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Ocean Warming May Increase the Abundance of Marine Consumers

Warmer ocean temperatures could mean dramatic shifts in the structure of underwater food webs and the abundance of marine life, according to a new study by researchers at the University of North Carolina at Chapel Hill.

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, due to be published online Aug. 25, 2009 in the journal PLoS Biology, 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 zooxplankton or fish, but a reduction in the overall mass of living creatures in the sea.

Mary O’Connor, Ph.D., the study’s lead author, said 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,” said 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, Calif. “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,” said John Bruno, Ph.D., associate professor of marine sciences in the UNC College of Arts and Sciences and a co-author 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 said.

Other study authors are Michael F. Piehler, Ph.D, assistant professor at the UNC Institute of Marine Sciences in Morehead City, N.C.; Dina M. Leech, Ph.D., formerly a postdoctoral researcher at the institute and now a research scientist at DePauw University; and Andrea Anton, a doctoral student in the UNC curriculum in ecology.

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Bruno can be reached at (919) 360-7651 or
O’Connor can be reached at (805) 892-2522 or
Dina Leech and Virginia Schutte collect zooplankton from Bogue Sound using a plankton tow net. Plankton from the net were rinsed into a sieve and then added to the experimental microcosms. Photo: M. O’Connor

Mary O’Connor checks temperatures in the food web experiment. Eight water tables each contain five microcosms, which are shielded from UV and full sunlight by plexiglass and window screen. The water bath maintains the temperature, and air tubes going into each microcosm deliver oxygen. Photo: A. Anton

Final microcosm communities. Phyto- and zooplankton are microscopic, but high density of phytoplankton colors the water green. Low densities of phytoplankton, as a consequence of low productivity or heavy grazing by zooplankton, result in clear water. Top to bottom: cold to warm treatments (21 C, 23 C, 25C and 27 C); Left to right: Nutrient additions and nutrient controls. Photo: M. O’Connor

Concentrated phytoplankton at the end of the experiment. Phytoplankton from 50 mls (1/60) of each microcosm are filtered onto a white filter before the concentration of chlorophyll is measured. Higher density of phytoplankton results in deeper green color. Top to bottom: cold to warm treatments (21 C, 23 C, 25C and 27 C); Left to right: Nutrient additions and nutrient controls. Photo: M. O’Connor and W. Eaton

Patric Lane | Newswise Science News
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