One of the most important types of water pollution , cultural eutrophication describes human-generated fertilization of water bodies. Cultural denotes human involvement, and eutrophication means truly nourished, from the Greek word eutrophic. Key factors in cultural eutrophication are nitrates and phosphates , and the main sources are treated sewage and runoff from farms and urban areas. The concept of cultural eutrophication is based on anthropocentric values, where clear water with minimal visible organisms is much preferred over water rich in green algae and other microorganisms .
Nitrates and phosphates are the most common limiting factors for organism growth, especially in aquatic ecosystems. Most fertilizers are a combination of nitrogen , phosphorus , and potassium. Nitrates are key components of the amino acids, peptides , and proteins needed by all living organisms. Phosphates are crucial in energy transfer reactions within cells. Natural sources of nitrates (and ammonia) are more readily available than phosphates, so the latter is often cited as the crucial limiting factor in plant growth. Nitrates are supplied in limited quantities by decaying plant material and nitrogen-fixing bacteria, but phosphates must come from animal bones, organic matter, or from the breakdown of phosphate-bearing rocks. Consequently, the introduction and widespread use of phosphate detergents , combined with excess fertilizer in runoff, has produced a near ecological disaster in some waters.
In ecosystems, there is a continuous cycling of matter, with green algae and plants making food from chemicals dissolved in water via photosynthesis ; this provides the food base needed by herbivores and carnivores. Dead plant material and animals are then decomposed by aerobic (oxygen using) and anaerobic decomposers into the simple elements they came from. Natural water bodies are usually well-suited for handling this matter cycling; however, human impacts often inject large amounts of additional nutrients into the system, changing them from oligotrophic (poorly nourished) to eutrophic water bodies. Once present within a relatively closed body of water, such as a lake or estuary , these extra nutrients may cycle numerous times before leaving the system.
Green algae and trashy fish may thrive in eutrophic water, but most people are offended by what they perceive as "scum." Nutrient-poor water is usually clear and possesses a rich supply of oxygen, but as the nutrient load increases, oxygen levels drop and turbidity rises. For example, when sewage is dumped into a body of water, the sewage fertilizes the algae, and as they multiply and die the aerobic decomposers multiply in turn. The increased demand for oxygen by the decomposers outstrips the system's ability to provide it. As a result, dissolved oxygen levels may fall or sag even below 2.0 parts per million , the threshold below which even trashy fish cannot a survive (trout need at least 8.0 ppm to survive). Even though the green algae are producing oxygen as a byproduct of photosynthesis, even more oxygen is consumed by decomposers breaking down the dead algae and other organisms.
As water flows downstream the waste is slowly broken down, so there is less for the decomposers to feed on. Biological oxygen demand slowly falls, while the dissolved oxygen levels rise until the river is finally back to normal levels. Most likely, the nutrients recycled by decomposers are either diluted, turned into biomass by trees and consumer organisms, or tied up in bottom sediments. Thus a river can naturally cleanse itself of organic waste if given sufficient time. Problems arise, however, when discharges are too large and frequent for the river to handle; under extreme conditions it becomes "dead" and is suited only for low-order, often anaerobic, organisms. Municipalities using river water locate their intakes upstream and their sewage treatment plants and storm drains downstream. If communities were required to do the reverse, the quality of river water would dramatically improve.
The major sources of nitrates and phosphates in aquatic systems are treated sewage effluent ; excess fertilizer from farms and urban landscapes; and animal wastes from feedlots , pastures, and city streets. In some areas with pristine waters, such as Lake Tahoe , tertiary sewage treatment has been added; chemicals are used to reduce nitrate and phosphate levels prior to discharge .
Runoff from nonpoint sources is a far more difficult problem because they are harder to control and remediate than runoff from point sources. Point sources can be diverted into treatment plants, but the only feasible way to reduce nonpoint sources is by input reduction or by on-site control.
Less fertilizer more frequently applied, especially on urban lawns, for example, would help reduce runoff. Feedlots are a major concern; runoff needs to be collected and treated in the same manner as human sewage; this may also apply to street runoff. Green cattle ponds are a sure sign of the abundant nutrients supplied by these mobile meat factories.
Phosphate detergents are superior cleaning agents than soap, but the resultant wastewater is loaded with this key limiting factor. Phosphate levels in detergents have since been reduced, but its impact is so powerful that abatement may require tertiary treatment.
The battle between Oklahoma and Arkansas over Illinois River pollution provides a useful case study of the debate over cultural eutrophication. This river has its headwaters in Arkansas and has become a prime tourist attraction in Oklahoma. Enthusiasts come from all over to canoe the river; summer use is especially heavy. However, economic development within the basin has resulted in a steady decline in river quality.
Arkansas started a legal war with Oklahoma when it sought and obtained Environmental Protection Agency (EPA) approval to dump half of the treated sewage from a new plant in Fayetteville into a tributary of the Illinois. Arkansas argued that its state-of-the-art treatment plant would produce an effluent having little impact on water quality by the time it reached the border. Oklahoma countered that it could not risk the potential economic loss if the river became polluted.
This controversy has had a salutary impact on research into the causes of cultural eutrophication within this basin. Scientists from Oklahoma State University and the University of Arkansas are collaborating on a long-term study of river quality and pollution sources. It is highly likely that nonpoint sources within both states will be identified as the key culprits, especially from livestock operations.
There is one major success story in the battle to overcome the effects of cultural eutrophication. The Thames River in England was devoid of aquatic life for centuries. Now a massive cleanup effort is restoring the river to vitality. Many fish have returned, most notably the pollution-sensitive salmon , which had not been seen in London for 300 years. However, much work remains, especially in former Warsaw Pact countries and those in the Third World .
[Nathan H. Meleen ]
Pettyjohn, W. A. Water Quality in a Stressed Environment. Minneapolis: Burgess, 1972.
Canby, T. "Water: Our Most Precious Resource." National Geographic 158 (August 1980): 144-179.
Harleman, D. R. F. "Cutting the Waste in Wastewater Cleanups." Technology Review 93 (April 1990): 60-69.
Herber, L. "Cool, Refreshing--and Filthy." In The Human Habitat: Contemporary Readings, edited by David L. Wheeler. New York: Van Nostrand Reinhold, 1971.
Maurits la Riviere, J. W. "Threats to the World's Water." Scientific American 261 (September 1989): 80-84+.