What Can Leaf Pack Data Tell Us About Stream Health?

Leaf pack macroinvertebrate data

To study water quality, scientists study both chemistry (what is dissolved in water) and biology (what is alive in the water). Chemical monitoring provides a “snap shot” of the condition of the stream at the time the sample is collected. In contrast, the living organisms show what is happening in the stream over a period of time.

Certain types of plants and animals are more tolerant than others to changes in habitat and water quality. The most commonly studied organisms in streams are fish, algae, and macroinvertebrates. Of these groups, the macroinvertebrate community is the most commonly used because:

  • They are fairly easy to sample and identify.
  • They are sensitive to pollution and changes in their habitats.
  • They have a relatively long life cycle and so are indicators of water quality over a period of time.
  • They are common in most streams or rivers.

Monitoring for Pollution Sensitivity

Although the Leaf Pack Experiment is not designed to monitor water quality, some basic assessment measurements may still yield interesting information. Two commonly used indices are percent of EPT and biotic index.

The EPT and Biotic Index are used to monitor the sensitivity of taxa to pollution. Some insects are more tolerant of pollution than others. Although these pollution tolerant groups, which include certain true flies and worms, can be found in the cleanest streams, their numbers should not dominate the community. On the other hand, the presence of mayflies, stoneflies, and caddisflies—sometimes referred to as the “canaries of the stream” because they are so sensitive to pollution—indicates good water quality.

Percent EPT

Percent EPT is short for the total number of Ephemeroptera (mayflies), Plecoptera (stoneflies), and Trichoptera (caddisflies). Many species within these three groups are sensitive to changes in water quality.

In general, the more EPT taxa, the better the water quality. There are exceptions: certain netspinning Trichoptera (Hydropsychidae) are more tolerant to pollution than other caddisflies and are therefore not counted as part of the total Percent EPT.

The following two tables show an example macroinvertebrate sample and how to calculate its percent EPT.

Example Macroinvertebrate Data

Taxon Total Number
Ephemeroptera (mayflies) 2,184
Plecoptera (stoneflies) 32
Trichoptera (caddisflies)
    Hydropsychidae (common netspinners) 667
    Other caddisflies 73
Anisoptera (dragonflies)
Zygoptera (damselflies)
    Corydalidae (hellgrammites)
    Sialidae (alderflies)
Coleoptera (beetles) 409
Diptera (true flies)
    Athericidae (watersnipe flies)
    Chironomidae (midges) 1517
    Simuliidae (black flies) 22
    Tipulidae (crane flies) 22
    Other Diptera
Amphipoda (scuds)
Isopoda (aquatic sowbugs)
Decapoda (crayfish)
Oligochaeta (aquatic worms) 54
Hirudinea (leeches)
Turbellaria (planarians)
Gastropoda (snails)
Total 4,980

Example Percent EPT Calculations

Step 1
Add the total number of mayflies, stoneflies, and other caddisflies.
Remember: Hydropsychidae don’t count towards percent EPT!
2,184 + 32 + 73 = 2,289
Step 2
Divide the number of EPT individuals by the total number of individuals in the samples.
EPT ÷ total # of individuals = 2,289 ÷ 4,980 = 0.46
Step 3
Convert to percentage.
0.46 x 100 = 46%

Biotic Index

Biotic Index is a comparison of the abundance of taxa and their tolerance to environmental stress. This widely used index can indicate organic and nutrient pollution. Organisms are assigned tolerance values which range from 0 to 10, depending on the organism’s sensitivity to changes in water quality and habitat (tolerance values increase as water quality decreases).

Although originally designed for use at the species level, this index is now more commonly used at the family level. A modified version at the order/family level is provided. While the modified version is less accurate than its unmodified counterpart, it is useful for quick assessments.

In contrast to the percent EPT index, the lower the biotic index, the better the water quality.

The following two tables show an example macroinvertebrate sample, how to calculate its biotic index, and how to use the biotic index to evaluate water quality.

Example Macroinvertebrate Data

Taxon Total Number Pollution Tolerance Value Total Tolerance Value
Ephemeroptera (mayflies) 2,184 3.6 7862.4
Plecoptera (stoneflies) 32 1 32
Trichoptera (caddisflies)
    Hydropsychidae (common netspinners) 667 5 3335
    Other caddisflies 73 2.8 204.4
Anisoptera (dragonflies) 4
Zygoptera (damselflies) 7
    Corydalidae (hellgrammites) 3
    Sialidae (alderflies) 4
Coleoptera (beetles) 409 4.6 1881.4
Diptera (true flies)
    Athericidae (watersnipe flies) 2
    Chironomidae (midges) 1517 6 9102
    Simuliidae (black flies) 22 6 132
    Tipulidae (crane flies) 22 3 66
    Other Diptera 6
Amphipoda (scuds) 6
Isopoda (aquatic sowbugs) 8
Decapoda (crayfish) 5
Oligochaeta (aquatic worms) 54 8 432
Hirudinea (leeches) 8
Turbellaria (planarians) 8
Gastropoda (snails) 7
Total 4,980 23,047.2

Example Biotic Index Calculations

Step 1
Total # of each taxon x Pollution Tolerance Value =
Total Tolerance Value
Step 2
Sum the Total Tolerance Value column
Step 3
Divide Total Tolerance Value by Total # of Individuals
23,047.2 ÷ 4,980 = 4.63
Step 4
Look up the Biotic Index Value in Table 3
Biotic Index = 4.63 = Good Water Quality

Evaluation of Water Quality from Biotic Index

Biotic Index Water Quality Degree of Organic Pollution
Less than or equal to 3.75 Excellent Organic pollution unlikely
3.76 to 5.0 Good Some organic pollution
5.1 to 6.5 Fair Substantial pollution likely
6.6 to10.0 Poor Severe organic pollution likely

How Healthy is Your Stream?

Enter the macroinvertebrate counts from your Leaf Pack Experiment into the Biotic Index and Waters Quality Calculator to find out.