Macroinvertebrates as bioindicators
Now that you have collected a standardized sample of the stream benthic community, we
will now evaluate that sample to calculate an Index of Biotic Integrity (IBI). An IBI is a
scored evaluation of biotic (living) components. Compared to other measures of stream
quality which are physical or chemical in nature, a biotic measurement uses the diversity
of organisms at your site to determine quality. The most common biotic indices include
fish, vegetation, herpetofauna (reptiles and amphibians), and benthic macroinvertebrates.
The battery of physical and chemical measurements that we perform cannot account for
every possible source of impact, but the benthic index can provide an indication of the
overall picture of stream quality.
How and why do we use macroinvertebrates to measure stream quality?
1. The macroinvertebrate community is extremely diverse, represented by thousands of
different species with a variety of feeding strategies.
2. The pollution tolerance levels of macroinvertebrates range from very high (10), being
able to withstand considerable pollution levels, to very low (0). The relative ratio of
sensitive and tolerant macros forms the basis of the IBI.
3. Sampling macroinvertebrates can be performed with relative ease by a single
individual with simple equipment.
4. The aquatic life spans of macroinvertebrates range from several weeks to several
years. This long life span provides an indication of stream quality over a period of time,
not just the sampling window.
5. Unlike fish, macroinvertebrates are fairly limited in mobility, meaning they cannot
avoid polluted areas. The adults will lay the eggs where they may, and the benthic larvae
are dependent upon the water quality and habitat to survive.
6. The methods for collecting, subsampling, preserving, and identifying
macroinvertebrates are well established; facilitating comparison of data between sites.
7. Macroinvertebrates can be found in any aquatic habitat as long as the water quality is
high enough to sustain them. Fish can be restricted by insufficient habitat quality such as
cover and depth, or by fish barriers such as dams.
8. Finally, macroinvertebrates communities can recover rapidly from repeated sampling
events, providing the ability for repeated sampling.
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Materials:
 White grid trays
 Tweezers
 Dissecting scopes
 Red and white dice
 Ethanol
 Benthic sample
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Specimen vials
Separating trays
Macro ID keys
Plastic spoons
Note cards
Parafilm
Procedure:
Preserving and accurately identifying macroinvertebrates can become quite time
consuming, especially if the sample is quite large. For these reasons, you will be
subsampling the sample we collected at your stream site and calculate the IBI from a
subsample of 100+ organisms. You employed a variety of standardized methods during
our collection to ensure that the macroinvertebrate sample you collected was
representative of the community at the site. You will now include even more scientific
standardization to ensure that the subsample you use is also representative. You cannot
just pick out the first 100 organisms you see, because you may be biased in which
organisms you notice. Instead, you will be randomly subsampling the sample. To do
this, you will be spreading out the sample on a grid tray, and randomly selecting which
grids will be used to collect and identify at least 100 macroinvertebrates.
* See TTW Subsampling Instructions
Kingdom
Phylum
Class
Order
Family  Macros will be identified to this level.
Genus
Species
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Macroinvertebrate IBI Calculation
Now that you have identified all the macroinvertebrates from the standardized subsample,
you will use them to calculate the IBI (Index of Biotic Integrity). There are many
different metric indices which can be used to evaluate a macroinvertebrate sample. The
majority of the metrics used in the MBSMP protocol have been adapted from the
Maryland Biological Stream Survey (MBSS). This allows the data that you collect to be
used in a comparable fashion to other stream study data gathered in the state.
Considerable effort has been applied in identifying and creating metrics that can
accurately explain and assess differences in water quality through the collection of
macroinvertebrates.
Table 1 shows the variety of metrics that will be used to calculate the IBI, the values
associated with each metric, and the scores each value receives.
Metrics *
Score
5
>14
3
9 – 14
1
<9
EPT families
>10
4 – 10
<4
Ephemeroptera families
>3
2–3
<2
Diptera families
>3
2–3
<2
% Ephemeroptera
>20.3
5.7 – 20.3
<5.7
Intolerant families
>8
4–8
<4
>12.5
8 – 12.5
<8
Number of families
Beck’s Biotic Index
Table 1. Metric Indices for the Benthic IBI
* Indices and scores from CBWP-MANTA – EA-98-3
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Each metric will be determined separately, and then all the values averaged to produce
the final IBI score. The following list defines each of the metrices and explains how and
why they are used to compute the IBI.
1.) Number of families – The total number of families of macroinvertebrates identified
from the sample. This metric gives an indication of the overall variety of the
macroinvertebrate community. Report the total number of families detected.
2.) EPT families – The total number of families within the three insect orders
Ephemeroptera (Mayflies), Plecoptera (Stoneflies), and Trichoptera (Caddis flies)
identified in the sample. This metric gives an indication of the variety of these pollution
sensitive orders. Report all of the EPT families detected.
3.) Ephemeroptera families – The total number of families collected and identified
within the pollution-sensitive Ephemeroptera (Mayflies) order. Report all of the
Ephemeroptera families detected.
4.) Diptera families – The Diptera (True flies), are an extremely diverse order with
some 18 semi-aquatic families represented in Maryland. This metric gives an indication
of their diversity within the sample. Report all of the Diptera families detected.
5.) % Ephemeroptera – The Mayflies are a typically sensitive group. The relative
percentage of Mayflies within the sample can give strong indications of water quality.
Report the % of the sample (individuals) that are mayflies.
6.) Intolerant families – Within each macroinvertebrate order, there are tolerant (4-10)
and intolerant (0-3) families. The number of intolerant families identified within the
sample will give an indication of water quality. Sum all of the intolerant families
collected and report that value. *See the Tolerance Values Table.
7.) Beck’s Biotic Index – This index is a weighted sum of intolerant taxa. [(2 x number
of Class 1 taxa) + (number of Class 2 taxa)]. Class 1 taxa have tolerance values of 0 and
1, Class 2 taxa have tolerance values from 2 to 4. Calculate the metric as shown above
and report that value. *See the Tolerance Values Table.
Once each metric has been calculated, enter the values and scores on Table 2.
Average all of the scores to determine the final overall IBI.
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Table 2. Scoring the IBI Metric Indices
Averaged IBI Score Range
4.0 – 5.0
3.0 – 3.9
2.0 – 2.9
1.0 – 1.9
Metric
Associated Rating
Good (Non-impacted)
Fair (Somewhat impacted)
Poor (Impacted)
Very Poor (Severely impacted)
Value
Score (1, 3, or 5)
Number of families
EPT families
Ephemeroptera families
Diptera families
% Ephemeroptera
Intolerant families
Beck’s Biotic Index
Averaged IBI Score
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Table 3. Qualification of the IBI Value
1. What rating did you come up with for your class’ stream site? __________________
2. How did this rating compare to the qualitative and quantitative surveys your class
performed? _____________________________________________________________
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Funded by a grant through NOAA
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