Eutrophication - a case of too much of a good thing?
Background:
In an ecosystem, all populations depend ultimately on the Primary Producers for
their source of energy. The process of transforming kinetic light energy into usable
chemical energy in the form of glucose sets up EVERYTHING else. If we take the
primary producers out of the system, the rest will collapse (think of dinosaur
extinction).
What determines the amount of primary production in an ecosystem? It depends
where you are. Think about the biomes below. What are the limiting abiotic factors
for each?
Desert Biome


limiting abiotic factor –
Characteristics of life -
Rainforest
 Limiting abiotic factor –
 Characteristics of life Aquatic ecosystems are unique in that water is not really a limiting factor. With the
surfaces of bodies of water receiving lots of sunlight, one might think that primary
production should be happening EVERYWHERE on the surfaces of oceans, lakes,
ponds and streams. But this is not the case. Why?
CO2, H20 – Provide the producers with C, H, O (carbs/lipids)
Light – Provide producers with Energy
What elements are missing?
Why are these important?
Amino acids contain –
DNA contains -
Both of these are necessary
for all living organisms!
So, to summarize, without important nutrients like nitrogen, sulfur and phosphorus,
primary production couldn't happen (even with lots of light and water.) In natural
ecosystems, these nutrients are returned to the soil by decomposition of leaves,
wood, animals that die and then decompose. Farmers figured this process out early
on and realized that by putting nitrogen rich substances like manure and fertilizer
on their fields they can increase crop yields.
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Too much of a good thing?
So, by increasing the amount of N,P,S in an ecosystem that already gets plenty of
light and water we can see an increase in primary production. One would assume
that we would then see an increase in the amount of life in all other trophic levels
with the increase in available energy. In aquatic ecosystems, things can be much
more complex.
Eutrophication
- In aquatic ecosystems  (bays, lakes, ponds, estuaries, streams etc.)
- Primary producers  algae, plankton, aquatic plants.
What happens:
1. Influx of nutrients (nitrogen and phosphorus) causes an rapid increase in
algae growth.
2. As the algae die and decompose, high levels of organic matter and the
decomposing organisms deplete the water of available oxygen, causing
the death of other organisms, such as fish.
3. Result is an anoxic body of water unable to support most complex life.
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Intro Questions:
1. Why would the decomposing algae cause a drop in the oxygen levels of
the lake? (hint: think about WHO is doing the decomposition).
2. What would be possible sources of the excess nitrogen/phosphorus?
Case Study: What’s killing the fish?
Lake Gordon is a large recreational lake in New Jersey, home to abundant amounts of
fish. Once a remote area, the south shore of the lake is now a newly developing
community. This summer, residents thought they noticed large numbers of dead fish on
the lake. Further investigations confirmed that, indeed, an unusually large number of fish
were dead. Environmental Protection Agency (EPA) investigators were called in to help
identify and solve the problem. They found the following:
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The homes along the south shore have very primitive septic tanks. Excess sewage
from these have been leaking into the water along the south shore of the lake.
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The cabin owners on the southern edge of the lake have very green lawns which
are treated regularly with chemical fertilizers.
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What’s Killing the Fish?
High School Science/Biology
Student Prompt Booklet
There is an increase in the number of aerobic organisms in the sediment of the
lake bed along the south shore.
Diagram C:
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Map of Lake Gordon.
The map illustrates the area at the south end of the lake where homes line the shore of the lake, and most dead
A noticeable algae bloom on the surface
waters of the lake around the south
fish have been collected.
shore.
Forest
Map of lake
Forest
Forest
Lake
N
x
x
x
x
Homes
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Table A: Fish deaths and growth of algae over time
Table B: Percent dissolved oxygen in lake water
observed at 3-hour intervals over a 3-day period.
Table B:
Dissolved oxygen levels in lake
over three-day period.
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Title of graph _____________________________________________________________________________
Time--
Key:
______= Day 1
Explain the trends:
A. Between what intervals in the amount of
dissolved oxygen increasing?
______= Day 2
______= Day 3
B. Between what intervals in the amount of dissolved oxygen decreasing?
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Discussion Questions.
1. In class we have discussed the relationship between photosynthesis and
cellular respiration. In the diagram below, fill in the appropriate molecules.
___________ and ____________
___________ and ____________
2. Identify the organisms that have:
a. Chloroplasts
b. Mitochondria
3. Bringing it all together:
a. Explain the biological processes responsible for the resulting data
b. Recommend a reasonable solution to the current environmental problem;
c. Predict how this solution would change your graph.
Your response is on the next page. This will count as a quiz grade for this unit. Good
luck!
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What’s killing the fish?
Name _________________________________
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