Assessing Seagrass Resources of the Great Barrier Reef Marine Park

Abstract he Great Barrier Reef Marine Park Authority is committed to maintaining the health of seagrass ecosystems in the Great Barrier Reef region. The Authority has recognised the importance of seagrass as a food source for dugongs and turtles, a nursery ground for juvenile fish and prawns, a stabiliser of coastal sediments and as a buffer to nutrients in land run-off that can potentially harm coral reefs. Conserving seagrasses and associated marine animals while providing for reasonable use of the Marine Park is a task for the Great Barrier Reef Marine Park Authority. With World Heritage values, fishing interests, foreshore development and the existence of a dugong protection area, the Hinchinbrook region has gained much attention from biologists, resource managers, politicians, fishers and the broad community. For efficient management of seagrass resources of the region, information on existing seagrasses is used by the Great Barrier Reef Marine Park Authority and other management agencies for regulating activities (e.g. foreshore developments and fishing practices) that can potentially harm seagrass habitats and associated fauna. To obtain baseline information on existing seagrass resources, the Great Barrier Reef Marine Park Authority contracted the Queensland Department of Primary Industries Seagrass Ecology Group to map seagrass habitats in the Hinchinbrook region (Dunk Island to Cape Cleveland) in detail. The seagrass survey was undertaken in October 1996 with divers describing intertidal and subtidal seagrass habitats. Aerial photography was used to aid mapping of seagrass meadows in some intertidal areas. Video and satellite images have aided with mapping intertidal seagrasses elsewhere in tropical Queensland and underwater video has been used to investigate deepwater seagrasses in the Great Barrier Reef region.

Figure 1. Seagrass presence and absence at sites sampled during the dive survey of the Hinchinbrook region, October 1996 Click on thumbnail to view this map A total 258 ± 30 km2 of seagrass habitat (an area approximately five to six times larger than Magnetic Island) was mapped from Dunk Island to Cape Cleveland. Large seagrass meadows in the sheltered bays (Missionary, Shepherd and Cleveland Bays) formed the most prominent seagrass features in the region. Figure 1 shows the meadows found in the Hinchinbrook region. Other important features were dense meadows of mostly Halophila and Halodule species along the Cardwell and Townsville foreshores and in the lee of the large continental islands: Dunk, Palm and Magnetic. The seagrass habitats mapped between Dunk Island and Cleveland Bay are likely to be regionally important to fisheries and dugong/turtle populations because the next substantial areas of seagrass habitat occur large distances to the north (Cairns) and south (Upstart Bay). The large subtidal seagrass habitat areas (e.g. Missionary Bay, Shepherd Bay, Townsville foreshore and Cleveland Bay) are probably very important alternative food sources for dugongs and sea turtles when the narrow intertidal habitat areas are inaccessible at low tides. This survey has identified the areas of seagrass important to coastal processes in the region. The survey results will be used in developing regional management plans for a sustainable coastal system. The information was recently used directly for establishing dugong protection areas which aimed to minimise conflicts between dugong conservation and commercial fishing interests. New methods for mapping seagrasses While free-diving in the Hinchinbrook Channel is not recommended to the general public, it is still the most efficient method to sample seagrass in shallow, turbid coastal areas. Alternative techniques are being sought to reduce costs of surveying and to minimise risks to divers where there are dangerous marine animals, abandoned fishing nets and other diving hazards. An alternative technique using acoustic technology was trialled on seagrass habitats in Cairns Harbour during May 1996. Acoustic survey techniques have an advantage over visual surveys and photographic remote imagery because sound (acoustic) signals are not affected as much as light is by water turbidity and depth. Data is also collected at higher spatial resolution than is usual with dive-based surveys and large areas can be surveyed quickly. Data is recorded digitally on computer in the field, and can be linked with a Global Positioning System (GPS). Trials of acoustic techniques for mapping seagrass were carried out at three sites within Cairns Harbour, and two configurations of acoustic hardware were tested against the results obtained through a dive based survey.

Figure 2. Set up of fan beam and conical beam acoustic systems used for mapping seagrasses habitat boundaries Click on thumbnail to view this figure A fan beam acoustic system was trialed as a technique for mapping of seagrass habitat boundaries. This technique uses a beam of sound that is very narrow (2°) in the horizontal plane and broad (60° - 90°) in the vertical plane (figure 2). This geometry has the effect of a sonar 'sweep' of a sea-floor area typically 1 m wide by 70 m long in a direction perpendicular to the vessel track. A conical beam acoustic system was also trialed for measuring seagrass biomass and sediment type. For determining seagrass biomass, the conical beam uses a very narrow beam of sound at a low grazing angle (10°) emitted from the transducer and reflected from above-ground biota (figure 2). Echo intensity (decibels) received by the transducer is affected by the density of seagrass (i.e. plant density results in higher echo amplitude). The fan beam acoustic system successfully mapped boundaries of seagrass meadows when combined with diver ground-truthed information. Meadow boundaries interpreted from fan beam data are at a higher resolution than is possible from dive-based surveys, however edges of 'patchy-cover' low-density seagrass habitat cannot be mapped with confidence. Figure 3. Acoustic response measured in decibels (line) and mean sediment grain-size (squares) along a survey transect, Cairns Harbour, May 1996 Click on thumbnail to view this figure We found that acoustic techniques can provide reliable sediment mapping information at spatial resolutions better than normally available from traditional mapping methods using sediment grabs (figure 3). Acoustic data can be used in some situations as a proxy for per cent mud - a useful sediment parameter in marine ecology studies. However, acoustic data alone cannot be used to interpret particle size composition (i.e. the proportions of each sediment fraction). Figure 4. Mean acoustic intensity (decibels) and ground-truthed seagrass biomass (g DW m-2) examined at one survey area in Cairns Harbour, May 1996 Click on thumbnail to view this figure Seagrass biomass in Cairns Harbour could not be determined with any accuracy by the conical beam mapping technique in this survey (figure 4). Although correlations with biomass greater than 5 g DW m-2 were detected at one site, the technique could not replicate the results using identical methods at the other survey sites. The lack of any significant correlation between acoustic data and seagrass biomass data is probably the result of a combination of many sources of error. Spatial errors in the data (smearing) can be caused by GPS position-fixing and the influence of surface chop on the orientation of the transducer. Irregular bottom topography and variation (patchiness) in seagrass species composition also create 'noise' in the acoustic data and affect the ability to detect differences in seagrass biomass. Acoustic techniques used to survey seagrass habitats will always require dive-based sampling to ground truth and interpret the acoustic signal, as well as collect information on species composition and faunal use (e.g. dugong feeding trails) of seagrass habitats. Each seagrass survey provides only a snapshot of seagrass resources - their distribution and abundance. To understand changes in seagrass meadows over seasons and years, carefully planned monitoring programs need to be conducted. Repeated surveys which monitor the health and abundance of seagrass meadows allow us to identify impacts of natural climatic events as well as impacts from agricultural and urban land-use, foreshore developments and fishing and boating.

Great Barrier Reef Marine Park Authority
PO Box 1379 TOWNSVILLE QLD 4810. Phone: (07) 4750 0700, Fax: (07) 4772 6093
E-mail: registry@gbrmpa.gov.au