1.8 Primary Productivity

Gross vs. Net Primary Productivity

Gross primary productivity (GPP) is the total rate at which producers convert solar energy into chemical energy (biomass) through photosynthesis. Producers use some of that energy for their own respiration. Net primary productivity (NPP) = GPP − respiration, representing the energy actually stored as new biomass and available to consumers at higher trophic levels.

Factors Limiting Productivity

Productivity varies across ecosystems due to limiting factors like light, temperature, water, and nutrient availability (especially nitrogen and phosphorus). Tropical rainforests and estuaries have high productivity due to abundant sunlight, warmth, water, and nutrients. Deserts and open oceans have low productivity due to water or nutrient scarcity, despite sometimes abundant sunlight.

Measuring and Comparing Productivity

Productivity is often measured as biomass or energy produced per unit area per unit time (e.g., kcal/m²/yr or g/m²/yr). Comparing NPP across ecosystems reveals which biomes support more life and food webs. High NPP ecosystems can sustain larger populations of consumers, making productivity a key indicator of an ecosystem's capacity to support life.

Key terms

Gross Primary Productivity (GPP)

Total rate at which producers convert solar energy into chemical energy via photosynthesis.

Net Primary Productivity (NPP)

GPP minus energy lost to respiration; represents energy stored as new biomass available to consumers.

Limiting factor

A resource (like light, water, or nutrients) that restricts growth or productivity when in short supply.

High productivity ecosystems

Tropical rainforests and estuaries, due to abundant sunlight, warmth, water, and nutrients.

Low productivity ecosystems

Deserts and open oceans, limited by water or nutrient scarcity despite available sunlight.

Try an AP-style question

What is the gross primary productivity?

1. 4 mg O₂/L
2. 1 mg O₂/L
3. 3 mg O₂/L
4. 2 mg O₂/L