Science of Water : Concepts and Applications

Environmental Biomonitoring, Sampling, and Testing 221
support a wide variety of algae, fi sh, and other organisms, but in the section of the water body where
oxygen levels are low (below 5 ppm), only a few types of worms survive. As stream fl ow courses
downstream, oxygen levels recover, and those species that can tolerate low rates of oxygen (such
as gar, catfi sh, and carp) begin to appear. In a stream, eventually, at some further point downstream,
a clean water zone reestablishes itself, and a more diverse and desirable community of
organisms returns.
During this characteristic pattern of alternating levels of dissolved oxygen (DO) (in response to
the dumping of large amounts of biodegradable organic material), a stream, as stated above, goes
through a cycle called an oxygen sag curve. Its state can be determined using the biotic index as an
indicator of oxygen content.
The biotic index is a systematic survey of macroinvertebrates organisms. Because the diversity
of species in a stream is often a good indicator of the presence of pollution, the biotic index can be
used to correlate with stream quality. Observation of types of species present or missing is used as
an indicator of stream pollution. The biotic index, used in the determination of the types, species,
and numbers of biological organisms present in a stream, is commonly used as an auxiliary to BOD
determination in determining stream pollution.
The biotic index is based on two principles:
1. A large dumping of organic waste into a stream tends to restrict the variety of organisms
at a certain point in the stream.
2. As the degree of pollution in a stream increases, key organisms tend to disappear in a
predictable order. The disappearance of particular organisms tends to indicate the water
quality of the stream.
There are several different forms of the biotic index. In Great Britain, for example, the Trent
Biotic Index (TBI), the Chandler score, the Biological Monitoring Working Party (BMWP) score,
and the Lincoln Quality Index (LQI) are widely used. Most of the forms use a biotic index that
ranges from 0 to 10. The most polluted stream, which, therefore, contains the smallest variety
of organisms, is at the lowest end of the scale (0); the clean streams are at the highest end (10).
A stream with a biotic index of greater than 5 will support game fi sh, whereas a stream with a biotic
index of less than 4 will not.
As mentioned, because they are easy to sample, macroinvertebrates have predominated in biological
monitoring. In addition, macroinvertebrates can be easily identifi ed using identifi cation keys
that are portable and easily used in fi eld settings. Present knowledge of macroinvertebrate tolerances
and response to stream pollution is well documented. In the United States, for example, the
Environmental Protection Agency (EPA) required states to incorporate a narrative biological criteria
into its water quality standards by 1993. The National Park Service (NPS) has collected macroinvertebrate
samples from American streams since 1984. Through its sampling effort, the NPS has
been able to derive quantitative biological standards (Huff, 1993).
Macroinvertebrates are a diverse group. They demonstrate tolerances that vary between species.
Thus, discrete differences tend to show up, containing both tolerant and sensitive indicators.
The biotic index provides a valuable measure of pollution. This is especially the case for species
that are very sensitive to lack of oxygen. An example of an organism that is commonly used
in biological monitoring is the stonefl y. Stonefl y larvae live underwater and survive best in cool,
well-aerated, unpolluted waters with clean gravel bottoms. When stream water quality deteriorates
due to organic pollution, these larvae cannot survive. The degradation of stonefl y larvae has an
exponential effect upon other insects and fi sh that feed off the larvae; when the larvae disappears,
so in turn do many insects and fi sh (O’Toole, 1986).
Table 8.1 shows a modifi ed version of the BMWP biotic index. Considering that the BMWP
biotic index indicates ideal stream conditions, it takes into account the sensitivities of different macroinvertebrate
species, which are represented by diverse populations and are excellent indicators of