We have recently discovered that aquatic mercury from a legacy point source on the headwaters of the majestic Shenandoah River in Virginia is contaminating songbirds living in forests nearby.  Mercury has long been of concern as a persistent contaminant with wide-ranging and potentially severe effects on animals.  When converted in the environment to its organic form, metylmercury, this toxic metal biomagnifies up the food web, accumulating in tissues to the point that it causes mortality, reproductive failure and other health effects in predatory wildlife and humans.  Atmospheric mercury can move long distances and contaminate broad swaths of habitat far from its source.  Aquatic mercury contamination, by contrast, often ends up concentrated in a single river system.  The aquatic mercury that commonly contaminates fish could invade adjacent terrestrial food webs through trophic transfer to wildlife or deposition of contaminated sediment during floods.  The extent to which point source mercury in this river is transferred to adjacent terrestrial food webs through biological or hydrological processes was unknown until our recent discovery.  Our finding has implications for the health of unstudied terrestrial ecosystems bordering thousands of mercury-contaminated rivers worldwide.

In the Shenandoah Valley, during the two decades prior to 1950, a synthetic fiber manufacturing facility deposited mercuric sulfate into the South River, one of three major tributaries of the South Fork Shenandoah River (location: 38.06°N, -78.88°W).  In 1977, elevated mercury in fish muscle triggered a human fish consumption advisory on the South and Shenandoah Rivers, just one of over 3000 mercury consumption advisories in the United States.  Bass along 77 kilometers of the South and South Fork Shenandoah Rivers in 2005 averaged 1.57 ± 0.87 (SD) ppm mercury (n = 67).  It is obvious that fish-eating birds and mammals feeding on these contaminated fish would be contaminated at even higher levels due to biomagnification.  We decided to look instead at songbirds that don't eat fish, both aquatic and terrestrial feeders.  We soon found that birds ranging from swallows to bluebirds to wrens had high levels of mercury.  A longterm swallow reproduction and survival study was established in 2005.  In 2006 we began sampling the food that these floodplain-dwelling birds were eating, and noticed that 25% of their diet was comprised of spiders.  Spiders, being predators, sit atop long food chains and thus potentially biomagnify mercury to high levels when it is present.  Indeed, we found that over 75% of the mercury ingested by wrens and bluebirds came from spiders.

Our current focus is on how the spiders are getting their mercury.  It could be ingested each year in the form of emergent insects, like mayflies, that carry mercury straight out of the river sediment.  Alternately, mercury from sediment may have entered the floodplain decades ago during floods, and simply be moving around teh terrestrial food chain from soil to plants to herbivores to spiders and birds.  In 2008-2009 we intend to solve this riddle and also to determine what the effects of mercury at 3-4 ppm in blood (wet weight) actually does to swallow reproduction and survival.  Stay tuned.