Depth Profile and Compensation Point Exercise
Background:
Primary production is the chemical energy generated by autotrophs, derived from fixation of CO2 in photosynthesis and
chemosynthesis.
Gross primary production (GPP) is the total amount of carbon fixed by autotrophs in an ecosystem. It is the overall rate
of biomass production by producers. GPP depends on the influence of climate on photosynthetic rate.
Net Primary Productivity (NPP) represents the biomass gained by the plant. It is the remaining fraction of biomass
produced after accounting for energy lost due to cellular respiration and maintenance of plant tissue. NPP is the energy
left over for plant growth and consumption by detritivores and herbivores. NPP represents storage of carbon in
ecosystems. Net Primary Productivity can be calculated using the following equation:
NPP = GPP (Gross Primary Productivity) – RL (Respiration Loss)
The productivity of an aquatic ecosystem is the rate at which sunlight is stored by plants in the form of organic
molecules using the following equation: 6CO2 + 6H2O + energy sun C6 H12 O6 + 6O2
Because of respiration, some of the organic molecules made by photosynthesis are used to supply energy according to
this equation: C6 H12 O6 + 6O2 6CO2 + 6H2O + ATP
A depth profile shows how the productivity of a lake varies with
depth. Typically, productivity is plotted as a gross primary
productivity expressed as milligrams of organic carbon (C6 H12
O6) produced per cubic meter of water per day (mg C/m3/day).
Respiration Loss is also plotted as mg C/m3/day. The following is
a typical depth profile:
Pre-Exercise:
1. Using the lake diagram, describe the different life zones found in freshwater lakes in terms of depth and
distance from shore, types of organisms, productivity, and nutrients.
2. How do the limiting factors (Water Depth, Clarity/turbidity, Chemical Composition, Sedimentation, Excess Nutrients,
Temperature, Water Movement, & Excess Plant Growth) vary between the surface and the bottom of a lake?
3. What does Gross Primary Productivity (GPP) and Net Primary Productivity (NPP) measure?
Exercise:
In this exercise you will compare Tiger Paw Lake to Bulldog Pond. These two bodies of water are comparable in terms of
overall size, depth, state of eutrophication, and species. The only major difference is turbidity, the amount of suspended
particles in the water column.
The following two data charts show how the gross primary productivity, expressed as mL O2/ liter / hour, varies with
depth. In the space provided calculate gross primary productivity as milligrams (mg) carbon (C) / cubic meter / day using
the following conversion units:
1 mL O2 = 0.536 mg C
and
1 m3 = 1000 liters
Please SHOW YOUR WORK on a separate sheet of paper.
DEPTH
(meters)
Tiger Paw
A
0.0
B
0.5
C
1.5
D
2.5
E
4.0
Bulldog
F
0.0
G
1.5
H
4.0
I
7.0
j
11.0
GPP
mL O2 / liters / hour
0.15
0.15
0.13
0.10
0.06
0.16
0.15
0.14
0.10
0.07
GPP
mg C / m3 / day
After you have made the conversions, construct a depth profile for each lake, using the sample on the first page as a
guide. The respiration loss for both lakes is 0.09 ml O2 / liter / hour = ________ mg C/m3/day.
**SHOW YOUR WORK!
This should also be plotted on your graphs. Draw a best fit line.
Tiger Paw Lake
Bulldog Pond
Analysis:
1. Calculate the Net Primary Productivity in each lake at the depth of 1.5 meters:
SHOW YOUR WORK!
NPP Tiger Paw
NPP Bulldog
2. Find the compensation point for each lake. This is the depth where Gross Primary Productivity equals
Respiration Loss. At this depth, therefore, NPP will be zero.
Compensation Point Tiger Paw
Compensation Point Bulldog
3. Which lake has a greater turbidity? Explain your answer.
4. Which lake is more productive? Explain your answer.
5. Describe the effect of turbidity on productivity in a lake, citing support from your graph and calculations.