Freshwater species may be restricted to the upper reaches of the estuary while marine species inhabit the estuarine mouth. Some species tolerate only intermediate levels of salinity while broadly adapted species can acclimate to any salinity ranging from fresh water to seawater. As water becomes saltier, its weight increases although its volume does not measurably rise. This change of weight results in a greater specific gravity since salt water is more dense than fresh water. By using a hydrometer, which measures the specific gravity of a water sample, one can calculate salinity.
Hydrometers are a fairly simple and inexpensive means of obtaining salinity.
The pH quantifies the acidity or alkalinity of a water sample. Most marine organisms prefer conditions with pH values ranging from about 6.5 to 8.5. The pH of the water is critical to the survival of most aquatic plants and animals. Many species have trouble surviving if pH drops under 5.0 or rises above 9.0 Changes in pH can alter other aspects of the water's chemistry, usually to the detriment of native species. Even small shifts in the water's pH can affect the solubility of some metals such as iron and copper.