Why can natural phosphorus resupply limit growth?

• Most rock-bound phosphorus is already dissolved and ready for uptake
• A large atmospheric phosphorus gas reservoir is unavailable for direct uptake
• Rock-bound phosphate can enter usable supplies slowly

What does limiting nutrient mean?

• The nutrient present in the smallest total amount, regardless of demand
• The nutrient whose usable supply is lowest relative to demand
• The nutrient with the largest total supply, regardless of organism needs

When a nutrient limits growth, extra light alone cannot replace that missing nutrient.

One biomass unit needs 10 moles of nitrogen and 2 moles of phosphorus. A model lake initially has 20 moles of nitrogen and 8 moles of phosphorus. Then 30 moles of nitrogen are added. All supplies are usable and other resources are adequate. What happens to the biomass limit?

• It rises from 2 to 5 units, because the new nitrogen supply sets the limit.
• It rises from 2 to 4 units, with phosphorus becoming limiting.
• It stays at 2 units, because nitrogen was the initial limiting nutrient.

In a model, one biomass unit needs 12 moles of nitrogen and 1 mole of phosphorus. There are 6 moles of nitrogen and 2 moles of phosphorus available. All other resources are adequate. Which nutrient limits growth?

• Nitrogen, because its supply supports only 0.5 biomass units
• Phosphorus, because its smaller raw amount must set the growth limit
• Neither, because the presence of both nutrients prevents any shortage

In a model, decomposers need 8 moles of nitrogen per mole of phosphorus. They process material with 24 moles of nitrogen and 1 mole of phosphorus. Carbon and energy are adequate. If all phosphorus enters new microbial tissue, what remains available for mineral release?

• 8 moles of nitrogen, the amount matched to the retained phosphorus
• 24 moles of nitrogen, the amount originally supplied by dead material
• 16 moles of nitrogen, after 8 moles enter microbial tissue

In nitrogen-limited coastal water, nitrogen from fertilizer runoff can increase algal growth. Microbial decay of the extra algae can then lower dissolved oxygen.

In a model lake, phosphorus alone limits algal growth. A student adds only nitrogen and predicts increased production. Why does the model not support this prediction?

• Phosphorus remains scarce relative to demand after nitrogen is added
• Nitrogen could raise growth whenever its raw supply is smaller, despite phosphorus limitation
• Adding either nutrient should raise production because the total nutrient supply increases