Eutrophication Experiments
Eutrophication Experiments
Eutrophication Experiments
EnvInquiry@cornell.edu
http://ei.cornell.edu
Eutrophication Experiments
Background
"Eutrophication" is derived from two Latin words meaning "good" and "food". In studies
of freshwater ecosystems, the term is used to refer to the process by which lakes, ponds,
and streams become enriched with inorganic plant nutrients, especially phosphorus and
nitrogen. This process happens naturally over a long period of time as dead organic
matter accumulates, and is one step in the normal succession of the freshwater ecosystem.
However, when the nutrient enrichment is due to the activities of humans fertilizer
runoff from agriculture or private homes, for example the rate of this natural process is
greatly intensified. Human-induced eutrophication is sometimes referred to as "cultural
eutrophication."
In freshwater systems, phosphorus and nitrogen serve as the limiting nutrients the
levels of these two nutrients determine the biological productivity of the lake, pond, or
stream. A sharp increase in one or both of these elements can result in an "algal bloom"
a sudden population explosion of algae. There are several direct and indirect costs
associated with algal blooms. As the algae population proliferates, waters become
increasingly more turbid. This turbidity is due in part to the single-celled algal organisms
present in the water and in part to dead and decaying organic material. The turbidity
coupled with the appearance of algal mats, which can sometimes reach from shore to
shore, render eutrophic bodies of water unappealing to swimmers and boaters.
An algal bloom can also set off a chain of events whose effects are more far-reaching and
more grave than these aesthetic considerations. As the algae begin to die, bacterial
populations move in to decompose the organic matter. These aerobically respiring
microbes can cause significant and sudden drops in the level of dissolved oxygen.
During the daylight hours, there is a great deal of photosynthetic (oxygen-generating)
activity in a eutrophic body of water, so this affect may not be apparent. At night,
however, when photosynthesis ceases (but plant and animal respiration continues), the
resulting net drop in dissolved oxygen can be devastating, sometimes leading to massive
fish kills.
In these experiments, pond water samples are enriched with varying levels of inorganic
nutrients and are cultured in the classroom under fluorescent lights. By looking at such
parameters as time of day, turbidity, pH, and dissolved gases, the dynamics of eutrophic
systems may be better understood.
Eutrophication Experiments
Environmental Inquiry
http://ei.cornell.edu
Suggested Steps
Engagement
Exploration
Eutrophication Experiments
Environmental Inquiry
http://ei.cornell.edu
Evaluation
Eutrophication Experiments
Environmental Inquiry
http://ei.cornell.edu
Materials
Pond water
Test kits for dissolved phosphate, nitrate, and CO2
1-liter soda bottles
Six milk crates
Three digital timers
Three fluorescent light fixtures
Three 20 Watt fluorescent bulbs
Liquid houseplant fertilizer
Dissolved oxygen meter
Spectrophotometer (turbidity)
Eutrophication Experiments
Environmental Inquiry
http://ei.cornell.edu
Assessment Items
Traditional Items
1.
A pond in the field behind your school is covered virtually from shore to shore with
a mat of algae. There are several patches of water lilies on the surface, as well, and
duckweed lines the pond's edges. At 2:00pm on a sunny summer afternoon, you
would expect the pH to be
(a) 5.5
(b) 7.0
(c) 8.5
(d) It is impossible to say from the information given (you may assume the pond
has not been affected by acid precipitation).
2.
Although you measured the dissolved oxygen content of the pond when you
measured the pH, you can't fall asleep later that night because you can't get your
mind off that pond! You decide to go back out to the pond at 11:30pm and test
your hypothesis that
(a) The dissolved oxygen concentration will be higher than it was in the
afternoon.
(b) The dissolved oxygen concentration will be lower than it was in the
afternoon.
(c) The dissolved oxygen concentration will be about the same as it was in the
afternoon.
(d) It is impossible to make a reasonable prediction.
3.
There is a second pond behind your school differing from the first one only in that it
has been stocked with perch and catfish. The dissolved oxygen concentration in the
fish pond will be
(a) Greater than the dissolved oxygen concentration in the non-fish pond during
the day but less than the non-fish pond at night.
(b) Greater than the dissolved oxygen concentration in the non-fish pond at
night but greater than the non-fish pond during the day.
(c) Always lower than the dissolved oxygen concentration in the non-fish pond.
(d) Always higher than the dissolved oxygen concentration in the non-fish
pond.
Eutrophication Experiments
Environmental Inquiry
http://ei.cornell.edu
Write a paragraph clearly describing your experiment and use schematic drawings,
flowcharts, graphs, and/or tables to enhance your design, as you see fit.
Part II. Trade your experimental design with another student. Write a short paragraph
analyzing the strengths and weaknesses of your classmate's design. Think about ways in
which your classmate's design is better than your own and also ways in which yours is
better. Whether you constructively criticize or compliment a certain aspect of the design,
be specific -- provide details. Finally, suggest a plan for strengthening any weak aspects
of the design.
Credits
This exercise was written by staff of the Institute on Science and the Environment for
Teachers at Cornell University under NSF Award #9454428.