Productivity, Nutrient Limitation and Ecosystem Stoichiometry · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

What truly limits growth

Science · Ecology & Conservation · ages 21-22
Name ______________________   Date ____________

  1. Why is phosphorus often the limiter in ecosystems?

    • It is poisonous to all plants
    • It evaporates within hours
    • It cycles slowly with no atmospheric shortcut

  2. What does limiting nutrient mean?

    • The nutrient with the longest chemical name
    • The nutrient in shortest supply relative to what organisms need
    • The nutrient found only in rocks

  3. Ecosystem production is usually set by the scarcest nutrient relative to need, not by energy alone.

    Circle one:   True   False

  4. You add plenty of that limiting nutrient. What happens next?

    • Growth rises forever with no new limit
    • Growth rises until the next scarcest nutrient limits
    • Growth falls because nutrients poison algae

  5. Algae need nitrogen and phosphorus at 16 to 1, but water offers 4 to 1. Which limits?

    • Phosphorus, since supply falls shortest of demand
    • Nitrogen, since 16 exceeds 4
    • Neither, since both are present

  6. Dead leaves hold a high nitrogen to phosphorus ratio. What do decomposers release first?

    • Nitrogen, since there is more of it
    • Carbon dioxide only
    • Phosphorus, since microbes keep the nitrogen they need

  7. Fertilizer runoff that doubles nitrogen delivery to coastal waters can trigger algal blooms and dead zones.

    Circle one:   True   False

  8. A student adds nitrogen to a phosphorus-limited lake and predicts a bloom. What is wrong?

    • The wrong ceiling was lifted, so phosphorus still caps growth
    • Nitrogen can never enter a lake
    • Lakes cannot host algae at all
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Answer key

For grown-ups. Fold this page away before handing over the rest.

What truly limits growth W1-mt_QWdZj1G9iT-s1

  1. It cycles slowly with no atmospheric shortcut · With rock as its main reservoir, natural resupply lags behind use.
  2. The nutrient in shortest supply relative to what organisms need · Growth stops at whatever runs out first against demand.
  3. True · Even with ample light, growth halts at the missing element.
  4. Growth rises until the next scarcest nutrient limits · Removing one ceiling exposes the next, so the limiter shifts rather than vanishes.
  5. Phosphorus, since supply falls shortest of demand · Divide supply by demand: phosphorus covers one quarter of need, nitrogen covers all of it.
  6. Phosphorus, since microbes keep the nitrogen they need · Microbes keep what they need for themselves and dump the surplus.
  7. True · Where nitrogen limits, a sudden surplus unlocks explosive growth that then rots.
  8. The wrong ceiling was lifted, so phosphorus still caps growth · Only the truly limiting nutrient unlocks growth; extra of the rest changes nothing.
Worksheet · LightMySky