Salinity is a measure of how salty the water is. It is measured using a device called a salinometer. Salinity is important because the amount of salt in the water affects the plants and animals living in it. Plants and animals need different levels of salinity to survive. Too much or too little salt can be harmful. Too much salt can make the water too salty for plants and animals to survive, while too little salt can make it too fresh, meaning the water is not salty enough. Salinity levels can also affect the amount of oxygen available in the water, which is important for many plants and animals.