What does a dissolved oxygen (DO) level indicate in relation to water quality?

Dissolved oxygen (DO) levels are a critical indicator of water quality because they directly reflect the ability of water bodies to support aquatic life. Aquatic organisms, including fish, invertebrates, and microorganisms, rely on dissolved oxygen for respiration. A healthy level of oxygen in the water is essential for the survival and growth of these organisms. When the DO level is too low, it can lead to hypoxia, which may cause stress or death to aquatic life and disrupt the ecosystem balance.

In healthy water bodies, dissolved oxygen levels typically range from 6 to 8 mg/L for most fish species. Factors such as temperature, salinity, and water movement can influence DO levels; warmer water holds less oxygen compared to cooler water. High levels of organic matter decomposition can also consume oxygen, potentially causing adverse conditions for aquatic life.

The other options, while relevant to water quality, do not directly relate to the role that dissolved oxygen plays in supporting aquatic organisms, which is why they do not capture the core significance of DO levels accurately.