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Cultural Eutrophication
When humans release large amounts of nutrients into the environment, it is called cultural eutrophication.
Different
sources of nutrients such as nitrogen and phosphates are found in fertilizers that are in the runoff from farms, human
wastes and animal wastes.
Phosphorus and nitrogen, just like hydrogen or oxygen, are elements found in nature. Both of these nutrients are used up
quickly in nature because plants and animals need them to grow.
Algae don’t need much nitrogen or phosphorus to live, so when lots of nutrients are in the environment, the algae can
grow at a very fast rate. The large amount of algal growth is called an algal bloom. The lakes become green and scummy
looking. This is a form or water pollution.
In addition, when the excess algae die and decomposes, the dissolved oxygen is used up by bacteria in the water.
Sometimes the dissolved oxygen levels become so low that aquatic animals cannot survive and we say the lake is dead.
How can nutrients be sources of pollution?
Purpose
In this investigation you will analyze the effect of increased concentrations of fertilizer on the biological oxygen demand of
decomposers in an ecosystem.
Materials
 Yeast (microorganisms)
 Test tube rack
 Whole milk (fertilizer/nutrients)
 Stop watch
 Methylene Blue
 1 dropper
 1 graduated cylinder
 Stir rod
 6 test tubes and stoppers
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PRE LAB 2
Answer the following questions before beginning the investigation:
BOD Level
Water Quality
(in ppm)
Very Good. There will not be much organic waste
1–2
present in the water supply.
3–5
Fair. Moderately Clean.
Poor and Somewhat Polluted.
6–9
Usually indicates organic matter is present and bacteria
are decomposing this waste.
100 or greater
Very Poor and Very Polluted.
Contains organic waste.
1. Which BOD level indicates a low level of decomposer activity?
2. Which BOD level would indicate that an excessive amount of nutrients are entering a water
ecosystem?
3. Where do the organic wastes indicated in the chart above come from?
INVESTIGATION
In this investigation milk will represent the nutrients or source of nitrogen and phosphorus. The yeast will be our source
of microorganisms.
Procedure
1) Label the test tubes #1 through #6. Place them in the test tube rack.
2) Using a graduated cylinder, add 5 ml of tap water to tubes #2 through #6.
3) Add 10 ml of milk to tube #1.
Steps 4 through 7 are an example of a simple serial dilution. Refer to Diagram 1 as you follow the procedure.
CULTURAL EUTROPHICATION: Lab Activity | 2
Diagram 1
4) Using a graduated cylinder, remove 5 ml of milk from tube #1 and place it in tube #2. Mix the contents well.
5) Remove 5 ml from tube #2 and place it in tube #3. Mix the contents well.
6) Remove 5 ml from tube #3 and place it in tube #4. Mix the contents well.
7) Remove 5 ml from tube #4 and place it in tube #5. Mix the contents well, and then discard 5 ml of the solution.
8) Add 3 drops of methylene blue to each tube. Be careful to hold the dropper/bottle upright so that the drops are uniform.
Mix each tube well.
9) The contents of each tube should now be as below:
TEST
TUBE #
mL Milk
and/or
H2 0
Methylene
Blue
% Milk
Dilution
1
5
3 drops
100%
1:1
2
5
3 drops
50%
1:2
3
5
3 drops
25%
1:4
4
5
3 drops
12.5%
1:8
5
5
3 drops
6.25%
1:16
6
5
3 drops
0%
--
MAKE A HYPOTHESIS – WHAT DO YOU THINK IS GOING TO HAPPEN?
Hypothesis
I think that when the concentration of fertilizer goes up the oxygen content of the water will
__________________________________________________________________.
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Prediction
Methylene blue is an indicator for oxygen. When oxygen is present the solution is blue. The blue color is lost when
oxygen is removed.
If the oxygen content decreases then the mixture will turn from ___________________ to ___________________.
10) Noting the time, quickly add 1 ml of yeast mixture to each test tube. Invert each test tube 4 times and place in the test
tube rack.
11) Carefully observe each tube and record the time at which the color change from blue to white is complete (no more
color change).
12) When the color has changed from blue to white in each test tube, record the exact time at which the change is
complete. The surface of each test tube will remain blue.
TEST TUBE
#
MIXING START TIME (A)
TIME OF COLOR CHANGE TO
TOTAL TIME FOR COLOR
WHITE (B)
CHANGE (B-A)
1
2
3
4
5
6
13) BEFORE YOU CLEAN UP, SHAKE ONE OF THE TEST TUBES THAT HAS TURNED WHITE. Record your
observations.
ANALYSIS
1) Which test tube was the first to use up the oxygen? Which was the last to use it up?
2) Which test tube represented the test tube with the greatest BOD? Why?
3) What was the purpose of test tube #1?
4) How can it be harmful to provide an ecosystem with too much food?
5) How would a large number of dead fish killed by pollution affect the environment?
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6) Methylene blue is used by the milk industry to test the quality of milk. What would decolorization indicate?
7) What is the gas taken in by the microorganisms?
8) What is the gas given off by the microorganisms?
9) Where do microorganisms living in water get the oxygen that they use in decomposition?
10) Where do the green plants living in water get the carbon dioxide that they need?
11) What happened in the tube you shook after it had turned white (step 13)? Why?
12) Why is the oxygen in this experiment used up?
13) Which part of your experiment represents the decomposers?
14) Which part of your experiment represents the sewage dumped into the water?
15) In which test tube did you have the most "sewage"?
16) Graph your results (using Excel if you’d like). What are the labels for your axes?
17) What does this graph tell you about the relationship between the biodegradable waste in water and the amount of
dissolved oxygen in the water?
18) What would be the effects of dumping a great deal of raw sewage into a river as regard to the dissolved oxygen in the
same river?
19-20) Write a conclusion as to what you learned by performing this lab. Be sure to include the following:

Was the hypothesis supported or refuted? Why or why not?

What were the sources of error in this experiment?
Read the article below on organic farming and eutrophication and answer the follow-up questions.
Organic Farming and Eutrophication
One source of excess nutrients that get into our waterways comes from farming. Many of the practices of organic
farming have been shown to reduce the effects of eutrophication.
For a food crop to be certified as organic, it must be grown without synthetic fertilizers or harmful pesticides.
Organic farmers must plant in such a way that the land will be left in better condition after farming than it was before
farming and that faming will not cause harm to the surrounding environment.
Non-organic farmers use synthetic fertilizers that contain nitrogen and phosphorus which are added to the soil.
The excess nutrients that are not consumed by the crop, buildup in the soil. When it rains heavily, the water rich in these
nutrients runs off into streams and rivers and accumulates in lakes. Organic farmers do not use synthetic fertilizers.
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Instead they may add nutrients to their crops by using compost.
Compost is much higher in nutrients than synthetic fertilizers, so less of it needs to be used. The nutrients are
released slowly from the organic matter and, therefore, they don’t accumulate in the soil as fast as synthetic fertilizers.
Many organic farmers practice low tillage farming. Only areas that are planted are tilled rather than the entire
field. This reduces erosion and runoff into waterways. Composting also reduces the effects of erosion which, as stated
above, contributes to excess nutrients in our watershed.
Another means practiced by many organic farmers is crop rotation. Crops are planted in an area. When the soil becomes
low in nutrients the farmer plants legumes the next season. Legumes are any plant with a bean type seed pod that
enhances the levels of nutrients in the soil without adding fertilizers by a process called nitrogen fixation.
Follow-up
After reading Organic Farming and Eutrophication. Answer the following questions.
1) List three ways that organic farming differs from traditional farming?
2) What are some of the nutrients that are found in synthetic fertilizers?
3) Why do synthetic fertilizers accumulate in the soil?
4) How do these fertilizers make it to lakes?
5) After completing the eutrophication lab, describe what happens when excess nutrients pollute lakes
and rivers?
6) Many organic farmers use what instead of synthetic fertilizers?
7) List two ways that compost prevents nutrients from collecting in the soil:
8) Describe two methods that are used to reduce erosion:
9) What type of plants actually add nutrients such as nitrogen to the soil?
10) What is crop rotation?
11) How does crop rotation reduce the need to add fertilizers?
Tahoe is experiencing a phenomenon known as cultural eutrophication—excessive algal growth due to
excessive nutrient levels. Nitrogen and Phosphorus from automobile emissions and urban and forested
areas act like fertilizer to accelerate algal growth.
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