Nutrient Loading

An excess of dissolved substances in a river that de-regulates the limiting nutrients controlling the amount of bio-productivity of that system. Nutrient loading can result from anthropogenic sources, such as effluent containing phosphate detergents dumped into rivers from sewage treatment plants. Natural sources of nutrient enrichment also exist, as in the case of the Green River in northern Utah. Nitrates and phosphates that were deposited in the strata above the normal level of the river were flooded by the impoundment created by the Flaming Gorge Dam project. As the result, these nutrients now leach into the lake, permitting large assemblages of algae to exist. A portion of these single cell plants are exported to the river below. Filter feeding macro invertebrates there take advantage of this wind-fall, enabling the Green River below the dam to become one of the world’s greatest producers of trout per linear mile (est. 22,000). Similar results occurred with the damming of the Madison River in Montana, and numerous other coldwater fisheries. Negative effects on the growth of macro invertebrates and trout result when nutrient loading over-produces the wrong kinds of algal species (e.g., blue-green), or occurs in warm water situations in which algal blooms literally smother the life forms by depleting the already limited amounts of oxygen during periods of darkness.

Links
  http://www.co.missoula.mt.us/measures/nutrient.htm
http://www.umt.edu/biology/flbs/Research/Nutrientloading.htm