Net Primary Productivity

By the process of photosynthesis, plants synthesize organic compounds. In this process, the light energy is converted into chemical energy and is stored in the plant cells. This energy is consumed by the organisms at the next trophic level.

Primary producers are also called autotrophs to absorb the light energy from sunlight and the stored carbon dioxide and water are used to form carbohydrates. All the living organisms on the earth depend on primary productivity either directly or indirectly. 

Primary Productivity

In ecology, primary productivity means the rate at which the energy is converted into organic substances with the help of photoautotrophs and chemoautotrophs.

It is further divided into 2 types: 1) Net primary productivity, 2) Gross primary productivity.

1. Net Primary Productivity

Among all the ecosystem processes NPP is the most measured one because it stores and accumulates the carbon present in ecosystems. 

NPP is calculated based on the increase in the biomass per unit area in unit time. 

NPP is dependent on various factors present in the ecosystem, they are:

• Temperature

• Humidity

• Sunlight

• Nutrients

What is Net Primary Productivity?

NPP definition is as follows, the rate at which the energy is accumulated. 

• NPP full form is Net Primary Productivity and is equal to the carbon taken up by the vegetation through photosynthesis (called Gross Primary Production or GPP) minus the carbon that is lost to respiration.

Net Primary Productivity of an ecosystem is = GPP - respiration

• In terrestrial systems, NPP is calculated by determining the annual carbon storage increment. Therefore, using the biomass and carbon storage estimates from the sample site, aboveground NPP can be calculated by taking the difference in the stored carbon in-between years.

Net Primary Productivity of an ecosystem is = Carbon stored in year 2 - Carbon stored in year 1.

2. Gross Primary Productivity

Gross primary productivity is the amount of carbon fixed by all the producers in an ecosystem during photosynthesis. 

About 40 - 85% of the GPP is not used during the respiration process which inturn becomes NPP. The highest NPP in the terrestrial environment is found on swamps and marshes and tropical rainforests. In aquatic ecosystems, the highest NPP is found estuaries, algal beds, and reefs. The lowest NPP is found in desserts. 

In mid-latitudes, productivity is related to seasonal change, with productivity peaking in each hemisphere's summer. The boreal forests of Canada and Russia experience high productivity in June and July and then a slow decline is found through fall and winter.

Tundra and alpine environments restrict plants from growing to a very high range since the desiccation caused by wind and bending of the stem induces ice damage. This means there will be very few layers of vegetation and less efficiency of light capture. Cold weather limits the NPP by reducing the rate at which enzymes can catalyze reactions. Even when the air temperature increases sufficiently it allows the photosynthesis process to occur, then the soil nutrients can be locked up permanently in the frozen soil.

Human activities such as land use can affect the magnitude of the global NPP and the flow of the biomass through the ecosystems. In order to measure the impact of land use on biomass which is available for ecosystems, a metric is known as human appropriation of net primary production (HANPP) has been put forward.