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How to calculate net primary productivity

Biologists use primary productivity, or primary production, to establish how efficiently plants convert carbon dioxide, water and light energy into glucose and oxygen through the process of photosynthesis, according to the Canadian Forest Service. The opposite of photosynthesis is respiration, during which plants consume oxygen and release carbon dioxide and water. The net effect is net primary productivity (NPP). Monitoring this figure over time illustrates the effect of climate and other changes on the environment.

Measure the rate of photosynthesis and respiration. Create a closed system, such as a stoppered clear glass bottle containing seawater. Measure the increase in oxygen over a defined period of time. For example, the water in the bottle contains 8 mg of oxygen per litre at the start of the experiment and 10 mg of oxygen per litre at the end of the experiment, one hour later. Both photosynthesis and respiration have taken place and NPP, which measures the net effect of both, is therefore 10 - 8, or 2 mg per litre per hour.

Confirm your result. Measure the rate of respiration by performing the same experiment in a dark glass bottle over the same period of time. As photosynthesis only occurs during the presence of light, it will not take place in this bottle. Therefore, the amount of oxygen will decrease. For example, the water in the bottle contains 8 mg of oxygen per litre at the beginning of the experiment as in Step 1. It contains 5 mg of oxygen per litre at the end of this experiment. The rate of respiration is therefore 8 - 5, or 3 mg per litre per hour.

Calculate the rate of photosynthesis by comparing the amount of oxygen in the two bottles at the end of the experiment. Only respiration took place in Step 2. Both photosynthesis and respiration took place in Step 1. Therefore, the difference in oxygen between them is due to photosynthesis. The clear bottle in Step 1 contains 10 mg of oxygen per litre. The dark bottle in Step 2 contains 5 mg of oxygen per litre at end of the hour. The rate of photosynthesis, or primary production, is 10 - 5, or 5 mg per litre per hour. Photosynthesis minus respiration equals NPP. Therefore, NPP is 5 - 3, or 2 mg per litre per hour, which is the same as the rate of NPP measured in Step 1.

Things You'll Need

• Clear glass bottle with stopper, 1 litre
• Dark glass bottle with stopper, 1 litre
• Seawater