When the Whole is Less than the Sum of Its Parts

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Environmental Ratios of Cadmium and Zinc are less Toxic to Aquatic Insects than Expected

At typical ratios that occur in the environment, cadmium and zinc combined were less toxic to aquatic insects than was expected from adding up their individual effects, according to a new study by the U.S. Geological Survey. The study was conducted on aquatic insects in a mesocosm meant to mimic conditions in mountain streams.

“This was our first study to look at metal mixtures in mesocosms, as opposed to our previous work that used field data,” said USGS scientist Chris Mebane, lead author of the study. “By allowing us to deliberately control the concentrations and exposures, this study helps bridge the gap between standard laboratory toxicity tests and real-world environments.”

Cadmium and zinc are elements that almost always occur together in nature—where cadmium is found, zinc will be there too, usually at 200 times the concentration of cadmium. Both cadmium and zinc can end up in streams after energy or mineral resource development disturbs nearby rocks with the two elements in them.

USGS scientist Travis Schmidt, designer of the experiment, examines the artificial streams used in the research. Credit: Chris Mebane, USGS

To determine what effects that might have on aquatic insect communities, USGS scientists tested varying concentrations of cadmium and zinc, individually and then in ratios likely to be found in nature. Aquatic insects often form the backbone of aquatic food webs, so what happens to them can have ripple effects throughout the whole ecosystem.

Cadmium was far more toxic to aquatic insects (particularly mayflies) than zinc, with effects at very low cadmium concentrations of less than 1 part-per-billion. Yet in combination at typical ratios that occur in the environment, cadmium and zinc were less toxic than was expected from adding up their individual effects.

“We believe that the reasons for this are while cadmium is much more toxic than zinc, zinc is naturally about 200 times more abundant in water than cadmium,” said USGS scientist Laurie Balistrieri, a co-author on the study​. “Further, zinc has a higher chemical affinity to attach to the surface of the insect’s gills and is able to out-compete cadmium from attaching to the gill.” 

Image shows small insects on a rock in a white plastic bucket
Brachycentrus caddisflies swim in an artificial stream used in the research. Credit: Chris Mebane, USGS.

Even though zinc is itself moderately toxic, because it is more abundant it can block more highly toxic cadmium from interfering with calcium uptake. Thus the overall toxicity of the cadmium and zinc mixture was reduced, relative to the sum of the expected damage that each alone could produce.

Because of their chemical nature, both cadmium and zinc can interfere with an organism’s ability to process calcium, which can result in toxic effects.

This study is part of a broader research project examining the environmental effects of water- draining mineralized rock. Future research will focus on copper, nickel and cobalt mixtures in drainage from platinum group element deposits.

More information about the study can be found here. More information about USGS mineral research can be found here. More information about USGS environmental health research can be found here. Sign up for our environmental health newsletter here, and follow us for more mineral studies on Twitter!