Primary Productivity and Energy Dynamics Manuel Tzul
Primary Productivity and Energy Dynamics Manuel Tzul
Primary Productivity and Energy Dynamics Manuel Tzul
Ecology
20/07/16
Primary Productivity and Energy Dynamics - Key
Introduction to Primary Productivity
(http://kingfish.coastal.edu/biology/sgilman/778PrimProd.htm)
The flow of energy through any ecosystem starts with the fixation of sunlight by plants and
other autotrophic organisms. In this way the plant accumulates energy and this energy is
called primary production. The rate at which this energy accumulates is called primary
productivity. The total energy accumulated is gross primary productivity, however, since
plants use some of this energy themselves, it is not all available for the food web. The difference
between what is accumulated and what is available for the food web is called net primary
production expressed in kilocalories or grams m-2 y-1 or kcal or g/m2/y. This is measured by
sequentially measuring growth of the biomass over time by marking the plants somehow, or
measuring a total at the end of the growing season. Alternatively you can measure oxygen
production or CO2 consumption both of which equal grams C produced.
In general, swamps and marshes have the highest primary production of all the world's
ecosystems. Primary production of all wetland types varies from 600-2000 gC/m2/y.
To review a little:
In general, the "openness" of a wetland to hydrological fluxes is probably one of the most
important determinants of primary productivity. So wetlands that are stagnant are less
productive than those that flow or are open to flooding rivers. This makes sense because a flowthrough system constantly gets more nutrients. This isn't 100% though because wetlands get
most of their nutrients from recycling rather than from the outside. This is what allows them all
to be fairly productive.
Experimental Scenario: (from old AP Bio Lab 12 and PH School Lab Bench)
1. Fill six bottles with water containing a culture of photosynthetic algae.
2. Measure the initial dissolved oxygen content of the water.
3. Leave one bottle alone, cover one bottle in tin foil and use increasing numbers of screens
to cover the remaining bottles. Then place all of the bottles under a light source for 24
hours. This will simulate a decrease in the amount of light available to the algae since
each additional screen will block out more light and the tin foil will completely obscure the
light from the algae.
4. Wait 24 hours and then measure the dissolved oxygen level of each bottle.
5. Use the O2 readings to determine the primary productivity conducted in each bottle.
a.
b.
Manuel Tzul
Ecology
20/07/16
c.
Manuel Tzul
Ecology
20/07/16
Analysis:
1. Why is dissolved oxygen a good measure of primary productivity?
Its a direct measurement of how much photosynthesis has been done.
2. What process(es) occur in each of the following bottles:
a. 100% light bottle : photosynthesis and cellular respiration
b. Dark bottle: cellular respiration
3. For each of the calculations show in Step #5 of the Procedure, explain how it provides the
given measurement. (Example, for calculation a, why does subtracting the amount of
dissolved oxygen in the dark bottle from the amount of dissolved oxygen in the initial
sample provide a measure of respiration?)
a. The dark bottle has only been doing respiration so the difference in the amount of
O2 in the beginning and the amount in the dark bottle at the end is all as a result of
respiration
b. The light bottle has done both photosynthesis and respiration, by subtracting the
dark bottle, which is a negative number, you add in what is lost from respiration and
gives you the gross amount made by photosynthesis
c.
This calculation gives you the amount of oxygen gained over the experiment by
photosynthesis, since respiration was also occurring we get the net productivity
4. The following dissolved oxygen readings were taken. Assume the initial DO was measured
at 4 mg O2/L. Complete the first three columns of the data table below, the last column
will be done after Part Two.
Light
Bottle
Gross
Net
O%
E=1
Productivity
11=0
Productivity
1-4 = -3
-3 x .698 x .536 =
2%
F=2
21=1
2 4 = -2
-1.12
-2 x .698 x .536 =
10%
C=3
31=2
3 4 = -1
-.74
-1 x .698 x .536 =
-.37
Manuel Tzul
Ecology
20/07/16
25%
A=5
51=4
54=1
1 x .698 x .536 = .
65%
B = 10
10 1 = 9
10 4 = 6
37
6 x .698 x .536 =
100%
D= 14
14 1 = 13
14 4 = 10
2.24
10 x .698 x .536 =
3.7
5. Under what circumstance would an autotroph have no net productivity? For the above
problem, at what light intensity do you expect that to happen? Please explain.
When the amount of photosynthesis done exactly equals the amount of respiration. In our
experiment it would be at about 17.5% light in between 10% and 25%.
Manuel Tzul
Ecology
20/07/16
Part Two: Energy Dynamics
Primary Productivity matters to ecosystems because it allows us to determine how much energy
is available for the higher trophic levels. In order to determine how much mass is available for
the next trophic level, the amount of O2 produced can be used to determine the amount of CO 2
assimilated.
1 ml of O2 = .536 mg of Carbon assimilated
To convert: ppm O2 = mg O2/L
mg O2/L x 0.698 = ml O2/L
ml O2/L x 0.536 = mg carbon fixed/L
Explanation of carbon assimilation:
(http://www.globalchange.umich.edu/globalchange1/current/lectures/kling/energyflow/energyflow.html) A
Manuel Tzul
Ecology
20/07/16
Manuel Tzul
Ecology
20/07/16
Refer to the following diagram for #4 and 5 (POGIL Ecological Pyramids)
4. Why can most ecosystems support no more than 4 levels in a trophic pyramid?
If we look at the pyramid above we can notice that as the energy is assimilated most of
the energy not passed on to the next trophic level. Therefore this causes that at the 4th
trophic level the organisms at that level receive low amounts of energy.
5. Explain why a vegetarian diet is said to be more energy efficient for humans than one
based on meat.
Based on the pyramid producers pass on 4,000 kcal to the herbivore. On the other hand
Carnivores just receive 24 kcal upon omnivores. Therefore humans diet would be more
energy efficient if they consumed food directly from plants because of the higher amount
of energy available.