Water quality is measured by several factors, such as the concentration of dissolved oxygen, bacteria levels, the amount of salt (or salinity), or the amount of material suspended in the water (turbidity). In some bodies of water, the concentration of microscopic algae and quantities of pesticides, herbicides, heavy metals, and other contaminants may also be measured to determine water quality. Although scientific measurements are used to define water quality, it is not a simple thing to say “that water is good” or “that water is bad.” So, the determination is typically made relative to the purpose of the water – is it for drinking or to wash a car with or for some other purpose?.  Poor water quality can pose a health risk for people. Poor water quality can also pose a health risk for ecosystems. In the Florida Keys, good water quality is essential to a healthy marine ecosystem. Seagrass and coral reef communities thrive in clean water that is relatively low in nutrients. Too many nutrients in the water can cause excess growth of algae, which can smother corals and seagrass. Pollutants such as metals, oils, pesticides, and fertilizers run off from land into the waters, causing excess algae growth and other harmful impacts.