Seagrass mapping using Landsat TM data
Peter J. Lennon
Sunmap Remote Sensing centre,
Department of Geographic Information,
Queensland, Australia
Paul Luck
Fisheries Branch,
Department of Primary Industries,
Queensland, Australia.
Abstract
Seagrass communities are highly productive coastal systems which form important nursery grounds for fishes and crustanceans and provide a primary source of food for dugong and turtles. Using image processing techniques, Landsat TM data was processed to map seagresses In a 95 km long estuarine habitat on the east coast of Australia. Area measurements were made and mapping accuracy assessment was undertaken.
Introduction
Seagresses are so called because many of them have ribbon-like grassy leaves. Some bear no resemblance to grasses at all and none of them are a true grass. All have prostrate stems buried in sand or mud and produce leaves on erect branches which vary in length from less than a millimetre to half a metre or more. Seagrasses grow in an area between the mid intertidal zone and down to a depth depending largely on clarity of water and form an important part of coastal ecosystems. The leaves of the larger species shelter underlying sediments from erosion and provide a habitat and in some cases food for resident animals such as shrimp and fish, particularly juvenile species. The meadows act as nurseries to a wide range of crustaceans ( especially shrimp). Seagresses are also the primary food resource for dugongs and the green turtle ( Chelonia midas) (Poiner et. al. 1987).
Several scientists have demonstrated a clear relationship between the size of nursery area and fishery catches (Taylor and Saloman 1968, Staples 1984). The larger the onshore nursery area the greater the fishery catch. Poiner ( 1989) reported the loss of >100 km1 of dense seagrass meadows in the Gulf of Carpentaria, Queensland, after a tropical cyclone. Examination of fish and prawn catches in subsequent months indicated a significant drop in the size of the catch as a result of the loss. Even lower density seagrass beds are quite important since they appear to be favoured by herebivorous species such as dugoing ( Preen et al. 1989). The production of accurate maps of these valuable ecological areas may better facilitate the development of management strategies and enable the protection of seagrasses for both commercial and environmental reason.
The area of study for this seagrass mapping exercise encompasses Great Sandy Strait between the continental mainland and Frasen Island on the east coast of Queensland, Australia. It is an extensive 95 Kilometre long estuarine environment relatively undisturbed by human activity. There are vast areas of intertidal seagrasses, as well as submerged seagrass beds, some of which are not exposed at low tides at all. The environment of Landsat TM data to map seagrass area.
Traditional methods of mapping Seagrasses
In Australia the mapping of seagrasses by remote sensing techniques has, in the past , involved the use of aerial photography, in particular , colour and colour infrared aerial photography. Sometimes this involves special flights for this purpose. If the coast of these special flights is prohibitive, then the first problem encountered may well be the age of the existing photography of any exists. Other problems with the use of aerial photography include the effects of sun flare and the need to mosaic photographs covering large areas. Extensive field surveys are undertaken in conjuction with the photography. Current field survey methods involve many hours of sampling on the banks, identifying and mapping different species and densities. It is a difficult task to perform especially at low tides. Mud banks are often quite soft and almost impossible to traverse. Tides, currents, and water clarity can of conditions, can be an exhausting task. Quantitative measurements underwater of biomass, densities of cover and species cohabitation, are a less accurate and more time consuming task than measurements in the exposed intertidal areas. Just to look at permanently underwater seagrass meadow can be difficult. If the waters are relatively clear it is sometimes possible to look at the beds by viewing through the bottom of waters are turbid, it is often necessary to scuba dive and in such cases visibility is still limited and so is assessment. Coarse sampling methods can be undertaken by using certain devices that can be dropped overboard and lowered to the bottom to grab a sample of the substrate. It is often necessary to synchronise the survey with tide times as well as good weather. All of these methods are not entirely accurate methods of sampling and assessment.
These traditional methods of study are costly and time consuming, involving numerous staff, vehicles and vessels. The relatively small areas that are surveyed are not easily monitored on a regular basis and the relevance of the information dates quickly. Financial resources usually limit the amount of estuarine areas that can be surveyed and monitored using these traditional techniques. The results are not usually quantitative and area not regularly repeated on a monitoring basis. The cost of performing these surveys and of flying to obtain new aerial photography is an important consideration and can be prohibitive. As a consequence, researchers have begun to assess the coasts and utility of other methods and sensors, such as satellite remote sensing using the Landsat satellites.
