigricaulis with smaller areas of the intertidal species
Zostera muelleri (Hirst et al. 2009a, b, 2010, 2011). The aquatic macrophyte Lepilaena
marina is also present growing amongst the intertidal seagrass (Hirst et al. 2012).
Figure 2. Mud Islands aerial photography 7 April 2011 (left) and 23 January 2012 (right).
Seagrass Changes
Changes in seagrass habitat between 2011 and 2012 are presented in Figure 5. While the
majority of seagrass remained unchanged during this period, seagrass losses were observed
on the outer edges of beds along the western shore. This seagrass loss was most likely due
to sand deposition in the shallow regions on the western shores shown in Figure 6. A sand
shoal encroaching into the western boundary of the seagrass bed on the northwestern shore
between 2008 and 2011 continued to extend eastward in 2012 smothering seagrass in its
path. Other seagrass losses were observed within the central lagoon area, although this area
has not been previously ground-truthed and some of the vegetation loss may be due to
changes in macroalgae cover.
Seagrass cover increased between 2011 and 2012 in the deeper regions seaward of the
main seagrass beds on the northern and eastern shores in areas which appear to have less
wave energy and lower rates of sand movement.
Figure 3. Mud Islands habitat mapping 7 April 2011.
Figure 4. Mud Islands habitat mapping 23 January 2012.
Figure 5. Seagrass cover and beach changes at Mud Islands between 2011 and 2012.
Coastline Changes
Coastline changes at Mud Islands between 2011 and 2012 are shown in Figure 5. The major
coastline change affected the southern entrance to the central lagoon. A sand spit gradually
extended from the southern tip of the western island across the southern entrance from
sometime between 1990 and 2008 until 2011 (Figure 7). This spit had nearly blocked the
southern entrance by April 2011, reducing it to a narrow channel approximately 5–10 m wide.
The spit was breached between April 2011 and January 2012, most likely due to storm
waves hitting the southern shore. A new southern entrance approximately 15 m wide is now
located where the sand spit was breached and the coastline at this location has returned to a
position similar to 1990 before the spit formed (Figure 7). The breaching of the spit has also
created a small lagoon on the southern shore of the eastern island.
The other main coastline change observed between 2011 and 2012 was a reduction in the
beach width at places on the southwestern shore of the western island (Figure 5).
Figure 6. Sand deposition in the shallow region on the northwest shore of Mud Islands
between 1990 and 2012.
Figure 7. Coastline changes at the southern entrance to Mud Islands’ central lagoon
between 1990 and 2012.
Historical Changes
The sandy islands that make up Mud Islands have undergone large changes in their position,
size and orientation since the earliest historical survey in 1836 (Bird 1993). These changes
were attributed to erosion and deposition, with phases when sand from the neighbouring
shoals of the Great Sand were washed ashore.
Changes in the Mud Islands’ coastal topography and seagrass habitat since the earliest 1939
aerial photography are presented in Figure 8. The total areas of seagrass and land at Mud
Islands since 1939 are summarised in Table 1.
Table 1. Seagrass and coastal topography area changes at Mud Islands.
2
2
Year*
Number of Islands
Land area (km )
Seagrass area (km )
1939
3
0.61
1.05
1984
3
0.65
1.48
2009
2
0.66
0.82
2011
2
0.69
0.85
2012
2
0.69
0.81
*1939 to 2009 data from Ball et al. (2009).
The earliest aerial photograph of Mud Islands flown in 1939 showed that Mud Islands
consisted of three islands surrounding a shallow inner lagoon (Figure 8). Subsequent aerial
photography showed a continuing pattern of coastline changes up to the early 1980s. The
topography of the islands has been relatively stable since 1984 (Figure 8), with a gap in the
inner lagoon between the two western islands filling with sand to form a single western island
by 2009 (Figure 8). The inner lagoon remains open to Port Phillip Bay via narrow southern
and northern entrances separating the two islands.
The changes in coastal topography at Mud Islands have influenced the location and size of
seagrass habitat. Most of the seagrass habitat in 1939 was located in the inner lagoon
between the two western islands, and to the north and west of these islands (Figure 8). The
greatest area of seagrass was observed in 1984 when seagrass was distributed around all
the islands and within the central lagoon. Since 1984 the seagrass habitat has reduced in
total area with most of the seagrass decline occurring on the western and southern shores,
with additional declines on the outer edge of seagrass beds on the eastern shore. Most of
the seagrass at Mud Islands is now located on the northern and eastern sides of the eastern
island and in the enclosed inner lagoon (Figure 8).
Figure 8. Mud Islands coastal topography and seagrass changes 1939 to 2012 (1939 to
2009 data from Ball et al. (2009)).
Mapping Accuracy
Previous seagrass mapping at Mud Islands applying the same methods as the current study
achieved a 100% mapping accuracy in distinguishing vegetated areas from bare sediment
(Hirst et al. 2009a, b, 2010, 2011). The high mapping accuracy was due to clear water and
shallow depths which characterise the site combined with distinct colour differences between
shallow vegetated areas and light coloured bare sediment being clearly visible in aerial
photography. Small beds of Caulerpa spp., including Caulerpa brownii and Caulerpa
scalpelliformis, were observed intermixed with the seagrass beds on the eastern coast during
video ground-truthing surveys in 2009 and 2010 (Hirst et al. 2009b, 2010). It was not
possible to separate these areas of green algae from the surrounding seagrass through the
aerial photography interpretation method applied in this study, although they only
represented a small proportion of the total vegetated area in 2009 and 2010.
The vegetation in the inner lagoon was not ground-truthed during the previous video surveys
and there may be stands of macroalgae amongst the areas classified as seagrass resulting
in an overestimate of the total area of seagrass.
The increased seagrass cover at the deep region on the seaward edge of the northern
seagrass beds mapped in 2012 was not ground-truthed and there are no field observations
to confirm that the increase in vegetation was seagrass. However, the colour/texture of the
vegetation increase visible in the 2012 aerial photograph at this location was consistent with
medium-dense seagrass elsewhere in the previous aerial photography.
SECTION 4 DISCUSSION
This project demonstrated that aerial photography interpretation is an effective method to
monitor coastline and seagrass changes at Mud Islands. The aerial photography highlighted
the dynamic coastal system at Mud Islands and the impact of changes in coastal topography
on seagrass habitat over recent decades. The ongoing process of sand deposition observed
on the western shores in recent years and the resulting reduction in overall seagrass habitat
at this region appears likely to continue.
The southern entrance to the central lagoon almost closed in 2011–12 due to the formation
of a sand spit over previous years. This spit was breached by January 2012, re-establishing
a new wider entrance close to its previous position in 1990. The closure of one or both
entrances to the central lagoon due to sand deposition and/or the formation of sand spits
may cause large changes to seagrass habitat in the lagoon area. Any changes to water
exchange to the central lagoon is also likely to affect shorebird habitat at Mud Islands.
Ongoing monitoring of the status of these entrances will provide important information for the
management of Mud Islands by Parks Victoria.
It is recommended that Parks Victoria continue to monitor coastline and seagrass changes at
Mud Islands through aerial imagery interpretation, with a particular focus on the status of the
two entrances to the central lagoon and the effect of sand deposition on seagrass beds along
the western shores. Aerial photography is a cost-effective imagery type for this monitoring,
but is limited to defining the total extent of vegetated areas. New multi-spectral highresolution satellite imagery (e.g. WorldView2) offers a potential means to differentiate major
subtidal vegetation categories through aerial imagery interpretation combined with field
measurements with a spectroradiometer. A joint project between DPI and Deakin University
to map seagrass at Yaringa and French Island Marine National Parks with funding from
Parks Victoria is currently investigating the application of WorldView2 satellite imagery to
subtidal habitat mapping. The outcomes of this work are likely to be applicable to future
habitat monitoring at Mud Islands.
It is also recommended that additional ground-truthing of the vegetated areas in the shallow
central lagoon and the deeper regions on the northern shores of Mud Islands be undertaken
to support any future mapping from aerial imagery and to confirm the presence of seagrass
and other vegetation types in these areas.
ACKNOWLEDGMENTS
This project was coordinated by Steffan Howe at Parks Victoria. Peter Young, Mariela SotoBerelov, Simon Heislers and Sean Blake contributed to seagrass mapping and video groundtruthing at Mud Islands between 2008 and 2011 for the Baywide Seagrass Monitoring
Program. Michael Conroy at the Department of Sustainability and Environment provided
access to the 2012 aerial photography through the DSE Coordinated Imagery Program.
REFERENCES
Anon. (1998) ER Mapper User Guide. 10th rev. edn. Earth Resource Mapping Pty Ltd, West
Perth.
Ball D., Soto-Berelov M., Young P. & Coots, A. (2009) Baywide Seagrass Monitoring
Program - Historical Seagrass Mapping. Fisheries Victoria Technical Report Series No. 70.
Department of Primary Industries, Queenscliff, Victoria.
Bird E.C.F. (1993) The Coast of Victoria. The Shaping of Scenery. Melbourne University
Press, Melbourne.
Finkbeiner M., Stevenson B., Seaman R. (2001) Guidance for Benthic Habitat Mapping: An
Aerial Photographic Approach. U.S. NOAA Coastal Services Center, Charleston, SC.
Hirst A., Ball D., Heislers S., Young P., Blake S., Coots A. (2009a) Baywide Seagrass
Monitoring Program, Milestone Report No. 2 (2008). Fisheries Victoria Technical Report No.
29, January 2009. Department of Primary Industries, Queenscliff, Victoria.
Hirst A., Ball D., Heislers S., Blake S., Coots A. (2009b) Baywide Seagrass Monitoring
Program, Milestone Report No. 6 (July-August 2009). Fisheries Victoria Technical Report
Series No. 71, November 2009. Department of Primary Industries, Queenscliff, Victoria,
Australia.
Hirst A., Ball D., Heislers S., Young P., Coots A., Blake S. (2010) Baywide Seagrass
Monitoring Program, Milestone Report No. 10 (July–August 2010). Fisheries Victoria
Technical Report Series No. 115, November 2010. Department of Primary Industries,
Queenscliff, Victoria.
Hirst A., Ball D., Heislers S., Kent J., Young P., Blake S., & Coots A. (2011) Baywide
Seagrass Monitoring Program, Milestone Report No. 14 (July–August 2011). Fisheries
Victoria Technical Report Series No. 151, November, 2011. Department of Primary
Industries, Queenscliff, Victoria.
Hirst A., Heislers S., Ball D., Blake S. and Coots A. (2012) Baywide Seagrass Monitoring
Program, Milestone Report No. 15 (October–November 2011). Fisheries Victoria Technical
Report Series No. 164, January 2012. Department of Primary Industries, Queenscliff,
Victoria.
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parks and includes both marine and terrestrial components.
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A broad range of environmental research and monitoring activities
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management decisions. This Technical Series highlights some of
the environmental research and monitoring activities done within
Victoria’s protected area network.
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For more information contact the Parks Victoria Information Centre
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