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Open-File Report 94-378
Nashville, Tennessee -- 1994

Biomonitoring Our Streams

What's It All About?


A USGS scientist in preparation for a biomonitoring experiment

Prepared by the

in cooperation with the


What is Biomonitoring?

Testing for chemical pollution in our nation's streams has traditionally meant using analytical chemistry. In recent years, environmental agencies have endorsed biological monitoring to enhance or replace chemical monitoring. The theory behind biological monitoring (biomonitoring) is to use the organisms living in the aquatic system as a measure of water quality. This concept was applied to air quality and used by miners who took canaries into deep mines with them. If the canary died, the miners knew the air was bad, and that they had to leave the mine.

Biomonitoring an aquatic system uses the same theoretical approach. Aquatic organisms are subject to pollutants in the stream as it flows by, day and night. Consequently, the health of the organisms reflects the quality of the water they live in. If the pollution levels reach a critical concentration, certain organisms will migrate away, fail to reproduce, or die, eventually leading to the disappearance of those species at the polluted site. Normally, these organisms will return if conditions improve in the system.

The three general components of an aquatic ecosystem that influence the biological community are: water chemistry, geomorphology, and hydrology. Each component influences the health of the biological community individually and together. Toxic chemicals are only a single factor within the water-chemistry component. The relation of these three components to each other can be shown on a triangle (figure 1).

Figure One
FIGURE 1 -- Three general components that influence the biological community composition

The relative importance of one component may change with time and location. At certain times of the year or in different geographic settings, a single factor may exert primary control on the well-being of the biological community. For example, straightening a stream and removing all woody debris drastically alters a stream's geomorphology. This results in loss of natural habitat and shelter for certain organisms. The organisms that require this shelter will disappear from the modified system. Separating the influence of one component from the others is difficult. Consideration of all three components and their interactions is critical when interpreting biomonitoring data.

Factors that control toxicity of a chemical

Two important factors that must always be considered when discussing chemical toxicity are the concentration and the length of exposure. The concept of concentration and exposure are practiced routinely by farmers when they apply pesticides. First, the farmer calculates the concentration required to cover and protect a crop based on the manufacturers' recommendations. Second, the farmer contemplates the length of exposure needed when considering the weather conditions. Prediction of rain often causes a farmer to postpone spraying because the rain would wash the pesticide off the crop, reducing the length of exposure. A sufficient concentration must be applied over an adequate exposure period for the chemical to be effective. Concentration and length of exposure are the basic factors that control toxicity of any chemical.

What is involved in a biomonitoring project?

A biomonitoring project begins with an experimental design and site selection. The next step includes selecting the organisms appropriate for the stream and types of contamination of concern. After the organisms have been selected, the appropriate collection technique must be applied. Finally, once the samples have been collected, identified, and counted, the data must be interpreted. These steps are described in more detail in the following tables.

Collecting a sample

TABLE 1 -- Approaches to site selection and some of the benefits and limitations of each
Approach              Expectation and limitation of results                 

Paired Basin  Two basins with similar features are compared to determine
             differences in water quality that result from differences in
             land use. Benefits: Good for comparison purposes. Limitations:
              Difficult to find two basins that are similar in all ways
              except the one variable being evaluated.

Upstream-     Quality of water entering at a site is evaluated to
 downstream   establish a known baseline condition.  Then, the water quality
              is evaluated again after it has traveled through a defined
              test site (past a factory, a landfill, a farm, or a wetland).
              Benefits: Changes in water quality that are the result of the
              test site can be distinguished.  Limitations: Can only be used
              on a moderate scale; often the test site (stream or river) has
              other inputs of water that enter through springs or ditches
              which contribute to changes in water quality.  All of the
              inputs must be considered before inferring a cause and effect.

Trend         Trend analysis is used to determine changes in water quality
 analysis     over time as a result of changing management practices.  For
              example, implementing conservation practices or improving a
              waste-treatment facility should improve water quality over
              time.  Benefits: Provides an opportunity to see and quantify
              improvements in water quality because of changes in operating
              procedures.  Limitations: Difficult to see any significant
              change in water quality over time in large areas because of
              variability resulting from natural events.  Thus, many years
              of data must be gathered before a statistically significant
              improvement in water quality is demonstrated.  This problem
              can be minimized by running a paired basin study concurrently.

TABLE 2 -- Target organisms and some general applications in water-quality monitoring
Category                          Application                                 

Bacteria         Detection of certain bacteria indicates sewage contamination.

Algae and        Exceptionally rapid growth of aquatic plants and algae
 aquatic plants  typically indicates nutrient
                 enrichment of the surface water.  Death during the growing
                 season may indicate acute turbidity (blocking sunlight) or
                 herbicide exposure.

Zooplankton     Microscopic animals that are indicative of toxins, turbidity,
                 and nutrient enrichment. Useful because these organisms have
                 a brief life-cycle, providing a fast response.  However, use
                 has been limited by their patchy distribution and the
                 inconvenience of their microscopic size.

Macro-           Good general indicators of overall water quality. They are
 invertebrates   sensitive to water chemistry, hydrology, and geomorphology.
                 Various species have a distinct range of sensitivity to
                 pollutants.  Thus, they are often used to indicate various
                 degrees of environmental degradation.

Fish             Good indicators of the ecosystem since fish rely on the other
                 categories for food. Thus, they reflect the overall health of
                 the entire food chain.  Their use is limited by their patchy

Collecting an artificial substrate colonized by aquatic insects

TABLE 3 -- Sampling techniques and the criteria for use
Sampling device               Good for use in the following situations       

Grab or scoop     Devices that are designed to penetrate bottom sediment and
 samplers         collect a sample in standing or flowing water. Samples are
                  sifted through a sieve to sort the organisms.  Large items
                  like twigs or stones are searched by hand.  This technique 
                  is primarily used to collect macroinvertebrates and aquatic

Nets:             Used primarily to collect macroinvertebrate, fish, and 
                  zooplankton samples.

    Kick net      Useful in small streams with gravelly bottoms and good flow
                  velocity.  Nets are stretched across the stream and the bed
                  upstream is disturbed by kicking; organisms flow into the

    Stream net    Long tapering nets with a wide, rigid opening. Nets are
                  secured in place and organisms passively drift or swim
                  into the net over a 24-hour period.  Used for fish and
                  macroinvertebrates, depending on net design and mesh size.

    Gill net      Used to sample fish of a certain size range. Useful in
                  large rivers and lakes.
    Seine net     Used to sample fish in shallow ponds or streams. A wide net
                  that reaches from top to bottom of the water and is pulled
                  at each end; used to sweep across a shallow body of water
                  and capture fish.

Artificial        Sampling devices made of natural or artificial material.
 substrates       The configuration of the substrates vary with the target 
                  organisms.  These substrates can consist of ceramic tiles,
                  glass slides, wood plates, leaf packs, cement balls, or
                  other items. Organisms are quantified per a known surface
                  area.  This technique is used to collect macro-
                  invertebrates or algae in rivers, lakes, and streams.
                  Substrates should be placed at the same depth in the water
                  column because different organisms will colonize the
                  substrates at different depths.  

Electroshocking   Used in streams, rivers, and lakes.  Strong electrical
                  pulses are sent between two electrodes.  Fish will move
                  toward the positive pole.  This is a nonlethal method to
                  capture fish.  Extremely dangerous technique and should
                  only be done by trained professionals. 

Interpreting the data

Data collected during a biomonitoring study are interpreted in different ways. Usually, the total number of organisms and the number of different species present are determined. Then these numbers are applied to an index that lists organisms according to their sensitivity to pollution. This produces an "index of biotic integrity." Other statistical procedures can be used to help interpret the data. In general, a greater diversity of organisms is usually an indicator of a healthy stream. The total number of organisms is not always indicative of the health of the stream. Sometimes a polluted stream can be filled with undesirable species.

A tremendous amount of data and information is required from a biomonitoring program before a direct cause and effect can be assigned. In essence, biological monitoring allows us to look at the effect of changing water quality without necessarily knowing what caused the change. Still, environmental scientists often refer to biomonitoring as more relevant than chemical monitoring in spite of interpretation limits. This is because biomonitoring measures the environmental consequence of water quality and not simply the chemical concentration.

In conclusion, biomonitoring provides a quick, pertinent overall environmental picture. This information is important to people who make management decisions aimed at maintaining and protecting the quality of our rivers and streams. Biomonitoring is particularly valuable for alerting people to deteriorating aquatic conditions just as canaries alerted the miners to deteriorating air conditions.

Sorting organisms from the sediment sample.

Questions to consider when conducting a biomonitoring study:

  1. What is the objective of the biomonitoring study?

  2. What monitoring strategy should be used to collect the data: an upstream-downstream approach, a paired basin approach, or a trend analysis approach? Refer to table 1 for more information.

  3. What target organisms were selected? And, is a seasonal fluctuation in population associated with the target organisms?

  4. Has the appropriate sampling technique been selected for the study?

  5. Does the reference site have a similar hydrologic and geomorphic setting compared to the test site?

  6. Has the stream bed been modified to change the geomorphology or hydrology? If yes, how will you distinguish between the effects of chemistry and hydrology or geomorphology on the biologic response?

  7. Was the biologic index designed for this particular geographic region used? If not, was the standard adapted to the area being studied?

The Cumberland River, mile marker 175, Nashville, Tennessee

Photographs by A. Webbers, U.S. Geological Survey

This material is based in part upon work supported by the U.S. Department of Agriculture, Extension Service, under special project number 91-EHUA-1-0063.

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