Pollution Tolerance Index

Invertebrates can be sorted into three pollution categories

After collecting, identifying and counting invertebrate samples, the results can be checked against the pollution tolerance index. A majority of invertebrates tending falling into any one category will indicate a certain level of water quality.

Organic Pollution Tolerance Index For Aquatic Macroinvertebrates
Decreasing Pollution Tolerance --->
Pollution Tolerant	Moderately PollutionTolerant	Pollution Intolerant
Increasing Water Quality --->
Presence in great numbers may indicate poor water quality but can be found in any type of water	Presence in great numbers may indicate fair water quality	Presence in great numbers may indicate good water quality
blackfly larvae bristleworms clams flatworms leeches midge larvae round worms lunged snails	amphipods caddisfly larvae cranefly larvae dragonfly nymphs damselfly nymphs gilled snails	mayfly nymphs stonefly nymphs

Keep in mind the following points when using this pollution tolerance index:

• Organic nutrient pollution refers to higher concentrations of phosphorus and nitrogen. This index does not reflect the effects of toxic chemical pollution.

• Benthic invertebrate populations are most useful as pollution indicators during periods of stable water flow. Large increases in river flow may cause an unusual scour that results in areas being washed clean of invertebrates.

• Invertebrates are most sensitive to pollution during the summer when high temperatures and low river flows are already causing environmental stress. Fish may also die under these stressful conditions.