Unit 1 · Topic 1.8 Beta

Primary Productivity

Primary productivity is the rate at which producers capture energy by photosynthesis.

Practice 1: Concept ExplanationPractice 6: Mathematical Routines

Part 1 · Hook

Why this matters

A cornfield and a rainforest both turn sunlight into plant matter, but at very different rates, and only part of what they capture is ever available to animals. Plants spend much of their energy just staying alive. What is left over, net primary productivity, is the food supply for every animal in the ecosystem.

Part 2 · Before you start

What this builds on

Part 3 · Prerequisite check

Quick check before you start

1. Which process releases CO2 as living things break down sugar for energy?

  1. Cellular respiration
  2. Photosynthesis
  3. Sedimentation
  4. Nitrogen fixation
Show the answer

Respiration breaks down sugar and releases CO2.

  • Correct: Cellular respiration:
  • Photosynthesis:
  • Sedimentation:
  • Nitrogen fixation:

2. A rate in "kcal per m² per year" means...

  1. energy divided by area and by time
  2. energy multiplied by area
  3. area divided by energy
  4. time divided by energy
Show the answer

Each "per" divides.

  • Correct: energy divided by area and by time:
  • energy multiplied by area:
  • area divided by energy:
  • time divided by energy:

3. What is a limiting nutrient?

  1. The nutrient in shortest supply compared with need
  2. The nutrient used the most
  3. A poison
  4. A gas
Show the answer

Adding it raises growth.

  • Correct: The nutrient in shortest supply compared with need:
  • The nutrient used the most:
  • A poison:
  • A gas:

Part 4 · See it

See it first

Invented data for a grassland in kilocalories per square meter per year. Gross primary productivity, GPP, is 12,000. Of that, 7,000 is used by the plants in respiration to stay alive. The rest, net primary productivity, NPP, is 12,000 minus 7,000 = 5,000, stored as new plant matter and available to the animals that eat plants.
Gross primary productivity split into respiration and net primary productivity. LevlPrep original diagram.

Part 5 · Step by step

How it works, step by step

  1. Producers capture light energy by photosynthesisthat total rate is gross primary productivity (GPP)
  2. Producers use part of it for their own cellular respirationthat energy is lost as heat
  3. What is left is net primary productivity: NPP = GPP - RNPP is the energy available to the animals that eat plants

Part 6 · Key ideas

Key ideas

  • Producers (autotrophs) make their own food by photosynthesis.
  • Primary productivity is the rate producers capture energy, in units such as kcal/m²/yr or g/m²/yr.
  • NPP=GPP−R\text{NPP} = \text{GPP} - \text{R}, so GPP=NPP+R\text{GPP} = \text{NPP} + \text{R}.
  • Productivity is highest where light, water, warmth and nutrients are all plentiful (rainforests, wetlands, estuaries, reefs) and lowest in deserts, tundra and the open ocean.

Part 7 · Misconception

A common mistake

The wrong idea: NPP is the energy plants capture, and GPP is what animals eat.

What actually happens: GPP is the total captured (gross, like a paycheck before costs). NPP is what is left after the plants’ own respiration (net, like take-home pay); animals can eat only from NPP.

Part 8 · Check yourself

Check yourself

Exam-style questions. Anything you miss goes into your review queue.

1. A salt marsh has a GPP of 9,000 kcal/m²/yr, and its plants use 5,500 kcal/m²/yr in respiration. What is its NPP?

Type a number, then its unit (kcal/m^2/yr). Scale words (2.5 million) and powers of ten (2.5 × 10^6) are read.

Show the answer

NPP\text{NPP}=GPP−R= \text{GPP} - \text{R}=9,000−5,500= 9{,}000 - 5{,}500=3,500= 3{,}500 kcal/m²/yr.

  • Answer: 3500 kcal/m^2/yr

2. A forest’s NPP is 2,000 kcal/m²/yr, and its plants use 4,000 kcal/m²/yr in respiration. Write the setup for its GPP, then give the answer.

Type the setup as on the exam, with brackets and units, then the answer.

Show the answer

GPP\text{GPP}=NPP= \text{NPP}+R+ \text{R}=2,000= 2{,}000+4,000+ 4{,}000=6,000= 6{,}000 kcal/m²/yr.

  • Setup: 2000+40002000 + 4000; answer 6000 kcal/m^2/yr

Data

Gross productivity and respiration in five ecosystems

Gross primary productivity (GPP) and plant respiration (R) in five ecosystems, in kcal per square meter per year (invented, rounded data for practice; real values vary widely)
EcosystemGPP (kcal/m²/yr)Respiration R (kcal/m²/yr)
Tropical rainforest7,6004,400
Temperate deciduous forest5,0003,000
Temperate grassland2,4001,200
Desert scrub400250
Open ocean1,000600

3. What is the NPP of the temperate grassland?

Type a number, then its unit (kcal/m^2/yr). Scale words (2.5 million) and powers of ten (2.5 × 10^6) are read.

Show the answer

NPP\text{NPP}=2,400−1,200= 2{,}400 - 1{,}200=1,200= 1{,}200 kcal/m²/yr.

  • Answer: 1200 kcal/m^2/yr

4. Which ecosystem has the highest NPP?

  1. Tropical rainforest
  2. Temperate deciduous forest
  3. Temperate grassland
  4. Open ocean
Show the answer

NPP: rainforest 3,200; deciduous forest 2,000; grassland 1,200; desert scrub 150; open ocean 400 kcal/m²/yr.

  • Correct: Tropical rainforest: Right: 7,600 - 4,400 = 3,200.
  • Temperate deciduous forest: 5,000 - 3,000 = 2,000.
  • Temperate grassland: 2,400 - 1,200 = 1,200.
  • Open ocean: 1,000 - 600 = 400.

5. What percent of its GPP does the tropical rainforest use in respiration?

Type a number, then its unit (%). Scale words (2.5 million) and powers of ten (2.5 × 10^6) are read.

Show the answer

4,4007,600×100\frac{4{,}400}{7{,}600} \times 100=57.9%= 57.9\%.

  • Answer: 57.89 %

6. Which ecosystem has the largest share of its GPP left as NPP?

  1. Temperate grassland
  2. Tropical rainforest
  3. Desert scrub
  4. Open ocean
Show the answer

NPP as a share of GPP: grassland 1,200/2,400=50%1{,}200/2{,}400 = 50\%; rainforest 42%; deciduous forest 40%; desert scrub 37.5%; open ocean 40%.

  • Correct: Temperate grassland: Right: half of its GPP is left.
  • Tropical rainforest: Highest NPP, but only about 42% of its GPP is left.
  • Desert scrub: About 37.5% of its GPP is left.
  • Open ocean: 40% of its GPP is left.

Model or diagram

Light and oxygen bottles in a lake

024681012BottleDissolved oxygen (mg/L)StartClear bottle, 24 hDark bottle, 24 h

7. Oxygen fell from 8.0 to 7.4 mg/L in the dark bottle in 24 hours. What was the rate of respiration, in mg/L of oxygen per day?

Type a number, then its unit (mg/L/day). Scale words (2.5 million) and powers of ten (2.5 × 10^6) are read.

Show the answer

In the dark there is no photosynthesis, so the drop is respiration alone: 8.0−7.4=0.68.0 - 7.4 = 0.6 mg/L per day.

  • Answer: 0.6 mg/L/day

Part 9 · Summary

Summary

Primary productivity is the rate at which producers capture energy by photosynthesis. Gross primary productivity (GPP) is the total; producers use part of it in respiration (R); the rest, net primary productivity, NPP = GPP - R, becomes plant matter that animals can eat. Productivity is highest where light, water, warmth and nutrients are all plentiful.

Part 10 · Up next

What comes next

Part 11 · Connections

Connections

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