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…