- In aquatic ecosystems, producers and primary consumers are largely microscopic plankton suspended in the water. By measuring the rates of oxygen production and of oxygen consumption in a known volume of water, estimates of the rate of gross photosynthetic productivity, net productivity, and respiration rate can be calculated.
- Using the Light and Dark Bottle Method, samples of water are placed in two glass bottles of the same volume. One bottle is left clear, and the other bottle is covered to exclude light. The initial O2 concentrations are measured, then the two bottles are returned to the light conditions from which they were taken for 24 hours. After 24 hours, the bottles are retrieved, and the final O2 concentrations are measured.
MATERIALS:
Two clear glass bottles of identical volume with lids or stoppers
- One left clear and the other covered to exclude light
Paint, tape, aluminum foil, or black plastic may be used.
- Other sets of bottles may be used to measure at different depths.
- if depths other than surface are sampled.
A float and line or pole apparatus
- to hold the bottles in place at the depth the samples were taken.
Pre-lab preparation (in the field)
- Chose a pond convenient for multiple trips to study. After deciding how many test sample pairs you will be doing, and at what depths, construct The apparatus to suspend the test bottles. The bottles need to be held in place at depth. A weight at the bottom of the line or lines will help to hold the bottles steady.
Pre-lab preparation (in the lab)
- Prepare your sample bottles. Each pair of bottles should consist of one clear, and one covered to keep light out. The bottles need to be of identical volume, and should be labeled for the intended depth. Construct all attachments for the bottles before you head to the field.
In the field
1. Take water samples just below the surface by directly holding the sample bottle in the water. Use a third bottle to collect some water as before to top-off the sample bottles, if necessary, after O2 readings are taken.
2. Measure the O2 concentration of each bottle according to the instructions for your meter. This should be the same as the samples are identical initially. If water is lost in the measurement, replace from the third bottle of sample that is identical to the sample pair. Secure the caps and suspend the sample bottles in the pond with the apparatus you constructed. Record the date and time, and the initial O2 concentration as mg / L (this is also parts per million ppm), in the DATA TABLE.
*If you are doing more than one depth, repeat this procedure for the other sample pairs. Collect the samples according to the instructions for your particular sample collection apparatus. Suspend them at the depth from which they were taken and record the appropriate data.
3. Allow respiration and photosynthesis to proceed for 24 hours.
4. Retrieve the sample bottles and measure the O2 concentrations of each bottle. Record the concentrations, along with the date and time, in the appropriate place on theDATA TABLE.
DATA TABLE: Oxygen concentration data. Samples Initial
Collection
date / timeDepth collected
(M)Initial O2
Concentration
(mg / L)Second
Collection
date / timeFinal O2
Concentration
(mg / L)
#1 Light
#1 Dark
#2 Light
#2 Dark
#3 Light
#3 DarkCALCULATIONS:
5. Calculate the respiration rate (R) in terms of oxygen consumption as follows:
t is the period of time over which respiration took place. If t is measured in days, as in the procedure given, then R will be mg O2/liter/day; if t were in hours, then R would be mg O2/liter/hr.
Respiration rate = _________ mg O2/L/day
6. Calculate the gross photosynthetic productivity of oxygen (PG) in mg O2/liter/day, as follows:
Gross photosynthetic productivity = _________ mg O2/L/day
7. The net productivity of oxygen (PN), expressed in mg O2/liter/day, is calculated as:
Net productivity = _________ mg O2/L/day
Extended Calculations
- It is sometimes desirable to express respiration and production in cubic meters of water, rather than liters. Since one cubic meter is 1000 liters, any of the results above may simply be multiplied by 1000 to obtain values in cubic meters.
- Using the balanced chemical equations for photosynthesis and cellular respiration, you can calculate values for carbon dioxide (CO2) fixed or respired.
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- Using the Light and Dark Bottle Method, samples of water are placed in two glass bottles of the same volume. One bottle is left clear, and the other bottle is covered to exclude light. The initial O2 concentrations are measured, then the two bottles are returned to the light conditions from which they were taken for 24 hours. After 24 hours, the bottles are retrieved, and the final O2 concentrations are measured.
ESTIMATION OF PRIMARY PRODUCTIVITY AND
RESPIRATION IN A POND
Light and Dark Bottle Method
INTRODUCTION: