Mercury in our study site, and how does it move?

OVERVIEW

This is the essential activity within Unit 2 and has students visit (or virtually visit) their field site in order to better understand its ecosystem and the food web that exists there.  Diagramming these characteristics helps lead to a richer understanding of how mercury is transported in the environment.

BACKGROUND

A review of mercury cycling in the environment

  • Elemental mercury (Hg(0)) is released from natural and manmade sources- it is a gas, fairly insoluble, and has an atmospheric half-life of six months to a year.
  • Oxidized mercury (Hg(II)) can either be emitted from point sources or can be formed from the elemental mercury mentioned above via oxidation in the atmosphere. This form of mercury does not last very long in the atmosphere but rather is deposited in terrestrial and aquatic ecosystems.
  • Oxidized mercury is very reactive. Because of this reactivity when Hg (II) enters the ecosystem it can be methylated to form methylmercury (CH3Hg+). Although the entire process of methylation is not completely known, we do know that for methylation to occur there must be an anoxic environment where sulfur-reducing bacteria are abundant. 
  • Methylmercury may adsorb onto phytoplankton, be absorbed by phytoplankton, or be consumed by bacteria eating microorganisms- mercury has now entered the food web. From a health perspective, human or other organism, methylmercury is the mercury form that is of highest concern.  This is the form of mercury that organisms are most likely to be exposed to and it is the most toxic form. See the diagrams and other materials in Unit 1 for a review of the mercury cycle.

GETTING READY

Materials

  • Student notebooks for note taking at field site
  • Handouts or Internet access
  • Medium for student diagram - paper, whiteboard, poster board, or computer

Time Needed

ACTIVITY PROCEDURE

Diagramming the mercury cycle for the study site(s)

Ideally you are able to visit the field site, student notebooks in hand, and spend at least part of a class period looking around, sketching, or taking notes about the field site in general. Is it forested? Are there a lot of houses and industrial buildings around, or is it rural? Are there farms nearby? How big is the stream or pond? Are there wetlands? Is it hills or flat? What types of trees and plants are around? Get students to have a mental picture of the site so they will have a frame of reference for the following units. Take photos or videos to help remember the details later on. (We will also be able to post these on a data sharing site soon).

If you are unable to visit the site as a class, share some photos of the site, consult hard copy maps, or use Google Earth to get a view of some of the site’s characteristics.

Getting Ready:

Get together the documents referenced below in hardcopy, or gain access to computers for research. These reports and information sources focus on mercury in the environment. They are all resources that we reference in Unit 1, so they may be things that you have already distributed to the students.

    Hubbard Brook Research Foundation's Mercury Matters

    Biodiversity Research Institute: Mercury Connections

    US EPA's mercury site

Doing the Activity

Mercury Cycle Diagram

 

Given 2-3 key literature references from the above resources, plus the dragonfly fact sheets, and some key words, have the students construct a mercury cycle diagram that is specific to your study area and organisms. The diagram should reflect the kind of system that the students will be working with—field, stream, pond, coast, and so on. What would matter at students' research sites? The diagram can be done on paper, collaboratively on a whiteboard, on a computer, or in poster formats. The important thing is that they begin to think about what is living there, about the food chain, and about the way that mercury might enter the site and then accumulate.

Consider having students develop a number of different versions of this diagram as they move through the entire program of inquiry and research. You would expect the diagrams to get more detailed over time. The goal, at this first stage, is to prepare the students for the next step in which they will be formulating research questions.

Here are some suggestions and considerations:

  • Make an initial visit to the field site to look at what types of media (leaves, soil, water, fish, aquatic plants, algae, etc.) are components of the site.
  • Have the students use their judgment to decide which sources and steps are appropriate for their own study system. For example, volcanoes are not a large source of mercury in Maine; neither is mining. But rain and snow are important as are forest processes and wetlands.
  • Encourage students to make a cartoon of what they see. Stick figures and rudimentary shapes are fine—this is not a work of art.
  • If students are stuck, ask them to take a ‘dragonfly larva’s-eye view’ – visualize themselves as a dragonfly larva in the stream or lake. What do they see and experience? Have them think of mercury moving in and out of contact with the larva along with food, water, and other necessities.
  • Since we are focusing on research that the students will do, they should not include information that is not relevant to the questions they will study. For example, they should not focus on terrestrial birds if they working with an aquatic system, except perhaps to note that they could be affected by the part of the system under investigation.
  • The students should reference and cite the material from other sources that they use in their mercury diagram.
  • We would like you to post the mercury cycle diagram(s) (scan or digital photo) on the Photo Gallery so that students can understand each other's work.

REFLECTION/ASSESSMENT IDEAS

Formative:

Below are a number of questions that you might use, either as homework or to stimulate some additional discussion.

Question Set for Activity B

  1. Why are there more fish mercury consumption advisories on the east coast of the United States than the west? (Answer: Power plant emissions in the Midwest are an important source of mercury. Other answers would include higher levels of acidity in Northeastern lakes, higher levels of sediment deposited in lakes by eastern rivers.)
  2. There are two ponds in your watershed - one has 0.5 acres of wetland next to it, the other has 40 acres of wetland next to it. Do you think the difference in amount of wetland will affect mercury concentrations in the fish of these two ponds? If so, how? (Answer: more wetland = more opportunity for methylation by anaerobic bacteria.)
  3. In an adjacent watershed there are two other ponds - one with highly oxygenated (oxic) sediments and one with areas of poorly oxygenated (anoxic) sediments. Do you think the difference in sediment types will affect mercury concentrations in the fish of these two ponds? If so, how? (Answer: poor oxygenation = more methylation by anaerobic bacteria.)
  4. Mercury bioavailability is influenced by a number of factors. What factors do you think are most important at your site? (This is a very important question for the students to spend some time with. You may want to consider engaging the students in a structured debate on the question.)
Summative:
  • Pass out a blank piece of paper and ask the students to draw the mercury cycle.
EXTENSION IDEAS

Teachers focused primarily on preparing students to investigate mercury levels in insects or fish might find that this first activity provides all of the introduction to toxicology that students need. Teachers in health occupations, however, might usefully follow this lesson with the next two lessons in the Science NetLinks Toxicology series: Toxicology 2: Finding the Toxic Dose and Toxicology 3: Toxicology and Human Health.

Biology teachers might want to connect this toxicology study to lessons on the structure of the cell.  There are also potential connections to the study of anatomy.

The Society of Toxicology suggests the following topics for more extensive student work on its Topics in Toxicology page:

  • Gulf War Syndrome
  • Thalidomide
  • DDT
  • Aflatoxin
  • Lead
  • Domoic Acid

Each topic is associated with a number of questions that require students to perform independent research. The questions range from factual to interpretive to evaluative. The goal is for students to gain enough information on a particular topic to be able to form an opinion about it. The site also provides Teacher’s Notes with a brief description of each of the topics and suggestions for use in a classroom toxicology discussion.

TEACHER'S CHEAT SHEET

These documents can be downloaded by clicking the links below. Individual PDFs are also on the Lesson Resources page.

HANDOUTS

These documents can be downloaded by clicking the links below. Individual PDFs are also on the Lesson Resources page.