What is the critical limiting factor for plants below the photic zone in an aquatic ecosystem?

1. Analyze the question The question asks which factor limits plant growth below the photic zone in water. The key word is photic zone, so the answer should relate to what that zone is.

2. Explain the concept

• The photic (euphotic) zone is the upper layer of a lake or ocean where sunlight penetrates enough for photosynthesis.
• Below it is the aphotic zone, where little or no light reaches.
• Light is absorbed and scattered as it moves down through water, so its intensity falls with depth.

3. Step-by-step solution

• A. Nutrients: Nutrients can limit growth in the photic zone, and they are often plentiful in deeper water because dead organisms sink and decay there. This is not the critical factor.
• B. Water: Aquatic plants are surrounded by water, so it is never limiting.
• C. Light intensity: Below the photic zone, light is too weak for photosynthesis. Plants cannot make food however much of the other factors is available, so light is the critical limiting factor.
• D. Carbon dioxide: CO₂ dissolves in water and is generally available, and it is often more concentrated at depth because of respiration and decay.

4. Answer The correct option is C. intensity of light.

5. Exam tip Whenever a question mentions the photic zone, think "photo" meaning light. Photosynthesis needs light, so plant distribution in water is controlled mainly by how deep light can reach.