Q1. Which of the following best defines ‘Plankton’?
- Organisms that swim actively in water
- Microscopic organisms that drift in surface waters
- Organisms that live only on the sea floor
- Organisms that live only in freshwater bodies
Correct Option: 2. Microscopic organisms that drift in surface waters
Explanation: Plankton are tiny, mostly microscopic organisms that float or drift in the upper layers of water bodies (oceans, lakes, rivers). They include:
• Phytoplankton (plant-like)
• Zooplankton (animal-like)
Plankton are not strong swimmers and rely on currents for movement. They are present in both freshwater and marine ecosystems, except in fast-flowing streams.
Q2. What limits the depth at which phytoplankton can live in the ocean?
- Water salinity
- Presence of predators
- Sunlight availability
- Ocean temperature
Correct Option: 3. Sunlight availability
Explanation: Phytoplankton are photosynthetic organisms and depend on sunlight to produce energy. Hence, they are restricted to the euphotic zone (the upper, light-penetrated layer of water). Below this zone, light is insufficient for photosynthesis, and they cannot survive.
Q3. Which of the following nutrients is NOT essential for most phytoplankton growth?
- Nitrogen
- Phosphorus
- Silicate
- Calcium
Correct Option: 4. Calcium
Explanation: Phytoplankton primarily require nitrogen, phosphorus, and in some cases, silicate (especially diatoms) for cell wall formation. Calcium is not a limiting or essential nutrient for their growth in aquatic environments.
Q4. In which regions is the concentration of marine phytoplankton the highest?
- Tropical open oceans
- Subtropical gyres
- High latitude regions and upwelling zones
- Deep ocean basins
Correct Option: 3. High latitude regions and upwelling zones
Explanation: Regions with cold, nutrient-rich waters like polar areas and upwelling zones support dense phytoplankton populations. These nutrients fuel photosynthesis and rapid growth. In contrast, tropical and subtropical areas often lack nutrients despite abundant sunlight.
Q5. What role do zooplankton play in the aquatic food web?
- Sediment stabilizers
- Primary producers
- Decomposers
- Primary consumers and nutrient recyclers
Correct Option: 4. Primary consumers and nutrient recyclers
Explanation: Zooplankton consume phytoplankton, making them primary consumers. They also recycle nutrients by excreting waste and decomposing after death, which replenishes the nutrient pool for phytoplankton growth.