Hydrobionts with special requirements for the state of the environment. Their presence, condition and behaviour are used to assess changes in the habitat, including the presence and concentration of pollution. 

An indicator of environmental condition is often not only their presence or absence in a water body, but also the intensity of their development. Such indicators can be macrophytes, some crustaceans, molluscs, worms (oligochaetes), insects (mayflies, stoneflies, caddisflies, etc.) and fish. For example, mirror carp and goldfish become a nuisance in the presence of oil and chemical industry effluents in the water. The minnow's high sensitivity to pollution makes it a reliable indicator of drinking water quality. The Siberian stone loach, blue loach, trout and other fish can be used as indicators of water purity. The paddlefish and other small aquatic crustaceans found in many water bodies can be monitored for changes (biochemical, physiological or behavioural) that may indicate a problem in their ecosystem. 

Bioindicators can tell us about the cumulative effects of different pollutants on an ecosystem and how long a problem may have existed, which physical and chemical tests cannot do. 

Bioindicators (from bio... and lat. indico, indicating, determining), organisms whose presence, number or developmental characteristics serve as indicators of natural processes, conditions or anthropogenic changes in the habitat. 

Many organisms are very sensitive and selective to various environmental factors (chemical composition of soil, water, atmosphere, climatic and weather conditions, presence of other organisms, etc.) and can only exist within certain, often narrow, limits of change in these factors. For example, aggregations of seabirds indicate the arrival of shoals of fish. 

Specific plankton and benthic organisms indicate the origin of water masses and currents and characterise certain habitat parameters (salinity, temperature, etc.). 

An important aspect of the use of bioindicators is their ability to assess the level of pollution of the natural environment and to monitor its quality and changes. For example, the composition of flora and fauna, the numerical ratio of their individual representatives are used to assess the degree and nature of pollution, the suitability of water for drinking and domestic purposes, as well as the effectiveness of treatment facilities. 

Microalgae have attracted attention in recent years for several reasons, including their greater sensitivity to pollutants than many other organisms. In addition, they are abundant in nature, they are an important component of many food webs, they are easy to grow and use in analyses, and there are few, if any, ethical problems with their use. 

Euglena gracilis is a motile freshwater flagellate capable of photosynthesis. Although Euglena is relatively tolerant of acidity, it reacts rapidly and sensitively to environmental influences such as heavy metals or inorganic and organic compounds. Typical reactions are inhibition of movement and changes in orientation parameters. In addition, this organism is very easy to handle and grow, making it a very useful tool for ecotoxicological assessments. A very useful feature of this organism is its gravireceptor orientation, which is very sensitive to pollutants. Gravireceptors are affected by pollutants such as heavy metals and organic or inorganic compounds. The presence of such substances is therefore related to the random movement of cells in the water column. In short-term tests, the gravitational orientation of E. gracilis is very sensitive. 

Other species, such as Paramecium biaurelia, also use gravitational orientation. Automated bioassays using Euglena gracilis flagellates in a device that measures their motility at different dilutions of a potentially contaminated water sample to determine the EC50 (concentration in the sample that affects 50 per cent of the organisms) and G-value (the lowest dilution factor at which a minor toxic effect can be measured) are possible. 

Benthic macroinvertebrates live in the benthic zone of a stream or river. They consist of aquatic insects, crustaceans, worms and molluscs that live in vegetation and river channels. Macroinvertebrate species can be found in almost all streams and rivers, except in some of the world's most hostile environments. They can also be found in almost any size stream or river, except those that dry up very quickly. This makes them useful for many studies because they can be found in regions where river channels are too shallow to support larger species such as fish. Benthic indicators are often used to measure the biological components of freshwater streams and rivers. 

In general, if the biological functioning of a stream is considered to be in good condition, it is assumed that the chemical and physical components of the stream are also in good condition. 

Benthic indicators are the most commonly used water quality test in the United States. Although benthic indicators should not be used to track sources of stress in rivers and streams, they can provide information on the types of sources that are often associated with observed stressors.