Common measures of water quality
Chemical/physical measures
Physical measurements include heat, suspended solids, colour and clarity, and acidity or alkalinity (pH).
Chemical contaminants include nutrients, hydrocarbons, pesticides, organic wastes, metals, cations, and anions. The properties of chemical contaminants and their effects vary widely. Some effects are immediate and readily observable, such as oil spills, toxic effects for humans, or stock or aquatic biota. Other effects may be ongoing and insidious, such as the accumulation of persistent compounds in the food chain.
Some chemical contaminants such as nutrients (e.g., nitrogen and phosphorus) are needed for aquatic plant growth, but when they occur in excess and under the right physical conditions they can cause algal blooms and prolific growths of aquatic plants with associated adverse effects on other aspects of water quality. Some forms of nitrogen, such as nitrate or ammonia, are also toxic in excess. Nitrate can cause health effects in human infants and ammonia can be toxic to aquatic biota.
Dissolved oxygen is also measured, not because it is a contaminant, but because it is a requirement for most living things and some contaminants can reduce its concentration.
Biological measures
Microorganisms, including bacteria, viruses and protozoa, can cause diseases in humans and in stock. Because the disease-causing organisms are often difficult to measure directly, an indicator is usually measured, such as E. coli for freshwater, or enterococci for marine water.
See Microbiological Water Quality Guidelines for further information.
Macroinvertebrates are useful measures of water quality because the composition of species present at a site can be used as an indicator of water quality. Sensitive species are only found at sites with high water quality, while tolerant species tend to be dominant at sites with degraded water quality and few species survive under very poor water quality.
See Macroinvertebrates as Indicators at for further information.
Periphyton can also be a useful indicator of water quality because the composition of species and total biomass can reflect water quality. Typically, periphyton growth is greatest in water with a high concentration of nutrients (e.g., nitrogen and phosphorus) as long as other physical factors (e.g., light, flow and flood disturbances) are favourable. However care must be taken with interpretation because these physical factors often limit periphyton growth in waters – even when nutrient concentrations are high.
See New Zealand Periphyton Guideline for further information.
Other measures
Another common method, which measures the effect of effluents on aquatic organisms directly, is known as Whole Effluent Toxicity Testing (WETT).
A simple and user-friendly set of chemical and biological measures is summarised in the NZ Stream Health Monitoring and Assessment Kit.
