What does dissolved oxygen (DO) indicate in aquatic ecosystems?

Dissolved oxygen (DO) is a critical measurement in aquatic ecosystems as it represents the amount of oxygen that is present in water, available for use by aquatic organisms such as fish, invertebrates, and microorganisms. The health of an aquatic ecosystem is often assessed by the concentration of DO since most aquatic life relies on this oxygen for respiratory processes. High levels of dissolved oxygen typically indicate a healthy ecosystem with abundant aerobic organisms, whereas low levels may suggest poor water quality and can lead to hypoxic conditions, making it difficult for aquatic life to survive.

In contrast, while temperature can affect dissolved oxygen levels—the warmer water holds less oxygen—it is not directly indicated by DO readings. The presence of toxins can negatively impact aquatic life and oxygen demand, but DO itself does not measure toxins directly. Similarly, the amount of organic matter can influence oxygen levels through decomposition, but dissolved oxygen is specifically about the gas itself available for respiration, not a measurement of organic content. Thus, the best understanding of dissolved oxygen in this context is its role in indicating the availability of oxygen for aquatic organisms.