Assessing mangrove dieback in the Gulf

Australia is home to 7% of the world’s mangroves, with the majority located in the tropical regions.

During the summer of 2015–16, one of the worst mangrove dieback events ever recorded devastated around 7400 hectares of mangrove forests along more than 1000 km of Gulf of Carpentaria coastline.

Field visits suggest that a relatively low percentage of trees have recovered and most are dying or dead, but there is no current formal assessment of the condition of affected forests and what proportion are recovering. There is also little understanding of the patterns of impact across the extent of dieback – in some areas, all mangrove species in all tidal elevations have been affected but in other areas only some species in specific locations have been affected. The dieback was not discovered for nearly five months, and individual reports did not recognise the scale of the event, demonstrating shortcomings in coastal monitoring capability. A better understanding of extent, patterns, condition, trend and the likely cause of dieback-affected mangroves will inform monitoring and management responses.

The dieback event has caused concern amongst local communities and industry bodies such as commercial and recreational fishing groups that rely on many ecosystem services provided by mangroves. The wider national and international community are also concerned about the dieback event due to its scale, diverse impacts and possible link to climate change.

Biodiversity is likely to have been significantly impacted by the dieback. For example, epibiont communities – the small plants and animals that live on mangrove trunks and aerial roots – have been shed, and leaf litter supplies, an important part of aquatic food chains, have almost ceased.

Of the affected mangroves, approximately 200 km are shoreline mangroves whose death would expose the coast to erosion and storm surge effects causing extensive geomorphic and ecological disturbance.

Project activities

  • Initial Rapid Assessment of dieback recovery using the Shoreline Video Assessment Method and LiDAR – an airborne technology which takes precise, three-dimensional measurements of the shape of the mangrove forest and forest floor.
  • Field verification and detailed monitoring including building the capacity of local Indigenous ranger teams to help monitor mangroves
  • Formal assessment of the condition of affected forests and what proportion are recovering
  • Analysis of the patterns of impact across the extent of dieback – in some areas, all mangrove species in all tidal elevations have been affected but it can be different in other areas

Field work will cover the entire dieback zone along southern Gulf shorelines from Weipa in the east to north of Numbulwar in the west with more detailed ongoing monitoring near Karumba, Burketown, Borroloola and Numbulwar townships.

Anticipated outcomes

  • A best practice standard baseline assessment of the affected shoreline to underpin future assessments of mangrove forests and/or associated environmental changes
  • More informed management options in response to mangrove dieback events, i.e. deciding on whether to intervene in the recovery process, and if so, where and how
  • Better informed management policies for fishery species and biodiversity that depend on mangrove habitats (e.g. catch quotas, seasonal closures), for carbon accounting, and for water resource planning where freshwater flows influence mangrove health
  • Increased coastal monitoring and reporting capability in the Gulf, including increased capacity of local Indigenous ranger groups (e.g. extension of MangroveWatch training courses and joint fieldwork) to continue coastal monitoring after the project has ceased.
  • Improved understanding of the effects of the dieback on shoreline erosion and geomorphology
  • Improved understanding of the values and ecological services provided by mangroves and associated coastal ecosystems

Anticipated outputs

  • A map and report assessing the extent of the dieback, its condition, the patterns of processes across the full range of the dieback in detail
  • Contributions to an online, interactive portal for displaying the dieback imagery collected by rangers, scientists and others, including a baseline dataset and associated condition metrics for each assessment location for monitoring change and recovery into the future
  • An evaluation of risks to shorelines and recommendations for the on-going standardised monitoring of tidal wetland vegetation bordering Gulf dieback areas
  • An assessment of the amount of carbon lost from dead and rotting trees, and the likely consequences
  • A review of mangrove habitat recovery and whether mitigation interventions might facilitate shoreline restoration
  • Fact sheets, peer-reviewed scientific publications and other products

Field work will cover the entire dieback zone along southern Gulf shorelines from Weipa in the east to north of Numbulwar in the west with more detailed ongoing monitoring near Karumba, Burketown, Borroloola and Numbulwar townships.

mangrove dieback study area

The mangrove dieback study area in the Gulf of Carpentaria

Climate drivers of the 2015 Gulf of Carpentaria mangrove dieback (report)

Harris T, Hope P, Oliver E, Smalley R, Arblaster J, Holbrook N, Duke N, Pearce K, Braganza K, Bindoff N. 2017. Climate drivers of the 2015 Gulf of Carpentaria mangrove dieback. Earth Systems and Climate Change Hub Technical Report No. 2, NESP Earth Systems and Climate Change Hub, Australia.

The project is led by Professor Norm Duke from James Cook University (JCU). Professor Duke will be assisted by additional researchers from JCU and Indigenous ranger groups in the region.

For more information, visit the Mangrove Research Hub and Australian Mangrove and Saltmarsh Network websites.

Contact
Norm Duke, James Cook University
E: norman.duke@jcu.edu.au
T: 0439 191 952

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This project is also being supported by the Tropical Water Quality Hub, the Marine Biodiversity Hub and the Earth Systems and Climate Change Hub of the Australian Government’s National Environmental Science Program.

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  • Dead mangroves line a Gulf shoreline near Karumba & Norman River mouth (Qld), photo Norm Duke
  • Aerial views of mangrove fringes showing sections of extreme damage (left) and minor damage (right) near Limmen Bight River mouth (NT), photos Norm Duke
  • Dead mangrove near Karumba & Norman River mouth (Qld), photo Norm Duke
  • Aerial views of seaward mangrove fringes showing foreshore sections of minor damage (left) and extreme damage (right) near Limmen Bight River mouth (NT), photos Norm Duke
  • Increasing coastal monitoring and reporting capability in the Gulf is a critical component of this project, photo Norm Duke.
  • Indigenous rangers will continue to be trained in shoreline assessment methods such as MangroveWatch, photo Norm Duke.