Warmer Oceans ‘Support only smaller Fish’

By 2100, warmer oceans with more carbon dioxide may no longer sustain one of the world’s most productive fisheries, warn marine ecologists at the University of Southern California, USC and the University of Delaware.

At present, the Bering Sea provides roughly half the fish caught in U.S. waters each year and nearly a third of all the fish caught worldwide.

But during this century, the Bering Sea’s rich food web, stretching from Alaska to Russia, could fray as algae adapt to global warming conditions, the scientific study shows.

“All the fish that ends up in McDonald’s fish sandwiches – that’s all Bering Sea fish,” said USC marine ecologist Dave Hutchins, whose former student at the University of Delaware, Clinton Hare, led research.

“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.

While the study must be interpreted cautiously, Hutchins said, its implications are “harrowing,” especially since the Bering Sea is already warming.

“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 said.

“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.”

Carbon dioxide’s direct effects on the ocean are often overlooked by the public, said Hutchins. “It’s all a good start that people get worried about melting ice and rising sea levels. But we’re now driving a comprehensive change in the way Earth’s ecosystem works and some of these changes don’t bode well for its future.”

The study done by Hutchins and Hare examined how climate change affects algal communities of phytoplankton, the heart of marine food webs.

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.

The Bering Sea is highly productive due mainly to diatoms, a large type of phytoplankton. “Because they’re large, diatoms are eaten by large zooplankton, which are then eaten by large fish,” Hutchins explained.

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 also would become scarce.

“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 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 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.”

Hutchins and colleagues are doing related experiments in the north Atlantic Ocean and also in the Ross Sea near Antarctica.

“We’re trying to make a contribution by doing predictive experimental research that will help us understand where we’re headed,” he said. “It’s unprecedented the rate at which things are shifting around.”

Source: ENS