Primary Productivity

You’ll be able to

Define gross and net primary productivity and relate them through respiration
Calculate NPP from GPP and respiration
Compare the productivity of major ecosystems and identify limiting factors

Productivity is the rate energy enters life

Primary productivity is the rate at which producers capture and store energy as biomass — the base that feeds every other trophic level. It is measured as energy per area per time, for example kcal·m⁻²·yr⁻¹, or as grams of biomass per area per time. A more productive ecosystem can support more consumers.

Gross vs. net: what the plant keeps

Gross primary productivity (GPP) is the total energy producers capture through photosynthesis. But producers must run their own cellular respiration (R) to stay alive, which burns some of that energy. What is left over and stored as new biomass — the part actually available to consumers — is the net primary productivity (NPP).

Net primary productivity

NPP = GPP − R

R is the energy producers spend on their own respiration. NPP is what remains as biomass; GPP is always larger than NPP.

Worked example

A forest has a gross primary productivity of 20,000 kcal·m⁻²·yr⁻¹, and its plants use 12,000 kcal·m⁻²·yr⁻¹ in cellular respiration. Find the net primary productivity, and state what fraction of GPP it represents.

1.Write the relationship: NPP = GPP − R.
2.Substitute the values: NPP = 20,000 − 12,000.
3.NPP = 8,000 kcal·m⁻²·yr⁻¹.
4.Fraction of GPP stored: 8,000 ÷ 20,000 = 0.40, or 40%.

Answer: NPP = 8,000 kcal·m⁻²·yr⁻¹, which is 40% of the gross primary productivity.

Checkpoint

An ecosystem has a GPP of 15,000 kcal·m⁻²·yr⁻¹ and the producers respire 6,000 kcal·m⁻²·yr⁻¹. What is the net primary productivity?

Tip

Think of it like a paycheck: GPP is your gross pay, respiration (R) is the taxes taken out, and NPP is your take-home pay — the energy actually available to build biomass and feed consumers. Take-home is always less than gross.

What limits productivity

Productivity is not the same everywhere. On land it is highest where warmth and water are abundant — tropical rainforests lead — and lowest in deserts and tundra, limited by water or cold. In aquatic systems the usual limits are sunlight (which fades with depth) and nutrients like nitrogen and phosphorus. This is why estuaries and coral reefs, shallow and nutrient-rich, are extraordinarily productive while the open ocean is a relative desert.

Checkpoint

The open ocean covers most of Earth’s surface yet has low primary productivity per square meter. What most directly limits it?

On the exam

Watch the wording on GPP/NPP problems. "Total energy captured" or "total photosynthesis" means GPP; "energy stored as biomass" or "available to consumers" means NPP. If a question gives you NPP and R and asks for GPP, rearrange to GPP = NPP + R.

Answer the 2 checkpoints as you read.