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Primary
Productivity

Understand how ecosystems capture and convert solar energy into organic matter: the foundation of every food web on Earth.

AP Environmental Science

What Is Primary Productivity?

Primary productivity is the rate at which autotrophs (mainly plants, algae, and cyanobacteria) convert inorganic carbon into organic compounds through photosynthesis or chemosynthesis. It is the entry point for virtually all energy in an ecosystem.

GPP, NPP, and Respiration

GPP
Gross Primary Productivity: the total rate of photosynthesis, including the organic matter used up in cellular respiration.
NPP
Net Primary Productivity: the rate of energy storage as biomass that is available to consumers. \( \text{NPP} = \text{GPP} - R \).
R
Cellular Respiration: the energy producers use for their own metabolism, released as heat.

Distinguish GPP, NPP and NEP system boundaries.

Use this illustration to explore the concept described in its caption.

A grass plant and roots accompany a carbon ledger: GPP 12,000 kg C minus plant respiration 7,200 kg C equals NPP 4,800 kg C, equivalent to 48 g C/m² over one growing season.
Over one growing season, 12,000 kg C of gross primary production minus 7,200 kg C of plant respiration leaves 4,800 kg C of net primary production. Across 10 ha, that is 48 g C/m² for the season; decomposer respiration is outside this plant budget. The graphic reports one growing season. The native worked example explicitly assumes this approximates one year before converting to annual biomass and energy.

Why NPP Matters

Net primary productivity determines how much energy is available for consumers (herbivores, carnivores, decomposers). Higher NPP means more biomass to support larger, more diverse food webs.

1
Carrying capacity: NPP sets the upper limit on how many organisms an ecosystem can support.
2
Carbon cycling: NPP removes \( \text{CO}_2 \) from the atmosphere and stores it in biomass.
3
Human resources: Agriculture, fisheries, and forestry all depend on NPP.

The Core Equation

GPP Total photosynthesis − R Respiration = NPP Biomass available

Energy Flow: Sun to Biomass

Sun Light energy GPP 100% R (autotroph) ~50-70% of GPP Heat lost NPP ~30-50% of GPP Consumers Herbivores, etc.

GPP vs NPP: Key Distinction

GPP NPP
Definition Total carbon fixed by photosynthesis Carbon stored as new biomass
Includes respiration? Yes No (subtracted out)
Measurable directly? Difficult (must estimate R) Easier (harvest method, remote sensing)
Ecological meaning Total energy captured Energy available to consumers
Analogy Gross salary Take-home pay

Factors That Control NPP

Sunlight
Solar radiation is the energy source: more light means more photosynthesis (up to a saturation point).
Water
Essential for photosynthesis and nutrient transport. Deserts have low NPP mainly due to water scarcity.
Temperature
Enzymes have optimal temperature ranges. Extreme heat or cold reduces productivity.
Nutrients
Nitrogen and phosphorus are often limiting. Fertilization can dramatically boost NPP.

Four bottles test which nutrient is limiting

A researcher beside a lake checks four groups of three matching culture bottles under white lamps.Compare a control, added nitrogen, added phosphorus, and both nutrients, with three independent vessels per treatment. Keep starting conditions, light, handling and measurement time consistent; infer nutrient limitation from measured responses.

Nutrient limitation is a comparison

Nutrient limitation is a comparison. Compare a control, added nitrogen, added phosphorus, and both nutrients, with three independent vessels per treatment. Keep starting conditions, light, handling and measurement time consistent; infer nutrient limitation from measured responses.Compare a control, added nitrogen, added phosphorus, and both nutrients, with three independent vessels per treatment. Keep starting conditions, light, handling and measurement time consistent; infer nutrient limitation from measured responses.

A field design for nutrient limitation

A three-by-four layout identifies twelve independent plots with C, N, P and NP labels, three of each, alongside the matching experimental controls.Randomly assign sham, nitrogen, phosphorus, or combined nitrogen-plus-phosphorus additions to three independent contained plots each. Match water, light, starting vegetation, disturbance and additions unrelated to nutrients; compare dry-biomass change per area over the same interval.Open full diagram

NPP Across Biomes

\( \text{NPP (g C/m}^2\text{/yr)} \)
 0 500 1000 1500 2000 1800 Tropical Rainforest 1200 Temperate Forest 1500 Estuary 700 Savanna 125 Open Ocean 90 Desert

Aquatic vs Terrestrial NPP

Terrestrial
Produces about 55% of global NPP. Dominated by tropical forests. Limited by water and nutrients.
Aquatic
Produces about 45% of global NPP despite covering 71% of Earth. Limited by light penetration and nutrient availability.

How to Calculate NPP

1
Identify GPP: Determine the total amount of carbon fixed or energy captured (often given in a problem as total photosynthesis).
2
Identify R: Find the respiration rate of the producers (cellular respiration by autotrophs).
3
Subtract: Apply \( \text{NPP} = \text{GPP} - R \). Make sure both values share the same units.
4
Interpret: NPP tells you how much energy or biomass is available for consumers and decomposers.

Example 1: Forest Ecosystem

A temperate forest has a GPP of \( 2{,}500 \; \text{g C/m}^2\text{/yr} \). Autotroph respiration accounts for \( 1{,}300 \; \text{g C/m}^2\text{/yr} \). What is the NPP?

Example 2: Efficiency Calculation

An ecosystem receives \( 1{,}000{,}000 \; \text{kcal/m}^2\text{/yr} \) of solar energy. Its GPP is \( 20{,}000 \; \text{kcal/m}^2\text{/yr} \) and NPP is \( 8{,}000 \; \text{kcal/m}^2\text{/yr} \). Find the photosynthetic efficiency and the fraction of GPP lost to respiration.

Example 3: Comparing Two Ecosystems

Coral Reef Open Ocean
GPP (\( \text{g C/m}^2\text{/yr} \)) 2,500 250
R (\( \text{g C/m}^2\text{/yr} \)) 1,000 125
NPP (\( \text{g C/m}^2\text{/yr} \)) 1,500 125
R as % of GPP 40% 50%

Common Mistakes

✕ Confusing GPP and NPP
GPP includes respiration. NPP is what remains after respiration is subtracted. They are not interchangeable.
✕ Ignoring Units
NPP can be expressed in \( \text{g C/m}^2\text{/yr} \) or \( \text{kcal/m}^2\text{/yr} \). Mixing units leads to errors.
✕ Ocean = Low Total NPP
Open ocean has low NPP per unit area, but its enormous total area means it contributes significantly to global NPP.

Summary of Key Concepts

Concept Key Idea
GPP Total photosynthesis by all autotrophs
NPP \( \text{GPP} - R \): energy available to consumers
R (autotroph) Energy used by producers for metabolism
Limiting factors Light, water, temperature, nutrients (N, P)
Highest NPP biome Tropical rainforest, estuaries, coral reefs
Lowest NPP biome Open ocean (per \( \text{m}^2 \)), deserts, tundra
Photosynthetic efficiency ~1-2% of incoming solar radiation

Question 1 of 5

An ecosystem has a GPP of \( 5{,}000 \; \text{kcal/m}^2\text{/yr} \) and autotroph respiration of \( 3{,}000 \; \text{kcal/m}^2\text{/yr} \). What is the NPP?

Question 2 of 5

Which factor is most likely to limit primary productivity in the open ocean?

Question 3 of 5

If a forest's GPP is \( 3{,}000 \; \text{g C/m}^2\text{/yr} \) and its NPP is \( 1{,}200 \; \text{g C/m}^2\text{/yr} \), what percentage of GPP is lost to autotroph respiration?

Question 4 of 5

Which biome typically has the highest NPP per unit area?

Question 5 of 5

A student claims: "Since the ocean covers 71% of Earth, it must produce 71% of global NPP." Which statement best explains why this claim is incorrect?