GPP, NPP, and NEP: How We Measure Ecosystem Productivity

Gross primary production (GPP), net primary production (NPP), and net ecosystem production (NEP) are used to quantify ecosystem productivity, which refers to the rate of autotrophic production. They can be measured in units of biomass/area/time or energy/area/time; thus, productivity can be defined in terms of mass or energy. This is possible because consumption of biomass across trophic levels correlates to energy transfer; thus, the two entities can be regarded synonymously.

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GPP is the total production of energy or biomass by the primary producers.

NPP is the energy or biomass available to higher trophic levels. NPP can be mathematically represented as follows:

NPP = GPP - Ra,

where NPP = net primary production,

GPP = gross primary production,

and Ra = rate of autotrophic respiration.

It is often the case that NPP is approximately equal to GPP/2, which means that the rate of autotroph respiration is approximately equal to the amount of energy available to higher trophic levels.

High-NPP ecosystems include tropical forests, estuaries, and coral reefs. Low-NPP ecosystems include open oceans.

NPP is different from standing crop, which is the total biomass of the primary producers in an ecosystem. NPP is the rate of biomass or energy production, while standing crop is like a "progress check" of NPP's effects at any given time. The units of standing crop are mass/area. (Sidenote: I suck at calculus, but uh, I think, uh, NPP is the derivative of standing crop???? Kind of????????)

NEP is the energy or biomass remaining after upward transfer to all trophic levels in the ecosystem. Mathematical representation:

NEP = GPP - Rt,

where NEP = net ecosystem production,

GPP = gross primary production,

and Rt = rate of all organisms' respiration (total).

NEP can be evaluated by measuring CO2 and O2 entry/exit rates. If the CO2 entry rate into the ecosystem is higher than the CO2 exit rate, the ecosystem is storing carbon, and thus may be growing in size (biomass). If O2 entry rate is higher than O2 exit rate, the ecosystem's producers are not photosynthesizing much (as O2 is an output of photosynthesis) so the ecosystem is not taking in carbon, and thus is not expanding.

NPP cannot be measured with CO2/O2 levels because NPP does not account for respiration rates of other trophic levels; thus, a measurement for only plant respiration cannot be taken, since one can only get samples for whole-ecosystem respiration.

Measuring ecosystem productivity is important to understand the world around us.

The main resource I used to understand this concept is my FAVE book, Campbell's Biology. (Chapter 55)