Nutrients are necessary for the survival and growth of aquatic plants which are the base of the food chain for all other aquatic organisms. Although a number of nutrients (such as nitrogen, phosphorus, silica, carbon, potassium, calcium, and magnesium) are needed by plants for growth and reproduction, nitrogen and phosphorus are the two of particular interest that are more commonly monitored by volunteer monitoring programs. Nitrogen and phosphorus are the nutrients that limit plant growth in most aquatic systems. Nutrient levels in an aquatic system vary depending upon temperature, rainfall, runoff, biological activity, and the flushing of the aquatic system. Nutrient levels are generally higher in the spring and early summer and impact the aquatic system in several ways. High nutrient levels can accelerate eutrophication of a waterway. Eutrophication is characterized by abundant growths of phytoplankton (microscopic plants and algae) called algal blooms that may block sunlight from submerged aquatic vegetation (see Chapter 10). These algal blooms result in lower dissolved oxygen levels as decomposition of their organic matter consumes the dissolved oxygen.

Developing nutrient criteria for the nation’s waters is currently a hot issue. The debate centers on determining the limiting nutrient for a particular type of water in a particular ecoregion. Currently, Virginia has not yet adopted water quality standards for nutrients except for total ammonia as it relates to the toxicity to aquatic animals and nitrate for public drinking water supplies. Nitrate levels in public water supplies should not exceed 10,000 ug/l (micrograms/liter), or 10 mg/l. The Virginia Department of Environmental Quality (DEQ) currently designates “nutrient enriched waters” where there is degradation due to excessive nutrients. For free-flowing streams, the maximum concentration for total phosphorus is 200 ug/l, or 0.20 mg/l; while it is only 50 ug/l, or 0.05 mg/l, for lakes. DEQ has recently begun the process of developing nutrient water quality standards. Nutrient water quality standards are scheduled to be adopted as follows: in 2005 for tidal tributaries to the Chesapeake Bay; in 2006 for lakes; and in 2007 for freshwater streams.

 Nutrient concentrations in aquatic systems are influenced by both natural and human sources. Natural sources of nitrogen and phosphorus include decomposition of organic matter, nitrogen fixation of atmospheric nitrogen by certain bacteria and algae, and geologic formations rich in nitrogen or phosphorus. Human sources include discharges from wastewater treatment plants, stormwater runoff, livestock wastes, fertilizer runoff from lawns and agricultural fields, groundwater seepage from failing septic systems, planting of nitrogen fixing plants (such as clover or beans) in agricultural fields, and atmospheric deposition (including acid rain) from the burning of fossil fuels.

 All of the above information quoted from the "Virginia Citizen Water Quality Monitoring Program Manual" Published July 2003
by the Virginia Department of Environmental Quality and prepared by the Virginia Citizen Water Quality Monitoring Program.