Environmental science

Page 1“Tuna for Lunch?” by Caralyn B. Zehnder

Part I – The Problem Amanda and Tara had been friends since the eighth grade. Th ey kept in touch through college and Tara was Amanda’s maid-of-honor at her wedding. Now with careers and hectic schedules, they didn’t get to see each other as often as they would like but they still tried to get together every so often. Th is afternoon was one of those lucky occasions when they were meeting for lunch at Th e Garden, a restaurant they regularly visited.

Amanda was sitting at a table near the window when Tara walked into the restaurant. Amanda jumped up and the two friends hugged.

“How have you been? You look great!” Tara exclaimed to her friend.

“I’m well. How are you?”

“I’m fi ne. Why are you grinning like that? What secret are you hiding?” said Tara as the two friends sat down.

“I wanted to wait to tell you in person—I’m pregnant!”

“Congratulations, Amanda! You and Chad must be so excited. When are you due? Do you have morning sickness? Do you know if it’s a boy or girl? What about names?

Just then their waitress came over and asked them if they were ready to order.

“I’ll have what I always have,” said Tara. “Pan-seared tuna with lime pepper crust and a garden salad, please.”

“Th at sounds great,” said Amanda. “But I don’t think I am going to order tuna like I usually do. My doctor said that I shouldn’t eat certain types of fi sh more than once or twice per week and I think tuna is one of those fi sh. I’ll have the Fettuccini Alfredo instead.”

Th e waitress wrote down their orders and walked away.

“I knew I would have to cut back on coff ee and not drink alcohol because of the baby, but I didn’t know about this fi sh restriction,” said Amanda. “I’m not sure why I can regularly eat some fi sh but not others. I thought fi sh was healthy and good for you. You have a degree in biology; do you know why my doctor doesn’t want me to eat tuna?”

“I’m not sure,” said Tara. “It’s been a few years since my last biology class, but I’ll try to track down some information about fi sh and pregnancy and see what I can fi nd.”

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by Caralyn B. Zehnder Department of Biological and Environmental Sciences Georgia College and State University, Milledgeville, GA

Tuna for Lunch? A Case Study Examining Mercury Bioaccumulation and Biomagnification

NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCE

“Tuna for Lunch?” by Caralyn B. Zehnder Page 2

Part II – Why Would Some Fish Be Restricted? Th at evening, Tara decided to do a little online investigation for her friend. She Googled “pregnancy” and “fi sh restriction” and came up with a list of articles, blogs, and websites. She scrolled through the list and settled on the following website from the U.S. Food and Drug Administration.

Questions 1. What is in some fi sh and shellfi sh that has caused the EPA and FDA to issue the restriction? 2. Why is there a restriction for pregnant women and young children, but not the rest of the population? 3. Do pregnant women have to avoid all fi sh? Explain your answer. 4. Should Amanda have avoided the pan-seared tuna for lunch?

2004 EPA and FDA Advice for: Women Who Might Become Pregnant, Women Who are Pregnant, Nursing Mothers, Young Children

Fish and shellfi sh are an important part of a healthy diet. Fish and shellfi sh contain high- quality protein and other essential nutrients, are low in saturated fat, and contain omega-3 fatty acids. A well-balanced diet that includes a variety of fi sh and shellfi sh can contribute to heart health and children’s proper growth and development. So, women and young children in particular should include fi sh or shellfi sh in their diets due to the many nutritional benefi ts.

However, nearly all fi sh and shellfi sh contain traces of mercury. For most people, the risk from mercury by eating fi sh and shellfi sh is not a health concern. Yet, some fi sh and shellfi sh contain higher levels of mercury that may harm an unborn baby or young child’s developing nervous system. Th e risks from mercury in fi sh and shellfi sh depend on the amount of fi sh and shellfi sh eaten and the levels of mercury in the fi sh and shellfi sh. Th erefore, the Food and Drug Administration (FDA) and the Environmental Protection Agency (EPA) are advising women who may become pregnant, pregnant women, nursing mothers, and young children to avoid some types of fi sh and eat fi sh and shellfi sh that are lower in mercury.

By following these 3 recommendations for selecting and eating fi sh or shellfi sh, women and young children will receive the benefi ts of eating fi sh and shellfi sh and be confi dent that they have reduced their exposure to the harmful eff ects of mercury.

1. Do not eat Shark, Swordfi sh, King Mackerel, or Tilefi sh because they contain high levels of mercury.

2. Eat up to 12 ounces (2 average meals) a week of a variety of fi sh and shellfi sh that are lower in mercury. • Five of the most commonly eaten fi sh that are low in mercury are shrimp, canned

light tuna, salmon, pollock, and catfi sh. • Another commonly eaten fi sh, albacore (“white”) tuna has more mercury than

canned light tuna. So, when choosing your two meals of fi sh and shellfi sh, you may eat up to 6 ounces (one average meal) of albacore tuna per week.

3. Check local advisories about the safety of fi sh caught by family and friends in your local lakes, rivers, and coastal areas. If no advice is available, eat up to 6 ounces (one average meal) per week of fi sh you catch from local waters, but don’t consume any other fi sh during that week.

NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCE

“Tuna for Lunch?” by Caralyn B. Zehnder Page 3

Part III – How Does Mercury Get into Fish? Tara decided to share this information with her friend. She picked up her cell phone and texted Amanda: “Some fi sh have mercury in them which is bad for your baby. I’m emailing you a brochure.” A few minutes later she had a new message from Amanda: “How does mercury get into fi sh and why are some fi sh high in mercury and others low?”

Since these questions were a little more complicated, Tara decided to e-mail her friend Michelle, who was a scientist working for EPA. Th e next day there was a reply from Michelle in her inbox.

From: Michelle Lapensa [mlapensa44@epa.gov] To: Tara Trepanski [ttfortwo@hotmail.com] CC: Subject: re: question for you about mercury Attachments:

Hi Tara,

Mercury can be highly toxic – it can cause nerve damage in young children and developing fetuses, which is why your friend’s doctor recommended that she limit her fi sh intake. The fetus’s developing nervous system is more vulnerable to mercury than an adult nervous system. Mercury is a naturally occurring element, and volcanoes and rock weathering can introduce mercury into the environment. However, human activities have been increasing mercury concentrations globally. Coal-burning power plants are the most common source of mercury pollution. Coal is naturally contaminated with mercury and, when it is burned, the mercury simply goes up the smokestack and into the air. Then the mercury is deposited in rain (wet deposition) or it falls from the sky (dry deposition) onto the ground and, more importantly, in water. I’ve attached a map showing mercury wet deposition across the U.S. Aquatic bacteria convert inorganic mercury (Hg) to the organic form methylmercury (MeHg), which is highly toxic. So it’s the methylmercury in fi sh that is of concern. The U.S. Geological Survey recently completed a mercury survey of freshwater environments and I’ve attached a summary of their report.

National Atmospheric Deposition Program/Mercury Deposition Network

Total Mercury Wet Deposition, 2008

2 (μμg/m ) Hg

4 4 – 6 6 – 8 8 – 10 10 – 12 12 – 14 14 – 16 16 – 18 > 18

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Sites not pictured: AK98 5.2 μg/m2

Figure 1: Mercury deposition map. Colors indicate mercury deposition levels (green = lowest and orange = highest). Points indicate monitoring stations. White areas were not monitored.

NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCE

“Tuna for Lunch?” by Caralyn B. Zehnder Page 4

Below are the data from a 2009 USGS report, Mercury in fi sh, bed sediment, and water from streams across the United States. Th e objectives of this report were to describe the nationwide occurrence and distribution of mercury in fi sh tissue and evaluate mercury in streambed (bed) sediment and stream water.

Methods • Fish: 291 fi sh from streams nationwide. Largemouth bass were targeted for

collection, but 34 diff erent fi sh species were collected. Fish were caught by electrofi shing, rod & reel, and gill nets. Fish fi llets were analyzed for mercury.

• Sediment: A plastic scoop was used to remove the upper 2 to 4 cm of bed sediment from 5 to 10 depositional areas; samples were composited into a single sample for each site. Each sample was homogenized and mercury levels were measured.

• Stream-water samples were collected by dipping Tefl on® or PETG (Nalgene) bottles in the center of streamfl ow by use of trace-metal clean techniques. Samples were analyzed for mercury.

National Water-Quality Assessment Program Toxic Substances Hydrology Program

Mercury in Fish, Bed Sediment, and Water from Streams Across the United States, 1998–2005

U.S. Department of the Interior U.S. Geological Survey

Scientific Investigations Report 2009–5109

Figure 2: Mercury concentrations (ug/g) found in fi sh tissues of commonly sampled fi sh species. Bars are means + one standard error. Fish were sampled from 291 locations around the U.S.

Table 1: Conversion factors. Mass Units is equal to Factor Kilogram (kg) 1000 grams 103 Milligram (mg) 0.001 grams 10-3 Microgram (ug) 0.000001 grams 10-6 Nanogram (ng) 0.000000001 grams 10-9

NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCE

“Tuna for Lunch?” by Caralyn B. Zehnder Page 5

Questions 1. What human actions lead to increased mercury levels in the environment? 2. How does the mercury end up in fi sh? Draw a fl ow chart following the mercury path. 3. Where in the United States are mercury wet deposition levels highest? What do you think explains this pattern? 4. Th e EPA criterion for human health is 0.3 ug/g. Which fi sh species have average mercury concentrations that

exceed the EPA limits? 5. Th e concern level for piscivorous (fi sh-eating) mammals is 0.1 Hg ug/g. Which fi sh species have average

mercury concentrations that exceed this limit? Why is the mercury level for piscivorous mammals lower than the level for human health?

6. Should you be concerned about mercury toxicity if you catch and eat a largemouth bass in a local lake? Why or why not?

7. In which samples were mercury concentrations the highest (fi sh, streams, or sediment)? Why do you think this is?

Table 2: Mean, median, standard deviation (Std Dev), minimum values and maximum values from fi sh tissue, sediment samples and water samples analyzed for methylmercury. n = the number of total samples collected. Mean Median Std Dev Minimum Maximum n Units Fish 0.261 0.169 0.278 0.014 1.95 291 ug/g Sediment 1.65 0.51 2.54 0.01 15.6 344 ng/g Water 0.19 0.11 0.35 <0.010 4.11 337 ng/L

NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCE

“Tuna for Lunch?” by Caralyn B. Zehnder Page 6

Part IV – Mercury in Aquatic Food Chains Tara spent the next evening looking over the information that Michelle had sent her. She was getting ready to send Amanda a follow-up e-mail when she received another message from Michelle.

From: Michelle Lapensa [mlapensa44@epa.gov] To: Tara Trepanski [ttfortwo@hotmail.com] CC: Subject: re: question for you about mercury Attachments:

Hi Tara,

I’ve been thinking about the questions you asked last night and I realized that I didn’t help you much with the second part of your questions. I’ve included a table from a recent paper about the relationship between trophic position (where an animal is along the food chain) and mercury in the food web of Lake Washington. For this paper, the scientists collected fi sh, crustaceans, and zooplankton and measured methylmercury levels in these organisms.

Hope this helps!

Cheers,

Michelle

*Arthropods are organisms with segmented bodies, hard exoskeletons, and multiple pairs of jointed legs. Aquatic examples include shrimp, crayfi sh, crabs, and insect larvae including caddisfl ies.

Questions 1. Draw a food web for Lake Washington using the species and food preferences given in Table 3. Start with phytoplankton (algae) as the base of your web and then build up the food chain.

2. Label the species in your food chain as either high (>100 ug/kg), medium (20-100 ug/kg), or low (below20 ug/kg) mercury concentrations. Which types of animals have the highest levels of mercury? Which types of animals have the lowest? Why do you think this is?

Table 3: Animal species, feeding preference, and methylmercury concentrations of organisms collected from Lake Washington.

Species Food Methylmercury (ug/kg)

Fish

Northern pikeminnow Other fi sh 413 ± 45

Cutthroat trout Other fi sh 194 ± 32

Smallmouth bass Other fi sh 261 ± 13

Juvenile sockeye Zooplankton and arthropods 46 ± 2

Stickleback Zooplankton and arthropods 39 ± 1.3

Arthropods*

Mysids (small shrimp) Phytoplankton 15 ± 2

Signal crayfi sh Phytoplankton 23 ± 4

Caddisfl y larvae Phytoplankton 6 ± 0.6

Zooplankton Bulk zooplankton Phytoplankton 4 ± 0.4

NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCE

“Tuna for Lunch?” by Caralyn B. Zehnder Page 7

Image credits: Photo of tuna in title block ©Tommy Schultz | Fotolia.com. Figure 1 courtesy of National Atmospheric Deposition Program, (nrsp-3). 2007. nadp Program Offi ce, Illinois State Water Survey, 2204 Griffi th Dr., Champaign, IL 61820. http://nadp. sws.uiuc.edu/maps/Default.aspx. Figure 2 by author based on data from B.C. Scudder et al., 2009, Mercury in fi sh, bed sediment, and water from streams across the United States, 1998–2005, U.S. Geological Survey Scientifi c Investigations Report 2009-5109.

Case copyright held by the National Center for Case Study Teaching in Science, University at Buff alo, State University of New York. Originally published December 14, 2010. Please see our usage guidelines, which outline our policy concerning permissible reproduction of this work.

Final Activity 1. Imagine you are Tara. Write an email to your friend Amanda explaining what you have learned about mercury.

Be sure to convey the aspects of your learning that will be most useful to Amanda. 2. Find two other examples of compounds that biomagnify. Explain how each compound and/or toxin enters the

biosphere and what impacts it has on living organisms in general and humans in particular. 3. Distinguish between bioaccumulation and biomagnifi cation and design a mnemonic device for the distinction.

References / Resources U.S. EPA: What You Need to Know about Mercury in Fish and Shellfi sh.

http://www.epa.gov/fi shadvisories/advice/. Last accessed: 10/11/10. U.S. FDA: What You Need to Know about Mercury in Fish and Shellfi sh.

http://www.fda.gov/Food/FoodSafety/Product-Specifi cInformation/Seafood/FoodbornePathogensContaminants/ Methylmercury/ucm115662.htm. Last accessed: 10/11/10.

NOW Science and Health – Th e Mercury Story. http://www.pbs.org/now/science/mercuryinfi sh.html. Last accessed: 10/11/10.

USGS—Mercury. http://www.usgs.gov/mercury/. Last accessed: 10/11/10

U.S. EPA—Mercury. http://www.epa.gov/hg/. Last accessed: 10/11/10.

Mercury Deposition Network. http://nadp.sws.uiuc.edu/mdn/. Last accessed: 10/11/10.

U.S. CDC—Agency for Toxic Substances and Disease Registry: Mercury Fact Sheet. http://www.atsdr.cdc.gov/toxfaqs/tf.asp?id=113&tid=24. Last accessed: 10/11/10.

Scudder, B.C., Chasar, L.C., Wentz, D.A., Bauch, N.J., Brigham, M.E., Moran, P.W., and Krabbenhoft, D.P. 2009. Mercury in fi sh, bed sediment, and water from streams across the United States, 1998–2005. U.S. Geological Survey Scientifi c Investigations Report 2009–5109, 74 p.

McIntyre, J.K., and Beauchamp, D.A. 2007. Age and trophic position dominate bioaccumulation of mercury and organochlorines in the food web of Lake Washington. Science of the Total Environment 372: 571-584.

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