EXAMPLE PROBLEM
In the surface waters of lakes and oceans, producers (plants) are mainly
unicellular algae, and most consumers are microscopic crustaceans and
protozoans. If you take a sample of surface water and place it in a bottle and
turn on the lights, you get photosynthesis (from the producers). If you turn off
the lights, you turn off the primary production, with darkness having no effect
on cellular respiration. When calculating the amount of energy that a plant
stores as biomass, which is then available to heterotrophs, you must subtract
plant respiration costs from the total primary production.
Procedure: Take a series of small glass bottles with stoppers and wrap half of
them with foil (label them “dark”) and do not wrap the other half (label these
“light”). Fill the bottles with water taken from a particular place and depth in
a water column; this water contains tiny plants and animals of the aquatic
ecosystem. Before suspending the bottles, determine the initial O 2
concentration in the water through chemical titration expressed as mg of O 2
per liter of water (mg/L). Then close the bottles with stoppers to prevent any
exchange of gases or organisms with the surrounding water. Then suspend the
bottles in the water column at the depth you took the samples from for one
hour. Finally, determine the oxygen concentration in both the light and dark
bottles after one hour of incubation.
ANSWER
Light bottles: In the light bottles, there is photosynthesis occurring, or Gross
Primary Production (GPP), and there is respiration occurring (R). The
difference between these two processes is Net Primary Production = NPP =
(GPP – R).
Dark bottles: In the dark bottles, there is no photosynthesis occurring, only
respiration.
After 1 hour, the measured oxygen concentrations were determined to be:
Initial bottles = 8 mg O 2 /L; Light bottles = 10 mg O 2 /L; Dark bottles = 5 mg
O 2 /L