UV damage: some surprises under the sun.

Ozone depletion and the concomitant strengthening of ultraviolet radiation can harm natural ecosystems in unanticipated ways, according to Canadian ecologists who tested how river organisms react to various types of light.

When researchers from the National Hydrology Research Institute in Saskatoon, Saskatchewan, started their study, they presumed that abundant single-celled plants in the river would grow slower when exposed to UV radiation -- an effect previously documented in studies of algae removed from oceans and lakes. But algal communities in the intact river ecosystem actually fared better when subjected to UV radiation.

It was the more complex -- and supposedly more resilient -- organisms that suffered in the experiment.

"This was quite unexpected. There is nothing in the literature to suggest we would get these results," says Max L. Bothwell, who discussed the team's findings in the July 1 SCIENCE.

Bothwell and his colleagues pumped river water through sets of open conduits, which they shaded with mylar, acrylic, and other materials. Some of the coverings let through all wave-lengths of solar radiation; others selectively blocked low-energy ultraviolet (UVA), mid-energy ultraviolet (UVB), or both. Thinning of the ozone layer will boost levels of UVB, but not UVA, striking Earth.

Over several weeks, the researchers analyzed how well algae and midge larvae grew under various conditions. While UV radiation did slow algal growth, the light hurt the algae-eating larvae even more, leading to overall algal increases in the river experiments.

In the past, ecologists presumed that increases in UV light would hit single-celled organisms such as algae hardest because they lack protective layers of cells. But the river experiments yielded the opposite result.

Because the algae-eating larvae form an important link in the food chain, their sensitivity to increasing levels of UV light could send ripples moving up the chain toward larger animals. "The effect of UVB at the ecosystem level is probably magnified higher in the food chain than it is at lower levels in the food chain," Bothwell says.

Deneb Karentz of the University of California, San Francisco, lauds the Canadian team for focusing on the interactions between different ecosystem levels. Karentz, who studies how UV light damages ocean algae, says the well-designed experiment leaves little room for criticism.