Q1. Which of the following aquatic ecosystems has the highest salt concentration?
- Freshwater
- Marine
- Brackish water
- Estuarine
Correct Option: 2. Marine
Explanation: Marine ecosystems have a salt concentration of 35 ppt or more, which is higher than freshwater (<5 ppt) and brackish water (5–35 ppt).
Q2. Which group of aquatic organisms are free-floating and have very limited locomotory power?
- Nekton
- Plankton
- Benthos
- Periphyton
Correct Option: 2. Plankton
Explanation: Plankton includes microscopic floating organisms (algae and small animals) with limited ability to move, and their distribution largely depends on water currents.
Q3. What is the term for organisms that live at the air-water interface, like floating plants and insects?
- Benthos
- Nekton
- Neuston
- Periphyton
Correct Option: 3. Neuston
Explanation: Neuston refers to organisms living at the surface of the water (air-water interface), including floating plants and small insects like water striders.
Q4. In which zone of an aquatic ecosystem does photosynthesis primarily occur?
- Benthic zone
- Aphotic zone
- Profundal zone
- Photic zone
Correct Option: 4. Photic zone
Explanation: The photic zone is the lighted upper layer of water where sunlight penetrates, allowing photosynthesis by aquatic plants and phytoplankton.
Q5. What is the phenomenon called when fish die during winter due to oxygen depletion under ice-covered lakes?
- Summer kill
- Algal bloom
- Oxygen shock
- Winter kill
Correct Option: 4. Winter kill
Explanation: Winter kill occurs when snow and ice block sunlight, stopping photosynthesis. Meanwhile, respiration continues, depleting oxygen and killing fish.