Key points
- Marine heatwaves are predicted to become more frequent due to climate change.
- These prolonged oceanic warm water events can have severe impacts on ecosystems.
- Our research has revealed the impacts of a severe 2015/16 MHW off Tasmania.
As the world warms so too will our oceans, as climate change leads to longer, more frequent, marine heatwaves (MHWs).
Much of Australia’s east coast has been experiencing MHWs this summer, stretching from Queensland to the south-east of Tasmania. These prolonged oceanic warm water events can have significant impacts on marine life, such as mass coral bleaching. They could even explain why a sea turtle was recently spotted in Hobart’s River Derwent. These turtles are rarely seen in waters below 20°C.
MHWs can last many months. They can be caused by a range of factors including arrival of warm water in ocean currents and the warming of the ocean surface from the atmosphere. Climate drivers like the El Niño declared for Australia in 2023 can also impact their frequency, intensity and duration.
Scientists are racing to understand the drivers and impacts of MHWs to help minimise the fallout of these extreme events. Our new study has revealed the consequences of MHW off Tasmania in 2015/16, which is considered the worst on record.
Altering the marine food chain
Our researchers found MHWs are altering the microorganism communities that form the base of the marine food chain. They analysed the unprecedented 2015/16 MHW, which occurred in the Tasman Sea off Tasmania’s east coast, and found the microbial community had been transformed.
Dr Mark Brown is the lead author of the paper. He said the study was only possible because researchers had established a long-term baseline of more than 12 years, describing what microbes usually look like around Australia at different times of year.
He said the changes to the microbes off Tasmania during the MHW event were glaring.
"We looked at this big, marine heatwave event and we saw that the organisms changed really profoundly to look more tropical, like organisms we’d see up around Queensland, northern New South Wales and even in the East Australian Current," Mark said.
"The reshaping could lead to the occurrence of unusual species and the development of unique combinations of organisms."
The study showed a shift away from normal phytoplankton species towards smaller cells that aren’t easily consumed by larger animals.
Mark said the MHW impacted the type and quality of food for copepods and krill, which in turn are eaten by fish.
“So the impacts can cascade up the food chain,” Mark said.
A window into the future
Dr Lev Bodrossy is a principal research scientist in the Environmental Genomics team in our Coasts program. He said MHWs represent a window into the future.
"Marine heatwaves could be more frequent, more extensive and last longer, and eventually, the oceans will be warmer which will change marine life,” Lev said.
"If we see what we saw in 2015 and 2016 become the norm, our marine life will change. First the phytoplankton, the basis of the food chain will shift southwards. This will then cause a follow up shift of the next levels of the food chain, cascading all the way up to the fish. This shift is likely to occur over time and with significant disruptions.
“We have already seen many species of fish moving further south into Tasmanian waters.”
Since June 2023, our forecasting has suggested above-average summer ocean temperatures around Australia, particularly off Tasmania, and these forecasts were realised - marine heatwaves did occur.
Lev said the current MHW around Tasmania, while not as large or long-lasting as the 2015-16 event, has resulted in similar maximum temperatures.
"Studies like these are crucial for understanding and forecasting the future status of the marine ecosystem," Lev said.
Lev said the study will enable them to evaluate the health of the marine ecosystem and they’ll be able to better predict the future of fish stocks and marine carbon sequestration in different regions of the global ocean.