A scientist from James Cook University says unusual ocean upwelling events could be putting some marine animals at risk of mass die-off.
Climate change is likely pushing shifts in pressure systems and ocean currents that cause such events.
But rather than being a cause of animal deaths due to the general phenomenon of warming oceans, these upwelling ‘cells’ are instances of sudden surges of cold water being pushed to the surface from the bottom of the ocean.
It’s a deadly brew, though, for certain species. As warming waters allow tropic species to fan out in the world’s oceans, a sudden cold spike can be lethal.
The study, led by JCU shark ecologist Nicolas Lubitz, was prompted by the wash-up of 260 marine animals along South Africa’s south and eastern coastline in 2021.
He says satellite imagery indicated a cold upwelling occurred in the region the day before carcasses began appearing on shore.
“We examined trends in the frequency and intensity of upwelling in the Agulhas Current in the Southern Indian Ocean and the East Australian Current— and found an increasing trend in frequency and intensity of cold events,” Dr Lubitz says.
The Agulhas Current extends along the east coast of Africa: through the gap of ocean between Mozambique and Madagascar towards the continent’s southern tip, transporting warmer waters.
Lubitz and his colleagues from Australia, France and South Africa, particularly investigated the impact of these upwelling events on bull sharks (Carcharhinus leucas), which typically live near the surface in tropical parts of Australia, southeast Asia, the Americas and Africa.
In their study, published today in the journal Nature Climate Change, they suggest a ‘bait and switch’ phenomenon could impact tropical fish species like bull sharks, where warming waters lure them into new, previously uninhabitable frontiers, only to shock them with a sudden return to lethally cold conditions.
“Our results suggest that climate change-driven intensification of upwelling could increase the frequency at which migratory, subtropical marine megafauna experience killer cold events,” Lubitz says.
“This highlights the potential impacts of increased cold events, and also the complexities of climate change on marine ecosystems since the oceans are generally warming but climate change can simultaneously shift currents and winds to produce short but intense cold snaps.”
