10. During organic pollution in lakes
    (1) BOD decreases
    (2) Amount of oxygen decreases
    (3) Amount of oxygen increases
    (4) Nutrient decreases

Organic pollution is a major threat to freshwater ecosystems, especially lakes. When large amounts of organic matter—such as sewage, agricultural runoff, or decaying plant material—enter a lake, they set off a chain reaction that can drastically alter water quality and aquatic life. Among the key changes, the most significant is the dramatic decrease in dissolved oxygen levels. This article explores what happens during organic pollution in lakes, why oxygen levels fall, and why other changes—like increased oxygen or nutrient decrease—are not typical outcomes.

What Is Organic Pollution in Lakes?

Organic pollution occurs when excessive organic matter enters a lake from sources like untreated sewage, stormwater runoff, or agricultural activities. This organic matter includes proteins, carbohydrates, fats, and other compounds that are rich in carbon. When these materials enter the water, they become food for bacteria and other microorganisms, which begin to break them down through the process of decomposition.

The Decomposition Process and Oxygen Demand

As microorganisms decompose organic matter, they consume large amounts of oxygen from the water. This process is measured as Biochemical Oxygen Demand (BOD), which indicates how much oxygen is required by microorganisms to break down organic material in water. The higher the BOD, the greater the potential for oxygen depletion.

• BOD Increases: During organic pollution, BOD increases because there is more organic material for microbes to decompose.

• Oxygen Consumption: Microbes use up dissolved oxygen in the water as they break down organic pollutants.

• Oxygen Depletion: If oxygen is consumed faster than it can be replenished by diffusion from the air or photosynthesis by aquatic plants, the amount of dissolved oxygen in the water drops sharply.

What Happens When Oxygen Levels Drop?

A decrease in dissolved oxygen has severe consequences for aquatic life:

• Fish Kills: Many fish and aquatic invertebrates require high levels of dissolved oxygen to survive. When oxygen levels drop, these organisms can suffocate and die.

• Shift in Species: Only species that can tolerate low oxygen (like some worms and certain bacteria) survive, leading to a loss of biodiversity.

• Anaerobic Conditions: In extreme cases, oxygen levels can drop to zero, creating anaerobic (oxygen-free) zones where only specialized, often harmful, bacteria thrive.

• Release of Greenhouse Gases: Under anaerobic conditions, bacteria produce methane and other greenhouse gases, which can escape into the atmosphere and contribute to climate change.

Why Do Oxygen Levels Decrease, Not Increase?

Some might wonder if the decomposition of organic matter could lead to an increase in oxygen. In reality, the opposite is true:

• Photosynthesis vs. Decomposition: While aquatic plants and algae produce oxygen during photosynthesis, this is often insufficient to offset the massive oxygen demand from decomposing organic matter, especially in polluted lakes.

• Nighttime Oxygen Drop: At night, when photosynthesis stops, oxygen levels can drop even further as respiration continues.

• Algal Blooms: In some cases, excess nutrients (not just organic matter) can cause algal blooms. While algae produce oxygen during the day, when they die and decompose, they consume oxygen, often leading to net oxygen loss.

Nutrient Dynamics During Organic Pollution

Contrary to what some might expect, nutrients like nitrogen and phosphorus do not decrease during organic pollution. In fact:

• Nutrient Increase: Organic pollution often introduces more nutrients into the lake, which can fuel algal blooms and further disrupt the ecosystem.

• Sediment Release: Under low oxygen (anoxic) conditions, nutrients stored in lake sediments can be released back into the water, exacerbating the problem.

Common Misconceptions

• BOD Decreases: Incorrect. BOD increases during organic pollution because more organic material is present for microbes to decompose.

• Oxygen Increases: Incorrect. Oxygen levels decrease as microbes consume it during decomposition.

• Nutrient Decreases: Incorrect. Nutrients generally increase or remain high, especially under anoxic conditions when sediments release stored nutrients.

The Vicious Cycle of Oxygen Loss

Recent research shows that oxygen loss in lakes is a growing global problem, driven by both organic pollution and climate change:

• Warming Waters: Warmer water holds less oxygen, and increased stratification (layering) makes it harder for oxygen to mix into deeper waters.

• Nutrient Runoff: Excess nutrients from agriculture and urban areas fuel algal blooms, which can further deplete oxygen when the algae die and decompose.

• Sediment Effects: Low oxygen at the lake bottom causes sediments to release more nutrients and greenhouse gases, creating a feedback loop that worsens water quality.

Monitoring and Mitigation

Monitoring dissolved oxygen levels is a key tool for assessing the health of lakes and detecting organic pollution. When oxygen levels drop, it is a clear sign that the lake is under stress and may require intervention.

• Wastewater Treatment: Proper treatment of sewage and industrial effluent can reduce the amount of organic matter entering lakes.

• Nutrient Management: Reducing fertilizer runoff from farms and lawns can help prevent algal blooms and oxygen depletion.

• Restoration Efforts: Aeration systems and wetland restoration can help replenish oxygen and improve water quality.

Summary Table

Key Takeaways

• Organic pollution in lakes leads to increased BOD and decreased dissolved oxygen.

• Oxygen levels drop because microbes consume oxygen as they break down organic matter.

• Nutrient levels generally increase or remain high, not decrease.

• Low oxygen causes fish kills, loss of biodiversity, and can trigger the release of greenhouse gases.

• Monitoring oxygen levels is essential for detecting and managing organic pollution in lakes.

Conclusion

During organic pollution in lakes, the amount of oxygen in the water decreases as microorganisms decompose the added organic matter and consume dissolved oxygen. This process increases BOD and can devastate aquatic ecosystems. Nutrients do not decrease; in fact, they may increase as sediments release stored nutrients under low-oxygen conditions. The correct answer to the question is therefore: the amount of oxygen decreases.

Understanding these dynamics is crucial for protecting our lakes and ensuring clean, healthy water for people and wildlife. By addressing the sources of organic pollution and managing nutrient inputs, we can help restore and preserve the health of freshwater ecosystems.