Eutrophication
Eutrophication is the process by which water bodies become enriched with nutrients, primarily due to human activities such as agriculture, wastewater discharge, and urban runoff. This influx of nutrients, especially nitrates and phosphates, accelerates the growth of algae, leading to a subsequent decline in water quality. The process typically progresses through stages, starting from oligotrophy, characterized by clear water and low productivity, to mesotrophy, and ultimately to eutrophy, where algal blooms become prominent and oxygen levels in deeper waters diminish. The final stage, known as hypereutrophy, results in excessive algal growth, often leading to the formation of "dead zones"—areas with severely low oxygen levels that can adversely affect aquatic life. Eutrophication is not limited to lakes; it can also occur in rivers and coastal waters, causing significant ecological and economic impacts, including detrimental effects on fisheries and tourism. Understanding eutrophication is vital for developing strategies to manage nutrient inputs and protect water quality in various aquatic environments.
On this Page
Subject Terms
Eutrophication
DEFINITION: Overenrichment of a water body with nutrients
The process of eutrophication, in which a body of water receives an excessive amount of nutrients that accelerates aquatic plant growth, leads to a reduction in the oxygen in the water that fish need to survive. Continued enrichment can lead to excessive algal growth, which in turn impairs fisheries, limits recreational uses of the water, and can create major problems for water-supply purveyors.
From a geologic perspective, lakes are relatively temporary features of a landscape. For example, the Great Lakes were formed over the last two million years by four or more major advances of huge ice sheets. Only relatively recently (about twelve thousand years ago) did these lakes develop into their current shapes.
Most lakes go through a series of trophic (nutrition-related) states that can take thousands of years before the lake basins eventually in with sediment. Oligotrophy is the first trophic stage; it is characterized by clear water, low plant productivity because of limited inputs, and high levels of dissolved oxygen throughout the water column. Crater Lake in Oregon is a good example of an oligotrophic lake. The next stage is mesotrophy, which has moderately clear water and moderate plant productivity and lower levels of oxygen in the hypolimnion, which is the lowest level in a lake. In the third stage, eutrophy, excess nutrients are present, transparency is reduced, algal scums start to appear, and oxygen (which fish need) is not present in the summer in the hypolimnion. Hypereutrophy is the final stage; in this stage algal scums dominate in the summer, few macrophytes (plants that can be seen without magnification) exist, and the hypolimnion is devoid of oxygen.
Anthropogenic (human-caused) changes in lakes result from excessive inputs of sediment, fertilizers (such as nitrates and phosphates), pesticides from agricultural practices, wastewater from animal-intensive industries such as livestock farming and meat processing plants, from urban sewage, and industrial effluents such as lead and mercury. The inevitable result is accelerated or cultural eutrophication, which can occur in decades as compared to the thousands of years seen under natural conditions.
Reservoirs that are used for water-supply purposes are particularly sensitive to algal blooms. If sufficient light and warmth are present, which is common in the high sun season, thermal occurs, and the lower zone of the (where the hypolimnion is located) becomes anaerobic. This lack of oxygen enables algal nutrients, such as silica, phosphorus, and ammonia, to come from bottom sediments, which in turn increases more algal growth, and the cycle continues. The reservoir surface becomes covered with algae, which then requires either physical removal or large applications of copper sulfate, which in itself can cause problems.
Eutrophication can occur in streams and marine environments as well as in lakes and reservoirs. Coastal waters, bays or estuaries that are partially enclosed, and shallow seas provide excellent opportunities for unbridled nutrient enrichment. For example, excess nutrients emanating from the extensive agricultural area in the midwestern United States flow down the Mississippi and Missouri rivers to where the rivers empty into the Gulf of Mexico. As a result, a hypoxic (depleted of oxygen) area known as a dead zone appears on a fairly regular basis off the mouth of the Mississippi River in the gulf. Although it varies in size, this dead zone has been known to cover an area of approximately 21,000 square kilometers (8,000 square miles), which is larger than the entire state of Massachusetts. In 2014 Texas A&M professor of oceanography Steve DiMarco, a leading expert on the dead zone, reported that the dead zone measured 2,600 square miles (about 6734 square kilometers).
Eutrophication has been linked to seaweed tides in which tons of seaweed are beached within a short period of time along coastal areas. Two of the most common seaweeds responsible for increasing numbers of such stranding events are green tides and golden tides. The green tides are due to macroalgae of the genus Ulva, which have affected coastal regions in the United States, Europe, and Qingdao, China. In 2011 a golden tide, due to Sargassum, affected Atlantic coastlines in west Africa as well as Trinidad and southern Caribbean islands. Seaweed tides have a negative impact on tourism, aquaculture, and fisheries.
Bibliography
Cech, Thomas V. Principles of Water Resources: History, Development, Management, and Policy. 3d ed. New York: John Wiley & Sons, 2010.
Cunningham, William P., and Mary Ann Cunningham. Principles of Environmental Science: Inquiry and Applications. 4th ed. New York: McGraw-Hill, 2008.
"Eutrophication." Britannica, 9 Jan. 2025, www.britannica.com/science/eutrophication. Accessed 15 Jan. 2025.
Gray, N. F. Drinking Water Quality: Problems and Solutions. 2d ed. New York: Cambridge University Press, 2008.
Laws, Edward A. “Cultural Eutrophication: Case Studies.” In Aquatic Pollution: An Introductory Text. 3d ed. New York: John Wiley & Sons, 2000.
North American Lake Management Society and Terrene Institute. Managing Lakes and Reservoirs. Madison, Wisc.: Author, 2001.
Smetacek, Victor, and Adriana Zingone. "Green and Golden Seaweed Tides on the Rise." Nature 504.7478 (2013): 84. MasterFILE Premier. Web. 15 Jan. 2015.
Texas A&M Univ. "Researchers Say Dead Zone in Gulf Smaller." Galveston County Daily News, The 16 July 2014: n. pag. NewsBank. Web. 15 Jan. 2015.