Aquatic Primary Productivity Lab
Sample Data—pH ANSWER KEY
Analysis of Results –
1. Describe the general results shown on the pH graph across the treatments. What happened to pH during the
light phase? What happened during the dark phase?
– the control showed very wide swings in pH between day and night.
– the 1 screen treatment showed a similar result as the control but with lower values.
– the 2 screen treatment showed little changes in pH and gradually declined over time.
– the 4 screen treatment showed little changes in pH and gradually declined over time.
(both 2 screen and 4 screen showed a response when the lights came on)
– the pH increased (became more basic) during the light phase.
– the pH decreased (became more acidic) during the dark phase.
2. Summarize the results of the light screen (pH) experiment. Which treatment appeared to have the highest
productivity at the conclusion of the experiment? Which treatment had the lowest productivity? Explain.
– Summary: The more light the wider the swings in pH (more productivity).
– highest productivity: the control treatment and/or 1 screen
– lowest productivity: the 4 screen treatment
– explanation: the more light the more photosynthesis and the more productivity
3. Were any of the treatments or ecosystems in the flasks light-limited? Explain.
– Yes, the 2 and 4 screen treatments were light limited. (or could also argue that all were light limited as
the artificial lights were not as strong as sunlight)
– Why: the screens cut down the amount of light available for photosynthesis.
4. Speculate on what would happen to productivity if the light intensity had been increased by placing the
flasks closer to the lights. Explain.
– speculation: the increased light should have increased the productivity in all of the treatments. Or, the
systems were already at max and only the 2 and 4 screen treatments would have increased.
– Explain: more light increases photosynthesis; or Ps is already at max
5. Interpret the results of this experiment in terms of primary productivity, respiration, and gross productivity.
Are the ecosystems in the flasks gaining biomass (organic compounds) or losing biomass? Explain.
- Primary Productivity: decreased from the control through the 4 screen treatment.
– Respiration: was assumed to be the same in all treatments (based on the dark bottle results).
– Gross Productivity: decreased from the control through the 4 screen treatment.
– Biomass: The control and the 1 screen treatment had positive net productivity values and were
increasing in biomass.
– Biomass: The 2 and 4 screen treatments had negative net productivity values and were decreasing in
biomass.
– Explanation: Systems with positive net productivity are making more food that is consumed by Rs.
They will gain biomass. Systems with negative net productivity are using more biomass in Rs than they
are making in Ps.
6. How could you tell if the rate of photosynthesis was exactly equal to the rate of respiration over the course
of the entire experiment? (Hint – ignore the daily day/night changes and look at the long term results) What
would happen to the biomass of an ecosystem in equilibrium over time? Explain.
– if Ps=Rs than the height of the pH curves and/or their slopes should be equal. (the pH line will NOT be
flat over time; this implies neither process is working over the day/night cycles)
– The biomass will not have a net change over time.
– Explanation: the system is making as much biomass in Ps as it is consuming in Rs.
7. What would happen to the biomass of an ecosystem whose respiration rate was higher than the
photosynthesis rate? Why?
– The biomass will decrease over time.
– why: Rs consuming food (biomass) at a faster rate than Ps can make biomass.
Sample Data—Dissolved Oxygen
All figures are in parts per million (ppm) of dissolved oxygen. (Hint: 1 ppm = 1 mg O2 /L)
Start – 3.0
Dark Bottle at end of experiment – 0.5
No Screens at end of experiment – 6.5
One screens at end of experiment – 5
Two screens at end of experiment – 2.5
Four screens at end of experiment – 1
Analysis of the Results –
1.
Use the provided class data set of dissolved oxygen readings to calculate the net productivity, respiration,
and gross productivity for each treatment. Create a table to show your results. (Hint: use the Light
Bottle/Dark Bottle comparison graph from the Dissolved Oxygen section & your AP Bio formula sheet)
Answers need to be in terms of mg carbon fixed/L!!
- The Rs value is the dark bottle: 3 ppm (start) – 0.5 ppm (final) = 2.5 ppm.
See graph in lab handout:
Net Productivity = Light bottle – initial (Gross Productivity – Rs)
Gross Productivity = Light bottle – Dark bottle (Net Productivity +Rs)
Treatment Net Productivity
Respiration Gross Productivity
6.5 – 3 = 3.5 ppm
2.5 ppm
6.5 -0.5 = 6 ppm
Control
5 – 3 = 2 ppm
2.5 ppm
5 - 0.5 = 4.5 ppm
1 Screen
2.5 - 0.5 = 2 ppm
2 Screens 2.5 – 3 = -0.5 ppm 2.5 ppm
2.5 ppm
1 - 0.5 = 0.5 ppm
4 Screens 1 – 3 = -2 ppm
The above is in terms of ppm dissolved oxygen. Next convert these numbers to mg Carbon fixed/L using
the conversions on your formula sheet. See below
Remember that: 1 ppm = 1 mg O2 /L
Mg O2 /L x 0.698 = mL O2 /L
mL O2 /L x 0.536 = mg carbon fixed/L
Gross Productivity
Treatment Net Productivity
Control
1.3 mg carbon fixed/L
2.24 mg carbon fixed/L
1 Screen
0.75 mg carbon fixed/L
1.68 mg carbon fixed/L
2 Screens -0.19 mg carbon fixed/L
0.75 mg carbon fixed/L
4 Screens -0.75 mg carbon fixed/L
0.19 mg carbon fixed/L
Please note: Net productivity is always smaller than gross productivity.
2.
Discuss how light appeared to affect the productivity of the flasks or ecosystems as evidenced by the
dissolved oxygen readings. Do the results of the pH graphs correlate with the DO data?
The DO readings followed the amount of light available. The control had the most light and the highest
DO readings (Ps>Rs). The DO readings decreased with increasing light screens. The 2 and 4 screen
treatments showed a negative net productivity. The gross productivity in these treatments was below the
Rs rate. (Ps<Rs). The values followed the trends seen in the pH graphs.
3.
Fill in the following table with predictions based on what you have learned. (Use: ↓ ↑ or ↔ )
Ratio
PS > RS
PS = RS
PS < RS
O2 levels
CO2 levels
Food/Biomass
Accumulation
pH change
↑
↓
↑
↑
↔
↓
↔
↑
↔
↓
↔
↓