Primary productivity at Higher Level: An HL-only statement. It takes 2.2.13's gross and net productivity to the producers: how fast they build biomass, what controls the rate, and how ecologists measure it in a lake and in a meadow.
Practise this as you read
- Calculate NPP = GPP - R, formula first, with units.
- Evaluate a method for measuring primary productivity.
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How fast producers grow: Primary productivity measures how fast an ecosystem's producers turn energy and simple chemicals into new living material.
The points to remember
- Primary productivity is the rate at which producers make new biomass.
- They use an external energy source: light, or chemical energy (2.2.22).
- They build it from inorganic carbon (carbon dioxide) and other elements such as nitrogen and phosphorus.
- Units: kg of carbon per m² per year (kg C m⁻² yr⁻¹); also g dry mass or kJ per m² per year.
- It is a rate, per area and per time; biomass is a store at one moment.
Remember it as: Productivity is a speed, not a size.
Real example: giant kelp off California can grow about half a metre in a day, one of the highest rates of primary productivity on Earth.
A rate needs a time: '2 kg per m²' is a biomass. '2 kg per m² per year' is a productivity. Always give both the area and the time.
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2.2.13 showed that any organism has a gross and a net productivity. For producers, HL uses these names and this formula.
GPP, R and NPP
- GPP (gross primary productivity): the total rate producers make biomass.
- Producers use part of it at once in their own respiration (R), lost as heat.
- NPP (net primary productivity): what is left as new plant biomass.
- Write the formula first: NPP = GPP - R.
- Keep the units the same for all three, and give them in the answer.
The formula: NPP = GPP - R
Warm, busy producers respire a lot, so a high GPP does not always mean a high NPP.
Worked example: Field studies of Amazon rainforest give, in round numbers, GPP = 3.3 kg C m⁻² yr⁻¹ and R = 2.2 kg C m⁻² yr⁻¹.
NPP = GPP - R = 3.3 - 2.2 = 1.1 kg C m⁻² yr⁻¹.
So the trees use two thirds of what they make just to stay alive.
The same sum works in dry mass: a rainforest plot with GPP = 9,400 g dry mass m⁻² yr⁻¹ and R = 6,100 has NPP = 9,400 - 6,100 = 3,300 g dry mass m⁻² yr⁻¹, the biomass left for herbivores and decomposers.
Respiration is a loss: NPP = GPP - R, never GPP + R. And always write the units: mass or energy, per m², per year.
Photosynthesis needs light, warmth, water, carbon dioxide and nutrients. Whichever is in shortest supply sets the rate, so these abiotic factors control productivity.
Abiotic factors that change NPP
- Light: sunlight, day length, cloud; in water, depth and turbidity (cloudiness).
- Temperature: photosynthesis is fastest when it is warm, slow in the cold.
- Water on land, and nutrients (nitrate, phosphate) everywhere.
- In the sea: currents and upwellings bring nutrients up; salinity, pH and CO₂ matter too.
- Pressure / buoyancy: plankton must stay afloat in the sunlit layer.
Real example: off Peru, cold upwellings feed so much phytoplankton that the anchoveta fishery there is one of the largest in the world. Estuaries are productive too: rivers bring nutrients and the water is shallow enough for light.
State a factor, then link it: In a 'state' question the factor alone is enough: 'temperature', 'nutrient availability'. In an 'explain' question, link it to the rate of photosynthesis.
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In the laboratory, a photosynthesising sample (such as pondweed) can be kept in water and the oxygen it gives off measured. In a lake, the same idea is used with bottles of lake water.
The light and dark bottle method
- Fill three bottles with lake water; measure dissolved oxygen in one at the start.
- Hang a clear (light) and a foil-covered (dark) bottle in the lake for a set time, e.g. 24 hours.
- Light bottle: photosynthesis minus respiration, so its oxygen gain = NPP.
- Dark bottle: respiration only, so its oxygen loss = R.
- GPP = NPP + R: the light bottle's gain plus the dark bottle's loss.
Worked example: NPP = light bottle end - start = 9.5 - 8.0 = 1.5 mg O₂ per litre per day.
R = start - dark bottle end = 8.0 - 6.8 = 1.2 mg O₂ per litre per day.
GPP = NPP + R = 1.5 + 1.2 = 2.7 mg O₂ per litre per day.
Example: students on a field course at Windermere, England's largest lake, can hang the bottles at 1 m depth from a jetty and compare them with bottles hung deeper down.
On land, the guide's method is to measure the change in biomass of the plants over time, for example in a grassland.
The harvest method
- Mark out three plots; cover one with opaque plastic so it cannot photosynthesise.
- Cut, dry (2.2.16) and weigh a sample at the start to get the dry biomass per m².
- Repeat at the end of a set time, e.g. four weeks.
- Open plot: its gain in dry biomass = NPP. Covered plot: its loss = R.
- GPP = NPP + R, per m² per unit time.
Start
Cut, dry and weigh samples from the plots.
Wait
Leave the plots for a set time, one covered.
End
Cut, dry and weigh again.
Calculate
Gain = NPP; loss under cover = R; GPP = NPP + R.
Real example: in the Park Grass Experiment at Rothamsted, England, the hay from each plot has been cut and weighed every year since 1856, the longest record of grassland productivity in the world.
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An 'evaluate' question wants the strengths and the limits of the method itself.
Strengths and limits
- Bottles, strengths: simple and cheap; ethical, as the water can be returned.
- Bottles, limits: zooplankton in the water also respire; only the submerged part is measured.
- Plots, strengths: easy to isolate the producers; works in any grassland.
- Plots, limits: plants are killed; roots are hard to collect; impossible for trees.
- Both: results depend on temperature and on how precise the instruments are.
Remember it as: Bottles: easy and kind, but mixed-up water. Plots: clear producers, but killing and no trees.
Example: in a tropical rainforest the harvest method fails, because the trees are too big to cut and weigh; scientists there measure tree growth and carbon dioxide exchange instead.
What does not count: 'The samples were too small' or 'the scales were inaccurate' are mistakes in doing it, not limits of the method. Name a real ecosystem, and write about productivity, not biodiversity.
How this comes up: Paper 2, Section A: state abiotic factors that influence NPP [2]. Section B (b): evaluate one method for measuring primary productivity in a named ecosystem [7].
Windermere is a large lake in the Lake District, England. Ecologists want to know how productive its phytoplankton are.
Evaluate one method for measuring primary productivity in a named ecosystem.
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