Material that becomes mixed and suspended in water will reduce its clarity and make the water turbid (dirty). In the summer months, plankton are growing and multiplying rapidly in the warm, nutrient-rich water. During periods of heavy rain, run-off from land can carry large amounts of silt into streams. Silt is often related to nutrient enrichment of a river because nutrients such as phosphorus cling to soil particles. Fine sediment can become re-suspended in more shallow waters during heavy winds and tidal action. In addition, unprotected shoreline will erode and contribute suspended particles to the water. In shallow areas, wind-generated waves and boat wakes stir up sediments. Wind and boat generated waves breaking on shore also contribute to turbidity.
Turbidity affects fish and aquatic life by interfering with the penetration of sunlight. Submerged aquatic vegetation (SAV) requires light for photosynthesis. If suspended particles "block out" light, photosynthesis, which produces oxygen for fish and aquatic life, will be reduced. SAV provides essential food, nursery areas, shelter and habitat for diverse communities of shellfish, waterfowl, and fish. If light levels become too low photosynthesis may stop altogether and algae will die. Sediment buries eggs and benthic (bottom dwelling) organisms' habitat. Large amounts of suspended matter may clog the gills of fish and shellfish and kill them directly. Fish cannot see very well in turbid water and so may have difficulty finding food.
Schematic adapted from "Turbidty: A Water Quality Measure", Water Action Volunteers, Monitoring Factsheet Series,
UW-Extension, Environmental Resources Center. It is a generic, un-calibrated impact assessment model based on Newcombe, C. P., and J. O. T. Jensen. 1996. Channel suspended sediment and fisheries: a synthesis for quantitative assessment of risk and impact. North American Journal of Fisheries Management. 16: 693-727.
Methods for measuring water clarity:
Currently, there are no water quality standards for water clarity. The Alliance uses two methods to measure water clarity; the Secchi disk and the turbidity tube. The Secchi disk is a circular disk that is attached to a graduated string and weighted down. Monitors lower the Secchi disk into the water body and observe when the Secchi pattern disappears. In cases where streams are too shallow or swift-moving, a Secchi disk is not appropriate and monitors measure water clarity using a turbidity tube. Monitors fill the turbidity tube with water and allow it to come out until they can see the Secchi pattern.
The above information was quoted from the Alliance for the Cheaspeake Bay's "River Trends Train the Trainer Manual" © April 2007.