The question asks to identify an example of a brackish water ecosystem. To answer this, we need to understand what brackish water is and which of the given options fits that definition.
C) Estuaries — Estuaries are defined as areas where freshwater from rivers and streams mixes with saltwater from the ocean, resulting in brackish water. This unique environment supports a diverse range of specialized flora and fauna.
The question asks why water at the bottom of a frozen lake remains liquid. This phenomenon is due to the anomalous expansion of water, specifically its density behavior with temperature.
Correct Option: C) The density of water is maximum at 4°C. This property, known as the anomalous expansion of water, is the primary reason why water at the bottom of a frozen lake remains liquid. Water at 4°C is the densest and sinks, while colder, less dense water (0-4°C) stays on top and freezes, insulating the warmer water below.
The question asks about the typical location of neuston organisms. To answer this, one needs to recall the definition and characteristics of neuston in aquatic ecosystems.
Correct Option: C) Floating or just below the air-water interface
The question asks to identify the causative factors for harmful algal blooms in seawaters. Algal blooms are primarily caused by an excess of nutrients, particularly nitrogen and phosphorus, in the water. We need to evaluate each given factor to see if it contributes to nutrient enrichment or creates conditions favorable for algal growth.
D) 1, 2, and 3 — All three factors (discharge of nutrients from estuaries, run-off from land during monsoon, and upwelling in the seas) contribute to nutrient enrichment in coastal waters, which is the primary cause of harmful algal blooms.
The question asks to identify the zone in an aquatic ecosystem where photosynthesis does not occur. This requires understanding the definitions of different aquatic zones and the conditions necessary for photosynthesis.
B) Aphotic zone — This zone is defined by the lack of sunlight penetration. Since sunlight is essential for photosynthesis, no photosynthetic activity can take place in the aphotic zone. Organisms here rely on organic matter that drifts down from the photic zone or chemosynthesis.
UPSC - 2021
The question asks to identify which of the given options is used in preparing a natural mosquito repellent. This requires knowledge of common plants and their uses, specifically in pest control or as repellents.
Correct Option: C) Lemongrass is well-known for its insect-repelling properties, particularly against mosquitoes, due to its essential oils like citronella. It is widely used in natural mosquito repellents, including candles, sprays, and lotions.