A team of researchers and Indigenous rangers have discovered why the Gulf of Carpentaria is such a dugong hotspot by carrying out a major survey of its seagrass beds, which are facing multiple threats from climate change.
New research conducted by the team has found that the seagrass meadows along the Gulf's Limmen area in the Northern Territory are more widespread and diverse than had been understood before.
Dugongs, which are classed as a vulnerable species, almost exclusively eat seagrass.
One of the leaders of the research project, Rachel Groom from Charles Darwin University, said scientists and the Limmen area's Marra people had long known that the Gulf was home to one of the world's biggest dugong populations, numbering up to 5,000, which is half of the total NT population.
That's dwarfed only by the Torres Strait, which has an estimated 15,000 dugongs.
"There are a lot of marine turtles that rely on the habitats of the Limmen region, and it's also the most important habitat for dugongs in the Northern Territory."
Dr Groom's colleague, Alex Carter from James Cook University, said the team had spent weeks surveying more than 3,000 seagrass bed sites over 2250 square kilometres.
"What we've found is extensive seagrass meadows that were growing right up in close to the mangroves in very shallow water, that exposes at low tide, and then right down into deep water at 20 metres," she said.
"We've mapped really large meadows and they were way more diverse than we expected – we had eight species in total."
The new research has been co-funded by both universities and the NT and federal governments, and also by the Li-Anthawirriyarra Sea Rangers who helped to carry out the survey.
Senior ranger Shaun Evans said the work to better establish the importance of the area's biodiversity was very important to his Marra people.
"All the animals that live in the ocean are important for our cultural side, like dugong and turtle, stingrays, [that] goes for all the ocean-dwelling animals," he said.
Scientists hope work will lead to increased protection
Only some of the area that was surveyed already has some protection from mining, shipping and commercial fishing because it falls within two NT and federal marine parks.
Dr Groom said the scientists hoped their research would demonstrate to both governments where protections weren't currently wide or strong enough.
"What this project has allowed is to really better understand where the priority areas are for protection, and I think some of those areas are just non-negotiable conservation areas," she said.
She said protecting the area was particularly important for its dugongs because they don't move elsewhere to feed.
Dr Groom's work tagging Limmen dugongs with the Indigenous sea rangers has helped to establish that.
"The Gulf, and particularly the Limmen and Sir Edward Pellew Islands areas, are really significant for these animals because they don't tend to move very far – they don't travel to Queensland's east coast or to Western Australia," she said.
Dr Carter said she hoped the team would be able to extend the research.
"This study is really one piece of the jigsaw puzzle of understanding seagrass habitats across Northern Australia, so we hope to continue these surveys across a larger area to better understand how they are connected, and we also hope that it leads to long term monitoring led by the rangers," she said.
Seagrass beds suffering climate change impacts
Dr Groom said the biggest threat to the dugongs and their seagrass habitat was climate change.
"The Gulf has one of the highest rates of sea level rise in Australia, at 9mm a year, and there is also some evidence that seagrass can be burnt, especially when it is exposed at low tide," she said.
"By all means we can act locally to take the pressure off some of these resources, but the bigger picture is, we need to have greater commitment from leadership across global governments."
The Gulf coast has suffered major climate change impacts in recent years including a mass die-off of thousands of kilometres of mangroves in 2016.
A researcher from the CSIRO's Oceans and Atmosphere section, Eva Plaganyi, has been studying how climate change impacts including sea temperature increases have been affecting the Gulf of Carpentaria and Torres Strait seagrass beds and the animals which depend on them for 13 years.
"In tropical waters a lot of the seagrass and the animals are near their thermal tolerance limits, for example seagrass can only cope with about 35 degrees Celsius, so there is a point at which seagrass starts dying," she said.
Dr Plaganyi said over the past century the Gulf's average sea temperature had increased by between one and 1.5 degrees Celsius.
"And there have been massive spikes in the temperatures at times, including in March 2016 when the average temperature rose by 2.5 degrees," she said.
"And of course if the seagrass dies, the dugongs which feed on it and other animals like the turtles, all the fish and crabs and other organisms that live on the seagrass, will also be impacted."
Dr Plaganyi said another big concern for these habitats was that stronger and more frequent cyclones would rip out seagrass beds, and changes in ocean currents would kill them by covering them up with sand.
