A survey of intertidal seagrass from Van Diemen Gulf to Castlereagh Bay, Northern Territory, and from Gove to Horn Island, Queensland
Anthony Roelofs, Rob Coles and Neil Smit
Report to the National Oceans Office
About the report
The distribution, structure and composition of intertidal and shallow sub-tidal seagrass communities from an area representing almost 10% of Australia’s mainland coastline are described in this report. Using a combination of helicopter survey techniques and sophisticated GPS systems, a joint Queensland/Northern Territory government team of seagrass scientists mapped intertidal seagrass meadows in coastal areas between eastern Van Diemen Gulf in the Northern Territory, and Horn Island in North Queensland. The information provided by this survey addresses a gap in biological knowledge identified as being important to northern marine regional planning processes. A Geographic Information System of the seagrass distribution layers is provided.
Seven seagrass species were recorded during the survey: Halodule uninervis, Syringodium isoetifolium, Thalassodendron ciliatum, Enhalus acoroides, Halophila decipiens, Halophila ovalis, and Thalassia hemprichii. Halodule uninervis and Halophila ovalis were by far the most common on the open sand and mud flats. Thalassia hemprichii and Thalassodendron ciliatum were found on reef platforms and around rocky islands.
Cymodocea rotundata, Cymodocea serrulata and Halophila spinulosa have been recorded in shallow sub-tidal waters in this region previously and Zostera capricorni has been recorded from Horn Island. These species were not seen in the present survey.
An area of just over 42,000 hectares of intertidal seagrass was mapped. Combined with data from previous surveys it is estimated that at least 70,000 hectares of intertidal and shallow sub-tidal seagrass are present in the survey region. The seagrass meadows are extremely patchy in their distribution and in their individual meadow structure. The seagrass meadows we mapped are similar to most of the historical seagrass data for the region however there are some differences. We suggest that these differences are due to the survey methods used and/or because of physical damage by tropical storms and cyclones for which the region is prone.
Almost all seagrass meadows observed showed evidence of turtle activity and/or dugong feeding. Some small patches were heavily grazed and may be vital for dugong travelling along the coast.
Seagrass and algae meadows have been rated the third most valuable ecosystem globally (on per hectare basis) for ecosystem services, behind only estuaries and swamp/flood-plains. The northern Australian seagrasses are extremely important and it is essential to understand, and to measure and monitor the parameters that control their distribution and abundance. The greatest threats of loss of seagrass in this area are likely to be from climate events or climate change. This report provides only a snapshot in time of the seagrass communities. Longer-term monitoring and seasonal comparisons would be desirable to better understand the dynamics of the distribution observed.
As part of this survey, the location of discarded fishing net and discarded or apparently abandoned crab traps were recorded. Large numbers of net pieces were identified despite the likelihood that the survey techniques, designed to map seagrass, almost certainly missed a large proportion. The density of observations was clumped with an area in the western Gulf and area to the north of Aurukun with high densities of beached net pieces. Large numbers of abandoned crab traps were recorded in the south western Gulf. Net and trap type and location are recorded in a Geographic Information System layer.