VANCOUVER — Dangling from their riverfront webs, riparian spiders may be useful watchdogs for waterway pollution.
By stockpiling the toxic baggage carried by their waterborne prey, these eight-legged monitors can reveal the type, quantity, exact location and potential biological harm of certain chemicals lurking in water and riverbeds. Spiders may even be better surveyors than some chemical tests, researchers reported in Vancouver November 11 at theannual meeting of the Society of Environmental Toxicology and Chemistry.
Along the banks of three rivers around the U.S. Great Lakes, researchers collected two types of spiders: long-jawed spiders (family Tetragnathidae) and orb weaving spiders (family Araneidae). Both feast on aquatic insects such as midges that spend their youth in the water, often mucking around in river sediments, where they can pick up polluting chemicals and metals. On the banks, the hungry spiders can build up high levels of chemicals and pass them on to bird predators.
The three rivers tested in the study —the Ottawa, Manistique and Ashtabula —are known to harbor contaminants, mainly polychlorinated biphenyls, or PCBs. Making up a family of more than 200 individual compounds, PCBs are neurotoxic and disrupt hormone systems in some higher animals and humans, but are fairly harmless to the river insects and their arachnid predators. Though PCBs were banned in the United States in 1979, their previous use in hundreds of industrial applications, including production of plastics, electrical transformers and dyes, has left a legacy of pollution. But once sunk in riverbeds, the threat that PCBs pose to animals and humans is tricky to predict from water and sediment samples alone.
“That’s the big challenge,” says environmental engineer Upal Ghosh of the University of Maryland, Baltimore County. “If we take the sediment, that doesn’t quite tell what the harm is.” Those chemicals could stay put, or some fraction could move into the food web. Figuring out the fraction in the food web is what’s most relevant for understanding the risk for other animals and humans. Using spiders to unravel that threat, he says, is intriguing.
Researchers led by ecologist David Walters of the U.S. Geological Survey in Fort Collins, Colo., collected upwards of 10,000 spiders living within a meter of the three rivers from 2009 to 2013. By measuring chemicals in five to 20 spiders at each section along contaminated stretches of the rivers, the researchers could gauge which PCBs were seeping out of the waters, at what levels, and how dangerous they might be to creatures on land.
“These do pose a risk to wildlife, like birds,” Walters said of the PCB levels found in the spiders. Declines in songbird populations, such as swallows, could be due in part to eating chemical-laden spiders. And, he said, the PCB levels in spiders mimic those seen in fish, which could pose health risks to people. All three rivers have fish consumption advisories, he said.
Because different manufacturing methods used different combinations of PCBs, the researchers could even use the spiders’ specific collection of chemicals to pinpoint the source of pollution. For instance, spiders downstream of a paper recycling plant along the Manistique River collected the same cocktail of PCBs used in paper recycling.
Walters hopes that the spider monitoring, which is now being used by other researchers around the country, could help keep tabs on polluted sites, track river clean-up methods that can cost millions and identify new hot spots of pollution.