成人大片

Under increasing global warming, tropical fish are escaping warmer seas by extending their habitat ranges towards more temperate waters.

But a new study from the 成人大片, published in , shows that the ocean acidification predicted under continuing high CO₂ emissions may make cooler, temperate waters less welcoming.

鈥淓very summer hundreds of tropical fish species extend their range to cooler and temperate regions as the waters of their natural habitat become a little too warm for comfort,鈥� says lead author Ericka Coni, PhD student in the University鈥檚 School of Biological Sciences. 鈥淔or at least two decades, Australian temperate reefs have been receiving new guests from the tropics.

鈥淎s a result of warming, we also see warm-temperate long-spined sea urchins increasing in numbers in southeast Australia, where they overgraze kelp forests and turn them into deserts known as 鈥榰rchin barrens鈥�. Coral reef fishes that are expanding their ranges to temperate Australia prefer these barrens over the natural kelp habitats.

鈥淏ut what we don鈥檛 know is how expected ocean acidification, in combination with this warming, will change the temperate habitat composition and consequently the rate of tropical species range-extension into cooler water ecosystems.鈥�

The researchers hypothesised that these two divergent global change forces 鈥� warming and acidification 鈥� play opposing effects on the rate of tropicalisation of temperate waters.

鈥淏ut what we don鈥檛 know is how expected ocean acidification, in combination with this warming, will change the temperate habitat composition and consequently the rate of tropical species range-extension into cooler water ecosystems.鈥�Ericka Coni, lead author

鈥淲e know that as oceans warm they also acidify, because they absorb about a third of the CO₂ emissions from fossil-fuel burning,鈥� says Ericka鈥檚 PhD supervisor and project leader Professor Ivan Nagelkerken from the University鈥檚 Environment Institute and Southern Seas Ecology Laboratories.

听鈥淲e also know that calcifying species like sea urchins are typically challenged by seawater with reduced pH levels resulting from elevated CO₂.鈥�

The research team, which also included Camilo Ferreira and Professor Sean Connell from the 成人大片, and Professor David Booth from the University of Technology Sydney, used two 鈥榥atural laboratories鈥� to study ocean warming (tropicalisation hotspots on the south-eastern Australian coast) and ocean acidification predicted for the end of this century (natural CO₂ vents off the coast of New Zealand) as an 鈥渆arly warning鈥� system to assess the combined consequences of ocean acidification and ocean warming.

They found that sea urchin numbers were reduced by 87% under elevated CO₂, leading to a reduction in number and size of urchin barrens. In their place turf algal cover increased which is less preferred by tropical species.

鈥淥ur study highlights that it is critical to study climate stressors together 鈥� we show that ocean acidification can mitigate some of the ecological effects of ocean warming,鈥� says Professor Nagelkerken.

鈥淔or south-eastern Australia, and likely other temperate waters, this means that ocean acidification could slow down the tropicalisation of temperate ecosystems by coral reef fishes.

鈥淏ut in the meantime, if left unabated, these tropical species could increase competition with local temperate species under climate change and reduce their populations.

鈥淚n the short-term we need to take steps to preserve kelp forests to help maintain the biodiversity and populations of temperate species and reduce the invasion of tropical species.鈥�