Question

There are many types of nonpoint source pollutants. When these accumulate in high enough concentrations in a waterbody, they can seriously affect the environment and the organisms living there. They can also affect human health.

The primary nutrients of concern in nonpoint source pollution are nitrogen and phosphorus. Both are essential for plant growth, but if too much of these substances enters a waterbody, it can lead to a condition called eutrophication (pronounced you-tro-fi-kay-shun). Eutrophication results in an overproduction of organic matter, particularly the microscopic plants called algae (Bricker et al, 1999).

You may have seen green masses of algae growing on a pond or lake. This excess algae blocks the sunlight needed by native bottom-dwelling plants, often killing them. As the algae and bottom-dwelling plants die, they sink to the bottom and are decomposed by bacteria, which use up oxygen in the water. Throughout most of the year, oxygen mixes completely throughout the waterbody. However, during the warmer months, or when ice forms on the surface, temperature differences cause the water to become stratified with warmer water at the surface and cooler water at the bottom. This layering prevents the complete mixing of the oxygen, which in turn can lead to a condition called hypoxia -- very low levels of oxygen in the water. Hypoxia makes it difficult for aquatic animals like fish and crabs to survive. A NOAA report notes that "potential consequences of eutrophication range from mere nuisances to serious human health threats" (NOAA, 2004).

A lake covered with algae.
Algae on a lake in Spain
In addition to hypoxia, eutrophication may be associated with conditions that result in harmful algal blooms (HABs). Harmful algae are often small, single-celled organisms that live in aquatic environments. Although these organisms are not harmful in small quantities, too many of them can negatively affect the environment and people's health. When fish and shellfish feed on HABs, they can accumulate toxins that the algae produce. Consequently, when people eat seafood with algal toxins in it, they may get sick. The distribution, frequency, and intensity of HABs appears to be increasing worldwide (National Research Council, 1999).

Nonpoint sources of nutrients often originate from agricultural activities (EPA, 2004c). Excess nutrients applied to crops in the form of fertilizers are washed away in runoff, typically during rainstorms. Nutrients also originate from urban and suburban areas, from sources such as lawn fertilizers, and even pet wastes.

Nitrogen and phosphorus also come from atmospheric inputs. Scientists believe that the combustion of fossil fuels like oil and coal by power plants, large industries, and automobiles is a major source of nutrients in the atmosphere (USGS, 2004). Controlling nutrient inputs is proving to be very difficult because the nutrients frequently originate from multiple sources that are challenging to identify and controlWhat are the consequences of increased nitrogen and phosphorus into water systems? (Site 3)

Answer 1

Increased inputs of nitrogen and phosphorus to waterbodies cause a cascade of ecological, human-health, and economic problems:

- Eutrophication and excess algal growth — nutrients fuel overproduction of algae and phytoplankton.
- Harmful algal blooms (HABs) — some blooms produce toxins that can sicken or kill wildlife, pets, and people and contaminate seafood.
- Light blockage and loss of submerged plants — dense algae reduce light reaching bottom plants, killing them and degrading habitat.
- Oxygen depletion (hypoxia) and “dead zones” — when algae and plants die, bacterial decomposition consumes dissolved oxygen, making water uninhabitable for fish and many invertebrates.
- Fish kills and reduced biodiversity — oxygen stress and habitat change lead to loss of sensitive species and altered food webs.
- Human health risks — exposure to algal toxins (through drinking water, recreation, or contaminated seafood) can cause illnesses.
- Reduced water quality and aesthetics — bad odors, unpleasant tastes, scums, and turbid water make water unusable for recreation and drinking without costly treatment.
- Economic impacts — losses to fisheries, tourism, and increased water-treatment and management costs.

Overall, excess nitrogen and phosphorus destabilize aquatic ecosystems, threaten wildlife and human health, and impose substantial social and economic burdens.