DUCKWEED POPULATION LAB

PURPOSE: The purpose of this lab is to provide students with the opportunity to observe logarithmic growth in a biological population.  Students will observe and record the growth of duckweed in a controlled environment.  Students will graph their observations and then discuss
the parameters and implications of logarithmic population growth.  Comparisons to the human population can be made.

STANDARDS:
 
 
 

MATERIALS: 0.5 l plastic water bottles, plant food containing nitrogen, potassium, and phosphorous, water, labels, marking pens, duckweed (available in a local pond or stream, or at a pet shop that sells fish), and a natural or artificial light source.

PROCEDURE:
1. Take a plastic water bottle, and using a knife carefully cut off the top
2.  Place 300ml of tap water into the bottle.
3.  Add an appropriate amount of plant fertilizer.  Be careful!  One of the most common
     mistakes is to add excessive amounts of fertilizer which can quickly kill the duckweed.
4.  Using tweezers, have the student place four duckweed  plants onto the water in the water
     bottle.
5.  Students should label the bottles with their name and the starting date of the experiment.
6.  Have the students place their bottles in an area with ample light or under a lamp designed for
     growing plants. You may need to build a cardboard rack to keep the bottles from falling
     over.
7.  Students can then observe their bottles everyday or every other day and count their
     duckweed. Since duckweed tend to clump together, students will need a probe or a pencil to
     separate the individual plants as they count.
8.  After four to five weeks of recording data, students can then use their data to create a graph
     of duckweed population growth.

DATA:
Date:7-19-97
Nitrate (ppm)
control
1:100
1:1000
1:10,000
1:100,000
1:1,000,000
pH
 7.5
 6.0
 7.0
7.5 
  7.5  7.5
tempoC
 30c
 30c
 30c
 30c
  29c  30c
phosphate (ppm)
 0.1ppm
0.1ppm 
 0.1ppm
 0.1ppm
  0.1ppm  0.1ppm
dissoved O2
 7.5mg/l
 7.5mg/l
 7.5mg/l
 7.5mg/l
  7.5mg/l  7.5mg/l
Number of Duckweed
 NA
 NA
NA 
 NA
 NA   NA
% Duckweed Coverage
 NA
NA 
 NA
 NA
  NA  NA
Date: 7-5-97   to  7-21-97
Nitrate (ppm)
control
1:100
1:1000
1:10,000
1:100,000
1:1,000,000
7-10-97
 4
 4
4 
 4
  4  4
7-13-97
 8
8 
 10
 12
  11  14
7-15-97
 11
 5
 11
 14
  11  12
7-17-97
13 
0 
 8
 15
  13  15
7-21-97
 12
 0
 8
 13
  13  15
 
 
 
 
 
    

 
 

CONCLUSION AND ANALYSIS: Ask students to discuss the nature of  the
duckweed growth curve.  Why does it become so steep over time?  Have the students identify the various factors that affect the growth rate of duckweed.  Do you think the growth curve will continue upward indefinitely?  Do you think other organisms such as humans could have a similar growth  curve?  Discuss limiting factors and how they affect the growth of an organism.

VARIATIONS:
1.  Have students vary the amount of plant food that they put in the water.
2.  Have students vary the amount of light the duckweed receives.
3.  Have students vary the amount of  individual nutrients such as nitrogen, potassium, and
     phosphorous the duckweed receives.
4.  Using an aquarium pump, have the students bubble in oxygen gas to see how increased amounts
     of oxygen will affect the growth of duckweed.
5. Vary the temperature of the water from bottle to bottle to see how duckweed growth is affected.
6.  Add soil to the bottom of the bottles to see how it influences the growth of duckweed.
7.  Use water from various sources (i.e. from ponds, streams, lakes, swimming pools, tap, bottled
     water, ocean, marsh etc.) and have students observe the affect on duckweed growth.
8.  Add a pollutant, such as detergent, to the bottles to see how it affects growth.
9.  Vary the ph level in the bottles to see how duckweed growth is affected.
 
PROPOSAL CONTENT STANDARDS RESOURCES
OUR DATA INPUT YOUR DATA MODELING
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