Turbidity is a characteristic of water that describes the amount of suspended solids in the water. Suspended solids can be phytoplankton, sediment, or detritus. Anthropogenic causes of turbidity include dredging activities, runoff from agricultural and urban areas, and shoreline erosion. Highly turbid water can prevent light from reaching plants on the bottom or phytoplankton in the water column, and can therefore reduce the amount of primary productivity in an aquatic system. High concentrations of suspended solids can settle onto submerged aquatic vegetation (SAV) and can smother shellfish beds and fish spawning grounds. Turbidity is measured as total suspended solids as milligrams of solids per liter, or with an instrument called a nephelometer, which measures the amount and angle of light scattering that is caused by particles suspended in the water.