Aim: To see how productivity of algae and Elodea changes with depth in a simulated lake.

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Example 9

Student work

How does primary productivity depend on depth in a Lake?

Aim: To see how productivity of algae and Elodea changes with depth in a simulated lake.

Prediction: As the depth (% of light decreases) increases the productivity will decrease.


Independent: depth – simulated by layers of clear plastic OHP sheets

Dependant: amount of dissolved oxygen

Controlled: reagent bottle, 250cm3; light source; amount of pond water and length of Elodea; time left to photosynthesis.


2 reagent bottles

Elodea – 12 pieces in 10cm lengths

Pond water

OHP film

Elastic bands to wrap film around bottles.

Light array

DO probe

aluminum foil


Collect 12 reagent bottles, 250cm3.

Label every two bottles as follows: Dark; 100% Light; 65% light; 25% light; 10% light and 2 % light.

The % of light was calculated using a light meter. The Lux reading under the light array was taken and then taken again under 1, 3, 5 and 8 layers of the OHP film. The difference between the first reading and the one under the OHP sheet was calculated and turned into a %.

Fill each bottle with pond water and add a strand of Elodea.

Secure the correct number of OHP films around each labeled bottle. Use the elastic bands to hold them in position. Take care not to cover the Elodea. Cover the dark bottles in aluminum foil.

Measure the DO in mg/l in each bottle and record in a table – these are the initial readings. Record each bottle three times after stirring the probe gently for 10 seconds. Place each bottle on its side under the light array. Leave for 5 days. Record the DO again, three times for each bottle. Record the results.
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Work out the averages of each set of bottles.

Using the formulas:

Respiration = initial bottle – dark bottle

Net productivity = light bottle – initial bottle

Gross productivity = light bottle – dark bottle

Calculate respiration, net and gross productivity of each light level.

Environmental systems and societies teacher support material 1


Example 9


Table 1: Initial Readings of DO mg/l in dark and light bottles containing algae and Elodea.

Amount of light in

DO mg/l

DO mg/l

DO mg/l


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