Environmental Factor, February 2008, National Institute of Environmental Health Sciences
Superfund Study of Toxins in China's Freshwater Ecosystem
By Melissa Fabiano
Scientists from Dartmouth College, Lakeland College, and the Chinese Academy of Sciences in Beijing investigated the bioaccumulation and trophic transfer of the toxic metals, mercury and arsenic, in Lake Baiyangdian, China. The study, "Mercury and Arsenic Bioaccumulation and Eutrophication in Baiyangdian Lake, China," published on-line in December 2007 in the journal Water, Air, Soil Pollution, targeted the largest lake in the North China Plains. The lake receives runoff from the industrial city of Baoding, which produces significant amounts of man-made pollutants that can affect the water and food supply of the region and enter the food chain.
In their field study, the team of investigators, which included Dartmouth biologists and NIEHS Superfund grantees Celia Chen, Ph.D., and Carol Folt, Ph.D., tested three separate sites, all of which were in proximity of Baoding's industrial outlets on Baiyangdian Lake. Each of these sites was associated in varying degrees with three pollution sources: coal emission, sewage release, and agricultural runoff. The researchers determined that the toxins of most concern at these sites due to their effect on the freshwater ecosystem were mercury in fish and arsenic in water.
"The mercury and arsenic in this system are high enough to be of concern to humans and wildlife that drink the water and consume the fish. For example, we saw arsenic levels in the water that represented more than 50 times the EPA-recommended limit for consumption in fish and shellfish," explained Chen, the study's lead author.
Because of the health risks posed by arsenic and mercury present in levels above EPA-established critical threshold in fish caught for human consumption, the researchers examined the quality of life of resident organisms in the lake with regard to these toxins and the effects of nutrient enrichment of lakes, a process known as eutrophication.
According to the study, eutrophication, which results from nitrogen and phosphorus in sewage and agricultural run-off entering a body of water and can eventually deplete the water's oxygen supply, promotes the proliferation of algal blooms that potentially mitigate concentrations of dissolved metals in the water due to uptake by the algae. However, the extent of mitigation varies due to the chemical characteristics of the toxin and ecological factors related to an organism's trophic level, or relative position in the food chain.
The team sampled and analyzed a group of 13 fish for mercury and found that all had levels of mercury above the critical threshold level and posed some risk to humans and wildlife. A second group of 39 fish were sampled and analyzed for arsenic content. These samples showed an arsenic content high enough to pose significant health risk.
The investigators also noted that total mercury bioaccumulation did not consistently increase with trophic levels at Baiyangdian Lake. Yet, they did find an increase in mercury concentration related to fish size, which they linked to the total bioaccumulation of mercury over a fish's lifetime and age-related shifts in diet, as older fish eat prey with a higher mercury concentration. Arsenic concentrations in the plant and animal life decreased with increasing trophic levels reflecting a trend of biodiminution in the food chain, which the researchers speculated may result from a higher level of arsenic excretion relative to assimilation.
According to the study, these results add further evidence that there are lower dissolved and particulate concentrations of mercury and arsenic when there is a presence of increasing algal blooms. The findings also shed light on how these toxins move through the food web in a freshwater ecosystem known to be polluted. The study concluded that "Our results suggest that both Hg and As contamination should be investigated further due to the potential health risks to humans from drinking lake water or consuming the fish."