Eutrophication Essay ⋆ Essays on Controversial Topics ⋆ EssayEmpire

Eutrophication, or cultural eutrophication, is the enhancement of the natural process by which streams, lakes, reservoirs, and estuaries become enriched with nutrients (phosphorous and nitrogen), enabling the ecosystem to support higher rates of production as measured by biomass or energy per unit area over time. This natural process of enrichment can take thousands of years and result in the succession of a glacial lake into a bog and, eventually, a prairie. However, this process can be greatly accelerated by human activities; while natural eutrophication occurs on geologic timescales, a reservoir undergoing cultural eutrophication can evolve into a bog in less than a hundred years.

This accelerated enrichment can have many detrimental ecological, aesthetic, and human health effects. Excess phosphorous and nitrogen can disrupt the natural balance of the aquatic ecosystem by spurring population explosions of nuisance algae and aquatic plants. As these algal populations sink and die, they create an oxygen demand in the underlying water where bacteria deplete oxygen supplies to decompose the dead algae. The oxygen-depleted bottom waters become poor fish habitat. In freshwater ecosystems, sudden, short-term episodes of low oxygen levels can cause fish kills, and extended periods of low bottom-water oxygen concentrations can cause a shift in fish populations from desirable sport fish to low-oxygen-tolerant species such as carp. In marine ecosystems, this phenomenon is seen in the Gulf of Mexico’s “dead zone,” where nitrogen from the Mississippi and Atchafalaya Rivers stimulates high rates of algal growth in the upper water layers, which cause oxygen depletion in the underlying waters.

Nutrient enrichment has also been cited as the cause of coral reef destruction, degrading both the ecological and recreational value of these marine resources. In freshwater ecosystems, nuisance algal blooms can diminish the aesthetic and recreational value of the water body by forming surface scums and producing earthy and musty tastes and odors, which can persist in finished drinking water. In addition to ecological and aesthetic degradation, some algal blooms can pose risks to human health. These harmful algal blooms (HABs) can produce potent toxins, and scientists have implicated them in wildlife, livestock, and pet deaths after the animals had drunk contaminated water. Long-term low dose exposures of a hepatotoxin, microcystins-LR produced by HABs of Microcystis, is suspected to contribute to high rates of liver cancer in certain parts of China. A short-term acute poisoning of the neurotoxin anatoxin-a, produced by an HAB of Anabaena, was the likely cause of death for a Wisconsin teenager in July 2002. In marine ecosystems, HABs have resulted in human exposures to the neurotoxins brevetoxin and saxitoxin, which are produced by the algae Karenia brevis and Alexandrium fundyense, a known cause of paralytic fish poisoning.

The causes of nutrient enrichment can be categorized into two main sources: point source pollution (i.e., from a pipe) and non-point source (i.e., diffuse) pollution. As all streams, lakes, reservoirs, and estuaries receive water from their respective watersheds (area of land that drains into a water body), any upstream or up-watershed sources of pollution can become pollution sources to the receiving water body. In the United States, the point source release of plant (and algae) nutrients, phosphorus and nitrogen, into the environment is controlled by the National Pollutant Discharge Elimination System (NPDES), a provision of the Clean Water Act.

Although the NPDES program has achieved much improvement in nutrient pollution control, recent assessments of the nation’s water quality have shown continued water quality degradation caused by these nutrients (e.g., a 1999 U.S. Geological Survey study showed that the nation’s median stream phosphorous concentration was still greater than the 0.100 mg L-1 threshold for reduced phytoplankton growth), demonstrating a need to manage both point and non-point sources of nutrient pollution. Non-point source pollutants are transported to the receiving waters via subsurface water, surface water (i.e., streams and rivers), or runoff (direct overland flow).

U.S. Environmental Protection Agency (EPA) studies have shown that in a natural forested area, 40 percent of rainwater returns to the atmosphere through evapotranspiration, 50 percent filters into subsurface flow, and 10 percent runs off the land surface into a receiving water body. However, with the removal of vegetation and its replacement with impervious surfaces (land cover that does not allow for the water to soak into the ground, e.g., rooftops, parking lots, and streets), the amount of water transported via runoff increases: Suburban land cover results in 30 percent rainwater runoff and urban land cover results in 50 percent rainwater runoff.

In 2000, EPA stated that the most common sources of pollution affecting U.S. streams and rivers were agricultural runoff, animal feeding operation runoff, hydrologic modification (e.g., channelization, dredging, and dam construction), habitat modification (e.g., removal of stream bank vegetation), urban runoff from lawns and impervious surfaces, erosion from urban development, and urban storm sewer overflows. These sources of non-point pollution can transport more than just nutrients to receiving waters: Pesticides from agricultural and suburban uses, bacteria and pathogens (e.g., E. coli and Cryptosporidium) from animal feeding operations, and oil and trash from parking lots can be transported as well.

Social Dynamics of Addressing Eutrophication and Runoff

EPA’s 2000 National Water Quality Inventory showed that, of the U.S. water bodies assessed, 47 percent of rivers, 53 percent of lakes, and 52 percent of estuaries were polluted or threatened. In 1996 and 2000, nutrients and siltation (sedimentation) were among the top five causes of impairment for streams, rivers, lakes, ponds, reservoirs, and estuaries. Both nutrient enrichment and siltation can lead to eutrophication, but in many cases, the nutrient enrichment and siltation caused by human manipulation of the environment can be prevented or minimized through the use of best management practices (BMPs). However, even though safe drinking and recreational waters are social necessities, a lack of understanding of causes and effects of pollution often inhibits stakeholder investment in BMPs.

Case studies by the Iowa Department of Natural Resources on Squaw Creek Watershed in 2002 and Cedar Lake Watershed in 2001 showed that perceptions of good water quality varied widely between water quality specialists and local landowners based on different ideas of how the water body should function. Whereas participants from Squaw Creek Watershed said that stream water quality was adequate (or not harmful) to wildlife but not okay for human consumption, water quality specialists measured good water quality on the basis of a broad set of physical, chemical, and biological characteristics. This discrepancy in determining the health of the watershed ecosystem impacted the importance stakeholders placed on water quality improvement efforts.

In the Cedar Lake study, water quality specialists identified agricultural operations as the main cause of excess nitrates in the lake water. (Studies implicate nitrates as a cause of the Gulf of Mexico’s dead zone.) However, the discrepancy between farmers’ and water quality managers’ perceptions of water quality led to only 50 percent of the farmer participants believing that agricultural activities were the cause of elevated nitrate levels. The farmers cited the many conservation practices they already employed as proof that their activities were not the cause of the high nitrate levels. As such, project managers focused efforts on educating stakeholders on the complexity and differences of different pollutant transport.

To be fair, this discrepancy in attitudes between water quality managers and stakeholders toward water resources is not unique to farming communities. Similar attitudes toward pollution exist in suburban and urban communities. From development practices that remove or redirect headwater streams and build retention ponds to homeowners who improperly fertilize their lawns or fail to maintain septic systems, many people do not recognize the link between an individual’s actions and the broader context of cumulative ecological effects. In an EPA 1997 study, urbanization was shown to have a direct impact on stream ecology. Therefore, recent efforts have focused on reducing runoff from suburban and urban areas, resulting in the implementation of urban BMPs such as EPA’s Low Impact Development program. This program, like agricultural BMPs, requires landowner buy-in, where the homeowner or homeowner’s association is responsible for maintaining the BMP.

Bibliography:

Committee on the Causes and Management of Eutrophication, Ocean Studies Board, Water Science and Technology Board, and National Research Council. 2000. Clean Coastal Waters: Understanding and Reducing the Effects of Nutrient Pollution. Washington, DC: National Academies Press.
Sharpley, A. N., T. Daniel, T. Sims, J. Lemunyon, R. Stevens, and R. Parry. 2003. Agricultural Phosphorous and Eutrophication. 2nd ed. ARS-149. Washington, DC: U.S. Department of Agriculture, Agricultural Research Service.
S. Environmental Protection Agency. 2000. “National Water Quality Inventory.” EPA-841-R-02-001. Washington, DC: U.S. Environmental Protection Agency, Office of Water.
Wetzel, Robert G. 2001. Limnology: Lake and River Ecosystems. 3rd ed. San Diego, CA: Academic Press.

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