Correct Option
The correct option is (b) 2 only.
Explanation
Aquatic ecosystems rely on Dissolved Oxygen (DO) for the survival of most organisms. The availability of oxygen in water is determined by physical factors such as temperature and the biological mechanisms of the animals inhabiting the ecosystem.
Statement-wise Analysis
- Statement 1 is Incorrect. While certain aquatic animals like marine mammals (whales, dolphins) and reptiles (turtles) possess lungs and must surface to breathe atmospheric air, the vast majority of aquatic animals (such as fish, crustaceans, and mollusks) possess gills. Gills allow these animals to extract oxygen dissolved directly in the water, eliminating the need to surface for respiration. Therefore, the statement is an incorrect generalization.
- Statement 2 is Correct. Water contains air (specifically oxygen) dissolved in it. Aquatic animals with gills utilize this dissolved oxygen for respiration. The concentration of dissolved oxygen is a critical factor in determining the health and diversity of an aquatic ecosystem.
- Statement 3 is Incorrect. The solubility of gases in liquids is inversely proportional to temperature. As the temperature of water increases, the kinetic energy of the gas molecules increases, causing them to escape from the solution. Consequently, heating water decreases the amount of dissolved air (oxygen) in it. This phenomenon is a key reason why thermal pollution (release of hot water) is detrimental to aquatic life.
Key Takeaway
Dissolved Oxygen (DO) levels in water decrease as temperature increases. Most aquatic animals respire using oxygen dissolved in water via gills, while only specific groups (like marine mammals) breathe atmospheric air.