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CHAPTER 2
MAPPING OF VICTORIA’S
NEARSHORE MARINE
BENTHIC ENVIRONMENT
Ralph Roob
Mapping of Victoria’s Nearshore Marine Benthic Environment
Chapter 2
CHAPTER 2
MAPPING OF VICTORIA’S NEARSHORE MARINE BENTHIC
ENVIRONMENT
2.1
Introduction
This chapter documents the mapping of Victoria’s nearshore sub-tidal marine habitats using a
combination of remote sensing and underwater survey techniques. Surveys were conducted in
stages over a four year period (1995-1999) employing a combination of Landsat Thematic
Mapper (TM) imagery, aeromagnetic and hydro-acoustic remote sensing techniques to
develop a 1:100,000 scale map depicting the distribution of broad substratum type classes for
Victoria’s open coast in waters generally < 30 m deep. Surveys were also extended into
deeper water (generally > 30 m deep) and out to Victoria 3 nm territorial waters for selected
offshore areas. The mapping was supplemented in places with ground truthing observations
from bounce dives, video deployment and collection of benthic samples.
The objectives of the mapping were to:
•
•
•
identify the broad substratum type classes (eg reef and sand) occurring in shallow subtidal waters across Victoria’s open coast using airborne and satellite remote sensing;
further refine the classification of substratum type classes from field surveys using hydroacoustic sonar techniques, video drops and benthic sampling; and
qualitatively describe the, geology and dominant epibiota of shallow subtidal reefs in
selected areas.
2.2
Technology Available to Map Marine Benthic Habitats
Modern advances in remote sensing technology, positioning systems, high-resolution video
and GIS technology have enabled the mapping of underwater marine habitats possible with a
relatively high degree of accuracy. This section introduces the range of technologies that
were employed during the project.
2.2.1
Positioning Systems
Global Positioning Systems
The Global Positioning System (GPS) developed by the US Department of Defence, employs
satellites to provide instantaneous 3 dimensional coordinates anywhere in the world. Civilian
GPS receivers utilise broadcast codes with introduced errors, termed “Selective Availability”,
from these satellites to return deliberately degraded positional accuracy of approximately 100
m (Frost and MacLeod 1991).
The full configuration of the GPS space segment comprises 24 satellites, orbiting every 12
hours at an altitude of 20,000 km. Each satellite transmits its own unique digital code
containing the orbital location. A GPS receiver determines its position by calculating pseudorange measurements to satellites in view, three are required for a two dimensional fix while
four are needed for a three dimensional fix (AMSA 1994).
The operation of GPS is the process of continuous coordination between the ground/control
segments and space segments. The ground/control segments transmit navigation messages,
that contain error corrections to the satellites of the space segment where the pseudo-range
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measurements are broadcast to GPS users. The user segment consists of a receiver that
consists of hardware and software for receiving, decoding, storing and processing collected
data for the determination of the receiver’s position (Mok 1995).
Differential GPS
Differential GPS (DGPS) can be used to improve the accuracy of position measurement to
within 5 m. DGPS receive corrections in real time by interfacing with a radio link between a
reference transmitter and receiver beacon. A reference transmitter / receiver beacon is placed
at a known location and the temporal variation between its computed position and the known
position is transmitted to improve the accuracy of another receiver beacon. Residual errors
are usually due to atmospheric conditions and differences between time clocks in the two
receivers.
There are two correction methods employed to provide DGPS positions. One determines a set
of pseudo-range corrections (PRC), then broadcasts them to the remote receiver where it is
applied to the pseudo-range measurements it is receiving. The other method known as block
shift (BS) determines the positional correction (delta Latitude, delta Longitude and delta
Height) based upon the known reference station position and the position determined using
satellite signals. These corrections are in turn transmitted to the remote station. The two
methods produce similar accuracies providing they are observing the same satellites. The
PCR method provides a more rigorous solution as only range measurements from satellites
common at both the base and remote sites are used in the computations.
2.2.2
Remote sensing
Satellite imagery – Landsat TM
Advances in remote sensing technologies enable the mapping of physical parameters to a
depth of up to 30 m in most areas. Landsat TM is a multi-spectral passive sensor that
provides imagery with pixel resolution of 30 m, and bands with wavelengths capable of
penetrating the water column (Fig 2.1). This sensor provides adequate detail to map substrata
at a scale of 1:100,000. Landsat TM is a highly effective and cost efficient remote sensing
system due to its water penetration capacity, relatively large spatial extent of scenes (185 x
185 km) and the regular overpass frequency (1 pass every 16 days) (Thulin and Lewis 1995).
The depth to which light penetrates the water column is strongly wavelength dependent
(Jerlov 1976). Figure 2.1 shows water penetration data plotted against wavelength for a range
of water types, including shallow coastal and deeper ocean waters (see Corner and Lodwick
1992). Longer wavelengths are capable of penetrating turbid waters while shorter
wavelengths in the 0.50 to 0.70 um are more suitable for penetrating clearer waters to a
greater depth (Tassan and Sturm 1986). Landsat TM band 1 registers the shorter wave lengths
of light that are in the visible blue range of the electromagnetic spectrum with wavelengths
between 0.45 and 0.52 um (Table 2.1). Short wave lengths are able to penetrate to greater
depths of water than bands with longer wave lengths, making it more effective in mapping of
underwater substrata.
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Waveband
Band 1
Band 2
Band 3
Range (nm)
450 – 520
520 – 600
630 – 690
Centre (nm)
485
560
660
Chapter 2
Spread (nm)
70
80
60
Table 2.1 Landsat Thematic Mapper wavebands.
35
30
25
20
Coastal Type 1
15
Coastal Type 3
10
Ocean Type III
5
0
310 350 375 400 425 450 475 500 525 550 575 600 625 650 675 700
Wavelength (nm)
Figure 2.1 Plot of depth of penetration against wavelength.
The quality of Landsat TM imagery is dependent upon variations in the emerging radiance
from an illuminated water column. These variations are most commonly a result of:
•
•
•
material within the water column (eg suspended sediments, chlorophyll based materials
and dissolved substances);
the nature of the seafloor substratum material; and
the depth of the water itself [(ie the attenuation of light energy in water increases
logarithmically as a function of depth. Light attenuation can be calculated using a simple
algorithm (Creasey and Fleming 1992)].
To maximise the effectiveness of the imagery for the mapping of substrate features, images
need to be registered on cloud free days and after a period of calm weather with low rainfall.
A period of calm weather allows suspended sediments to settle out of the water column
ensuring that suitable conditions exist to maximise light penetration (Tassan and Sturm
1986). The effects of high concentrations of suspended solids, dissolved organic substances
and phytoplankton on the remotely sensed signals make interpretation of substratum types
increasingly difficult because of the altered spectral reflectance and the reduced water depth
penetration (Thulin and Lewis 1995).
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2.2.3
Chapter 2
Hydro-Acoustic Devices
As some areas of Victoria’s open coast are greater than 30 m in depth, techniques other than
airborne and satellite remote sensing were required to determine the nature of the seafloor.
Acoustic sonar devices provide information to classify substratum types of the seafloor in
deeper water (generally > 30 m).
The initial development of sonar was triggered in 1912, by the loss of the Titanic, and the
need to detect the presence of large objects under water by means of the echo compressional
waves. The term sonar is derived from the words sound navigation and ranging. The most
important acoustic parameter of the ocean is the speed of sound in water. At different
geographic locations the behaviour of acoustic signals vary significantly as a signal proceeds
away from its source. Changing values of temperature and salinity (which effect seawater
density), pressure and depth, as well as objects and bubbles influence this behaviour (Figure
2.2) (Holme and McIntyre 1984).
Echo sounding processors
Echo sounding processors digitise the echo trace from an echo signal or ‘ping’. The digital
echo trace is processed using a series of algorithms. The results are subsequently analysed
then compared to calibration data sets in order to discriminate between substratum types.
Various echo sounding processors perform these tasks in different ways. Analyses of the
return echoes enable researchers to determine hardness and roughness, and in some
applications can even determine vegetation cover of the substratum.
Each of the commercially available systems is capable of determining a number of
substratum type classifications. The shape of the echo signal is influenced by the
characteristics of the seabed. The return signal from a rough rock bottom will exhibit a high
degree of scatter, whereas a smooth soft muddy bottom will return a weak narrow signal
(Collins et al 1996).
2.2.4
Submersible video
In order to undertake video transecting or still footage of the seafloor, camera configurations
are required that may be towed or positioned at consistent heights above the seafloor without
accumulating vegetation or catching on reef. Unique towable camera frame configurations
are designed to capture specific video footage.
Submersible cameras need to be housed in water tight casings that can be mounted in
enclosures that provide protection from the substrate. These enclosures may be frames, sleds
or towed bodies. Surveillance cameras are suitable to image the seafloor, while Super VHS
recording equipment provides broadcast quality footage of the substrate (Roob and Ball
1997).
Camera controllers and recording equipment are most suitably mounted on the survey vessel
where they can be efficiently operated. This permits the most expensive and fragile
components to be maintained in a relatively safe environment. Video images from the subsurface camera are transmitted via a Fibron umbilical cable to the recorder as well as carrying
power to the camera and lighting unit (Holme and McIntyre 1984).
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2.2.5
Chapter 2
Benthic sampling
A number of benthic sampling devices may be employed to collect information on a range of
features. The choice of equipment will depend on the type of sample required as well as
vessel limitations and environmental considerations. The Forster’s anchor dredge has an
inclined plate that digs in to the sediment upon contact with the seafloor (Figure 2.3). A small
tube with a self-sifting mesh is attached to collect sediment while netting attached to the rear
of the anchor will collect algal and seagrass specimens.
To collect quantitative samples of sediment as well as animals inhabiting them, grabs are
employed. The Smith-McIntyre grab has hinged buckets mounted within a stabilising
framework and powerful springs to assist penetration of the sediment (Figure 2.3). Trigger
plates on either side of the frame ensure that the grab releases as it makes contact with the
seafloor (Holme and McIntyre 1984).
12
17
Sound Speed (m /sec)
S a lin ity (% )
T em p erature (C )
37.
5
22
0
38
38.
5
1 50 0 1 52 0 1 54 0 1 56 0 15 8 0
0
39
0
200
200
200
400
400
400
600
600
600
1200
Depth (m)
1000
800
1000
1200
Depth (m)
800
800
1000
1200
1400
1400
1400
1600
1600
1600
1800
1800
1800
2000
2000
2000
Figure 2.2 Depth profiles of temperature and salinity, variables which influence the behaviour
of acoustic signals.
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Figure 2.3 Forster’s anchor dredge (left) and Smith McIntyre grab (right).
2.2.6
Scientific Divers
Self contained underwater breathing apparatus (SCUBA) was invented by Cousteau and
Gagnan in 1943. It is widely employed as a means of conducting in situ benthic research
underwater. Diving can be used to collect samples, film and record data. Scientific divers are
able to assess the variation in substratum types, biological assemblages and variation in
abundance (Holme and McIntyre 1984). However, there are a number of factors that limit the
use of divers, these include safe time limits, depth, temperature, visibility, oceanic conditions
and dangerous animals.
2.2.7
Power supply
The electronic instrumentation discussed in this chapter require either a 12 volt DC or 240
volt AC power supply. It is important to maintain the variation in voltage frequency to less
than 1%. Small portable generators are often unable to maintain sufficient stability and either
power level filters are employed or the power is supplied by batteries. Where batteries are
used they must either be charged or contain sufficient charge to account for the duration of
the survey. Battery supply of 12 volt DC can be converted to 240 volts AC by a thyristorinverter (Holme and McIntyre 1984).
2.2.6
Geographic information systems
Geographic information systems (GIS) incorporate digital databases which store spatially
referenced information that have topology ie. mathematical relationships exist between
spatial features. The information can be displayed and analysed using various components or
programs contained within the system. GIS provides a quantitative method of studying
environmental processes and the relationships between physical, chemical and biological
information (Roob et al 1995).
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2.3
Chapter 2
Mapping Design
The mapping of Victoria’s nearshore open coast marine habitats was undertaken in two steps
involving the processing, classification and ground truthing of data derived from airborne,
satellite and hydro-acoustic remote sensing techniques.
Step 1
Remotely sensed imagery from the Landsat TM satellite and aerial photography was digitally
captured to map an initial template of broad substratum types in shallow waters (generally <
30 metres) at a nominal scale of 1:100,000. Broad substratum types were delineated using six
predefined classes developed by Dr Hugh Kirkman (formerly of CSIRO in Western
Australia).
Step 2
Hydro-acoustic processors were utilised aboard survey vessels to provide acoustic data from
depths outside the range of satellite penetration (generally > 30 m), and to provide additional
data for areas classified in Step 1.
A submersible video camera was used from survey vessels to calibrate variations in acoustic
signals. The use of video to calibrate acoustic signals allowed further data on seafloor
attributes to be collected, and provided an optical image to characterise dominant epibenthic
biota.
Supplementary geological and biological information was also collected using benthic
sampling techniques and scientific divers. Details of the methodologies and interpretation
associated with both steps is described below.
2.4
Broad Substratum Type Classification of Landsat TM Satellite Imagery
2.4.1
Selection and Choice of Images
Landsat TM images (Table 2.2) were selected from microfiche reproductions provided by the
Australian Centre of Remote Sensing (ACRES), a division of the Australian Land
Information Group (AUSLIG). Information from the Bureau of Meteorology was interrogated
to ensure optimum weather conditions existed prior to the registration of imagery. Only
images registered between mid-October and April were chosen to take advantage of sun
angles greater than 45o. The weather pattern for the previous three days was then examined
from the Bureau of Meteorology data to determine if storms or strong winds might have
disturbed seafloor sediments, and there by reduced light penetration, in the target area.
The imagery (Band 1) was purchased pre-processed to Level 9, that is rectified to the
AUSLIG topographic map at a scale of 1:100,000, with Australian Map Grid positions and
checks at every 10 km on the image.
The imagery was enhanced to maximise the contrast and enable differentiation between
categories of substratum. Individual stretches were made for three segments of each Landsat
image. The processed images were then printed at 1:100 000 map scale. The printed image
also included the AUSLIG 1:25, 000 coastline that had been pre-classified according to the
presence/absence of intertidal reef (Figure 2.4a).
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2.4.2
Chapter 2
Interpretation of Landsat TM Imagery
Based on the texture and density of the grey scales the processed imagery was visually
classified by Dr Hugh Kirkman of CSIRO, David Ball of MAFRI and the author. Broad
substratum type classes were then delineated on stable base film overlays. Black and white,
colour aerial photography and existing substratum (habitat) maps for localised areas were
also used to assist with the interpretation and classification (Figure 2.4a).
Once the discernible underwater substrate classes had been traced onto the stable base film
they were digitised. ArcInfo™ GIS software was used to digitally capture these maps to
produce a library coverage called SUBSTRATA100 (Roob et al 1997, Figure 2.4b).
All areas were discrete polygons bounded on the landward edge by the 1:100,000 AUSLIG
topographic map coastline and the offshore limit by the extent of discernible substrata,
bounded by a straight line, or the 3 nm jurisdiction of Victorian waters. Except for Western
Port, the territorial base line was used to close off bays, inlets and estuaries. Interim
interpretative maps were then produced for field checking purposes (Figure 2.4b).
Path
Date
95
94
93
92
91
90
AMG Zone
3-3-95
24-2-95
10-11-93
21-10-94
13-2-93
12-2-95
54
54
55
55
55
55
Latitude of
Centroid
545020
619700
235650
365050
503570
664630
Longitude of
Centroid
5763200
5729700
5722940
5727565
5731060
5804650
Dimension
(km)
100 x 55
200 x 130
210 x 180
230 x 175
230 x 175
230 x 175
Table 2.2 Landsat TM imagery utilised for 1:100,000 substratum mapping.
Substratum
Sand
Field characteristics
Substrate with no apparent reef or seagrass.
Sparse seagrass
Density of seagrass where a hand can be placed between shoots.
Medium seagrass
Density where two fingers held together can be placed between shoots.
Dense seagrass
Seagrass that completely covers the bottom.
Low profile reef
Flat platform reef, less than 1 m in relief, that is easily covered in mobile sand.
High profile reef
Rugose reef with a relief predominantly greater than (or equal to) 1 m. Reef
often covered in large brown seaweeds such as Ecklonia and Phyllospora.
Table 2.3 Broad substratum type classes used to classify Landsat TM imagery (classes originally
developed by Dr Hugh Kirkman, formerly of CSIRO in Western Australia).
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(a)
Sand
Low Profile Reef
(Seaward limit of interpretation)
Figure 2.4 Broad substratum type classes interpreted from Landsat TM image of
(b)
Discovery Bay, western Victoria, (a) shows original grey scale imagery, (b) shows classes
displayed as coloured polygons in the GIS information product SUBSTRATA100. Note
that Landsat TM imagery for this region could be interpreted to a depth of
approximately 50 m.
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2.5
Chapter 2
Substratum Classification Using Hydro-Acoustic Devices and Benthic
Sampling
The interpretation of Landsat TM imagery outlined above provided an initial template for the
distribution of six broad substratum type classes in Victoria’s nearshore waters. However, in
order to verify the interpretation and further refine the classification of substratum types, it
was necessary to develop a cost-effective field program to provide additional attribute data
and ground truth the overall reliability of the interpretation.
Additionally, as the depth limit of the Landsat TM classification was generally restricted < 30
m it was also necessary to extend the mapping at certain sites into deeper water, and extend
the substratum type mapping to the 3 nm limit of Victoria’s territorial waters for selected
offshore survey areas.
2.5.1
Survey Areas
All field surveys involving hydro-acoustic remote sensing were focused on discrete areas
listed below. This work has been reported in detail by Roob and Currie (1996), Roob and
O’Hara (1996) and Roob et al (1999a).
Discovery Bay
Lady Julia Percy
Port Campbell
Moonlight Head West
Point Nepean to Flinders
Cape Liptrap
South East Point
Cape Howe
2.5.2
Cape Nelson
Logans Beach
Moonlight Head East
Point Addis
The Nobbies
Shellback Island
Point Hicks
Cape Grant
Lake Gillear
Moonlight Head Central
Harold Holt
Bunurong
The Anser Group
Rame Head
Survey Design
A number of survey vessels were chartered to undertake acoustic surveys, they included: "M.V. Starfire” (17 m), “Orca II” (10 m), “Haliotis” (7 m) and “AB Hunter” (14 m). Due to
the number and configuration of vessels chartered, the survey equipment was installed in a
water proof cabinet that could be mounted on most vessels in a short period of time.
On initiation of each field day, the hydro-acoustic transducer was deployed, the differential
GPS was initiated and the signals calibrated. Survey vessels was then steered along a predetermined course. Changes in the return signals and depth of the underlying sea floor was
monitored to determine when to deploy a submersible video and benthic dredge in order to
collect representative samples of the substratum types encountered.
The extent to which the substratum type classes detected using acoustic signals can be
spatially resolved by the density of survey transects. Transecting speed was the determining
factor on the spacing of transects that could be conducted on any one day. In general,
transects were usually spaced between 200 - 500 m apart.
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2.5.3
Chapter 2
Application of Hydro-Acoustic Processors
Two echo sounding processors (RoxAnn and EchoListener) were employed to determine the
nature and extent of substratum types within the 22 survey areas.
These systems captured the profile of the seafloor while also providing an indication of
bottom hardness. The systems consist of a head amplifier connected to a Ratheon echosounder in parallel with the existing display/transceiver. The hydro-acoustic processors
digitise the return signals from the echo sounder and assign values to the return echoes for
use in post processing.
RoxAnn and EchoListener essentially perform the same function (ie they enable
classification of the seafloor), however, their approach is different. Both systems apply
algorithms to correct for fluctuations in the transmit pulse peak voltage. EchoListener is
transparent in the way that it digitises the signal from the echosounder and provides raw data.
RoxAnn internally processes the data by employing a range of algorithms and provides
quantified values for the first and second echoes. EchoListener also enables the operator to
view the echogram and therefore obtain further information such as a visual profile of the
seafloor, indication of algal abundance on reefs and biomass (fish) in the water column.
RoxAnn system
Data captured by the RoxAnn system includes values that quantify the first and second
echoes of the sounder as well as depth. The first echo (E1) provides an indication of
roughness while the second (E2) represents hardness. The strength of the first echo
diminishes with the dispersal of the signal caused by rougher bottoms, ie. the E1 value
applied by RoxAnn is large for rough, and small for smooth terrain. The strength of the
second echo is dependent upon its delay which is proportional to the amount the signal
penetrates the substrate, ie the E2 value applied by RoxAnn is large for hard, and small for
soft substrates (Chivers et al 1990).
These parameters together with time, date, position (from DGPS) and depth, were displayed
and logged on a PC running Microplot (Sea Information Systems, Aberdeen). The display has
four main components:
•
•
•
•
navigational chart with a plot of the vessels track;
scrolling depth profile of the vessels passage;
a numerical display of the Echo1.Echo2 values; and
display of Echo1.Echo2 values on a grid (x-y plot).
By plotting these values against each other on an X,Y scatter plot, groupings can be identified
that represent various substratum type classes.
In order to identify substratum type categories and calibrate, subsets of RoxAnn data
recorded during video deployments were used to “train” the complete RoxAnn data set.
Scatter plots of roughness (E1) versus hardness (E2) for all video deployments within a
survey area were produced (Figure 2.5). Using descriptive accounts of the substratum type
observed during each video deployment together with geological data derived from dredge
samples, discrete substratum type classes (eg sand, ‘high profile’ reef) were identified (Figure
2.6).
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Figure 2.5 Scatter plot of acoustic data set displaying roughness (E1) and hardness (E2) at the
Discovery Bay survey area.
Figure 2.6 Subsets of acoustic data recorded by video deployments at the Discovery Bay survey
area cropped at the 95 percentile range.
To clearly describe patterns within the data, a mid 95 percentile range is applied. This is a
common practice used to disregard extreme values or outliers within data sets. These ranges
were applied to the complete dataset in GIS, to interpolate the spatial extent of substratum
type classes. The interpolation produced homogeneous polygons by applying an inverse
distance weighting algorithm with a search radius of 300 m and a cell size of 50 m.
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For most survey areas, echo returns provided a clear distinction was between hard and soft
substrata (ie rock and sand), but lesser differences were encountered to clearly separate
structures and relief of hard surfaces (eg discriminating a flat ‘platform’ reef from a jagged
‘heavy’ reef) or sediment textures (eg discriminating ‘silt’ from ‘coarse sand’). In some cases
the differentiation of substratum type characteristics was confounded, when for example reef
was covered by a layer of sediment or kelp, or when reef was interspersed with sand. The
hardness value recorded for calcarenite reef in certain areas was less than that of hard packed
fine sand. This was most likely due to the porous nature of the lithified calcarenite sediments
in these areas.
Slight changes in the signature of particular substratum types varied from survey area to
survey area. This was attributed to geographic and temporal variations in seawater properties
and sea conditions. For example rough seas can increase the Echo 1 value, this was taken into
account when interpreting the data. In such circumstances the instrumentation was regularly
calibrated with video deployments.
EchoListener system
SonarData’s EchoListener is a passive device for “listening” to the transducer of an
echosounder. EchoListener records the (envelope detected) return signal from the transducer
for display as a high resolution echogram and for logging (Figure 2.8). EchoListener has a
high impedance input and has no effect on its host echosounder. EchoListener’s individual
ping correction algorithm allows the correction of every ping for fluctuations in the transmit
pulse peak voltage, thus enabling calibrated data to be logged from echo-sounders. The
Echoview software with the hydrographic module enables post processing and display of
digital echograms including automated bottom-picking and quality control of acoustic (depth)
data.
The geographic position of individual pings are determined by linear interpolation of
navigation data between fixes. A display of the cruise track with independently scaled axes
allows the “tuning” of the GPS quality assurance parameters. Positional fixes forming
sections of the cruise track that are given a bad navigation flag are rejected. Echograms that
were digitally recorded by EchoListener were analysed in EchoView and visually classified.
The positions of changes in substratum class were flagged using a feature in the software’s
functionality that provides data to be assigned with a category. The data points contain the
geographic location, depth and substratum type classification, which are read into GIS.
Figure 2.7 Echogram
displaying the first,
second and third
echoes with the
bottom trace
classified according
to substratum type.
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2.5.4
Chapter 2
Field Verification
Submersible Video
In order to undertake video transecting over reef and sand, camera configurations were
required that could be towed at consistent heights above the seafloor without accumulating
vegetation or being caught on reef. Consequently unique towable camera frame
configurations were designed to capture video footage.
A submersible Panasonic WC110 video camera was used to collect real time imagery of the
benthos. This provided qualitative information relating to substratum types and dominant
marine flora and fauna as well as ground truthing variations in the signature output from the
acoustic devices (ie characteristics of the substratum).
The camera was installed in a water tight housing rated to a pressure of 300 metres. This,
along with a powerful flood light was then mounted within a heavy stainless steel frame that
enabled the equipment to be deployed and landed directly beneath the survey vessel even in
strong currents. The frame also provided a degree of protection for the camera when
deployed over rocky substrata. A field of view was calibrated to provide a 1m2 image of the
benthos. Video images from the sub-surface camera were transmitted via an Fibron umbilical
to a Panasonic AG5300 recorder. Edited footage from each video deployment was
subsequently collated and title slides were inserted that describe location and depth of each
deployment.
On each deployment, the camera frame was lowered until it made contact with the seafloor
where it was left to collect about a minute of footage. It was then raised approximately 1 m –
2 m above the seafloor while the vessel drifted. Due to the varying strength of local currents
and different sea surface conditions, the time and distance drifted while the camera was
deployed varied.
Benthic Sampling
Benthic samples were collected at multiple points within each of the survey areas listed in
2.5.1. using a modified Forster’s anchor dredge. This particular dredge was preferred for
routine sampling and ground truthing because of its ability to sample in both reef and soft
sediment substratum types.
Additional verification sites for soft sediment substratum types were derived from the
Environmental Inventory Stage 4 (Ferns 1999). For this survey sediment samples were
obtained using a Smith-McIntyre Grab. A number of detailed analyses were performed on the
sediment samples, including grain size, carbonate content and identification of infauna (see
Roob et al 1999b for site selection and sample protocol).
Divers were also deployed to make qualitative descriptions, collect samples and photograph
substratum types and dominant epibiota (Table 2.4). All algae and seagrass collected were
pressed and dried for subsequent examination, while fauna samples were fixed in a 10%
formaldehyde solution for subsequent identification in the laboratory.
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Mapping of Victoria’s Nearshore Marine Benthic Environment
Site ID
D15
Latitude
Longitude
-37.803867 149.273950
Chapter 2
Depth (m) Observations
8
A low profile reef site with substrate geology consisting of 60% granite and
40% sand, in the form of 60% boulders and 40% sand gutters. Epibiota
cover consists of 30% crustose coralline algae, 55% other epifauna and
15% no cover. Upper storey cover consists of 30% Phyllospora sp, 15%
Ecklonia sp, 40% no algal cover and 15% other cover.
Table 2.4 Example of a recording obtained by scientific diver for ground truthing and refinement
of broad substratum type classes. Diver descriptions also provided valuable qualitative
information on dominant epibiota.
2.5.5
Refinement of Broad Substratum Type Classes
The application of hydroacoustic sonar devices, combined with field verification techniques
permitted refinement of the broad substratum classification derived from Landsat TM
imagery (Table 2.3) in survey areas listed in section 2.5.1. In total, 467 detailed observations
were collated from submersible video deployments, benthic samples and scientific dives
across these areas. Refinement was produced through:
•
•
empirical validation of Landsat TM image interpretation; and
provision of additional information on seafloor characteristics from field surveys.
Empirical data led to the adjustment of polygon boundaries in some areas, and allowed
seaward extension of those areas investigated using field survey techniques (section 2.5.1).
Characteristics of the seafloor could be refined by describing additional attributes pertaining
to the structure, relief and texture of substratum material. These attributes represent easily
identifiable components of hard and soft substratum materials and improve the resolution of
information for describing the physical nature of the seafloor (Table 2.5).
2.6
Information Products
2.6.1
Substratum Mapping
From the consolidation of data sets derived from remote sensing techniques and benthic
sampling it is now possible to produce mapping products showing broad substratum type
classes at a nominal scale of 1:100,000 for Victoria’s nearshore waters. Figure 2.8 provides
an example of a information product showing the distribution of the broad substratum type
classes for ‘reef’ and ‘sand’.
In areas listed in section 2.5.1 (Roob and Currie 1996; Roob and O’Hara 1996; and Roob et
al 1999a), the seafloor can be mapped and described in more detail by reference to
predominant substratum attributes. Figure 2.9 provides an example of an information product
that can be produced for such areas. The spatial scale and level of taxonomic resolution of the
substratum classification for the mapping product is dependant on the detail of attribute
information available for the particular area. Future information products and their
interpretation may also benefit from incorporation of other relevant data sets such as coastal
land form features and bathymetry models, an example of which is discussed below in
section 2.6.1.
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Mapping of Victoria’s Nearshore Marine Benthic Environment
Chapter 2
Substratum Attributes
Reef
Relief
1. Low profile
2. High profile
Textures
1.
2.
3.
4.
5.
Solid
Smooth
Broken (boulders / slabs / bommies)
Gutters
Outcrops
1.
2.
3.
4.
1.
2.
3.
4.
5.
6.
7.
Sediment / Unconsolidated
Flat
Ripples
Gently undulating ridges
Steeply undulating ridges
Larger material (Cobble / Pebble /
Granules)
Coarse / very coarse sand
Medium sand
Very fine / fine sand
Mud (silt / clay)
Muddy sand
Shelly rubble / grit
# Key
•
•
•
•
Low profile reef = flat reef (such as rock platforms) with relief predominantly < 1 m.
•
•
•
•
•
•
•
•
•
•
•
•
•
Gutters = gutter-like depressions or chutes in and between rock beds, often filled with sediment.
High profile reef = rugose reef with relief predominantly ≥1 m.
Solid = solid rock, not obviously broken into fragments.
Broken (Boulders / Slabs / Bommies) = rock fragments >30 cm diameter or expanses of broken reef termed ‘slabs’ or
‘bommies’.
Outcrops = protruding rock extensions, often found on edges of reef terraces.
Flat = surface predominantly smooth without noticeable rises or depressions.
Ripples = obvious rises up to 0.3 m in height.
Gently undulating ridges = rises > 0.3 m in height, gradually sloping between successive troughs and rises.
Steeply undulating ridges = rises > 0.3 m in height, steeply sloping between successive troughs and rises.
Larger material (Cobble/ Pebble/Granules) = 2mm – 30 cm diameter
Coarse / very coarse sand = 0.5 mm – 2 mm diameter.
Medium sand = 0.25 mm – 0.5 mm diameter.
Very Fine / fine sand = 0.25 mm – 0.063 mm diameter.
Mud / silt = < 0.063 mm diameter.
Muddy sand = mixture of sand and mud.
Shelly rubble / grit = sediment composed of shelly debris.
Table 2.5 Substratum attributes for relief and texture recorded using field surveys techniques.
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Mapping of Victoria’s Nearshore Marine Benthic Environment
Chapter 2
Figure 2.8 Information product showing substratum type classes for ‘reef’ and ‘sand’, geology
and field verification sites off the coast of Cape Bridgewater (western Victoria).
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representativeness of Victoria’s rocky reefs
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Mapping of Victoria’s Nearshore Marine Benthic Environment
Chapter 2
Figure 2.9 Information product showing marine habitats off the coast of Cape Paterson (central
Victoria) based on predominant substratum attributes. Attributes were derived from
combination of airborne and satellite and hydro-acoustic remote sensing techniques, video
transects, benthic grabs and underwater observations by scientific divers.
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Mapping of Victoria’s Nearshore Marine Benthic Environment
Chapter 2
2.6.2 Bathymetry Model
Detailed information of bathymetry is useful to combine with substratum mapping to explain
the distribution of substratum types at various depths. For biological attributes, depth is often
a determining factor of the extent of dominant biota such as kelps and seagrass. Due to the
importance of depth information, a Digital Depth Model (DDM) was generated for Victoria’s
coastal waters and further offshore into the Bass Strait. This bathymetry data was derived
from the RAN Hydrographic Office’s Bathymetric map series. This series contains isobaths
that are in metres and calibrated to the vertical datum of mean sea level. The coastline detail
was derived from the 1:100,000 AUSLIG topographic series. Part of this model is illustrated
in Figure 2.10.
The model contains information relating to relief features below the high water mark. A
vector model with depth zones was chosen in preference to raster or grid, to enable
compatibility with other vector GIS layers. The depth zones consist of 10 m intervals from 0
to 200 m and one zone from 200 to 300 metres. This was achieved by digitising the
bathymetry and closing off areas bounded by the 300 m isobath at the entrances to Bass
Strait, these polygons were then attributed with a code representing the depth zone.
Figure 2.10 Three-dimensional bathymetric model off Victoria’s west coast from Discovery Bay
to Portland Bay.
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Mapping of Victoria’s Nearshore Marine Benthic Environment
2.7
Chapter 2
Discussion
This survey has demonstrated the successful use of various complementary techniques for
capturing, processing and analysing data to map and characterise marine habitats based on
predominant substratum type attributes. These attributes can be represented either as broad
classes (eg reef and sand) or in more specific detail by combining attribute descriptions for
structure, relief and texture where this data is available.
The data capture and processing techniques developed for these surveys are repeatable and
deliver relatively accurate results, provided the data thus obtained is used under consistent
guidelines and data validation protocols. The methods are efficient and cost effective while
the resulting information product is in a form that is useable for marine management.
The geo-spatial data sets developed for this project have provided a strategic coverage of
Victoria’s nearshore waters. Some gaps exist for deeper waters (generally > 30 m) within the
3 nm limit of Victoria’s Territorial waters, however additional offshore surveys will only be
conducted where it is relevant to specific future management objectives.
2.8
Acknowledgments
The mapping component of the Environmental Inventory Project Stage 3 was carried out with
joint funding support provided by Environment Australia (Marine Protected Areas Program),
the Environment Conservation Council (Marine and Coastal Investigation) and Division of
Parks Flora and Fauna (Marine Strategy Unit). The author wishes to acknowledge
contributions by Dr Hugh Kirkman and David Ball towards the mapping. Those who
provided technical assistance, Allister Coots, John White and Ian Higginbottom. As well as
those who provided administration and other advice, in particular Dr Garth Newman, Nik
Dow, Neil Hickman and Dr Leanne Gunthorpe.
Environmental Inventory of Victoria’s Marine Ecosystems Stage 3 (2nd Edition) – Understanding biodiversity 2 - 20
representativeness of Victoria’s rocky reefs
Habitat mapping of Victoria’s nearshore subtidal marine waters
2.9
Chapter 2
References
AMSA (1994). Characteristics of AMSA Differential GPS Broadcast services. AMSA
Network planning, Navigational Services.
Benwell,G.L. (1979). Hydrographic Mapping: A Seminar Incorporating EG & G Side Scan
Sonar and Motorola Miniranger III Positioning System. Department of Surveying, University
of Melbourne.
Chivers, R.C., Emerson, N and Burns, D. (1990). New acoustic processing for underwater
surveying. The Hydrographic Journal. Number 56. pp. 9-17.
Clay, S.C. and Medwin, H. (1977). Acoustical Oceanography, Wiley and Sons.
Collins, W., Gregory, R. and Anderson, J. (1996). A Digital Approach to Seabed
Classification. Sea Technology. August 1996.
Corner, R. J., and Lodwick, G.D, (1992), Spectral Bathymetry using Landsat Thematic
Mapper data in Cockburn Sound, WA. In: Proceedings of The 6th Australasian Remote
Sensing Conference, Wellington, New Zealand. pp. 180-190.
Creasy, J.W., and Fleming, C., (1992). Shallow water mapping in the Philippines using
Landsat TM Imagery. In: Proceedings of The 6th Australasian Remote Sensing Conference,
Wellington, New Zealand. pp.191-199
Ferns, L.W. (Editor). (1999). Environmental Inventory of Victoria’s Marine Ecosystems
Stage 4 (Part 1) – Physical classification of soft sediment ecosystems. Parks Flora and Fauna
Division, Department of Natural Resources and Environment, East Melbourne, Australia
Frost, B. and MacLeod, R. (1991). Real-time differential accuracies of GPS receivers with
embedded differential software, University of South Queensland, Survey Industry Seminar
1991.
Hammond, L. S., Black, K., Jenkins, G. P. and Hodgkinson, R. S. (1994). The Influence of
Water Circulation Patterns in Bass Strait on Recruitment Success and Stock Differences in
Scallops. Victorian Institute of Marine Science Final Report to Fisheries Research
Development Corporation Committee on Project 87/117.
Holme, N. A. and McIntyre, A.D. (1984). Methods for the Study of Marine Benthos.
Blackwell Scientific Publications.
Jerlov, N.G. (1976), Marine Optics. Elsevier Oceanography Series 14. Elsevier, Amsterdam.
pp. 231.
Mok, E. (1995). Applying GPS Positioning Technique in GIS Data Collection, How does it
work? GIS Asia Pacific.
Roob, R. and Currie, D. (1996). Marine and Coastal Special Investigation: Offshore survey
of selected areas. Report to the Land Conservation Council by the Victorian Fisheries
Research Institute. Department of Natural Resources and Environment, Melbourne, Australia.
pp. 22.
Environmental Inventory of Victoria’s Marine Ecosystems Stage 3 (2nd Edition) – Understanding biodiversity 2 - 21
representativeness of Victoria’s rocky reefs
Habitat mapping of Victoria’s nearshore subtidal marine waters
Chapter 2
Roob, R. and O’Hara, T. (1996). Marine and Coastal Special Investigation: Additional
offshore survey of selected areas. Report to the Land Conservation Council by the Marine
and Freshwater Resources Institute. Department of Natural Resources and Environment,
Melbourne, Australia. pp. 16.
Roob, R. and Ball, D. (1997). Seagrass: Gippsland Lakes. Marine and Freshwater Resources
Institute Report to Fisheries Victoria, Department of Natural Resources and Environment,
Melbourne, Australia. pp. 48.
Roob, R., Lewis, A., Thurlin, S. and Pelikan, M. (1995). Development of a Marine and
Coastal Geographic Information System for Victoria Stage 1. Department of Conservation
and Natural Resources, Melbourne, Australia. pp. 32.
Roob, R, Hough, D., Mahon, G. and Dow, N. (1997). Information products. In: G. Mahon
(Ed.). Development of a Marine and Coastal Geographic Information System for Victoria
Stage 2. Parks, Flora and Fauna Division, Department of Natural Resources and
Environment, Melbourne, Australia.
Roob, R. Blake, S. and Parry, G. (1999a). Marine and Coastal Special Investigation:
Additional offshore survey of selected areas. Report to the Environment Conservation
Council by the Marine and Freshwater Resources Institute. Department of Natural Resources
and Environment, Melbourne, Australia.
Roob, R., Gunthorpe, L. and Turnbull, J. (1999b). Collection and physical classification of
sediments. In: Ferns, L.W. (Editor). (1999). Environmental Inventory of Victoria’s Marine
Ecosystems Stage 4 (Part 1). Parks Flora and Fauna Division, Department of Natural
Resources and Environment, East Melbourne, Australia.
Tassan, S. and Sturm, B. (1986). An algorithm for the retrieval of sediment content in turbid
waters from CZCS data. International. Journal of Remote Sensing 7: 643-655.
Thulin, S. and Lewis, A. (1995), Feasibility of Substrate Mapping using TM/SPOT image
data. Natural Resource Systems. Sub-project 2. In: Roob, R., Lewis, A., Thurlin, S. and
Pelikan, M. (1995). Development of a Marine and Coastal Geographic Information System
for Victoria Stage 1. Department of Conservation and Natural Resources, Melbourne,
Australia. pp. 32.
Environmental Inventory of Victoria’s Marine Ecosystems Stage 3 (2nd Edition) – Understanding biodiversity 2 - 22
representativeness of Victoria’s rocky reefs