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Dissolved Oxygen and Aquatic Primary Productivity
Concept 3: Primary Productivity
Primary Productivity

Primary productivity is a term used to describe the rate at which plants and other photosynthetic organisms produce organic compounds in an ecosystem. There are two aspects of primary productivity:

Gross productivity = the entire photosynthetic production of organic compounds in an ecosystem.
Net productivity = the organic materials that remain after photosynthetic organisms in the ecosystem have used some of these compounds for their cellular energy needs (cellular respiration).

Since oxygen is one of the most easily measured products of both photosynthesis and respiration, a good way to gauge primary productivity in an aquatic ecosystem is to measure dissolved oxygen. We cannot measure gross productivity directly because respiration, which uses up oxygen and organic compounds, is always occurring simultaneously with photosynthesis — but we can measure it indirectly. Let's see how to do this.

We can measure net productivity directly by measuring oxygen production in the light, when photosynthesis is occurring.
We can also measure respiration without photosynthesis by measuring O2 consumption in the dark, when photosynthesis does not occur.
Since net productivity = gross productivity respiration, we can calculate gross productivity.

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