Estimating Bioaccumulation

Estimating Bioaccumulation: DDT in a Lake

"A medium sized lake can serve as a major food source for a subsistence community. Suppose that a community of about 100 people gets much of its protein from fishing on such a lake. Every day, each person eats one or two fish, which weigh on average about 150 grams. The fish feed primarily on shrimp, as well as insects and other small animals. You have learned that an alga that is one of the primary food sources for shrimp in the lake is contaminated with DDT, a persistent organic pesticide that farmers upstream have begun using to control ants in food crops. You want to know how much of this DDT might eventually end up in members of the community."

The following shows how to calculate the concentration of DDT in the shrimp, given the concentration in algae and the eating habits of the shrimp.

Knowns:

• Algae contain about 0.0002 mg of DDT per gram of algae. That is about 0.2 parts per million (ppm).
• Over its (short) life, a shrimp will grow to weigh about 1 gram
• The average shrimp eats about 10 grams of contaminated algae during its life.

The amount of DDT consumed by the shrimp can then be calculated:

DDT consumed by shrimp = amount algae consumed × concentration of DDT in algae

So

• DDT consumed by shrimp = 10 galgae × 0.0002 mgDDT per galgae = 0.002 mgDDT

• Since a shrimp weighs 1 gram, the concentration of DDT in the shrimp is
• 0.002 mgDDT / gshrimp, or about 2 ppm

• This assumes that ALL of the DDT consumed by the shrimp stays in its body (a “worst case” assumption that is probably not true).

Now, use this same method to estimate the eventual concentration of DDT in fish and in humans, given the following information:

• The average fish eats about 1.7 kg of shrimp as it grows to weigh 150 g.
• The average person eats 1 – 2 fish per day over a 60 year life.
• The average adult weighs about 70 kg.
• Note that you will have to compute the total amount of fish eaten by multiplying the amount eaten per day by the number of days in the individual’s life.

Extra:

• If you have access to computers, these equations can easily be set up in a spreadsheet program.
• Consider varying the various consumption rates and body weights, consider the case of children, and / or plot the bioaccumulation in humans as they age.

Algae: 0.0002mg of DDT/ gram of algae = 0.2ppm
Shrimp: 0.002mg of DDT/ gram of shrimp = 2 ppm
Fish: eats 1.7kg of shrimp times 0.002mg DDT/g of shrimp = 23ppm