Primary Productivity (Cambridge (CIE) AS Environmental Management)
Revision Note
Written by: Alistair Marjot
Reviewed by: Bridgette Barrett
Primary Productivity
During photosynthesis, primary producers (such as plants and algae) convert light energy to chemical energy, which is stored within biological molecules
Gross primary production can be defined as the amount of chemical energy stored in the carbohydrates within plants (during photosynthesis)
Roughly only 1% of the light falling on a plant is used in photosynthesis to produce glucose:
99% of the light either passes through the leaf without hitting chloroplasts, is reflected off of the leaf or is transferred to heat energy
After that, 1% is successfully absorbed and used to form glucose
The quantity of energy now stored in glucose is the gross primary production
Gross Primary Productivity
The rate at which plants are able to store chemical energy via photosynthesis is referred to as gross primary productivity (GPP)
Gross primary productivity can be expressed in units of energy per unit area per unit time, for example:
J m⁻² yr⁻¹ (joules per square metre per year)
kJ km⁻² yr⁻¹ (kilojoules per square kilometre 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)
Gross primary productivity can also be expressed in units of mass per unit area per unit time, for example:
g m⁻² yr⁻¹ (grams per square metre per year)
kg km⁻² yr⁻¹ (kilograms per square kilometre per year)
In aquatic environments, it may be more suitable to measure gross primary production per unit volume:
For example, for aquatic algae, gross primary productivity could be given in kg m⁻³ yr⁻¹ (kilograms per cubic metre per year) or kJ m⁻³ yr⁻¹ (kilojoules per cubic metre per year)
Worked Example
The total chemical energy contained within the grass that grows in a 200 m² field over the course of one year is found to be 1 000 kJ. Calculate the gross primary productivity of the grass field. Give appropriate units.
Answer
Step 1: Calculate the total chemical energy contained within the grass in 1 m² of the field over the course of one year
1 000 ÷ 200 = 5 (kJ)
Step 2: Give the appropriate units
5 kJ m⁻² yr⁻¹
Worked Example
On average, a patch of arctic tundra covering an area of 1 km² is estimated to produce a total biomass of 1 500 kg per year. Calculate the gross primary productivity of this patch. Give your answer in g m⁻².
Answer
Step 1: Calculate the average yearly biomass of 1 m² of the arctic tundra patch (1 km² = 1 000 000 m²)
1 500 ÷ 1 000 000 = 0.0015 (kg yr⁻¹)
Step 2: Convert this into grams
0.0015 × 1 000 = 1.5 g m⁻² yr⁻¹
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
Net primary productivity can be calculated using the equation:
NPP = GPP - R
NPP is expressed in units of biomass or energy per unit area or volume per unit time, for example:
Using area: g m⁻² yr⁻¹ (grams per square metre per year) or J m⁻² yr⁻¹ (joules per square metre per year)
Using volume: kg m⁻³ yr⁻¹ (kilograms per cubic metre per year) or kJ m⁻³ yr⁻¹ (kilojoules per cubic metre per year)
As with GPP, volume would be used when calculating NPP in aquatic habitats
Worked Example
The grass in a meadow habitat converts light energy into carbohydrates at a rate of 17 500 kJ m⁻² yr⁻¹. The grass releases 14 000 kJ m⁻² yr⁻¹ of that energy during respiration. Calculate the net primary productivity of the grass in the meadow habitat.
Answer
Step 1: Work out which numbers correspond to which parts of the equation
The meadow grass converts 17 500 kJ m⁻² yr⁻¹ into carbohydrates; this is GPP
The meadow grass releases 14 000 kJ m⁻² yr⁻¹ of that energy in respiration; this is R
Step 2: Substitute numbers into the equation
NPP = GPP - R
NPP = 17 500 - 14 000
Step 3: Complete calculation
17 500 - 14 000 = 3 500
NPP = 3 500 kJ m⁻² yr⁻¹
The rate at which biomass is produced in an ecosystem is known as the ecosystem productivity:
This rate varies greatly between different ecosystems depending on climatic factors such as average temperature, rainfall and insolation, as well as the numbers of producers, consumers and decomposers they contain
Examiner Tips and Tricks
When answering questions on GPP or NPP, make sure you give the appropriate units. GPP and NPP can either be expressed in terms of biomass (per unit area per unit time) or chemical energy (per unit area per unit time). The biomass of an organism is effectively a measure of how much chemical energy is stored within it!
The worked example for calculating NPP uses the equation in its basic form, but you may also be expected to rearrange the equation e.g. to calculate GPP or R
If a question provides you with the NPP and R and asks you to calculate GPP, you will need to use the following equation:
GPP = NPP + R
If a question provides you with the NPP and the GPP and asks you to calculate R, you will need to use the following equation:
R = GPP - NPP
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