Aquatic Mercury in Food webs

A new paper came out in Science a while ago that I’ve been meaning to write about:BPR3

Cristol, D.A. et al. 2008. The movement of aquatic mercury through terrestrial food webs. Vol. 320. no. 5874, p. 335;  DOI: 10.1126/science.1154082
Along a mercury-contaminated river in Virginia, United States, terrestrial birds had significantly elevated levels of mercury in their blood, similar to their aquatic-feeding counterparts. Diet analysis revealed that spiders delivered much of the dietary mercury….These results show that any risk from mercury faced by the river’s aquatic birds exists for birds in adjacent terrestrial habitats as well. By preying on predatory invertebrates such as spiders, songbirds increased the length of their food chains, increasing the opportunity for biomagnification.

A very interesting study that shows how little things add up–literally.

In the 1970s, a serious mercury contamination problem was discovered in a river near the site of a fiber production plant. The authors of this study looked at where that mercury ended up.
What makes this study different from other aquatic food web studies is that they sampled land birds– birds that were not fish eaters, or feeding on aquatic vegetation.  Most past studies have sampled aquatic birds as “sentinels” for contamination.

Spiders made up 20 to 30 percent of the birds’ diets, yet delivered about 75 percent of the mercury. In fact, the spiders had higher levels of mercury than kingfishers, aquatic birds that are quite a bit larger than a spider.

Studies of bioaccumulation and biomagnification have produced Biomagnification graphicsome classic ecological studies over the years. (Tracing DDT from runoff into lakes up to the Bald Eagle, for example.) This graphic is a visual example of how mercury can move out of the water and into living animal tissue.  The mercury accumulates in each level of this food chain (or, trophic level), which is called bioaccumulation.

Generally, each step in the food chain (plant-> plant eater -> predator –> bigger predator) multiplies contamination by a factor of 10.  We have known for some time that mercury was a problem for fish and fish-eating birds. For humans, there are mercury warnings about fish eating in over 40 states, including Michigan.

The new information from this study suggests that this graphic needs a new arrow–one that points off to the side to land arthropods and birds.

If spiders eat mercury-laden insects, the birds eating those spiders get a bigger dose of Hg than birds eating caterpillars feeding on “clean” land plants.

And that is a Very Bad Thing for songbirds that are arthropod predators.

What insects might the spiders be eating? That wasn’t quantified in this study, but the obvious suspects are the many insects that have aquatic larvae, but terrestrial, free-flying adults.

That would include mosquitoes and midges, damselflies and dragonflies, Caddisflies, mayflies, and many other aquatic insect groups.

Apparently we need mercury warnings for warblers, as well as people.

Additional Reading:

Source of graphic

4 thoughts on “Aquatic Mercury in Food webs

  1. Hi,
    My dad use to put drugs in animal food (it was legal) and he had a clipping under the glass on his desk illustrating how small a part-per-million really was.

    The only one I can remember was a X ounce needle in a two ton haystack is 1 ppm.

    I wish I could find other illustrations of how small a ppm or ppb or ppTRILLION? really is? I can’t fathom how small an amount that would be! (Plus I can’t do the calculation!)

  2. Perfect! Both sites had great layman examples. Thank you very much, hope it wasn’t too much trouble. I drove myself nuts trying to come with search terms!

    It really is amazing when toxicity is measured in PPT!
    “one mile on a 2-month journey at the speed of light”

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