The Mercurial Path of Mercury to Aquatic Ecosystems

Fig. 1. Contoocook Lake in Jaffrey New Hampshire. This is an example of an ecosystem that may suffer from the impact of mercury. Photo credit: John Phelan, Wikipedia commons.

The trophic transfer of heavy metals fascinates me and has a direct bearing on my life, and the lives of other fish consumers. During my time working as a researcher on mercury accumulation with Dr. Celia Chen at Dartmouth College, I learned just how important it is to be a careful consumer of fish. I eat fish weekly, but select smaller fish species. Specifically, I choose to eat trout, tilapia, and sardines, but avoid the larger species such as tuna and swordfish. Let me explain why.

Fig. 2. Warning about fish contaminated with mercury. Have you ever seen one of these signs? Source: redjar, Flickr.

You might have seen the warnings about mercury in fish. There are a myriad of food advisories posted about which fish to eat and which to avoid to minimize your consumption of mercury. Mercury accumulation in foods is of concern because at high enough concentrations it is a neurotoxin (a poison that acts on our nervous system) and if ingested in high amounts has wide ranging effects on human and wildlife health and wellness [1, 2]. In particular, many advisories are specifically focused to protect pregnant women (fetuses are particularly vulnerable) and heavy consumers of sushi (large fish such as tuna tend to have higher levels of mercury) [1], but concerns around mercury apply beyond just these two groups. There are currently more than 4,821 fish consumption advisories in the U.S. [3] and the Northeast has some of the highest levels of mercury deposition in the United States [4]. This is a widespread issue. The consumption of fish, particularly of predators at the top of the aquatic food chain, exposes us to mercury.

So how and why is there mercury in the fish we’re consuming? Why don’t we hear about mercury in any other foods we eat? To answer this questions let’s start at the beginning.

What is mercury and where does it come from?

Mercury is a naturally occurring heavy metal found throughout the Earth’s crust and it is released at background levels by erosion, volcanic eruptions, and gas emissions from oceans [5]. However, atmospheric concentrations of mercury (Hg) have been increasing with industrialization due to the combustion of coal and the use of mercury in industrial processes, particularly gold mining. The naturally occurring mercury trapped in coal is released when combusted. Regulations were put into place in 2011 by the U.S. Environmental Protection Agency (EPA) to limit the anthropogenic (human-caused) release of mercury. These regulations have improved the situation; however, utility coal burners, waste combustors, medical incinerators, and industrial boilers still release a total of 100 tons of mercury a year in the U.S.** **[6]. The mercury, which was safely trapped in the earth’s crust, is now released to the atmosphere.

So how does mercury end up in my food?

Released mercury exists in the atmosphere as tiny particles. These eventually fall back down to the earth. Some land directly in the water, but other land on the surfaces of trees, rocks, grass, etc. When the mercury particles land on earth, they get washed off with the next rainfall. This starts their path through small streams, to larger streams, to lakes or out to the ocean. Now the mercury that was released from coal on land is in aquatic ecosystems, the home of fish. Under specific conditions, inorganic mercury (the form that was released into the atmosphere) gets converted to the organic form, methylmercury (MeHg) by microbes (bacteria and fungus too small to be seen without a microscope) [7]. This form of mercury is of particular concern as it is the more toxic form and more readily absorbed by biota (plants and animals) by nutrient uptake processes. Further, it bioaccumulates (meaning that it builds up in organisms over time), and it biomagnifies (meaning that it concentrates at higher and higher levels as it moves up the food chain). More specifically, the mercury binds to small organic particles in the water, which then get eaten by tiny plankton, which then get eaten by small insects, which get eaten by small fish, which get eaten by larger fish. Check out this website for a great diagram of the process! Eventually, the trophic transfer of mercury reaches the fish that we like to eat. This fish in your dish has the accumulated mercury from all of the prey it ate during its lifetime. This is why there is particular concern about large, predatory fish such as swordfish, king mackerel, pike, and tuna. As successful predators, they have particularly high levels of mercury.

Fig. 3. The release of mercury to the atmosphere and subsequent accumulation in fish, and recommendations for the frequency in which you should eat certain fish. Source: Bretwood Higman, Wikipedia Commons [8].

So, should I not eat fish then?

While this all sounds scary, you can still eat fish and in fact, you should; the omega-3 fatty acids, high quality protein, iron, selenium, and iodine in fish are good for you! Ultimately the advice is to eat fish, but to do so in moderation, to not consume fish from contaminated sites, and to avoid eating predatory, piscivorous (fish-eating) fish except in moderation. Generally, fish consumption advisories recommend small fish and fish at lower trophic levels. Check out the EPA’s guidelines and try eating more tilapia, trout, catfish, cod, and sardines. These smaller fishes pose less of a health risk in your dishes. I have accrued some great sardine recipes if you want them! One of my favorites is included below.

Penne with Fennel and Sardines

8 ounces dried penne pasta

4 cloves garlic, diced

1 head fennel, thinly sliced

2 cans sardines, keep oil

1 lemon, squeezed

Cook the pasta al dente. Drain, but retain a ladle of pasta water. Panfry the garlic until golden brown and set aside. Cook the fennel in the skillet until tender and slightly golden. Add sardines with their oil. Break up the sardines Add the lemon. Add the ladle full of pasta water to the skillet. Return the garlic. Cook it all for another couple of minutes. Serve with grated Parmesan cheese.

References:

[1] Learn about Mercury and Its Effects. Natural Resources Defense Council. http://www.nrdc.org/health/effects/mercury/effects.asp

[2] Mercury Connection: The Extent and Effect of Mercury Pollution in Northeastern North America. Biodiversity Research Institute. http://www.briloon.org/uploads/BRI_Documents/Mercury_Center/Mercury_Connections/BRIMercury-NE.pdf

[3] National Listing of Fish Advisories General Fact Sheet 2011. U.S. EPA. https://www.epa.gov/fish-tech/national-listing-fish-advisories-general-fact-sheet-2011

[4] NADP’s Atmospheric Mercury Network: Moving toward Total Mercury Deposition. http://nadp.sws.uiuc.edu/lib/brochures/amnBrochure.pdf

[5] Nriagu and Becker. 2003. Volcanic emissions of mercury to the atmosphere: global and regional inventories. The Science of the Total Environment. 304:3-12.

[6] Hidden Risk: Mercury in Terrestrial Ecosystems of the Northeast. Biodiversity Research Institute. http://www.briloon.org/uploads/BRI_Documents/Mercury_Center/Hidden%20Risk/HiddenRisk_lr.pdf

[7] Mercury Matters: a synthesis of mercury contamination in the Northeast. http://www.hubbardbrook.org/research/animals/mercury07.shtml

[8] Bretwood Higman, Ground Truth Trekking. Original:http://www.groundtruthtrekking.org/Graphics/MercuryFoodChain.html Wikipedia Commons: https://commons.wikimedia.org/w/index.php?curid=15761635

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