What does biochemical oxygen demand (BOD) measure?

Biochemical oxygen demand (BOD) is a critical measurement in assessing water quality, particularly in determining the level of organic pollution. It specifically quantifies the amount of oxygen that microorganisms will consume while decomposing organic matter in a water sample over a specific period, typically five days at a controlled temperature.

When BOD is measured, it indicates the degree of pollution in the water; higher BOD values imply a greater presence of organic material, leading to increased microbial activity and consequently higher oxygen consumption. This can be detrimental to aquatic life, as it can result in lower dissolved oxygen levels, which fish and other organisms rely on for survival.

The other options, while related to aspects of aquatic ecosystems, do not accurately describe what BOD measures. The focus is solely on the oxygen consumed by microbes during the decomposition process, making C the correct answer.