Mapping of marine habitats is essential for understanding and communicating the distribution of natural values within marine national parks and sanctuaries. It is particularly important to map the marine environment as it is not as easily visualised as the terrestrial environment.
Knowledge of the bathymetry, distribution and extent of habitats is necessary for effective and targeted management activities such as emergency response, monitoring and research. There are varied benefits of marine habitat and bathymetry mapping including the development of a baseline inventory, identification of suitable monitoring sites and possible tracking of environmental change. In addition, it helps park managers predict where pest invasions may be a problem and discover sites suitable for diving or snorkelling.
A variety of mapping techniques provide different information about the seabed. Multibeam sonar and airborne laser/Light Detection and Ranging (LiDAR) are excellent for mapping bathymetry, aerial photography is useful for assessing habitats and sidescan sonar is good for determining bottom structure.
Some form of marine mapping has been conducted in all 24 marine national parks and sanctuaries through partnerships with the Coastal CRC, University of Western Australia, Fugro Survey, Deakin University, DEPI and community groups. Many of the MPAs have had both shallow water and deep water areas mapped.
Multibeam sonar is a surveying tool used to determine the depth (bathymetry) of the seafloor and obtain seafloor hardness information which is useful for mapping marine habitats. The technique is extremely useful because the use of multiple beams allows for a large area of seabed under the ship to be mapped.
The multibeam sonar transducer is usually mounted rigidly to the hull of the vessel so that its location can be accurately calculated. The unit emits pulses of sound perpendicular to the direction of travel. The return pulses are detected by a receiver and two types of data are obtained. The duration between pulse emission and detection determines the depth and the strength of the signal indicates seafloor hardness which provides information on habitat.
Different water depths can be mapped, however the frequency emitted needs to be altered accordingly. In deeper water low frequencies are used and in shallow waters higher frequencies are used. A trade off occurs between frequency and resolution with lower frequencies having lower spatial resolution.
Multibeam sonar is the primary mapping tool used by Parks Victoria because it can map large areas of seafloor at one time. The information gained through multibeam sonar activities has been used to create detailed maps of bathymetry and biota in many of Victoria's marine national parks and sanctuaries. Furthermore, this technique has helped in discovering new shallow water dive sites which are similar in species composition to deep dive sites.
Sidescan sonar is a technique which creates an image of the seafloor similar to a low angle photograph taken with a flash. A transducer, towed behind a boat, emits pulses of sound (sonar) in a thin fan-shaped beam perpendicular to the direction of travel. The sonar pulses reflect off objects which project above the seabed and these reflections are recorded by the sidescan equipment. Since sound is used, this method is accurate in dark and murky waters unlike methods which use light.
The method can detect both distance and differences in material composition of submerged objects. Distance is related to the length of time it takes for the return sonar pulse to be detected by the receiver and material composition is related to the strength of the signal. For example, rocks absorb less and reflect more sound than sandy or muddy objects, thus the sensor records a stronger signal from rocky objects. Therefore, the information we can obtain includes the location of reefs, kelp forests, drop offs, sandy habitats, old river beds, anchor damage and ship wrecks.
The information Parks Victoria has from using sidescan sonar feeds back into the management of the parks. It is useful for detecting damage to the parks (e.g. anchor damage), discovering new habitats in Marine Protected Areas and understanding the processes and important structures of the park.
LiDAR (Light Detection and Ranging)
LiDAR is an optical remote sensing technique which can be used to map shallow habitats and bathymetry underwater down to a depth of approximately 20 metres. It has the potential to generate huge amounts of data about the bathymetry (seafloor surface features) and the types of habitat present.
The LiDAR instrument is mounted underneath an aeroplane and emits repeated pulses of light - usually laser pulses. These hit objects and are reflected back to a sensor and recorded. GPS is used to record the location and pitch of the aeroplane, thus ensuring the accuracy of the LiDAR data.
The time between emission and recording of the return signal indicates the distance the object is from the LiDAR instrument. Both the sea surface and the seafloor will reflect the light beam, thus the depth of the water can be easily calculated. Sediment or biological material can affect the returning signal and will influence the quality of the data. However, this information can be used to predict different habitat types of the area. For example, two return recordings within a couple of metres of each other and a third, higher recording could indicate a kelp bed. The first two recordings could be the rocks making up the sea floor and the canopy of the kelp forest. The third recording would be the water surface. Thus LiDAR can provide park managers with a range of useful information.
Parks Victoria has used LiDAR collected by DEPI at most of the marine national parks and sanctuaries. This data has been very useful for understanding the bathymetry of the parks and influencing management decisions.
Aerial photography is a common method of habitat mapping and is used widely across the globe. As the name suggests, photographs are taken from above and subsequently analysed to determine what habitat is present. It is very beneficial to use photographs from multiple time periods to confirm analyses and account for changes through time. High resolution satellite imagery can be useful if detailed photographs cannot be accessed.
Habitat mapping through aerial photography is limited to shallow habitats, where the benthos can be photographed with enough clarity to determine what habitat is present. The maximum depth at which habitats can be mapped using this technique varies depending on weather and sea conditions. Along the Victorian Bass Coast it is typically 5–10 m.
Parks Victoria has used aerial photography to map shallow habitats in all marine national parks and sanctuaries except The Arches and The Twelve Apostles (due to difficulties in acquiring suitable photographs). To validate the data obtained from this approach, ground truthing has also been performed.
Ground truthing involves collecting data ‘on the ground’ to validate data collected remotely such as by aerial photography. The collection of ground truth data calibrates, and helps with interpretation of, remotely gathered data.
The method of ground truthing can vary. In marine systems SCUBA divers or underwater video tend to be used. SCUBA divers provide more detailed and quantitative observations of seabed habitats, but this is restricted by the number of dives that can be completed in a day and safe diving depths. Underwater video can survey far greater areas of the seabed and requires a smaller crew.
Parks Victoria has performed ground truthing at most marine national parks and sanctuaries. The primary method of ground truthing used is geo-referenced towed underwater video. Position information (longitude and latitude), date, time, vessel speed and bearing are linked to the video footage. In intertidal zones towed video is not practical. Instead a scientist visits the site at low tide on a relatively calm day and records data onto a palmtop computer.