What does the use of coliform as an indicator species measure?

The use of coliform bacteria as an indicator species primarily measures the relationship between human health risk and the concentration of pathogens in water. Coliform bacteria, particularly Escherichia coli (E. coli), are commonly found in the intestines of warm-blooded animals and their presence in water is a strong indicator of fecal contamination. This contamination suggests that pathogens could also be present, potentially leading to waterborne diseases.

By monitoring coliform levels, water quality analysts can assess the likelihood that harmful microorganisms are present in a given water source. A higher concentration of coliform indicates a greater risk to human health, as it suggests the possibility of more harmful pathogens being present alongside them. This makes coliform bacteria a useful and practical tool for ensuring the safety of drinking water and protecting public health.

The other options focus on different variables that do not specifically tie human health risks to pathogen levels. For example, measuring water quality on a relative scale or assessing the levels of nutrients have different implications and do not address the direct correlation between coliform presence and potential health risks.