How is the Redfield ratio used to infer nutrient limitation (N vs P) in freshwater systems, and what typical thresholds indicate limitation?

How is the Redfield ratio used to infer nutrient limitation (N vs P) in freshwater systems, and what typical thresholds indicate limitation?

• A. Nitrates are always the limiting nutrient.
• B. If ambient N:P is below ~16:1, phosphorus is limiting.
• C. If ambient N:P is above ~16:1, nitrogen is limiting.
• D. The Redfield ratio indicates that below ~16:1 nitrogen limitation is typical; above ~16:1 phosphorus limitation is typical.

Answer: (D)

The concept here is using the ratio of dissolved nitrogen to phosphorus as a clue to which nutrient might be limiting phytoplankton growth. The Redfield ratio, about 16 parts nitrogen to 1 part phosphorus on a molar basis, represents a balance often needed for typical aquatic algae. When the ambient N:P in a freshwater body is lower than that ratio, nitrogen is the scarcer relative to phosphorus, so growth is typically limited by nitrogen. When the N:P is higher than that ratio, phosphorus becomes the limiting nutrient because there isn’t enough phosphorus to match the available nitrogen. So, roughly, if the water’s N:P is below ~16:1, nitrogen limitation is common; if it’s above ~16:1, phosphorus limitation is common. Keep in mind this is a general guide—the exact threshold can vary with the specific ecosystem, nutrient forms measured (nitrate, ammonium for N; phosphate for P), and other environmental factors.

The concept here is using the ratio of dissolved nitrogen to phosphorus as a clue to which nutrient might be limiting phytoplankton growth. The Redfield ratio, about 16 parts nitrogen to 1 part phosphorus on a molar basis, represents a balance often needed for typical aquatic algae. When the ambient N:P in a freshwater body is lower than that ratio, nitrogen is the scarcer relative to phosphorus, so growth is typically limited by nitrogen. When the N:P is higher than that ratio, phosphorus becomes the limiting nutrient because there isn’t enough phosphorus to match the available nitrogen.

So, roughly, if the water’s N:P is below ~16:1, nitrogen limitation is common; if it’s above ~16:1, phosphorus limitation is common. Keep in mind this is a general guide—the exact threshold can vary with the specific ecosystem, nutrient forms measured (nitrate, ammonium for N; phosphate for P), and other environmental factors.