Assessing mangrove dieback in the Gulf

  • Mass dieback of mangrove ecosystems in Australia’s Gulf of Carpentaria occurred over a period of three to four months in late 2015.
  • The event was synchronous along 2,000 km of southern Gulf shorelines with at least 551 km of shoreline showing notable dieback. A total of 76.5 km2 of mangrove ecosystems were lost, which was around 6% of mangrove cover in the area.
  • The primary cause of this mass dieback event was a sudden and temporary drop in sea level associated with a severe El Niño in 2015–2016. Sea levels dropped by up to 0.4–0.5 m for a period of 5–6 months (April to October 2015).
  • The dominant mangrove canopy species, Avicennia marina var. eucalyptifolia, was most severely affected by the dieback. These mangroves suffered from extreme moisture stress at higher elevation ecotones bordering the expansive saltpans between highest tide levels and mean sea level.
  • Approximately 39.4 million trees died, releasing an estimated 820,895 tonnes of carbon.
  • Rates of sea level rise are extreme in the Gulf, but mangroves in many areas had responded by progressively spreading up the tidal profile while maintaining their protective shoreline fringing stands. However, there were abundant signs of the consequences of sea level rise with shoreline erosion and loss of sea-edge mangrove trees coupled with saltpan scouring and terrestrial retreat.
  • Tropical cyclones had localised severe impacts hindering mangrove recovery by pushing wrack piles of dead mangrove wood across areas, destroying recovering trees and seedlings, coupled with destructive shoreline scouring and erosion.
  • A previously undetected, earlier mass dieback of mangroves was also discovered to have occurred in late 1982 under similar circumstances to that in 2015. Furthermore, its extent and widespread synchronicity were comparable. The 1982 dieback was associated also with a severe El Niño Southern Oscillation (ENSO) event co-incident with an extreme drop in sea level.
  • These new findings reveal the occurrence of a 33-year-interval collapse–recovery cycle defined by widespread mass dieback events in 1982 and 2015. Recovery from the earlier event took 10–15 years depending on the localised impacts from severe storms and flooding. However, there are serious questions surrounding future recovery trajectories.
  • The occurrence of these dieback events has allowed us to develop a sea level stress index with defined threshold levels for lethal and sublethal impacts on mangrove stands in the Gulf. This index is furthermore correlated with the Southern Oscillation Index from which there may be several weeks warning of imminent future mass mangrove dieback events in Australia’s remote Gulf of Carpentaria.

  • Aerial views of mangrove fringes showing sections of extreme damage (left) and minor damage (right) near Limmen Bight River mouth (NT), photos Norm Duke
  • Local Aboriginal rangers were kept informed and they were keen to participate in field data collection.
  • Field collection involves getting into remote selected sites to take measures and to sample the mud, vegetation and animals.
  • Impacted shorelines of dead vegetation are highly vulnerable to drift logs like the large Melaleuca trunk shown. These woody debris destroy and level both the standing dead mangrove vegetation along with any surviving saplings, and seedlings attempting to get established. Note the threatened, leafy plant caught to the left of the log – in the path of destruction.
  • The impacts on fauna are dramatic as shown in these images comparing surviving mangrove trees shading the ground with functional crab burrows, compared to dead mangrove trees with parched dry ground and piled aggregations of dead gastropod shellfish and fewer crabs
  • Key questions relate to recovery potential comparing severe damaged shoreline (right image) versus less-damaged shorelines (left image) where dieback was restricted to higher intertidal zones. Note how tree loss not only affects vegetation and habitat, but also sediment erosion exposing below ground roots.
  • Traditional Owners and MangroveWatch scientists monitor local shoreline health in the western Gulf of Carpentaria, photo Norm Duke.
  • Traditional Owners and MangroveWatch scientists monitor local shoreline health in the south eastern Gulf of Carpentaria, photo 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.

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