Between these two extremes, Dr. Nagelkerken and his colleagues have tried to create a happy medium. They filled 12 pools with 475 gallons of seawater apiece and built simple ocean ecosystems in each one.
They put sand and rocks on the bottom of the pools, along with artificial sea grass on which algae could grow. They stocked their small-scale ecosystems, called mesocosms, with local species of crustaceans and other invertebrates, which grazed on the algae.
For predators, they added a small fish known as the Southern longfin goby, which feeds on invertebrates.
To test the effects of climate change, Dr. Nagelkerken and his colleagues manipulated the water in the pools. In three of them, the researchers raised the temperature 5 degrees — a conservative projection of how warm water off the coast of South Australia will get.
The scientists also studied the effect of the carbon dioxide that is raising the planet’s temperature.
The gas is dissolving into the oceans, making them more acidic and potentially causing harm to marine animals and plants. Yet the extra carbon dioxide can be used by algae to carry out more photosynthesis.
To measure the overall impact, Dr. Nagelkerken and his colleagues pumped the gas into three of the pools, keeping them at today’s ocean temperatures.
In three others, the researchers made both changes, heating up the water and pumping in carbon dioxide. The scientists left the remaining three pools unaltered, to serve as a baseline for measuring changes in the other nine pools.
On its own, Dr. Nagelkerken and his colleagues found, carbon dioxide benefited all three layers of the food web. Algae grew faster, providing more food for the invertebrates. The invertebrates, in turn, provided more food to the gobies.
But the combination of extra carbon dioxide with warmer water wiped out that benefit.
Even with extra algae to eat, the invertebrates failed to grow faster, perhaps because the algae provide less nutrition when they grow at higher temperatures. It is also possible that the invertebrates are under too much stress in warmer water to grow more.
The invertebrates also faced more pressure from their predators. The warm water sped up the metabolism of the gobies, making them hungrier. They devoured more invertebrates. Hammered from above and below, the invertebrate populations collapsed.
Mary I. O’Connor, an ecologist at the University of British Columbia who was not involved in the Australian research, praised it as an ambitious advance on earlier studies. “It’s showed us something we haven’t seen before,” she said.
Dr. Nagelkerken and his colleagues published initial results from these mesocosm studies last month in the journal Global Change Biology. In a separate report published in the February issue of the journal Oikos, Dr. Nagelkerken and his colleagues reported evidence that acidification can interfere with the ability of fish to hunt.
In that study, the researchers raised a species of sharks in warm, acidified seawater. They found that the sharks hunted more for sea urchins, one of the species they eat because of higher temperatures.
But they were less successful at detecting prey, most likely because the altered chemistry of the seawater interfered with their nervous systems.
Dr. Nagelkerken said these experiments had ominous implications for ocean ecosystems — as well as for the 3.1 billion people worldwide who depend on fish for 20 percent or more of their protein.
“As you go further higher up the food web, you get more of a mismatch between the need for food and the availability of food,” Dr. Nagelkerken said. And it’s the species high in the ocean’s food webs that we fish for.
Just how vulnerable fish will be depends on their individual ecosystems. Dr. Nagelkerken said he hoped the studies he and his colleagues are carrying out will prompt other researchers to replicate them with species and conditions from other parts of the world.
“These kinds of experiments are essential tools for understanding change in nature,” Dr. O’Connor, the University of British Columbia ecologist, said.
Dr. Nagelkerken’s research, she said, “is not a prediction of the future, but it is nice proof that we can expect food web reorganization with continued ocean warming and acidification.”