ANS:B - 4 PPm

A dissolved oxygen (DO) concentration of 4 ppm (parts per million) is often considered a minimum threshold for supporting aquatic life, including fish, in freshwater ecosystems. Here's an explanation:

1. Respiration Requirements: Like all aerobic organisms, fish require oxygen to carry out respiration, which is the process of converting oxygen into energy. Dissolved oxygen in water is vital for fish to extract oxygen from their environment and breathe effectively. A minimum level of dissolved oxygen is necessary to meet the metabolic demands of fish for respiration.
2. Threshold for Survival: While different species of fish have varying tolerance levels to low oxygen concentrations, a DO level of 4 ppm is generally recognized as the minimum threshold for the survival of many freshwater fish species. At this concentration, fish can respire adequately to meet their metabolic needs and sustain healthy physiological functions.
3. Stress and Vulnerability: When dissolved oxygen levels drop below 4 ppm, fish may experience physiological stress, reduced growth rates, impaired reproduction, and increased susceptibility to diseases and environmental stressors. Prolonged exposure to low oxygen levels can lead to fish mortality and negatively impact aquatic ecosystems' health and biodiversity.
4. Environmental Factors: The availability of dissolved oxygen in water is influenced by various environmental factors such as temperature, water flow, aquatic vegetation, organic matter decomposition, and pollution levels. Factors like high water temperatures, excessive nutrient inputs, and pollution from human activities can decrease dissolved oxygen levels, posing challenges for fish survival and ecosystem health.

While a dissolved oxygen concentration of 4 ppm is often considered a minimum threshold, it's important to recognize that specific fish species may have different oxygen requirements, and optimal dissolved oxygen levels can vary depending on environmental conditions and habitat characteristics. Therefore, maintaining adequate dissolved oxygen concentrations in freshwater ecosystems is essential for sustaining healthy fish populations and overall ecosystem balance.