The last fish you ate probably came from the Bering Sea. At present, the Bering Sea provides roughly half the fish caught in U.S. waters each year and nearly a third caught worldwide.

While the basic dynamics of a 'greenhouse ocean' are not well understood, marine ecologists writing in Marine Ecology Progress Series expressed concern that, if their predictions are true, a warming ocean would lead to a much different ecology there.

“All the fish that ends up in McDonald’s, fish sandwiches — that’s all Bering Sea fish,” said USC marine ecologist Dave Hutchins. “The experiments we did up there definitely suggest that the changing ecosystem may support less of what we’re harvesting — things like pollock and hake,” Hutchins said.

The study examined how climate change might affect the algal communities of phytoplankton, the heart of marine food webs. The Bering Sea is highly productive thanks mainly to diatoms, a large type of phytoplankton. Phytoplankton use sunlight to convert carbon dioxide into carbon-based food. As small fish eat the plankton and bigger fish eat the smaller fish, an entire ecosystem develops.

“It's kind of a canary in a coal mine because it appears to be showing climate change effects before the rest of the ocean,” he noted. “It’s warmer, marine mammals and birds are having massive die-offs, there are invasive species—in general, it’s changing to a more temperate ecosystem that’s not going to be as productive.”

The scientists found that greenhouse conditions favored smaller types of phytoplankton over diatoms. Such a shift would ripple up the food chain: as diatoms become scarce, animals that eat diatoms would become scarce, and so forth.

“The food chain seems to be changing in a way that is not supporting these top predators, of which, of course, we’re the biggest,” Hutchins said.

A shift away from diatoms towards smaller phytoplankton could also undermine a key climate regulator called the “biological pump.” When diatoms die, their heavier carbon-based remains sink to the seafloor. This creates a “pump” whereby diatoms transport carbon from the atmosphere into deep-sea storage, where it remains for at least 1,000 years.

“While smaller species often fix more carbon, they end up re-releasing CO2 in the surface ocean rather than storing it for long periods as the diatom-based community can do,” Hutchins explained. This scenario could also make the ocean less able to soak up atmospheric carbon dioxide.

“Right now, the ocean biology is sort of on our side,” Hutchins said. “About 50 percent of fossil fuel emissions since the industrial revolution is in the ocean, so if we didn’t have the ocean, atmospheric CO2 would be roughly twice what it is now.”

The researchers collected the algae samples from the Bering Sea’s central basin and the southeastern continental shelf. They incubated the phytoplankton onboard, simulating sea surface temperatures and carbon dioxide concentrations predicted for 2100.

Each of these variables was tested together and independently. Ratios of diatom to nanophytoplankton in manipulated samples were then compared with those in plankton grown under present conditions.

The scientists found that photosynthesis in greenhouse samples sped up two to three times current rates. However, community composition shifted from diatoms to the smaller nanophytoplankton. Temperature was the key driver of the shift with secondary impacts from the increased carbon dioxide concentrations, according to the study.

Clinton Hare led the research and other collaborators were Karine Leblanc of the Centre National de la Recherche Scientifique, in France; Giacomo DiTullio, Peter Lee and Sarah Riseman of the College of Charleston; Raphael Kudela of the University of California at Santa Cruz; and Yaohong Zhang of the University of Delaware.

The National Science Foundation supported the research.