The level of dissolved oxygen DO in water bodies plays a crucial role in the survival of aquatic organisms, especially in polluted water. Dissolved oxygen is the amount of oxygen that is present in water and is available for use by aquatic organisms. It is a critical factor in maintaining a healthy aquatic ecosystem, as it is essential for the respiration of fish, invertebrates, and other aquatic organisms.In polluted water, the level of dissolved oxygen can be significantly affected due to various factors, such as the presence of organic matter, chemicals, and nutrients. Here's how the level of dissolved oxygen impacts the survival of aquatic organisms in polluted water:1. Organic pollution: When water bodies receive an excessive amount of organic matter such as sewage, agricultural runoff, or industrial waste , the decomposition of this organic matter by microorganisms consumes a large amount of dissolved oxygen. This process, called biochemical oxygen demand BOD , can lead to a decrease in the DO levels, causing stress or even death for aquatic organisms that require higher levels of oxygen for survival.2. Eutrophication: The presence of excessive nutrients, such as nitrogen and phosphorus, in polluted water can lead to eutrophication. This process causes rapid growth of algae and other aquatic plants, which eventually die and decompose. The decomposition of these plants consumes dissolved oxygen, leading to a decrease in DO levels. This can result in oxygen depletion, especially in the deeper layers of the water body, causing hypoxic or anoxic conditions that can be lethal for many aquatic organisms.3. Chemical pollution: Some pollutants, such as heavy metals, pesticides, and other toxic chemicals, can directly affect the survival of aquatic organisms by causing physiological stress, impairing their ability to obtain oxygen from the water, or interfering with their respiratory processes. Additionally, some chemicals can react with dissolved oxygen, reducing its availability for aquatic organisms.4. Temperature: Polluted water can also experience increased temperatures due to industrial discharges or other factors. Higher water temperatures can reduce the solubility of oxygen in water, leading to lower DO levels. Warmer water also increases the metabolic rates of aquatic organisms, which in turn increases their oxygen demand, further exacerbating the problem.Low levels of dissolved oxygen in polluted water can have severe consequences for aquatic organisms. It can lead to reduced growth, reproduction, and survival rates, as well as changes in species composition and distribution. In extreme cases, it can cause massive fish kills and the collapse of aquatic ecosystems. Therefore, maintaining adequate levels of dissolved oxygen in water bodies is essential for the survival and well-being of aquatic organisms, especially in polluted environments.