Zebra mussels reroute contaminant flow through food chain
New research into how invasive species impact food webs suggests that some little fish may have a higher PCB concentrations than a trophy bass.
Conventional wisdom holds that contaminants such as PCBs accumulate up the aquatic food web, reaching the highest concentrations in the predatory fish at the top of the food chain. But not in Lake Michigan’s Calumet Harbor—new research reveals that some smaller fish have higher PCB concentrations than the predators that eat them. The findings, presented at the Ecological Society of America meeting in Montréal in August, could mean that resource managers will have to revise fish advisories and sampling protocols for monitoring cleanups, experts say.
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Calumet Harbor, which runs from Chicago, Ill., into northern Indiana, is heavily polluted as a result of industrial and shipping activity. The harbor has a simple food web dominated by two invasive species, the zebra mussel and the round goby, says Carla Ng, a chemical engineer at Northwestern University and lead author of the study. Because of the food web’s simplicity, Ng thought the harbor was a perfect location to test a new bioaccumulation model for PCBs that is based on nitrogen-15 (15N) measurements. She and her colleagues, for the first time, applied a bioaccumulation model for PCBs to the stable 15N isotope study of food web structure in the harbor.
The ratio of heavy to light isotopes of nitrogen generally increases by 3.4% with each consumer up the food chain because of the selective loss of the light isotope as organisms grow, and thus the ratio is used to measure the position of each species in a food web, Ng says. But when the data were collected, the isotope analysis unexpectedly revealed that juvenile round gobies have a greater isotope signature and thus occupy a higher food web position than their predators, the smallmouth bass, she says. The researchers then calculated PCB concentrations and found that the juvenile round gobies, not the smallmouth bass, do indeed have the greater contamination levels. “The smallest round goby is as much as five times more contaminated with PCBs than the smallmouth bass,” Ng says.
The switch in food web positions is due in part to the often-ignored role of detritus, such as fish carcasses, zebra mussels’ feces, and the particles of undigested food wrapped in mucus that they excrete, which are known as pseudo feces, Ng says. When sediment-dwelling organisms such as insects and tiny crustaceans eat this detritus, they accumulate contaminant burdens linked to organisms at higher positions in the food web. Juvenile round gobies eat the sediment dwellers as well as the PCB-rich eggs of smallmouth bass and the eggs of other adult gobies. As a result, they receive a larger dose of PCBs than the adult gobies that mainly consume zebra mussels, which are lower on the food web, Ng says.
The lack of biomagnification of PCBs from gobies to bass can be explained by the bass’s slower growth rate and consumption of other species lower in PCBs than the gobies, Ng says. The scientists who develop bioaccumulation models assume a food web structure based on average diets of adult species; hence, they expect the bass to have higher PCB levels than the gobies. They have overlooked that young gobies are eating bass eggs and that the zebra mussel has added a positive feedback loop in the food web, Ng says.
Federal and state resource managers will have to rethink their fish advisories, which currently recommend that people eat smaller fish to minimize exposure to contaminants, says Marty Berg, an aquatic ecologist at Loyola University and a coauthor of the study. “When we did the PCB analysis for the U.S. EPA’s Great Lakes National Program Office, we interviewed anglers, and they said the round gobies were easy to catch and cooked up nicely,” he says. Now, in addition to sampling large fish, resource managers will have to sample smaller fish for both fish advisories and monitoring cleanups, Berg warns.
“The research shows that if you’re going to explain bioaccumulation of contaminants, you have to have a good understanding of the food chain and seasonal and year-to-year differences in diet,” adds Anders Andren, an environmental chemist at the University of Wisconsin. The findings are not unique to Calumet Harbor, and similar bioaccumulation patterns may occur in many other water bodies, he says. Although other studies of nitrogen isotope ratios have found the complexities uncovered by Ng, her study was unique as the first to combine nitrogen isotope analysis with biocontamination models.
