The physical and chemical characteristics of waterbodies affect the abundance, species composition, stability, productivity, and physiological condition of aquatic organism populations. Biological methods used to assess water quality include the collection, counting, and identification of aquatic organisms; biomass measurements; measurements of metabolic activity rates; measurements of pollutant toxicity, bioconcentration, and bioaccumulation; and processing and interpretation of biological data.
Information from these methods may serve one or more of the following purposes:
• To explain the cause of color, turbidity, odor, taste, or visible particulates in water;
• To help interpret chemical analyses (e.g., relating the presence or absence of certain biological forms to oxygen deficiency or supersaturation in natural waters);
• To identify the source of one water that is mixing with another;
• To explain the clogging of pipes, screens, or filters, and to help design and operate water and wastewater treatment plants;
• To determine optimal times for treating surface water with algicides and to monitor treatment effectiveness;
• To determine the effectiveness of drinking water treatment stages, to help determine the effective chlorine dose in a water treatment plant, and to indicate treatment problems or deficiencies;
• To identify the nature, extent, and biological effects of pollution;
• To indicate the progress of self-purification in waterbodies;
• To help determine the condition and effectiveness of unit processes and biological wastewater treatment methods in a wastewater treatment plant;
• To document short- and long-term variability in water quality caused by natural phenomena or human activities;
• To provide data on the status and trends of an aquatic system;
• To correlate the biological mass or components with water chemistry or conditions. Note: A statistical correlation may not always signify a cause-and-effect relationship because of the presence of confounding variables or unknown covariates.