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Lake shape a major factor in outbreaks of epidemics among plankton

James E. Kloeppel, Physical Sciences Editor
217-244-1073; kloeppel@uiuc.edu

8/7/2006

Photo of Carla Caceres
Click photo to enlarge
Photo by L. Brian Stauffer
Carla Cáceres, a professor of animal biology in the School of Integrative Biology at Illinois, is the lead author of a new paper that shows the shape of a lake's basin has a prominent role in epidemics affecting water fleas grazing on lakes in Michigan.

CHAMPAIGN, Ill. — The shape of a lake’s basin – in reverse of what researchers had theorized – has a prominent role, along with predation and weather patterns, in epidemics affecting water fleas grazing on lakes in Michigan, researchers say.

The findings, published in the June issue of the journal Ecology, however, may extend well beyond these minuscule crustaceans officially known as Daphnia dentifera Forbes. “The ecology of disease must move beyond the study of host-parasite interactions in isolation,” says lead author Carla E. Cáceres, a professor of animal biology in the School of Integrative Biology at the University of Illinois at Urbana-Champaign.

“Instead, ecologists sorely need a broader framework that jointly considers interactions among biotic drivers, such as selective predation, and abiotic/physical drivers of epidemics,” she and co-authors from three other institutions conclude.

The researchers had hypothesized that lakes with gently sloping sides, where there is increased contact with the sediment in the warm, upper layer of shallow water along the shoreline, would more strongly influence the ability of a common yeast parasite (Metschnikowia bicuspidata) to infect populations of Daphnia.

In an effort to learn something new about “why epidemics erupt in some host populations at some times whereas others show little or no infection,” Cáceres and the research team spent three years studying 18 lakes with glacial origin in Barry and Kalamazoo counties in southern Michigan. All the lakes feature Daphnia as a common member of the zooplankton but vary in surface area, depth, basin shape and productivity.

Daphnia are infected by eating asci, a club-like structure of spores formed through the sexual reproduction of the parasite. Once ingested, these needle-shaped spores eventually kill the host after knifing through its gut. When host die, asci return to the water.

Epidemics were more likely to occur after mid-August, peaking in September, following the passage of cold fronts, “but outbreaks were only common in the lakes with the steepest sides,” the researchers wrote. “Lakes with gradually sloping sides were devoid of epidemics.” Cold fronts, they noted, increase turbulence, allowing for a mixing in the water that increases the chances of spore resuspension and horizontal transport.

The physical environment of the lakes rather than the density of hosts or the prevalence of infection appeared to dictate the outbreaks of epidemics, Cáceres said. The researchers suggest separate but related mechanisms to explain their findings:

• Predation in shallow-sloping lakes in warm weather may reduce interactions between Daphnia and parasites. In deeper water, Daphnia escape predators by seeking deepwater refuge, Cáceres’ team believes. The variation in fish species also may be wider in deeper lakes. A companion study, published in the July issue and led by Spencer R. Hall of Indiana University, argues that warmer temperatures under global warming won’t necessarily translate to more epidemics for all host-parasite interactions. Hall’s findings suggest that higher densities of predators feeding on hosts such as Daphnia could serve to lower the risk of epidemics.

• Aquatic plants in shallow-sloping lakes are more numerous along the shore and may interfere with the transport and suspension of the spores into the feeding area of Daphnia. In deep-sloping lakes, Daphnia may swim closer to shore since deepwater refuge is available to them. Such movement, researchers suggest, would put Daphnia into shoreline regions that boast higher resuspension of the spores and allow more contact with parasites.

Co-authors on the Cáceres-led study were Hall, a former U. of I. postdoctoral researcher at Illinois; Meghan A. Duffy, formerly a doctoral student at Michigan State University and now a postdoctoral researcher at the University of Wisconsin; Alan J. Tessier, a zoologist and program director in the Division of Environmental Biology of the National Science Foundation; and Chad Helmle and Sally MacIntyre, both at the University of California at Santa Barbara.

Three National Science Foundation grants supported the research.

Editor’s note: This release was written by Jim Barlow when he was life sciences editor of the U. of I. News Bureau. He became director of science and research communications at the University of Oregon on July 10.