The minimum level of Dissolved Oxygen (DO) necessary in rivers and streams is 4 mg/l. Dissolved Oxygen refers to the amount of oxygen gas dissolved in water, which is crucial for the survival of aquatic organisms. It is one of the key indicators of water quality and is important for maintaining a healthy ecosystem in rivers and streams.

Importance of Dissolved Oxygen:
- Oxygen is essential for the respiration of aquatic organisms. Fish, insects, and other organisms that live in water require oxygen to survive. Without sufficient levels of dissolved oxygen, these organisms may suffocate and die.
- Dissolved oxygen also plays a vital role in the decomposition of organic matter in water bodies. Bacteria and other microorganisms that break down organic waste rely on oxygen to carry out this process. Insufficient DO levels can lead to the accumulation of organic matter, resulting in water pollution and the depletion of oxygen.
- Additionally, dissolved oxygen affects the solubility of other substances in water, such as nutrients and metals. Adequate DO levels help maintain a balanced and healthy aquatic environment.

Ecological Considerations:
1. Sensitive Organisms: Many aquatic organisms, including fish and macroinvertebrates, have specific DO requirements for their survival and reproduction. Sensitive species may not be able to tolerate low DO levels and can be negatively affected by oxygen depletion.

2. Oxygen Demand: Organic matter, such as leaves, algae, and animal waste, consumes oxygen as it decomposes. This natural process can lead to a decrease in DO levels. Additionally, human activities, such as agriculture and wastewater discharge, can contribute to increased organic pollution and oxygen demand in rivers and streams.

3. Temperature: Warmer water holds less dissolved oxygen than colder water. As water temperature increases, the oxygen-carrying capacity of the water decreases. Therefore, higher temperatures can exacerbate the effects of low DO levels on aquatic organisms.

4. Algal Blooms: Excessive nutrient pollution, often caused by agricultural runoff or sewage discharges, can lead to algal blooms. These blooms can result in reduced DO levels as the algae consume oxygen during their growth and decay processes.

Conclusion:
Considering the ecological requirements of aquatic organisms and the factors that affect dissolved oxygen levels in rivers and streams, a minimum DO level of 4 mg/l is necessary to support a healthy and diverse aquatic ecosystem. Maintaining adequate DO levels is crucial for the survival and well-being of aquatic organisms and is an important aspect of water quality management.