My sad visit to the Gulf of Carpentaria
Dr Norm Duke, James Cook University
To assess the damage to the Gulf mangroves, I recently flew by helicopter over 700km of coastline, where there had been reports of widespread mangrove die-offs.
I was shocked by what I saw. This is the worst mangrove mass die-off I have seen anywhere in the world. There have seen smaller instances of this kind of moisture stress before, but what is so unusual now is its extent, and that it occurred across the whole southern gulf in a single month towards the end of last year.
Mangrove forests played an essential role in the region’s ecosystem and are the nurseries for many fish species. Think of them as kidneys – as water filters and purifiers. As water from rivers and floodplains run into the ocean, mangroves filter a lot of sediment, and protect coral reefs and seagrass meadows. That service will be lost in the areas affected by die-off.
There are already anecdotal reports of marine life dying and piles of dead seagrass washing up on the shore. If that’s true, then turtles and dugongs will be starving in a few months. And it will get worse over the coming years as the roots of the dead plants rot.
The problem is that the mangrove growth rate isn’t high enough to stabilise the environment. In five or six years’ time, the roots will break down and those sediments will become destabilised. And that will threaten the near-shore habitats of seagrass and coral. Some zones have been completely removed of vegetation across the tidal profile and no amount of seedling growth will stop ongoing erosion.
Mangroves also protect the shoreline and coastal ecosystems from storms and tsunamis. Absorbing waves that hit the coast helps limit the impact of storms and rising sea levels. We need that resilience and protection of the shoreline so we can slow down the effects of sea level rise.
Mangroves die off naturally on a small scale, but I’ve never seen anything of this magnitude. Around the world there had been widespread destruction of mangroves, but usually as a result of direct local impacts such as clearing for the creation of shrimp farms. But the areas in northern Australia are relatively pristine.
The clear culprit in this case is climate change, which is warming waters and making rainfall more erratic. That puts the mangrove forests at their tolerance limit and when a strong El Niño hit the world at the end of last year – warming waters in northern Australia and drawing rainfall away – they were pushed past their tolerance thresholds.
Mangroves are good at adapting, but not to such severe changes that occur so quickly. It is difficult to say how well they will recover over the coming years. Some areas could transition completely away from being mangrove-dominated, and become salt pans – flat, unvegetated regions covered in salt and other minerals.
There has been very little monitoring of these relatively pristine mangroves in areas where very few people live. Such a program is urgently needed. We need to be able to form a rapid assessment response for these emergent situations. These habitats are retreating far more rapidly than any of the other endangered forest types we have.
We need to equip people to carry out independent assessments of what the local impacts are. We need to be monitoring these events so we can properly understand what these larger effects are. Are we, for example, reducing the resilience of mangroves thus making them more vulnerable to the climate?