In aquatic ecosystems, oxygen is dissolved in water, and its concentration varies constantly based on factors that influence oxygen input and emission. The average dissolved oxygen content in freshwater is 0.0010 percent by weight (10 parts per million or 10 ppm), which is 150 times lower than the oxygen concentration in an equivalent volume of air. This article will explain to you about Dissolved Oxygen which will be helpful in preparing the Environment syllabus for the UPSC Civil service exam.

What does Dissolved Oxygen (DO) mean?

• Dissolved oxygen (DO) is defined as the amount of oxygen that is present in water.
• Adequate dissolved oxygen is crucial for all forms of life and is required for excellent water quality.
• Aquatic life is stressed when dissolved oxygen levels fall below 5.0 mg/L. Lower concentrations result in more stress.
• Large fish kills can occur when oxygen levels fall below 1-2 mg/L even for a few hours.
• The decrease in the level of dissolved oxygen is termed as Biological Oxygen Demand (BOD).
• Because bacteria that feed on sewage multiply in numbers and respire, any release of raw sewage into a river results in a fall in dissolved oxygen level. This respiration depletes oxygen in the water quickly, resulting in fish deaths.

How does Water Pollution Affects Dissolved Oxygen?

• The dissolved oxygen level of water is reduced when organic and inorganic pollutants are present.
• These pollutants may arise due to municipal waste treatment outfall, industrial point sources, agricultural and urban runoff, removal of riparian vegetation, channel alteration and groundwater inflow.
• Water with a dissolved oxygen level of less than 8.0 mg/L is potentially polluted.
• Water with a dissolved oxygen level of less than 4.0 mg/L is considered extremely contaminated.
• The amount of dissolved oxygen in water is critical for aquatic creatures' existence.
• The amount of DO present in water is determined by a number of factors such as surface turbulence, photosynthetic activity, O2 consumption by organisms and organic matter breakdown.
• The greater the amount of waste, the faster it decomposes and consumes oxyge.
• As a result, the dissolved oxygen level in the water will be reduced.

Conclusion

Across all sites, the dissolved oxygen is substantially lower in the summer months (December to February) than in the winter months. However, they cause irreversible damages to the ecosystem irrespective of the seasons. Water pollution in general has the potential to disrupt entire food chains. Human-caused changes in water temperatures can also kill all aquatic life in a pond or lake.

FAQs

Question: What is dissolved oxygen (DO)?

Question: How is dissolved oxygen measured?

Question: What factors affect dissolved oxygen levels in water?

Question: Why is low dissolved oxygen a problem?

Question: What is the relationship between eutrophication and dissolved oxygen?

MCQs 

1. Dissolved oxygen is vital for:

A) Mineral formation in water

B) Respiration of aquatic organisms

C) Increasing water salinity

D) Reducing water clarity

2. What happens when dissolved oxygen levels drop significantly?

A) Water becomes clearer

B) Aquatic life thrives

C) Hypoxic conditions develop, leading to potential fish kills

D) Photosynthesis is enhanced

3. Which factor decreases the solubility of oxygen in water?

A) Low temperatures

B) High salinity

C) High temperatures

D) None of the above

4. Eutrophication primarily results from:

A) Excess nutrients

B) Oxygen-rich water

C) High mineral content

D) Algae removal

5. What term describes an area with extremely low oxygen levels in water?

A) Hypoxic zone

B) Aerobic zone

C) Benthic zone

D) Nutrient-rich zone

GS Mains Questions and Model Answers

Q1: Discuss the significance of dissolved oxygen in maintaining aquatic ecosystems and its relationship with water pollution.

Answer: Dissolved oxygen (DO) is vital for maintaining healthy aquatic ecosystems as it supports the respiration of fish and other aquatic organisms. Pollution, such as excess nutrients from agricultural runoff, leads to eutrophication, causing algal blooms. As algae decompose, they consume large amounts of oxygen, lowering DO levels and creating hypoxic conditions that threaten aquatic life. Effective water pollution control, such as reducing nutrient runoff, is critical for maintaining adequate DO levels and protecting biodiversity.

Q2: Analyze the impact of human activities on dissolved oxygen levels in water bodies.

Answer: Human activities, including agricultural runoff, industrial discharge, and wastewater release, introduce excess nutrients and pollutants into water bodies. This leads to eutrophication and oxygen depletion, creating hypoxic conditions. Elevated temperatures from thermal pollution can also reduce DO solubility. Addressing these impacts requires stringent pollution control measures, sustainable land use practices, and conservation efforts to protect aquatic ecosystems.

Q3: Explain the causes and consequences of eutrophication in water bodies.

Answer: Eutrophication occurs due to excess nutrients, such as nitrogen and phosphorus, entering water bodies from agricultural runoff, sewage, and industrial discharge. It results in rapid algal growth, which depletes oxygen levels during decomposition. The consequences include hypoxia, fish kills, loss of biodiversity, and the formation of dead zones. Effective management of nutrient inputs and pollution control can mitigate eutrophication, protecting aquatic life and water quality.

Previous Year Questions on Dissolved Oxygen 

1. UPSC CSE 2018

Question: Assess the impact of low dissolved oxygen levels on aquatic ecosystems.

Answer: Low dissolved oxygen levels lead to hypoxic conditions, causing stress or death in aquatic organisms, such as fish and invertebrates. This disrupts ecosystems, reduces biodiversity, and affects the food web. Hypoxia often results from human-induced pollution and eutrophication. Addressing nutrient runoff, wastewater management, and industrial discharges is crucial to maintaining healthy oxygen levels in water bodies, supporting aquatic life and ecosystem balance.

2. UPSC CSE 2020

Question: Discuss the role of eutrophication in reducing dissolved oxygen levels in water bodies.

Answer: Eutrophication, driven by excess nutrients like nitrogen and phosphorus, leads to rapid algal blooms. As algae die and decompose, microorganisms consume large amounts of dissolved oxygen, resulting in oxygen depletion (hypoxia). This harms aquatic organisms, causes fish kills, and disrupts ecosystems. Effective control of nutrient pollution through sustainable agricultural practices, wastewater treatment, and regulatory measures can mitigate eutrophication and maintain adequate DO levels.