A Citizen's Guide to Understanding and Monitoring Lakes and Streams

Chapter 3 - Streams

Nutrients in Streams

Why Are They Important?

Nutrients in streams serve the same basic function as nutrients in a garden. They are essential for growth. In a garden growth and productivity are considered beneficial, but this is not necessarily so in a stream. The additional algae and other plant growth allowed by the nutrients may be beneficial up to a point, but may easily become a nuisance.

The main nutrients of concern are phosphorus and nitrogen. Both elements are measured in several forms. Phosphorus can be measured as total phosphorus (TP), or soluble reactive phosphate (SRP) (also sometimes called phosphate (PO4) or orthophosphate (ortho-P). The last, three represent different terms used to describe the fraction of TP that is soluble or available to organisms for growth.

Nitrogen can be measured as total nitrogen (TN), total Kjeldahl nitrogen (TKN), nitrate-nitrogen (NO3), nitrite-nitrogen (NO2) [these are usually measured as nitrate-nitrite-nitrogen (NO3 –NO2)], or ammonia-nitrogen (NH4). TN is similar to TP and is used to represent the total amount of nitrogen in a sample. TKN represents the fraction of TN that is unavailable for growth or bound up in organic form; it also includes NH4. The remaining fractions (NO3 – NO2 and NH4) represent bioavailable forms of nitrogen. If they are summed they can be compared to the SRP fraction of phosphorus.

One chemical form of an element can be converted into another. The conditions under which the conversion occurs are influenced by many factors, such as pH, temperature, oxygen concentration, and biological activity.

The total concentration of a nutrient (e.g., TP or TN) is not necessarily the most useful measurement. For example, if a sample is analyzed for TP, all forms of the element are measured, including the phosphorus "locked up" in biological tissue and insoluble mineral particles. It may be more useful to know the concentration of phosphorus that is actually available for growth. SRP better reflects bioavailability.

Although there are many different forms of nutrients that can be measured there are only three commonly used combinations. These are: (1) measure all forms of both elements – TP, SRP, TN, NO3 - NO2, NH4; (2) measure only total nutrients – TP and TN; or (3) measure only available nutrients – SRP and NO3 - NO2, and NH4. (In the first example TKN could be exchanged for TN. In either case, the remaining fraction can be estimated by difference.)

Reasons for Natural Variation

The concentration of nutrients and the form they are found in changes continually. How and why they change is a very complex field of study. First, the total input of nutrients varies depending upon land use and other factors. During the summer, nutrient input may increase due to fertilization of cropland or lawns and gardens. During the winter, high rainfall causes increased wash-off of organic matter such as leaves, twigs, grass, and other debris. Because decomposition of this organic matter releases nutrients, it constitutes an important source of nutrient loading.

If the stream is fed by a lake or other water source with naturally high variations in nutrient concentrations, the stream will reflect the same variations. In the Puget Sound region, salmon carcasses from annual spawning migration represent a large seasonal source of organic matter and nutrients.

Whether the increase in total nutrient concentrations results in higher available nutrient concentrations, and therefore an immediate increase in growth or productivity, depends upon the original form of the nutrient and physical conditions. If nutrients enter as organic matter that first needs to be decomposed before it can be utilized for growth, temperature becomes important due to its effect on the rate of decomposition. (During warmer month, nutrients entering the system as intact organic matter would be decomposed relatively quickly as compared to cold, wet-weather months when decomposition is slow).

These dynamics are further complicated by the fact that increased growth leads to greater numbers of organisms that need even more nutrients. So, as nutrients become available they are immediately utilized. In this case, an increase in total nutrients would not be reflected by any measurable increase in available nutrient fractions. In short, clear, simple relationships between increases in organic matter or other sources of nutrients, and resultant increases in either total nutrient concentrations or available nutrient concentrations, become obscure.

Expected Impact of Pollution

Increased nutrient concentrations are almost always an impact of pollution. Municipal and industrial discharges usually contain nutrients, and overland flow from developed watersheds contains nutrients from lawn and garden fertilizers as well as the additional organic debris so easily washed from urban surfaces. Agricultural areas also contribute to nutrient increases through poor manure and fertilizing practices and increased erosion from plowed surfaces.

Nutrient loading can result in increased algae growth. In stream segments where conditions are right, algae take the form of an attached growth – called periphyton – on rocks, logs, and other substrate. You may have noticed long green filaments or masses of algae in streams or even stepped on rocks made slippery by these growths. Excessive growths of attached algae can cause low DO, unsightly conditions, odors, and poor habitat conditions for aquatic organisms.

Nutrient Concentrations (µg/L) Measured in Three Western Washington Streams During 1988-89. (No TN data are available).

    TP SRP NH4 NO3-NO2
Cedar River A 14 0 18 290
  S 7-30 5-16 1-111 126-195
  W 10-25 3-11 1-29 315-585
Newaukum A 100 83 100 1987
  S 43-65 37-59 1-28 1440-2350
  W 74-213 55-180 72-380 1560-3020
Springbrook A 194 147 451 608
  S 180-254 52-230 175-725 320-740
  W 105-293 80-234 74-1240 405-963

A = yearly average; S = summer range (May - Oct.); W = winter range (Nov.-Apr.)

Revised From: Metro 1990. Quality of Local Lakes and Streams, 1988-1989 Status Report. Municipality of Metropolitan Seattle, Water Resources Section.

Nutrient concentrations are reported in units of micrograms or milligrams of nutrient per liter of water (ug/L or mg/L). There are no WashingtonState water quality standards for nutrient concentrations. The table above summarizes nutrient data from three Western Washington streams.

The next section discusses total suspended solids and turbidity in streams.

Chapter Four provides information about how to measure nutrients in streams.

Stream Temperature | Stream Dissolved Oxygen | Stream pH | Stream Nutrients | Stream TSS and Turbidity | Stream Fecal Coliforms | Return to Table of Contents