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

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Introduction

Transfer-Encoding: chunked ﻿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. Variables: 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. Materials: 12 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 Method: 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 %. ...read more.

Middle

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 ________________ 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) ...read more.

Conclusion

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 ...read more.

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