The six drugs they tested-antihistamines, caffeine, an antibiotic, and an antidiabetic medicine-dampened algal growth by 4% to 22% compared with the controls. Respiration was more than cut in half, and in one stream, photosynthesis fell by 99%. The results are expected to be published online in Ecological Applications this week. The antihistamine diphenhydramine had a strong impact on algae, although other studies have not shown an effect for the crustacean Daphnia or plants at much higher concentrations. "We were surprised by how strong an effect there was," Rosi-Marshall says. "It's still a bit of a mystery."
"I think this is very important stuff," says Emily Stanley, a freshwater ecologist at the University of Wisconsin, Madison, who was not involved in the research. "The basic processes that rivers perform are slowing down."
A big limitation of the study is that the team does not know the concentrations of drugs to which the biofilms were exposed. That's not good enough for Joakim Larsson, an animal physiologist at the University of Gothenburg in Sweden. "It does not become interesting until you know something about the dose," he says. But Stanley thinks that the drugs in the experiment are "probably being delivered at a rate that's in the ballpark of realistic."
Bryan Brooks, an aquatic ecotoxicologist at Baylor University in Waco, Texas, says it's important that future studies further investigate the impact of pharmaceuticals on biofilms, as this could eventually be important for regulation under the U.S. Clean Water Act. Also crucial, he says, is figuring out their effect when mixed with fertilizer, pesticides, and all the chemicals in streams and rivers.