A new US Geological Survey study that was recently featured on Oregon Public Broadcasting is examining why fish in the Hells Canyon Complex of the Snake River have elevated levels of mercury. Mercury can enter freshwater ecosystems and accumulate in organisms through several processes. In Oregon, mercury occurs naturally in some places in deposits known as cinnabar. The mercury gets released into the environment through mining activities. Atmospheric deposition is another pathway by which mercury gets into the environment, primarily from coal burning. Although the Pacific Northwest has few coal-burning power plants, atmospheric mercury can travel long distances.
Once mercury arrives at a waterbody, it can be converted into the more toxic methylmercury. This methylmercury is incorporated into the body tissue of organisms either by feeding or across their body surface. What makes mercury so concerning from a human health perspective is that it ‘bioaccumulates’ as it moves up the food web. So, organisms like plankton at the bottom of the food web have relatively little mercury, but since the mercury ‘sticks’ to fatty tissues, it is found in increasingly higher concentrations in organisms at the top of the food web, like fish, birds, mammals, and eventually, humans. Because our exposure is through our diet, this frequently leads to fish consumption advisories (e.g., Washington, Oregon).
The USGS study has hypothesized that increased nutrients and temperature are affecting the mercury in the Snake River, making the fish unsafe to eat. This may sound familiar to followers of the Aquatic Ecology Lab, as lab member Meredith Jordan recently completed her Master’s thesis on the same topic. Meredith experimentally manipulated temperature and nutrients in large cattle tanks at Cottage Grove Reservoir and measured how much methylmercury was accumulating in zooplankton. She found that warmer temperature could increase the concentration of methylmercury, but that nutrients could actually somewhat mitigate this increase (though the controls always had the least amount of methylmercury). Read more about Meredith’s research here.
Meredith’s research poses an interesting question, as it suggests that the effects of one stressor (temperature) may be buffered by another stressor (nutrients). This ‘ecological surprise’ suggests that we still have much to learn about how stressors interact with each other in nature.