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
