This June, the world's oceans reached 17 degrees Celsius, their highest average temperature since record keeping for these data began in the 19th century. And a new experiment suggests that those balmier waters might mean big changes for the marine food chain.

Marine ecologist Mary O'Connor of the National Center for Ecological Analysis and Synthesis at the University of California, Santa Barbara, and colleagues at the University of North Carolina at Chapel Hill set up five four-liter "microcosms" of seawater filled with microorganisms from the Bogue Sound estuary on the North Carolina coast. Over the course of eight days last spring, the scientists then exposed the microcosms to varying degrees of warming and nutrient levels to mimic storm flow into an estuary.

Theoretically, increased nutrients and warmth should fuel the growth of tiny drifting plants known as phytoplankton—as evidenced by seasonal dead zones that form at the mouths of many rivers worldwide when the tiny plants bloom, die and, while decaying, suck up all the available oxygen in the seawater. But the researchers found that increasing temperatures, although initially enhancing the growth of phytoplankton, also allowed increased grazing by zooplankton (microscopic animals) and bacteria, according to the results published today in PLoS Biology.

"As temperature rises, the zooplankton start to grow faster than the phytoplankton," O'Connor explains. "The zooplankton are more abundant and faster-growing, and are able to eat all the phytoplankton in warmer water. This creates a bottleneck in the food chain that could have large implications for the ocean's food web."

Not only does that mean that there are fewer phytoplankton around to suck up carbon dioxide, but it could also mean less food for other grazers. But it does not necessarily mean that the zooplankton will gorge themselves to death; other research has shown that food webs with more animals (consumers) than plants (producers) is sustainable for at least five years. And higher on the food chain it is zooplankton, such as krill, that are feasted on by marine life ranging from fish to whales.

Boosting the number of zooplankton, however, means the overall mass of ocean life declines: the tiny animals metabolically burn 90 percent of the phytoplankton they consume, incorporating only 10 percent. All told, with a 6-degree Celsius rise in water temperature, total biomass in the warmest microcosm shrank by 50 percent, O'Connor reports.

This effect only holds, however, in areas that are rich in nutrients. In the experimental microcosms where the nutrients nitrogen and phosphorus were kept low, those limits defined the relative abundance of plants and animals. And other factors—ocean acidity or salinity—could also play large roles. "The ultimate effect of temperature on zooplankton and consumers higher in the food chain will depend on other ocean conditions that affect resource availability," O'Connor says.

That could mean that nutrient-rich waters in places like the Arctic Ocean will begin to see this food chain shift as the seas continue to warm—and a consequent rise in the number of fish. "Our experiments and current theory suggest that warming in nutrient-rich areas should increase [the number of] fish," O'Connor says. "I think we can figure out how and where climate change may lead to greater fish productivity and where it might reduce fish productivity."

But even in the Arctic, there is typically a nutrient limit, says phytoplankton ecologist Michael Behrenfeld of Oregon State University. "It's a very interesting idea," he says. But an increase in fish harvests "might be a bridge too far with this. There are other factors that need to be considered."

For example, his own satellite-imagery research on the phytoplankton in the North Atlantic reveals that bloom starts in wintertime as a result of deep, nutrient-rich water welling up to the surface. Warming is diminishing that upwelling and therefore the availability of nutrients. "We see a decrease in blooms," Behrenfeld says. "How much can we use [four-liter] microcosms to extrapolate to natural systems, especially natural systems at longer timescales?"

Nevertheless, the experiment provides a glimpse of how the marine food chain might be transformed by climate change. "Worldwide, ocean waters are warming and will continue to warm by several degrees," O'Connor says. "By understanding the effects of temperature in these ideal conditions, we can begin to apply this model to natural systems."