Primary & Secondary Productivity (HL) (HL IB Environmental Systems & Societies (ESS))
Revision Note
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
Measuring primary productivity
Common units for measuring productivity are kg carbon m-2 year-1(kilograms of carbon per square metre of ecosystem per year)
Protocols for estimating primary productivity can vary based on the ecosystem being studied
Laboratory techniques
In laboratory settings, scientists can estimate primary productivity by measuring the rate of photosynthesis in samples, such as plants or phytoplankton
Controlled experiments can provide data on how different conditions (like light intensity or CO2 levels) affect productivity
Field techniques
In field studies, changes in biomass can be measured over time to estimate productivity
For example, in grassland ecosystems, researchers may sample vegetation at different times to see how much biomass has increased
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)
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
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
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
Exam Tip
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 the quantity of carbon compounds it contains, which in turn provide a good estimate 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
Secondary Productivity
Gross secondary productivity (GSP) is the total energy/biomass assimilated by consumers and is calculated by subtracting the mass of faecal loss from the mass of food eaten
Gross secondary productivity can be calculated using the equation:
GSP = food eaten - faecal loss
Net secondary productivity (NSP) is calculated by subtracting respiratory losses (R) from GSP
Net secondary productivity can be calculated using the equation:
NSP = GSP - R
As with gross primary productivity and net primary productivity, GSP and NSP are expressed in units of biomass or energy per unit area or volume per unit time e.g.
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)
Volume would be used when calculating GSP or NSP in aquatic habitats
Worked Example
In a patch of woodland, caterpillars ingest 2 000 kJ m-2 yr-1 of chemical energy from the biomass of oak leaves. The caterpillars lose 1 200 kJ m-2 yr-1 of this energy in faeces. They lose a further 600 kJ m-2 yr-1 of this energy through respiration. Calculate the net secondary productivity of the caterpillars.
Answer
Step 1: Calculate GSP
GSP = food eaten - faecal loss
GSP = 2 000 - 1 200
GSP = 800 kJ m-2 yr-1
Step 2: Calculate NSP
NSP = GSP - R
NSP = 800 - 600
NSP = 200 kJ m-2 yr-1
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