# FOOD WEBS:  PRIMARY PRODUCTION

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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I.  Terms and definations.

A.  Primary production is the conversion of  light or abiotic chemical energy into organic chemical energy.

The principle mechanism by which this conversion takes place is photosynthesis.

B.  In case you forgot, photosynthesis converts light into high-energy carbon bonds.

12H2O + 6CO2 + energy = C6H12O6 + 6O2  + 6H2O

A total of  709 kilocalories of energy are needed to create 1 mole of sugar

C.  Because plants also use up energy through respiration, the photosynthetic rate will not equal the amount of energy stored.

Gross Primary Production (GPP) = the amount of sugar created through photosynthesis

Net Primary Production (NPP) = amount of stored sugar
or
GPP - respiration

II.  Measuring Gross Primary Production

By knowing the amount of sugar generated through photosynthesis, and multiplying by 709 kcal/mole of sugar, we can calculate the amount of sunlight which has been absorbed.

The rate of  photosynthesis can be measured by analyzing the chemicals unique to either side of the photosynthesis equation (e.g., CO2, O2, or C6H12O6)

A.  On Land.  Place a plastic bag around a plant.  Measure the change in atmospheric concentration of  CO2 during the daylight hours.  This will tell you NPP, as both photosynthesis and respiration are taking place.  Do the same at night, which will tell you the respiration rate, as photosynthesis is not taking place.  By adding these two numbers together (NPP+respiration) you will know GPP

B.  In Water.  Collect a bucket of water.  Mix it well and measure the oxygen cencentration in the sample.  Put it into two glass gars, one of which has been wrapped in aluminum foil so that no light will shine on it.    Place the jars back into the water and let sit for a few hours.  Measure the Oxygen levels of the two jars.  The amount of Oxygen generated in the clear bottle will tell you NPP (both photosynthesis and respiration taking place), while the amount of  oxygen used up in the shaded bottle will tell you Respiration.  Add both to get GPP.

C.  Sugar generation.  Place a plant into an air-tight plastic bag.  Pump radioactive CO2 into the bag, and let the plant briefly be exposed to it.  Take the plant out of the bag, grind it up, and determine how much radioactive sugar has been created.  This will tell you GPP directly.

III. Estimating Primary Production Efficiency.

A.  Primary production efficiency equals the fraction of sunlight energy which falls on an area which ends up being harvested to make sugar.

B.  GPP Efficiency is calculated by dividing GPP energy for a given area by the total sunlight energy that area is exposed to.

For example:

7.08 moles of sugar are generated via GPP per year for each square meter of Lake Mendota in Madison.

Multiplying this by 709, we calculate that 5017 kcal of sunlight is absorbed during this process.
By putting sunlight sensors out on the lake, we can determine that 1,188,720 kcal of sunlight falls on each square meter of Lake Mendota during that year.

GPP Efficiency for Lake Mendota = (5017 / 1,188,720) * 100 = 0.42%

GPP Efficiency for other ecosystems:

Forests   2 - 3½%
Grasslands  1 - 2%
Crop Fields   1½%

C.  NPP Efficiency is calculated by dividing NPP energy into total sunlight energy.

Because Respiration losses are greater in some systems, NPP and GPP efficiencies will not be the same.  For the communities listed above, NPP Efficiency equals approxil\matley 1%, even through GPP Efficiencies were up to three times greater in some communities.

Thus, approximately 1% of sunlight is made available by plants for other organisms to consume.

IV.  Limits on Primary Production.

Primary Production is limited by many of the same factors previously discussed.

On land, soil Nitrogen, water and light most often limit production

In water, Phosphorus and light most often limit production.

In aquatic systems, both too little sun, as well as too much sun, may limit produciton.

Areas with too little sun (deep in water) are photolimited

Areas with too much sun (at water surface) are photoinhibited

Maximum pproduction occurs a few meters deep into the ocean, where sunlight levels are not as high.  These areas are photosaturated.

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