In the state of California, there are 74 lakes listed as impaired from mercury pollution by the State Water Resources Control Board. These include the important reservoirs of Shasta Lake and Lake Oroville.
Studies in the past have looked at the effects that the toxic metal has on fish living in the lakes, in terms of how mercury bioaccumulates in them. But that approach doesn’t necessarily quantify how much mercury makes it all the way up the food chain. A better way, according to researchers at the U.S. Geological Survey, is to look at the mercury that gets passed along to birds that eat the fish.
Taking advantage of that method, USGS scientists embarked on a survey of California lakes to assess how much of the contaminant could be found in waterfowl living around them. The aim was to use those levels as a proxy for measuring mercury’s effects on wildlife living in the lakes.
To make sure that the sampling of lakes they used was representative, researchers chose 25 water bodies from the state’s North and South, with varying sizes, shapes and elevations. Each was stocked with different types of sport fish, but had to contain largemouth bass, with varying mercury exposure levels. Lastly, all of the lakes being considered had to have a history of use by grebes, a water bird.
The scientists wanted to know how much of a risk methylmercury posed to fish in the water bodies. But of additional concern was identifying an approach from there that would allow for estimating risks to waterfowl and other wildlife up the food chain.
Their methods for assessing those relied on sampling grebes along with sport fish and prey fish. Grebes were captured after they had been disoriented by a spotlight. Fish were gathered via electrofishing and nets.
Bird eggs and bird blood were assessed for mercury concentrations. For sport and prey fish, mercury was measured in the whole body for prey fish and muscle fillets for sport fish. Those data were then thrown into a statistical model that can estimate the risk of exposure to mercury for wildlife across lakes in California.
In 14 of the lakes studied, grebes were found to have blood mercury levels above 1 parts per million, a level thought to increase reproductive risks. But one lake, Lake Berryessa, had grebes with blood mercury of more than 3 parts per million, a much higher risk level.
Other lakes tested included Perris Reservoir and Lake Casitas in Southern California and Clear Lake and Lake Almanor in Northern California. Full results of the study are published online.
Do you think using grebes in this study was any more or less effective than just relying on fish? Why or why not? Please consider leaving a comment to share your thoughts!