Comprehension

Read the following passage and answer the next five questions:
Agricultural runoff, sewage, and industrial effluents are major causes of the deterioration of water quality in lakes and ponds. Productivity of lakes and ponds (for example, algal bloom and phytoplankton bloom) increases manifold due to agricultural runoff as well as due to excess inputs of nutrients including limiting nutrients, but this continuous increased productivity leads to the death of lakes and ponds. The dissolved oxygen BOD, temperature, turbidity, and salinity are important parameters of water quality in a lake and change naturally on a day and night basis as well as a seasonal basis. However, anthropogenic inputs change these parameters abruptly and adversely impact the water quality of the lakes. As a result, water in lakes and ponds does not remain fit for human consumption as well as for livestock. Immediate attention and affirmative actions are needed to protect lakes and ponds in our environment.

Question: 1

Continuous increase of productivity of lakes eventually causes:

Updated On: Mar 28, 2025
  • Biomagnification in lakes
  • Bioaccumulation in lakes
  • Eutrophication in lakes
  • Bioconcentration in lakes
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The Correct Option is C

Solution and Explanation

  • The passage states that increased productivity (e.g., algal blooms) due to agricultural runoff and excess nutrient inputs leads to the death of lakes.
  • This process is called eutrophication, where excessive nutrients cause dense plant/algal growth, leading to oxygen depletion and ecosystem collapse.
  • Biomagnification (1) refers to the increase in toxin concentration up the food chain, while bioaccumulation (2) and bioconcentration (4) refer to toxin buildup in organisms, but these are not directly related to nutrient-driven productivity increases.
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Question: 2

Dissolved oxygen in water______:

Updated On: Mar 28, 2025
  • Remains constant in winter and summer
  • Remains low in winter and high in summer
  • Remains high in winter and low in summer
  • Doesn’t depend on temperature of water
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The Correct Option is C

Solution and Explanation

  • The passage mentions that dissolved oxygen (DO) changes naturally on a seasonal basis.
  • Cold water (winter) can hold more dissolved oxygen than warm water (summer) due to lower metabolic rates and increased gas solubility.
  • Thus, DO is higher in winter and lower in summer.
  • Option (4) is incorrect because DO does depend on temperature.
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Question: 3

Excess suspended particles in lake water due to agricultural runoff result in:

Updated On: Mar 28, 2025
  • High salinity
  • High turbidity
  • High dissolved oxygen
  • High BOD
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The Correct Option is B

Solution and Explanation

  • Turbidity refers to water cloudiness caused by suspended particles (e.g., soil, silt from runoff).
  • Agricultural runoff introduces sediments and nutrients, directly increasing turbidity.
  • Salinity (1) refers to salt content, which is unrelated to suspended particles. Dissolved oxygen (3) may decrease due to turbidity blocking sunlight for photosynthesis. BOD (4) increases due to organic matter decomposition, but this is an indirect effect.
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Question: 4

Water in lakes having very high algal and phytoplankton growth is expected to have:

Updated On: Mar 28, 2025
  • High dissolved oxygen
  • High dissolved oxygen and high BOD
  • Low dissolved oxygen and high BOD
  • Low dissolved oxygen and low BOD
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The Correct Option is C

Solution and Explanation

  • During algal blooms, photosynthesis by algae may temporarily increase dissolved oxygen (DO) during the day, but at night, respiration consumes oxygen.
  • When algae die, their decomposition by bacteria further depletes oxygen, leading to low DO and high BOD (Biochemical Oxygen Demand, a measure of organic matter decomposition).
  • Options (1) and (2) are incorrect because sustained high DO is unlikely. Option (4) is incorrect because BOD increases with organic matter (dead algae).
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Question: 5

Which of the following is a limiting nutrient in lakes and ponds?

Updated On: Mar 28, 2025
  • Iron
  • Phosphorus
  • Magnesium
  • Sulphur
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The Correct Option is B

Solution and Explanation

  • Phosphorus (P) is typically the primary limiting nutrient in freshwater ecosystems (like lakes and ponds). Its scarcity restricts algal growth.
  • While nitrogen (not listed) is also limiting in some cases, phosphorus is more critical in freshwater systems.
  • Iron (1), magnesium (3), and sulphur (4) are micronutrients but do not typically limit productivity.
  • The passage mentions "limiting nutrients," implying phosphorus as a key driver of eutrophication.
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