Use of Aquatic Invertebrates in the Assessment of Water Quality

Pollution is essentially a biological phenomenon in that its primary effect is on living organisms. A biological investigation of a polluted lake or stream has several advantages over chemical analyses. It is less time-consuming because a single series of samples can reveal the status of the animal and plant communities which themselves represent the results of the summation of the prevailing conditions. The animals and plants provide a record of the prevailing conditions and are not affected by a temporary alleviation of a polluting effluent.

Intensive studies were carried out by Gaufin and Tarzwell in 1952 and 1956 to determine the effects of organic pollution on the aquatic communities of Lytle Creek and the value of these populations as indicators of pollutional conditions. The studies revealed that little reliance could be placed upon the mere occurrence of a single species in a given locality as an indicator of pollution. In the creek the nine species of macroinvertebrates which were most numerous in the septic zone also occurred in the recovery and clean water zones, but in much smaller numbers. The septic zone had less than one fifth as many species as the clean water zone, but the total number of organisms per unit area was many times greater. The septic zone was characterized by species adapted to live in low dissolved oxygen concentrations or those able to secure their oxygen directly from the air.

In the clean water zone there was a great variety of invertebrate communities, each consisting of many different species. Most of the species which occurred in the septic and recovery zones were also found in very limited numbers in the clean water zones. In addition, there was also present a wide variety of forms which were intolerant of conditions in the polluted zones. Most of these were the gill-breathing, immature stages of such insects as the mayflies, stoneflies, caddis flies, and alder flies.

In evaluating the reliability of aquatic organisms as indicators of pollutional conditions and water quality, one must consider the different indicator organisms not separately but as biological associations or communities. The organisms should be considered as groups according to their morphological adaptations and physiological requirements.

Several formal systems for the biological assessment of pollution based on community composition have been suggested. A reduction in community diversity has been noted in a number of rivers polluted by organic wastes. Diversity indices, based on information theory, are very useful for comparing changes in community composition in streams altered by pollution.