Question

In most freshwater lakes, the algal productivity is limited by the availability of which of the following inorganic ions ?

• A. Carbon
• B. Nitrogen
• C. Phosphorus
• D. Sulphur

Hint:

Remember: Phosphorus is the limiting nutrient in freshwater lakes, whereas Nitrogen is typically the limiting nutrient in marine (oceanic) ecosystems.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Primary productivity in aquatic ecosystems depends on the availability of solar energy and essential nutrients required for plant and algal growth.
According to Liebig's Law of the Minimum, growth is limited by the essential nutrient that is least abundant relative to the physiological requirements of the organisms.

Step 2: Detailed Explanation:
In the vast majority of freshwater ecosystems (lakes, reservoirs, and rivers), phosphorus (specifically in the form of soluble inorganic orthophosphate, \(\text{PO}_4^{3-}\)) is the primary limiting nutrient for algal growth.
The ratio of Nitrogen to Phosphorus (N:P ratio) in freshwater is typically higher than the Redfield ratio required by algae.
Consequently, even small additions of dissolved phosphorus (such as from agricultural runoff, domestic sewage, or detergents) can trigger massive, explosive growth of green algae and cyanobacteria, a process known as eutrophication.
Let us compare this with marine ecosystems:
- In marine (oceanic) ecosystems, nitrogen (as nitrate, \(\text{NO}_3^-\)) is generally the primary limiting nutrient.
- Carbon, sulphur, and other macronutrients are highly abundant in freshwater systems relative to algal requirements and rarely act as limiting factors.

Step 4: Final Answer:
Therefore, algal productivity in most freshwater lakes is limited by the availability of Phosphorus.