Excess Nutrients in Surface Waters

Highland streams in the southern Appalachians typically have low nutrient levels (Wallace and others 1992). Nutrients such as nitrogen and phosphorus are basic building blocks of tissues and organisms, and their availability is critical to ecosystem function. Biologically active, inorganic forms of these nutrients, including nitrate (NO3-N), ammonium (NH4-N), and soluble reactive phosphorus (SRP), are of particular importance. Dynamics of nutrients depend on transport in the water column, and on numerous processes linking the water column to the streambed, the streambanks, and the land.

Ecosystem processes can be seriously altered and degraded when nutrient concentrations become excessive. Photosynthesis by algae and plants (termed primary production) is stimulated by increased availability of nutrients, and other system processes such as decomposition of organic matter may be stimulated as well. In still water such as that in reservoirs, the increased production and decomposition rates can lead to problems of eutrophy, including oxygen depletion that kill fish.

Oxygen depletion rarely occurs in flowing water due to constant mixing, but enrichment may still change the community structure of organisms such as algae, invertebrates, and fish. Mountain streams are natural heterotrophic systems that depending on external energy inputs of leaf litter and wood. Excess nutrients shift the energy base toward an autotrophic system, driven by energy production instream. With this habitat change, organisms that are adapted to the low-nutrient environment of the highlands may be replaced with a new suite of organisms adapted to the new conditions. As a result, stream structure and function may be very different. Various human activities on the landscape contribute nutrients to surface waters:

• Deforestation-- The loss of forest cover reduces nutrient uptake by vegetation, and soil disturbance accelerates mineralization of organic matter-- the conversion of organic forms of nutrients to inorganic forms. As a result, concentrations of nutrients, such as nitrogen, in runoff to streams are elevated.
• Agriculture-- Fertilizer applications to land are carried in water and sediment as runoff.
• Livestock-- Wastes from livestock in streams and runoff from feedlots add nutrients in streams.
• Urbanization-- Municipal waste discharges and runoff of fertilizers applied to urban and suburban areas are nutrient sources.

Nutrient inputs can be reduced by adopting and implementing best management practices for land uses, and by more thorough wastewater treatment.