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

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

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Example 9

Results:

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

bottle

1st sample

2nd sample

3rd sample

Bottle 1

Bottle 2

Bottle 1

Bottle 2

Bottle 1

Bottle 2

100%

3.00

3.20

3.10

2.70

3.56

2.80

65%

3.10

3.79

2.90

3.43

2.97

3.60

25%

3.15

3.12

3.50

3.30

3.20

3.90

10%

3.25

3.10

3.40

3.50

3.25

2.99

2%

3.10

3.00

3.00

2.90

3.30

3.20

Dark (0%)

4.00

3.70

3.20

2.90

3.50

3.40

Table 2: Readings after 5 days 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

bottle

1st sample

2nd sample

3rd sample

Bottle 1

Bottle 2

Bottle 1

Bottle 2

Bottle 1

Bottle 2

100%

3.70

4.50

3.98

4.70

4.56

4.60

65%

3.70

4.50

3.50

3.90

3.80

4.20

25%

2.40

3.90

4.00

3.43

3.45

3.40

10%

3.10

3.20

3.38

3.40

2.40

2.79

2%

3.05

2.20

3.50

3.10

2.20

2.30

Dark (0%)

3.00

2.70

2.22

3.16

2.26

2.59

Table 3 : Averages DO mg/l readings of each light level.

Amount of light in bottle

Average DO mg/l (initial

Average DO mg/l (after 5

levels)

days)

100%

3.06

4.34

65%

3.30

3.93

25%

3.36

3.43

10%

3.25

3.05

2%

3.08

2.73

Dark (0%)

3.45

2.66

Table 4:

Calculations of respiration, net and gross productivity using DO mg/l

2 Environmental systems and societies teacher support material

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Example 9

Formulas are given in the method above.

Amount of light

Respiration,

Net productivity,

Gross productivity,

in bottle

DO mg/l

DO mg/l

DO mg/l

100%

3.06-2.66=0.40

4.34-3.06=1.37

4.34-2.66=1.68

65%

3.30-2.66=0.64

3.93-3.30=0.63

3.93-2.66=1.27

25%

3.36-2.66=0.70

3.43-3.36=0.07

3.43-2.66=0.77

10%

3.25-2.66=0.59

3.05-3.25= -0.20

3.05-2.66=0.39

2%

3.08-2.66=0.42

2.73-3.08= -0.35

2.73-2.66=0.07

Dark (0%)

3.45-2.66=0.79

2.66-3.45= -0.79

2.66-2.66=0.00

DO mg/l

________________

Graph to show the averages of DO

mg/l in different amounts of light

5

4

Average DO mg/l

3

(initial levels)

2

Average DO mg/l

1

(after 5 days)

0

00%

%

25%

10%

2%

)

65

%

0

(

ark

D

Amount of light

Graph to show respiration, net and gross productivity of algae and Elodea in different light levels

DO mg/l

________________

2

1.5

1

0.5

0

-0.5

0%

6

%

2

%

10%

-10

5

5

amount of

________________

2%

(0%)

Dark

light

________________

Respiration, DO

mg/l

Net productivity,

DO mg/l

Gross productivity,

DO mg/l

Environmental systems and societies teacher support material 3

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Example 9

Conclusion:

My prediction seems to be supported by the results that I collected. As the level of light decreases the amount of productivity also decreases. This can be seen in the results from 100% light to 25% light there is a steady decrease in the net productivity from 1.37 to 0.07 mg/l DO. After this the net productivity is negative indicating that very little photosynthesis is going on at these “depths”.

Discussion:

Photosynthesis needs light to work. Plants start to photosynthesis and produce excess oxygen, which we are measuring in this experiment when the level of light is high enough. At the same time all plants are respiring and so using up oxygen. As we are measuring oxygen amounts in the water then some of the oxygen produced by photosynthesis will be used up and some released to the water.

The respiration amounts for each sample do not vary too much: from 0.40 to 0.79 mg/l DO. So this should not have too much influence on the final results.

The gross productivity shows a steady decline as the light levels drop from 1.68 to 0.07 mg/l DO, having zero when the sample is in the dark – as theory predicts. Once the respiration factor is taken into account the net productivity shows a similar decline from 1.37 to –0.35 mg/l DO. Indicting that once a negative figure is reached for net productivity then photosynthesis is less than respiration so no excess oxygen is being released to the water.

Evaluation:

The main area of error is with the use of the probe. The DO levels constantly change so it is very difficult to know which number to record. The instructions state that to obtain an accurate reading the probe should be gently stirred for a number of seconds. This was done but was it done in exactly the same way each time – unlikely. This is an inherent error and as the data is so close it means that the whole data set must be suspected. More samples could be taken and possibly then the averages would be closer to the actual amount.

The amount of algae in each sample is uncertain and the length and health of the Elodea could vary enough to upset the results. This could be corrected by using a calorimeter to determine the amount of algae or by adding a known quantity of algae to artificial pond water. The Elodea should be equally healthy – very hard to determine objectively and exactly 10cm in length.

4 Environmental systems and societies teacher support material