Which of the following reveal how much oxygen is available in the water?

Dissolved oxygen is a critical parameter that indicates the amount of oxygen present in water, which is essential for the survival of aquatic organisms such as fish, invertebrates, and microorganisms. The concentration of dissolved oxygen is vital for their metabolic processes and overall ecosystem health.

High levels of dissolved oxygen typically indicate a healthy water system, while low levels can signal pollution or over-enrichment from nutrients, leading to excessive growth of bacteria or algae. In turn, these organisms can deplete oxygen levels, creating hypoxic conditions which are harmful to aquatic life.

The other options focus on different aspects of water quality. For instance, nutrients refer to the elements that can promote growth in organisms, but they don't directly measure oxygen levels. Bacteria are indicators of organic matter breakdown and can affect oxygen levels but are not a measure of oxygen themselves. pH is a measure of acidity or alkalinity, which can influence the solubility and availability of oxygen but does not directly measure the amount of oxygen in the water. Thus, dissolved oxygen is the only choice that specifically quantifies the available oxygen for aquatic life.