BiologyCBSE Class 12 · Ecosystem

Primary Productivity: GPP and NPP

Updated 15 October 2026 · Fact-checked

Primary production is the biomass made by plants in photosynthesis per unit area over a time period; its rate is productivity. Gross primary productivity (GPP) is the total rate of organic matter made; net primary productivity (NPP) is what remains after plant respiration: NPP = GPP − R. NPP is the food available to herbivores and decomposers.

Plants lose part of GPP in respiration; what remains, NPP = GPP − R, is the food available to herbivores and decomposers.

Illustration drawn for QuestionBuddy.

Understand Primary Productivity

Energy input. No ecosystem can keep working without a steady supply of solar energy; this is its basic input.

Definitions.

Primary production: the biomass (organic matter) that plants build by photosynthesis on a unit area during a given period. Expressed as weight (g m⁻²) or energy (kcal m⁻²).
Productivity: the rate of biomass production, expressed as g m⁻² yr⁻¹ or kcal m⁻² yr⁻¹, which lets us compare different ecosystems.
Gross primary productivity (GPP): the rate of production of organic matter during photosynthesis.
A considerable part of GPP is used by plants in respiration (R).
Net primary productivity (NPP): NPP = GPP − R. NPP is the biomass available for consumption by heterotrophs (herbivores and decomposers).
Secondary productivity: the rate of formation of new organic matter by consumers.

What controls primary productivity?

The plant species in the area.
Environmental factors (light, temperature, water).
Availability of nutrients.
The photosynthetic capacity of the plants.

So productivity varies greatly between ecosystems.

Global picture. The annual NPP of the whole biosphere is about 170 billion tons (dry weight) of organic matter. Although oceans cover about 70 per cent of the surface, their productivity is only 55 billion tons; the rest (about 115 billion tons) is produced on land. Main reasons for low ocean productivity: light decreases rapidly with depth, so photosynthesis is restricted to the surface layer, and nutrients (especially nitrogen and phosphorus) are limited in much of the open ocean.

Worked logic. If a grassland fixes 2000 g m⁻² yr⁻¹ (GPP) and respires 800 g m⁻² yr⁻¹, NPP = 1200 g m⁻² yr⁻¹; this 1200 is what grazers and decomposers can use.

Flow chart: sunlight → photosynthesis (GPP) → plant respiration (R) uses part → NPP (standing crop increase) → herbivores (secondary productivity) and decomposers.

Draw and label: productivity flow. Checklist: a sun icon with an arrow to a box 'producers'; inside the box 'GPP'; an arrow out labelled 'R (respiration loss, heat)'; a second arrow out labelled 'NPP' leading to two boxes 'herbivores' and 'decomposers'; a small box 'secondary productivity' under herbivores. Labels: solar energy, GPP, R, NPP = GPP − R, herbivores, decomposers, secondary productivity.

Primary Productivity: key facts

Net primary productivity: $N P P = GP P - R$; R = respiration losses of plants.
Units: $production: g m^{- 2} or k c a l m^{- 2}; productivity: g m^{- 2} y r^{- 1} or k c a l m^{- 2} y r^{- 1}$; Productivity is a rate.
Global NPP: $170 = 55 (oceans) + 115 (land) billion tons/yr (dry weight)$; Oceans cover about 70% of the surface.
Fraction of GPP respired: $\frac{R}{GP P} \times 100 %$; Useful in comparing ecosystems.
Secondary productivity: $rate of new organic matter formed by consumers$; Depends on NPP available.

How to answer Primary Productivity questions

Productivity questions are definitions, comparisons and simple calculations. Keep GPP, R and NPP straight and always write units.

1Write the relation NPP = GPP − R before substituting.
2Check the units: production is an amount (g m⁻²), productivity is a rate (g m⁻² yr⁻¹).
3To compare ecosystems, compare NPP per unit area per year, not totals for different areas.
4For 'why low productivity', list the limiting factors: light, nutrients, temperature, water, plant species and photosynthetic capacity.
5For oceans, give both reasons: light falls off with depth; nutrients are limited.
6Link NPP to food chains: it is the energy available to herbivores and decomposers.

Primary Productivity in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

GPP, R and NPP

Suppose a farm scientist measures that a maize field fixes 3200 g of organic matter per m² per year by photosynthesis, and the crop uses 1400 g m⁻² yr⁻¹ in its own respiration. A neighbouring grassland has GPP 1800 and R 700 (same units).

Calculate the NPP of each, the percentage of GPP lost in respiration, and which provides more food for consumers per m².

Try it first, then show the solutionSolution

Maize NPP = 3200 − 1400 = 1800 g m⁻² yr⁻¹; respired fraction = 1400/3200 × 100 = 43.75%.
Grassland NPP = 1800 − 700 = 1100 g m⁻² yr⁻¹; respired fraction = 700/1800 × 100 ≈ 38.9%.
The maize field has the higher NPP, so it provides more organic matter for consumers (and harvest) per m² per year, even though it respires a larger share of its GPP.

Answer: Maize NPP 1800 (43.75% respired); grassland NPP 1100 (≈38.9% respired); maize gives more food per m².

Memory hook: Gross minus what plants burn = net for everyone else.

Tricks to remember Primary Productivity

Short memory aids. Say each one aloud once, then use it on a question the same day.

Salary analogy
GPP = gross salary; R = household expenses; NPP = savings others can borrow.
Use it when: NPP = GPP − R.
170 = 55 + 115
Oceans 55 (with 70% area), land 115.
Use it when: Global productivity figures.
Rate means per year
Productivity always has 'per year' in its unit.
Use it when: Units questions.

Primary Productivity: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Production vs productivity · 2 marks · Very short answerBoard level

Distinguish between primary production and primary productivity, giving the unit of each. Why is productivity, and not production, used to compare two ecosystems?

Try it first, then show the solutionSolution

Primary production: the amount of biomass made by plants in photosynthesis per unit area over a time period, expressed as weight (g m⁻²) or energy (kcal m⁻²) (1 mark).
Productivity: the rate of biomass production, in g m⁻² yr⁻¹ or kcal m⁻² yr⁻¹; being per unit area per unit time, it lets ecosystems of different size and measuring period be compared (1 mark).

Answer: Production is an amount (g m⁻² or kcal m⁻²); productivity is a rate (g m⁻² yr⁻¹ or kcal m⁻² yr⁻¹), which makes comparison possible.

Example 2: Simple calculation · 1 mark · Very short answerBoard level

The GPP of a pond is 4000 kcal m⁻² yr⁻¹ and plant respiration uses 1500 kcal m⁻² yr⁻¹. Find the NPP.

Try it first, then show the solutionSolution

NPP = GPP − R = 4000 − 1500.

Answer: 2500 kcal m⁻² yr⁻¹.

Example 3: Ocean productivity · 3 marks · Short answerHarder board level

Oceans cover about 70% of the earth's surface but produce only 55 of the 170 billion tons of annual NPP. Calculate the share of land and give two reasons for the low productivity of oceans.

Try it first, then show the solutionSolution

Land NPP = 170 − 55 = 115 billion tons; oceans give 55/170 ≈ 32% of global NPP from about 70% of the area (1 mark).
Light intensity decreases rapidly with depth, so photosynthesis is limited to surface waters (1 mark).
Nutrients such as nitrogen and phosphorus are scarce in much of the open ocean, limiting producers (1 mark).

Answer: Land 115 billion tons; oceans limited by light with depth and by nutrients.

Example 4: Factors · 2 marks · Very short answerHarder board level

List the factors on which primary productivity of an area depends.

Try it first, then show the solutionSolution

The plant species inhabiting the area and their photosynthetic capacity (1 mark).
Environmental factors (light, temperature, water) and availability of nutrients (1 mark).

Answer: Plant species, photosynthetic capacity, environmental factors and nutrients.

Example 5: Respiration share · 3 marks · Short answerHOTS / competency

Suppose two forests have the same GPP of 6000 g m⁻² yr⁻¹. Forest A respires 70% of its GPP and Forest B 50%. Which has the higher NPP, by how much, and what might explain A's higher respiration?

Try it first, then show the solutionSolution

A: R = 0.70 × 6000 = 4200, NPP = 1800; B: R = 3000, NPP = 3000 (1 mark).
B has the higher NPP, by 3000 − 1800 = 1200 g m⁻² yr⁻¹ (1 mark).
A may be warmer (respiration rises with temperature) or have more old, non-photosynthetic biomass (large trunks) to maintain (1 mark).

Answer: B's NPP is higher by 1200 g m⁻² yr⁻¹; A may be warmer or have more maintenance biomass.

Example 6: Primary vs secondary · 2 marks · Very short answerHarder board level

Define secondary productivity. Why must secondary productivity in an ecosystem always be less than its NPP?

Try it first, then show the solutionSolution

Secondary productivity is the rate of formation of new organic matter by consumers (1 mark).
Consumers build their biomass from the NPP they eat, and they too lose much energy in respiration and as heat, so the new organic matter they form is less than the NPP available (1 mark).

Answer: Rate of new organic matter by consumers; it comes from NPP minus consumer losses, so it is smaller.

Where students lose marks in Primary Productivity

The slips examiners see most often, and what to write instead.

Mistake: Swapping GPP and NPP
'Net' and 'gross' are confused.
Wrong working
GPP = NPP − R.
Right working
NPP = GPP − R.
Do this: Gross is bigger; net is what remains.
Usually costs: 1 mark.
Mistake: Units without time
Production and productivity are confused.
Wrong working
Productivity = 1200 g m⁻².
Right working
Productivity = 1200 g m⁻² yr⁻¹.
Do this: Productivity is a rate.
Usually costs: ½-1 mark.
Mistake: Saying respiration losses are due to animals
R is misread.
Wrong working
R is the amount eaten by herbivores.
Right working
R is the plants' own respiration.
Do this: R belongs to the producers.
Usually costs: 1 mark.
Mistake: Only one reason for ocean productivity
Incomplete answers.
Wrong working
Oceans have less light.
Right working
Light decreases with depth and nutrients are limited.
Do this: Give both light and nutrients.
Usually costs: 1 mark.

Exam tips

Write NPP = GPP − R in every productivity answer.
Learn the global figures: 170 billion tons total, 55 billion from oceans covering about 70% of the surface.
Distinguish production (amount) from productivity (rate).
Define secondary productivity in one line when asked about consumers.
Note: ecological succession, nutrient cycles and ecosystem services are not in the 2026-27 syllabus.

Practice questions on Primary Productivity

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
NPP is equal to:
(a) GPP + R (b) GPP − R (c) R − GPP (d) GPP × R
Hint
Net after respiration.
Answer
(b) GPP − R.
Q2 · 2 marks · Very short answerBoard level
In what units is productivity expressed, and why are such units used?
Hint
Per area per year.
Answer
g m⁻² yr⁻¹ or kcal m⁻² yr⁻¹. Expressing production per unit area per unit time allows the productivity of different ecosystems to be compared.
Q3 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Oceans have low productivity despite covering about 70% of the earth's surface.
Reason (R): Light decreases with depth and nutrients are limited in the oceans.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Limiting factors.
Answer
(a).
Q4 · 2 marks · Very short answerHarder board level
A field has NPP of 2400 g m⁻² yr⁻¹, and plants respire 40% of their GPP. Find the GPP and R.
Hint
NPP = 60% of GPP.
Answer
NPP = GPP − 0.4 GPP = 0.6 GPP, so GPP = 2400/0.6 = 4000 g m⁻² yr⁻¹; R = 0.4 × 4000 = 1600 g m⁻² yr⁻¹.
Q5 · 4 marks · Case-basedHOTS / competency
Read and answer.
Suppose an ecologist estimates that the algae in a lake fix 900 g of organic matter m⁻² yr⁻¹ in photosynthesis and use 300 g m⁻² yr⁻¹ in their own respiration. Fish in the lake add 30 g of new biomass m⁻² yr⁻¹.
(a) Give the GPP and NPP. (1)
(b) What is the 30 g figure called? (1)
(c) Why is it so much smaller than NPP? Give two reasons. (2)
Hint
Secondary productivity; not all NPP is eaten.
Answer
(a) GPP = 900 g m⁻² yr⁻¹ (total fixed); NPP = 900 − 300 = 600 g m⁻² yr⁻¹. (b) Secondary productivity (of the fish). (c) Not all NPP is eaten by fish (much goes to other consumers and decomposers), and consumers lose much of the energy they take in through respiration (heat) and in undigested matter, so only a small part becomes new biomass.
Q6 · 2 marks · Very short answerHarder board level
Why is NPP, and not GPP, the energy available to the rest of the ecosystem?
Hint
What do plants use first?
Answer
Plants use part of their GPP in their own respiration (R) to run their metabolism. Only the remainder, NPP, is stored as plant biomass that herbivores and decomposers can consume.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Formula for NPP?Show answer
NPP = GPP − R.
2.Unit of productivity?Show answer
g m⁻² yr⁻¹ or kcal m⁻² yr⁻¹.
3.Annual NPP of the biosphere?Show answer
About 170 billion tons (dry weight).
4.Ocean NPP per year?Show answer
About 55 billion tons.
5.What is secondary productivity?Show answer
The rate of formation of new organic matter by consumers.
6.Who can use NPP?Show answer
Heterotrophs: herbivores and decomposers.

Primary Productivity: frequently asked questions

What is primary productivity?

The rate at which plants produce biomass (organic matter) per unit area by photosynthesis, expressed in g m⁻² yr⁻¹ or kcal m⁻² yr⁻¹.

What is the difference between GPP and NPP?

GPP is the total rate of organic matter production in photosynthesis; NPP is what remains after plants' respiration (NPP = GPP − R).

Why is ocean productivity low?

Light decreases with depth, restricting photosynthesis to surface waters, and nutrients are limited in much of the open ocean.

What affects primary productivity?

The plant species present, environmental factors, nutrient availability and the photosynthetic capacity of plants.

What is secondary productivity?

The rate of formation of new organic matter by consumers.