Given below are two statements, one labelled as Assertion (A) and the other labelled as Reason (R). Read the statements and choose the correct answer using the code given below.
Assertion (A) : In general a rise in temperature of water adversely affects the aquatic life.
Reason (R) : An increase in temperature causes hyperoxia among aquatic species.

Question

Given below are two statements, one labelled as Assertion (A) and the other labelled as Reason (R). Read the statements and choose the correct answer using the code given below.

Assertion (A) : In general a rise in temperature of water adversely affects the aquatic life.

Reason (R) : An increase in temperature causes hyperoxia among aquatic species.

Prepp Team · Accepted Answer

Analyzing the Effect of Water Temperature on Aquatic Life

This question asks us to evaluate an Assertion (A) and a Reason (R) regarding the impact of water temperature on aquatic life.

Evaluating Assertion (A)

Assertion (A): In general a rise in temperature of water adversely affects the aquatic life.

Let's analyze this statement. Water temperature is a crucial environmental factor for aquatic organisms. Most aquatic species have specific temperature ranges in which they can survive, grow, and reproduce. A significant rise in water temperature, often due to factors like climate change, industrial discharge (thermal pollution), or removal of riparian vegetation, can have several adverse effects:

Reduced Dissolved Oxygen: Warmer water holds less dissolved oxygen than colder water. Oxygen is essential for the respiration of fish, invertebrates, and other aquatic organisms. A decrease in dissolved oxygen can stress or even kill aquatic life, especially species with high oxygen demands.
Increased Metabolic Rates: Higher temperatures increase the metabolic rates of cold-blooded (ectothermic) aquatic animals. This means they need more oxygen and food at a time when oxygen availability might be lower. Increased metabolism can lead to stress and reduced growth or survival.
Increased Susceptibility to Disease and Toxins: Warmer temperatures can stress aquatic organisms, making them more vulnerable to diseases and parasites. The toxicity of some pollutants can also increase at higher temperatures.
Disruption of Life Cycles: Temperature cues are important for triggering events like spawning, migration, and hatching in many aquatic species. Altered temperature regimes can disrupt these critical life cycle stages.

Based on these points, the Assertion (A) is generally true. A rise in water temperature does adversely affect aquatic life.

Evaluating Reason (R)

Reason (R): An increase in temperature causes hyperoxia among aquatic species.

Now let's analyze this statement. Hyperoxia refers to a condition where there is an excess amount of oxygen. As discussed when evaluating Assertion (A), an increase in water temperature actually *decreases* the solubility of oxygen in water. This leads to lower levels of dissolved oxygen, a condition known as hypoxia (low oxygen) or anoxia (absence of oxygen), not hyperoxia.

Therefore, the Reason (R) is false. Increased water temperature leads to a *decrease* in dissolved oxygen, causing hypoxia, not hyperoxia.

Conclusion

We have determined that Assertion (A) is true, and Reason (R) is false.

Comparing this finding with the given options:

Option 1: Both (A) and (R) are correct... (Incorrect, R is false)
Option 2: Both (A) and (R) are correct... (Incorrect, R is false)
Option 3: (A) is true, but (R) is false. (Correct)
Option 4: (A) is false, but (R) is true. (Incorrect, A is true and R is false)

Thus, the correct answer is that Assertion (A) is true, but Reason (R) is false.

Statement	Evaluation	Explanation
Assertion (A): Rise in temperature of water adversely affects aquatic life.	True	Warmer water holds less dissolved oxygen, increases metabolic rates, and stresses organisms.
Reason (R): Increase in temperature causes hyperoxia among aquatic species.	False	Increased temperature decreases dissolved oxygen solubility, leading to hypoxia (low oxygen).

Revision Table: Temperature Effects on Aquatic Ecosystems

Increased Water Temperature Effect	Impact on Aquatic Life
Decreased Dissolved Oxygen Solubility	Stress, suffocation risk, especially for species with high oxygen demand (e.g., salmonids). Leads to hypoxia/anoxia.
Increased Metabolic Rate (for ectotherms)	Higher oxygen and food needs, increased stress, reduced energy for growth/reproduction.
Increased Susceptibility to Diseases/Toxins	Weakened immune systems, higher toxicity of certain pollutants.
Disruption of Reproduction Cycles	Incorrect timing of spawning, hatching, or migration.
Shift in Species Composition	Cold-water species may decline or disappear, replaced by more warm-tolerant species.

Additional Information: Dissolved Oxygen and Water Quality

Dissolved oxygen (DO) is a key indicator of water quality and ecosystem health. It is the amount of gaseous oxygen dissolved in water. Aquatic organisms like fish and invertebrates breathe this dissolved oxygen. The amount of oxygen that can dissolve in water depends primarily on temperature, pressure, and salinity.

Temperature: As temperature increases, the solubility of gases (including oxygen) in water decreases. This is a physical property of water.
Pressure: Higher atmospheric pressure allows more gas to dissolve.
Salinity: Water with higher salinity (more salt) holds less dissolved oxygen than fresh water.

Oxygen is added to water through atmospheric diffusion (oxygen from the air dissolving into the water surface) and photosynthesis by aquatic plants and algae. It is removed from water by respiration of aquatic organisms, decomposition of organic matter, and diffusion back into the atmosphere.

When water temperature rises, the rate at which oxygen dissolves from the air decreases, and the rate at which organisms consume oxygen increases (due to higher metabolism). This creates a double challenge for aquatic life, making sufficient dissolved oxygen levels difficult to maintain.