Non-point Source Pollution

Fertilizer runoff
Source: NOAA
Non-point source pollution arises when contaminants are carried into waterways by natural processes, like runoff or air currents. A common example is when runoff carries fertilizers from farms into waterways. Harder to pinpoint - because the source covers a large land area - and more difficult to regulate than point source pollution, especially because of the lack of a single responsible party, non-point source pollution is a serious and insidious threat to ecosystem health. Examples of major contaminants include:

Suspended sediment

suspended sediment
Source: USGS and Barbara Hite
Sediments occur naturally and are integral components of aquatic systems. Nearly all waters contain suspended sediments that may be of physical, chemical or biological origin, and the quantities of these sediments usually vary with season. This natural variation in suspended sediment concentrations occurs typically in response to natural events (i.e. rainfall and snow melting) which increase the flow and sediment levels of the waterways. As a result, in order to ensure their survival, aquatic species have adapted their life cycles to accommodate these natural variations in the environment (Birtwell 1999). However, the input of suspended sediment from catastrophic events such as floods and volcanic eruptions, and anthropogenic activities such as dredging, mining, and releasing water from dams, are recognized as potential threats to the well-being of marine biota.

Although sediment, and its associated effects on water clarity and turbidity, is a natural component of aquatic systems, it is apparent from scientific research that there is an increased risk to the survival of aquatic organisms when sediment levels exceed background values for a particular period of time. There are many ways which an excessive amount of sediment might be harmful to a fishery. These include:

  1. Acting directly on the fish swimming in the water in which solids are suspended, either by killing them or reducing their growth rate, resistance to disease, etc. Increased turbidity and decreased light penetration alter fish feeding and schooling practices, leading to reduced survival. The high concentrations of sediments also irritate the gills of fish and can cause death. In addition, sediment can destroy the protective mucus covering the eyes and scales of fish, making fish more susceptible to infections.
  2. Preventing the successful development of fish eggs and larvae. For example, especially under reduced flow conditions, settleable solids in river waters have the potential to be deposited in streams, where they may exert a detrimental influence on fish eggs in spawning beds.
  3. Modifying natural ecosystems. High concentrations of sediments can fill spaces in the river bottom, displacing or smothering plants, invertebrates, and insects in the river bed. This directly affects the food source of fish, and can result in smaller and fewer fish.
  4. Carrying toxic agricultural and industrial compounds, which can cause abnormalities or death in fish. (Environment Canada 2001) In order to facilitate the protection of aquatic organisms from elevated levels of sediment in their environment, guidelines and criteria have been formulated. As early as 1964, the European Inland Fisheries Advisory Commission (EIFAC) put forth such guidelines for the protection of fishery resources, which are as follows:
* Parts per million approximate (mg- L--1 ) Numerous criteria and guidelines have been formulated since then, and more recent ones have been based on the analyses of Newcombe and MacDonald (1991), Anderson et al. (1996), and Newcombe and Jensen (1996) and Caux et al. (1997). These authors state that aquatic biota respond to both concentration of suspended sediments and the duration of exposure to them, and relate the two through an "index of pollution intensity" or "stress index" (Birtwell 1999). Newcombe and MacDonald's 1991 paper recommended the use of a "stress index" that is "calculated by taking the natural logarithm of the product of concentration and duration" and would provide resource managers with a method to predict the effects of pollution episodes on aquatic biota. The British Columbia Ministry of Environment, Lands, and Parks (BCMELP) (1998), and the Canadian Council of Ministers of the Environment (CCME) (1999) guidelines are the most recent documents on this topic, and they are based, in part, on the publication by Caux et al. (1997).

It is recognized that there is some level of risk to aquatic organisms depending on the sediment levels discharged and the sensitivity of the organisms in the receiving stream. However, scientists have concluded that these impacts would be best assessed using the concentration of suspended sediment above background levels. The levels of risk and the corresponding concentrations of sediment follow:

Source: Birtwell 1999

It is concluded that elevated levels of sediment may be harmful to fish, and in addition, negatively impact their habitat. Criteria, guidelines and recommendations, though formulated by many different government agencies, tend to be mutually supportive. At the same time they have application limitations, especially relating to the protection of aquatic organisms from the effects of sediment concentrations of tens of mg- L-1. Application of the criteria must be done while recognizing potential impacts on aquatic organisms at both the lethal and the sublethal level. Particle size and nature of the sediment must be considered as well (Birtwell 1999).

Excess Nutrients

Nutrients are required by aquatic ecosystems for primary production; plants, often algae, absorb these nutrients and use them to grow. These plants form the base of the food chain in aquatic ecosystems. However, excess nutrients, especially nitrogenous compounds, are carried by runoff from agricultural areas and cause a phenomenon called eutrophication. The nutrients over-fertilize the ecosystem and cause an explosion in algae population--an algal bloom. When this huge mass of algae dies, however, it consumes oxygen in its decomposition, lowering the dissolved oxygen content for the waterway in general. Because aquatic organisms cannot remove oxygen from air or from water molecules, they rely upon oxygen dissolved in the water to survive; if this oxygen is depleted, the aquatic community essentially asphyxiates. Eutrophication has been a major problem in estuarine areas, like the Chesapeake Bay in Maryland, USA and continues to be a problem in freshwater lakes and ponds as well.


Trace metals are required for aquatic life but in higher concentrations heavy metals such as iron, lead, mercury, aluminum, and magnesium are toxic to fish, especially at low pHs (PA FBC). One reason metal toxicity is such a problem is that no natural processes exist to neutralize or remove them (Chapman, 1996). Metals also tend to accumulate in bottom sediments (Chapman, 1996), which presents a problem if those sediments are later disturbed. Industrial wastewater discharges (point source) and mining are common metal sources, although metals like lead (from automobiles) can also come from atmospheric deposition. Aluminum, cadmium, chromium, copper, iron, mercury, manganese, nickel, lead, zinc, arsenic, and selenium are the commonly monitored "metals" although beryllium, thallium, vanadium, antimony, and molybdenum are also very toxic and important to monitor if a pollutant source is likely to discharge them (Chapman,1996).

Detergents, pesticides, industrial toxins, pharmaceuticals, etc.

There are a variety of other toxins that can harm fish, even if present only in small quantities. Some toxins, such as PCBs and chlordane, are not only toxic but also tend to bioaccumulate, meaning that organisms high on the food chain ingest large amounts of the toxin through their prey and then have it build up in their bodies. Not only is this detrimental to fish and ecosystem health, but it is also a danger to consumers, who are at the top of the food chain. Health advisories are in place in many parts of the United States for high levels of mercury, PCBs, and chlordane in many fish and other aquatic species. Other contaminants, such as pesticides, can have severe effects on aquatic ecosystems by poisoning the most sensitive organisms. There is also evidence that pharmaceutical products, especially hormones, that are released into the water cause health problems in many species (Boxall et al, 2003). Other ecosystem-damaging contaminants like detergents, petroleum products, and industrial toxins also can be carried into waterways.