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 flow-through 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.
c.
Analysis:
1. Why is dissolved oxygen a good measure of primary productivity?
It’s 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 Productivity
Net Productivity
Gross Carbon fixed
O%
E=1
1–1=0
1-4 = -3
-3 x .698 x .536 = -1.12
2%
F=2
2–1=1
2 – 4 = -2
-2 x .698 x .536 = -.74
10%
C=3
3–1=2
3 – 4 = -1
-1 x .698 x .536 = -.37
25%
A=5
5–1=4
5–4=1
1 x .698 x .536 = .37
65%
B = 10
10 – 1 = 9
10 – 4 = 6
6 x .698 x .536 = 2.24
100%
D= 14
14 – 1 = 13
14 – 4 = 10
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%.
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 CO2 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 hare (or a population of hares) ingests plant matter; we'll call this ingestion. Part of this material is processed by the
digestive system and used to make new cells or tissues, and this part is called assimilation. What cannot be assimilated,
for example maybe some parts of the plant stems or roots, exits the hare's body and this is called excretion. Thus we
can make the following definition: Assimilation = (Ingestion - Excretion). The efficiency of this process of assimilation
varies in animals, ranging from 15-50% if the food is plant material, and from 60-90% if the food is animal material.
The hare uses a significant fraction of the assimilated energy just being a hare -- maintaining a high, constant body
temperature, synthesizing proteins, and hopping about. This energy used (lost) is attributed to cellular respiration. The
remainder goes into making more hare biomass by growth and reproduction (that is, increasing the overall biomass of
hares by creating offspring). The conversion of assimilated energy into new tissue is termed secondary production in
consumers, and it is conceptually the same as the primary production or NPP of plants. In our example, the secondary
production of the hare is the energy available to foxes who eat the hares for their needs. Clearly, because of all of the
energy costs of hares engaged in normal metabolic activities, the energy available to foxes is much less than the energy
available to hares.
Analysis:
1. Use the equations above to determine the Gross Carbon fixed in each bottle. (Column 4)
2. In which bottle is the most carbon available to the next trophic level? Explain why this is.
3. Use your calculations and your understanding of primary and secondary production to explain why a tropical
rainforest can support a larger food web than the arctic tundra.
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?
5. Explain why a vegetarian diet is said to be more energy efficient for humans than one based on meat.