Questions: Ecological Stoichiometry and Element Ratios

Question 1

A lake has a dissolved N:P ratio of 25:1. Farmers apply nitrogen fertilizer to the surrounding watershed, raising the N:P ratio to 35:1. What effect on phytoplankton productivity do you expect?

• A. Productivity increases proportionally because more nitrogen is always beneficial for phytoplankton growth
• B. Productivity decreases because excess nitrogen is toxic to phytoplankton at high concentrations
• C. Little to no productivity increase, because phosphorus is already the limiting nutrient and adding the non-limiting element does nothing
• D. Productivity increases because a higher N:P ratio shifts the community toward faster-growing species

Answer: (C)

The Redfield ratio is approximately 16:1 (N:P). At 25:1, nitrogen is already in relative excess compared to what phytoplankton need — phosphorus is the limiting nutrient. Adding more nitrogen widens the excess of the already-abundant element; it cannot be used because phosphorus constrains how much biomass can be built. This is the core stoichiometric principle: growth is limited by the element in shortest supply relative to organismal demand, and adding more of a non-limiting element has no effect.

Question 2

Daphnia (water fleas) are fed abundant phosphorus-poor algae, yet their growth rate drops sharply. What is the correct stoichiometric explanation?

• A. The algae contain a toxin that inhibits Daphnia metabolism at high feeding rates
• B. Daphnia are phosphorus-rich organisms and cannot build sufficient ribosomal RNA and biomass when their food lacks phosphorus, regardless of food quantity
• C. Overeating carbon-rich food causes energy toxicity that suppresses growth pathways
• D. Low phosphorus algae are less digestible, reducing caloric extraction per unit consumed

Answer: (B)

Daphnia are rapidly growing organisms that require large amounts of ribosomal RNA — which is phosphorus-intensive — to support their growth. When their food has a high C:P ratio (phosphorus-poor), they cannot obtain enough phosphorus to meet their biochemical needs even if they eat constantly. Food quantity is irrelevant; the elemental ratio is the constraint. This is the stoichiometric mismatch concept: the consumer's elemental composition determines what is limiting, independent of how much food is available.

Question 3

Adding fertilizer to an ecosystem can fail to increase productivity if the added element is not the one currently limiting growth.

• T. True
• F. False

Answer: True

This is the central practical prediction of ecological stoichiometry. Growth is limited by whichever element is in shortest supply relative to organismal demand. If an ecosystem is phosphorus-limited, adding nitrogen fertilizer accomplishes nothing because organisms cannot use additional nitrogen without sufficient phosphorus to pair it with in proteins and nucleic acids. This explains why nutrient addition experiments sometimes produce no response — and why correct diagnosis of the limiting nutrient is essential before any fertilization strategy.

Question 4

If the absolute amount of phosphorus in a lake doubles, primary productivity will typically double proportionally.

• T. True
• F. False

Answer: False

Productivity depends on the elemental ratio relative to organismal demand, not on absolute quantities. If nitrogen is simultaneously scarce, doubling phosphorus without adding nitrogen shifts which element is limiting but does not proportionally increase productivity. Furthermore, if phosphorus was already in excess relative to the Redfield ratio, doubling it again has no productive effect. Stoichiometric thinking requires ratio analysis, not just counting individual elements.

Question 5

Why does the Redfield ratio (106C:16N:1P) serve as a benchmark for predicting nutrient limitation in marine ecosystems, and what does deviation from it reveal?

Model answer: The Redfield ratio reflects the average biochemical composition of marine phytoplankton — the proportions of carbon, nitrogen, and phosphorus they need to build their biomass. Because organisms consume nutrients in these proportions, the dissolved N:P ratio of seawater can be compared to 16:1 to predict which element is relatively scarce. When the dissolved N:P ratio is lower than 16:1, there is relatively less nitrogen than phytoplankton need, so nitrogen limits growth; when higher than 16:1, phosphorus is relatively scarce and limits growth. Deviations from the Redfield ratio in seawater thus directly predict the identity of the limiting nutrient.

The Redfield ratio bridges biochemistry and oceanography: it translates organismal elemental needs into a testable prediction about ecosystem-level nutrient dynamics. It was one of the first demonstrations that elemental ratios — not absolute concentrations — are the relevant quantity for understanding biological constraint.