Primary Productivity in Ecosystems (College Board AP® Environmental Science): Study Guide

What is primary productivity?

• During photosynthesis, primary producers (such as plants and algae) convert light energy to chemical energy

  • This chemical energy is stored within biological molecules (biomass)

• Primary productivity can be defined as:

  • The rate at which biomass is produced using an external energy source (like sunlight) and inorganic sources of carbon and other elements

Gross primary productivity

• The rate at which plants are able to store chemical energy or biomass via photosynthesis is referred to as gross primary productivity (GPP)

  • In other words, GPP is the total amount of energy captured through photosynthesis in a given area over time

  • It represents the maximum energy available before any is used by the organism for its metabolic needs

Net primary productivity

• Net primary productivity (NPP) is the GPP minus plant respiratory losses (R):

  • Of the total energy stored in glucose during photosynthesis, 90% will be released from glucose during respiration

  • 90% of the energy originally converted by the plant will therefore not be stored as new plant biomass and will not be available to be passed on to herbivores (primary consumers)

• NPP can therefore be defined as the rate at which energy is stored in plant biomass, allowing for respiratory losses:

  • NPP is important because it represents the energy that is available to organisms at higher trophic levels in the ecosystem, such as primary consumers and decomposers

• NPP is the basis for food chains and food webs, as it represents the energy that supports all life forms in an ecosystem

  • NPP can be thought of as the amount of plant growth that can be harvested by:

    • Primary consumers in natural ecosystems

    • Farmers in agricultural systems

    • Foresters in silvicultural systems

• Net primary productivity can be calculated using the equation:

NPP = GPP - R

Measuring primary productivity

• GPP and NPP can be expressed in units of energy per unit area per unit time, for example:

  • J m⁻² yr⁻¹ (joules per square meter per year)

  • kJ km⁻² yr⁻¹ (kilojoules per square kilometer per year)

  • kcal m⁻² yr⁻¹ (kilocalories per square meter per year)

    • In this case, 'area' refers to the area of land that is being studied

    • This land contains the primary producers that are producing the biomass

    • If there are no primary producers present in this area of land, there will be no gross primary production

• GPP and NPP can also be expressed in units of mass per unit area per unit time, for example:

  • g m⁻² yr⁻¹ (grams per square meter per year)

  • kg km⁻² yr⁻¹ (kilograms per square kilometer per year)

• In aquatic environments, it may be more suitable to measure GPP and NPP per unit volume:

  • For example, for aquatic algae, gross primary productivity could be given in:

    • kg m⁻³ yr⁻¹ (kilograms per cubic meter per year)

    • kJ m⁻³ yr⁻¹ (kilojoules per cubic meter per year)

Primary productivity in aquatic ecosystems

• Light availability limits photosynthesis in aquatic ecosystems

  • Red light is absorbed within the upper 1 meter of water

  • Blue light penetrates deeper but usually no more than 100 meters in clear water

• Photosynthesizers in these ecosystems, such as algae and phytoplankton, have adapted to low light levels

  • Some have pigments like chlorophyll-a that absorb light more efficiently

  • Others live near the surface to maximize light exposure