Warmer oceans tangle food chain

UNC-CHAPEL HILL (US) — Even small increases in ocean temperatures could drive dramatic shifts in the structure of underwater food webs and the abundance of marine life, according to a new study.

Until now, little has been known about how changes in temperatures might affect the total productivity and growth of all marine consumers (such as animals, fungi and bacteria) relative to their prey (including algae and plants).

The study by researchers at the University of North Carolina at Chapel Hill looked at a simple underwater food chain and how temperature changes affect organisms’ growth and metabolism. In warmer temperatures, these processes happen faster. As a result, demands for food and nutrients increase with temperature.

Researchers placed tiny zooplankton (consumers in the food chain) and phytoplankton (which are photosynthesizing producers) in small containers and incubated them at different temperatures and in two nutrient scenarios reflecting low and high resource supply conditions for phytoplankton.

The results suggest that higher temperatures could lead to an increase in the number of consumers in the ocean, such as zooplankton or fish, but a reduction in the overall mass of living creatures in the sea. Results were published online Aug. 25 in the journal PLoS Biology.

Mary O’Connor, the study’s lead author, says the findings have implications for how marine and other ecosystems might respond to global warming.

“Small changes in ocean temperature, like those expected with climate change or even just a warmer summer, have fundamentally different effects on marine consumers and their food supply,” says O’Connor, who carried out the research while a graduate student at UNC and is now a postdoctoral fellow at the National Center for Ecological Analysis and Synthesis in Santa Barbara. “This means we may be able to understand some important consequences of ocean temperature change before we go out and study temperature effects on every single species.”

“The components of this theory have been around for decades, but I think we are just starting to comprehend the enormous range of processes and patterns in nature that are very strongly influenced by temperature,” says John Bruno, associate professor of marine sciences in the UNC College of Arts and Sciences and a coauthor of the study.

Ocean temperature averages about 30 C (86 F) in the tropics and 2 C (35.6 F) in the polar regions, and varies between summer and winter. Climate models predict ocean temperatures will rise between 2 C and 7 C (or between 1 F and 11 F) in different parts of the world in the next 100 years, and increases of 1 C to 4 C (1 F – 9 F) have already been observed. All of these types of changes would affect the food chains of the ocean, O’Connor notes.

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