Stream Bioassessment Laboratory
ZOO 4430
R. Hall
Introduction: One of the ways that an ecologist can examine the “health” of an
ecosystem is to look at the the organisms found there. This is called bioassessment. It is
simply a technique where the assemblage of organisms can give an index as to the degree
to which the ecosystem is stressed. The basic principle is that some species are more
tolerant of pollution or stress than others. If an ecosystem has nothing but tolerant species
then it may be polluted. If an ecosystem has intolerant species, then it is unpolluted, or
unstressed. Another way one can examine ecosystem health is to look at the diversity of
species present. Be aware that the information from bioassessment is only an index of the
degree of pollution. That is, it is not measuring the rate of pollution or overall ecosystem
heath in some defined units. An analogy would be measurement of rainfall. We can
directly measure rainfall easily with rain gauges. If measuring rainfall were not so easy,
we would have to rely upon an index, such as amount of dead grass in a lawn, as an
indicator of past rainfall.
Bioassessment has been applied to different systems using different species,
however much of the use is in aquatic habitats such as streams. Entomologists classify
different species into according to their tolerance of pollution. Some species (e.g. mayfly
nymphs) are intolerant of any sort of organic pollution, while others (midge larvae) are
tolerant of polluted streams. Thus finding mayflies in a stream means that the water
quality is probably good. However, finding midge larvae does not mean that water
quality is bad; midge larvae can live in both polluted an unpolluted streams. It is the lack
of any intolerant taxa that would lead one to believe that the stream is polluted.
Bioassessment is often better than chemical measures of water quality because
bugs integrate stress through a long time periods. Suppose some creep offloads a bunch
of sewage/poison etc. into a river once a month at 2 AM. You, the water quality monitor,
will never detect this stuff when you sample after your morning coffee break. The bug
populations on the other hand will not bounce back by 10 AM the next day (or even in 6
months); therefore, the absence of key taxa tells you that something is indeed wrong with
the river. These data will hold up in court, thus bioassessment is commonly practiced.
Recently it was shown that these bioassessment indices can correlate with changes in
ecosystem function. Wallace et al. (1996) poisoned a stream for 2 years. Bioassessment
dropped off dramatically, with concurrent changes in ecosystem function, i.e. litter
decomposition, carbon export, invertebrate secondary production. Bottom line is that
these indices work.
When using both of these methods, it is important to have some sort of reference.
What if you lived in a part of the country where there is only 3 tolerant and no intolerant
species living in pristine streams? (A good example would be the hot springs at
Yellowstone National Park). You cannot call them polluted if you know that they are
indeed unpolluted. When using a bioassessment index one must consider how many and
what kind of species live in an unpolluted stream. Thus it is important to compare results
with a stream in the same locale that you think is relatively pristine.
Objectives:
1. To compare the invertebrate assemblage, using bioassessment indices a
gradient of the Laramie river, presumed to follow a gradient of human impact.
2. Using a bioassessment index, determine the degree of pollution in the
polluted stream.
Methods:
Location: Four sites on the Laramie river , Jelm, Monolith, Downtown Laramie, Below
Laramie.
Sampling and analysis.
We collected, picked and sorted samples from each of the four sites. We will use 4
different bioassessment methods to assess the Laramie River
1. Izaak Walton League method. We will broadly follow the protocol given by the Izaak
Walton League “Save Our Streams” program. The Izaak Walton League is a
conservation organization in the U.S. Indeed, Izaak Walton was the first dry-fly
fisherman. We will use their protocol for several reasons:
1. It is relatively simple to use and it is not necessary to be an aquatic
entomologist to identify the samples.
2. It combines both diversity and taxon specific tolerance in a single index.
3. It is being used by people all over the country by people interested in streams.
Therefore the results we get can be put into a national database of hundreds of streams
assessed using the same methods.
Hilsenhoff FBI. This is a more in-depth analysis and requires that you know each of the
families. Sort the bugs into different piles, and determine the families. Calculate the
FBI using the techniques in the Hilsenhoff paper (it is very straightforward). Note that
this technique is a simplified version of the HBI, where one identifies organisms to
species, and calculates the biotic index.
Note that Hilsenhoff is an insect person and leaves oligochaetes off his list. Based
on Lenat (1993) I have calculated a value of 8.9 for Oligochaetes.
EPT index. EPT is one of the most powerful indices, believe it or not. It is simply the
number of Ephemeroptera, Plecoptera, and Trichoptera taxa found in the streams. We
should use genera, and not families, but all we have is families. No matter, we should still
see a trend.
Percent model affinity, developed by Novak and Bode for the New York DEC. Basically
one compares the invertebrate assemblage a stream of unknown water quality with a
“model” stream. The model stream is a compendium of many different streams that are
known to be unaffected by pollution. Thus higher quality streams will have a close
affinity to the model streams. We will calculate the percent model affinity using the the
Jelm site as the “model”. Note the problem that we have only one model stream. This is
not good, however in a five hour lab there is not time to calculate a model stream
database the size of the one they have in New York.
Calculate the percent model affinity using the same categories that Novak and Bode use
for each of the three downstream sites.
Which of these three methods work the best? Which reach is the most affected by
pollution? Which is easiest
References
Hilsenhoff, W. L. 1988. Rapid field assessment of organic pollution with a family-level
biotic index. Journal of the North American Benthological Society. 7:65-68.
Lenat D.R. 1993. A biotic index for the southeastern United States: derivation and list for
tolerance values, with criteria for assigning water-quality ratings. Journal of the
North American Benthological Society 12:279-290.
Wallace, J. B., J. W. Grubaugh, and M. R. Whiles. 1996. Biotic indices and stream
ecosystem processes: results from an experimental study. Ecological Applications
6:140-151.
Data Sheet (Taken from the Save Our Streams stream quality survey)
Names______________
____________________
Stream_________________ Date________________
Location of sampling_________________
Weather__________________
Sensitive
___caddisfly larvae
___hellgrammite
___mayfly nymphs
___gilled snails
___riffle beetle adult
Somewhat sensitive
Tolerant
___beetle larvae
___aquatic worms
___clams
___blackfly larvae
___crane fly larvae ___leeches
___crayfish ___midge larvae
___damselfly nymphs ___pouch snails
___stonefly nymphs ___scuds
___water penny larvae
___sowbugs
___fishfly larvae
___alderfly larvae
___atherix
_____ # of taxa times 3
_______ index value
_____ # of taxa times 2
+
______ index value
+
Add the three index values to get a total value ___________
Water quality rating
Excellent (>22)
Good (17-22)
Fair (11-16)
Poor (<11)
_____ # of taxa times 1
_______ index value