When considering the water quality of lakes and streams, two important questions come to mind: "What are nutrients?" And: "Why are nutrients a problem in lakes and streams?"
Nutrients are chemical elements critical to the development of plant and animal life. In healthy lakes and streams, nutrients are needed for the growth of algae that form the base of a complex food web supporting the entire aquatic ecosystem . The most common nutrients in lakes and streams are nitrogen and phosphorus.
Under the right conditions, including abundant nutrients, algae and aquatic plants will continue to grow and multiply well beyond the amount needed to support the food web. The excess growth then dies, and microorganisms break it down, consuming dissolved oxygen from the water in the process. Dissolved oxygen, which aquatic organisms need just as humans need oxygen from the air, can be completely used up by the breakdown process. When this happens, aquatic organisms die from lack of oxygen. Extensive fish kills can result.
Eutrophication is the process of enrichment of lakes and streams with nutrients, and the associated biological and physical changes. Eutrophication is a natural process, but human activity has dramatically increased its rate in many waterbodies. * Lakes and ponds are particularly vulnerable to eutrophication because the nutrients carried into them continue to buildup; in contrast, the nutrients can be carried away in moving water.
Some results of excessive eutrophication are visible: thick mats of algae in the water; scum and foam; odor and taste problems; and death and disease of fish and other aquatic organisms. Other effects, such as the reduction in dissolved oxygen, cannot be seen directly, although the conditions often produce visible results such as dead fish.
An increase in the water's pH as a result of the increased growth of algae is another impact that is not directly visible. High pH can be toxic to fish and other organisms, and it can also make other substances, such as ammonia, even more toxic than they are otherwise.
Excess nutrients not only affect stream health but also may impact human health and livestock. Although phosphorus is not toxic to human adults in moderate concentrations, high levels of nitrate in drinking water (10 milligrams per liter or greater) can injure or kill livestock or human infants. Nuisance species of algae, such as some cyanobacteria (also called blue-green algae), produce toxins that affect the nervous system and liver, posing a threat to animals and humans who ingest them.
The worldwide increase in red tides and other blooms of toxic algae in coastal ocean waters has been linked to nutrient enrichment coming from coastal rivers. Nuisance species such as these, in fresh water as well as coastal oceans, can increase and force out less tolerant species, resulting in a loss of aquatic biodiversity .
Several sources of nutrients are found in lakes and streams. Some are from natural sources, but many stem from human activities.
Nutrients are present naturally in lakes and streams, but human activity has greatly increased the amounts going into surface waters. Background levels of nitrogen and phosphorus are generally quite low and are normally measured in milligrams per liter. Background nitrate concentrations in streams are usually less than 0.6 milligrams per liter, whereas background phosphorus rates in streams are even lower, less than 0.1 milligrams per liter. Soil and rocks are the primary natural sources of phosphorus, usually in the form of phosphates . Natural nitrogen sources include leaves and other organic debris from riparian vegetation.
Wastewater (or sewage) treatment plants are point sources of nutrients by virtue of the effluent which they discharge directly to rivers and streams. Unless the effluent has received tertiary treatment, or treatment to remove nutrients, it can be a significant contributor.
In the United States, treatment plants are regulated under the federal Clean Water Act. Under the act, if nutrients are a problem, then more stringent controls can be imposed. Tertiary treatment is expensive, and requires new systems, so some plants spread their effluent on land during times when nutrients could cause water quality problems.
In the past, household detergents brought high loads of phosphorus to treatment plants, which then were discharged with the effluent. In the United States, however, laws restricting the phosphorus content of detergents have produced markedly reduced phosphate levels.
Septic systems may contribute large amounts of nutrients, particularly if located close to the water. Standard septic systems do not remove nitrates; however, special systems like sand filters that remove nutrients are now becoming more common.
Sediment from excessive erosion is a nonpoint source that transports phosphorus in particles attached to soil. Construction sites lacking effective erosion control systems can dramatically increase the amount of sediment reaching lakes and streams, bringing in large phosphorus loads.
Manure is a significant source of nutrient pollution in lakes and streams. Manure from livestock, if not properly managed, can reach streams through runoff or from direct deposits by animals in the water. The U.S. Geological Survey estimates that more than 7 million metric tons (nearly 16 billion pounds) of nitrogen and more than 2 million metric tons (more than 4 billion pounds) of phosphorus are applied to agricultural lands as manure each year. In the same way, pet waste is a nutrient source in urban and suburban areas, and aggregations of ducks, geese, and other waterfowl have also caused problems.
Commercial fertilizers are a major source of both phosphorus and nitrogen. According to the U.S. Geological Survey, about 12 million metric tons (26 billion pounds) of nitrogen and 2 million metric tons (4 billion pounds) of phosphorus are applied annually in commercial fertilizer in the United States. Depending on the composition of the soil in an area, irrigation amounts and application methods, and the amount of rainfall, nutrients not needed by crops either run off the land into lakes and streams, build up in the soil, or seep down into groundwater. Groundwater can seep into a stream and be a source of nutrients.
Atmospheric nitrogen comprises about 78 percent of the air that humans breathe. The burning of fossil fuels forms oxidized nitrogen compounds, which then reach the Earth when it rains or snows. In some parts of the United States, in particular the Northeast and the Upper Midwest, the so-called "acid rain" associated with these processes conveys large nitrogen loads to lakes and streams. The U.S. Geological Survey estimates that more than 3.5 million metric tons (nearly 7 billion pounds) of atmospheric nitrogen are deposited in the United States each year.
SEE ALSO Acid Rain ; Algal Blooms in Fresh Water ; Algal Blooms in the Ocean ; Chemicals from Agriculture ; Clean Water Act ; Estuaries ; Fresh Water, Natural Composition of ; Groundwater ; Lake Management Issues ; Lakes: Biological Processes ; Lakes: Chemical Processes ; Lakes: Physical Processes ; Pollution of Lakes and Streams ; Pollution of the Ocean by Sewage, Nutrients, AND Chemicals ; Septic System Impacts ; Stream Health, Assessing .
Roberta J. Lindberg
Carpenter, S. et al. "Nonpoint Pollution of Surface Waters with Phosphorus and Nitrogen." Issues in Ecology 3 (1998).
Mueller, D. K., and D. R. Helsel. Nutrients in the Nation's Waters—Too Much of a Good Thing? U.S. Geological Survey Circular 1136 (1996).
Puckett, L. J. Nonpoint and Point Sources of Nitrogen in Major Watersheds of the United States. Water-Resources Investigations Report 94-4001 (1994).
The Tualatin River, located southwest of Portland, Oregon, has a history of pollution problems dating back 100 years, when industry and sewage treatment plants dumped waste into the stream. These point sources of pollution were cleaned up and regulated under the federal Clean Water Act (enacted in 1972), yet pollution problems stemming from overenrichment continued.
Studies identified various nonpoint sources of nutrients, including soil erosion and manure from livestock. The state of Oregon helped landowners improve runoff control, while sewage plants upgraded their phosphorus removal technology.
Despite major progress, officials could not identify all the sources of phosphorus until continued study revealed groundwater as a nutrient source. Releasing more water from upstream dams in the summer to dilute the phosphorus has helped keep the Tualatin River from again being so alive it was dying.
* See "Lake Management Issues" for a schematic showing natural and cultural (human-induced) eutrophication in a lake.