Question

Continuous increase of productivity of lakes eventually causes:

• A. Biomagnification in lakes
• B. Bioaccumulation in lakes
• C. Eutrophication in lakes
• D. Bioconcentration in lakes
• A. Remains constant in winter and summer
• B. Remains low in winter and high in summer
• C. Remains high in winter and low in summer
• D. Doesn’t depend on temperature of water
• A. High salinity
• B. High turbidity
• C. High dissolved oxygen
• D. High BOD
• A. High dissolved oxygen
• B. High dissolved oxygen and high BOD
• C. Low dissolved oxygen and high BOD
• D. Low dissolved oxygen and low BOD
• A. Iron
• B. Phosphorus
• C. Magnesium
• D. Sulphur

Answer 1

The Correct Option is C

The continuous increase in productivity of lakes is primarily associated with a phenomenon known as eutrophication. Eutrophication is characterized by an excessive richness of nutrients in a lake or other body of water, frequently due to runoff from the land, which induces a dense growth of plant life, especially algae. This can lead to the following consequences: 

• Oxygen Depletion: The dense algal blooms eventually die and decompose, a process which consumes oxygen, leading to a decrease in oxygen levels in the water, creating hypoxic or anoxic conditions.
• Aquatic Life Impact: The lack of oxygen can suffocate aquatic life, leading to fish kills and the loss of biodiversity within the ecosystem.
• Water Quality Degradation: Water quality becomes poor, often resulting in murky water, unpleasant odors, and the presence of toxins that can be harmful to both aquatic life and humans.

Considering the passage: Nutrient input from human activities, like agricultural runoff, increases the productivity of lakes. Initially, this may seem beneficial, but in the long term, it harms the lake ecosystem by triggering eutrophication.

Therefore, the correct answer to the question concerning the continuous increase of productivity in lakes is eutrophication in lakes.

Answer 2

• 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.

Answer 3

Dissolved oxygen (DO) in water is an essential aspect of water quality. It indicates the amount of oxygen available in the water for aquatic organisms. The solubility of oxygen in water is influenced by temperature; colder water can hold more dissolved oxygen than warmer water. This means that the concentration of dissolved oxygen is typically higher in winter and lower in summer. 

This phenomenon occurs because, in colder temperatures, the kinetic energy of water molecules decreases, allowing more oxygen to be dissolved from the atmosphere into the water. Conversely, in warmer temperatures, the increased kinetic energy results in decreased solubility of oxygen in water. Therefore, aquatic environments tend to have higher dissolved oxygen levels in winter and lower levels in summer.

The correct answer to the question is: Remains high in winter and low in summer.

Answer 4

• 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.

Answer 5

Agricultural runoff can lead to an increase in suspended particles in water bodies, a phenomenon commonly seen in lakes and ponds near agricultural areas. One of the direct consequences of this influx of suspended particles is an increase in the water's turbidity. Turbidity refers to the cloudiness or haziness of a fluid caused by large numbers of individual particles that are generally invisible to the naked eye, similar to smoke in the air.

Given the options provided:

1. High salinity: This would typically be associated with high concentrations of salt, which is not directly related to suspended agricultural particles.

2. High turbidity: This is the most likely consequence since suspended particles directly contribute to water turbidity.

3. High dissolved oxygen: This is unlikely as it is generally reduced in nutrient-rich conditions caused by runoff due to increased biological demand.

4. High BOD: While agricultural runoff can lead to an increase in Biological Oxygen Demand (BOD), it is an indirect effect, through nutrient enrichment leading to higher productivity rather than suspended particles directly.

Therefore, the correct answer is clearly high turbidity. Increased turbidity can lead to reduced light penetration, which can adversely affect aquatic life and the aesthetic quality of water bodies.

Answer 6

• 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.

Answer 7

To understand why water in lakes with very high algal and phytoplankton growth is expected to have low dissolved oxygen and high BOD, we need to consider a few key environmental principles: 

Dissolved Oxygen (DO): The level of oxygen dissolved in water is critical for the survival of aquatic organisms. Oxygen enters water through diffusion from the air, by photosynthesis of aquatic plants, and through water movement. In lakes with high algal and phytoplankton growth, these organisms initially increase oxygen levels during daylight due to photosynthesis. However, at night, both algae and other bacteria consume oxygen for respiration, leading to oxygen depletion.

Biochemical Oxygen Demand (BOD): BOD is a measure of the quantity of oxygen used by microorganisms to decompose organic matter in water. Lakes with excessive algal and phytoplankton growth will have high levels of organic matter when these organisms die. The decomposition process significantly increases the BOD as more oxygen is consumed to break down the organic matter.

Impact of Eutrophication: The process of eutrophication, often resulting from agricultural runoff, introduces excessive nutrients like nitrogen and phosphorus, promoting algal blooms. This increased productivity alters the equilibrium of aquatic ecosystems. After initial oxygenation in daylight, the increased biomass demands more oxygen for decomposition, resulting in lower oxygen levels.

Parameter	Effect with High Algal Growth
Dissolved Oxygen	Initial increase due to photosynthesis; significant decrease during decomposition
Biochemical Oxygen Demand	Increased due to excessive decomposition of dead algae

Analyzing these factors, the correct answer is: Low dissolved oxygen and high BOD. High algal growth initially raises DO during photosynthesis but leads to depletion at night and during decomposition stages, thus raising BOD as the ecosystem struggles with increased organic matter breakdown.

Answer 8

• 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).

Answer 9

In aquatic ecosystems such as lakes and ponds, limiting nutrients are those nutrients that are in shortest supply relative to the needs of organisms. Excess or deficiency of these nutrients can significantly influence the productivity and health of the ecosystem. In the context of the passage provided, it is mentioned that agricultural runoff and excess nutrient input enhance the productivity like algal bloom, which suggests a role of certain crucial nutrients. 

Among the options provided:

• Iron: Although an essential micronutrient for photosynthesis and is required in trace amounts, it is not typically considered a limiting nutrient in freshwater ecosystems.
• Phosphorus: This is often the limiting nutrient in freshwater environments such as lakes and ponds. It is essential for plant growth and primary productivity. Excess phosphorus from runoff leads to eutrophication, causing algal blooms.
• Magnesium: A component of chlorophyll, magnesium is needed in plants but does not usually limit growth in aquatic systems.
• Sulphur: Necessary for protein synthesis, rarely limiting, and typically present in sufficient quantities through natural processes.

The correct answer is Phosphorus. It is commonly the limiting nutrient that controls the rate of growth and biomass production in freshwater ecosystems.

Answer 10

• 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.