Which substance is often measured to assess the salinity of water?

Chloride ions are commonly measured to assess the salinity of water because they are one of the major contributors to the overall ionic composition of saline water. Salinity refers to the concentration of dissolved salts in water, and chloride ions are prevalent in most natural waters, especially in seawater. Measuring the concentration of chloride ions provides a reliable estimate of the water’s salinity because their levels are proportionate to other salts in the water.

In contrast, substances such as ammonia nitrogen, carbon dioxide, and phosphates do not directly correlate with salinity as they typically relate to nutrient levels, aquatic health, and biological processes rather than the concentration of dissolved salts. Thus, while they are important for other water quality assessments, they do not serve as a primary indicator of salinity.