Experts agree that it is not sufficient to monitor the state of watercourses using exclusively physical-chemical variables. Instead it is considered necessary to complement them with monitoring the most important constituents of the biocoenosis. Changes in water quality can also be indicated through biocoenosis. Suitable eutrophication indices are the mean concentrations of phytoplankton (chlorophyll a) and of periphyton on artificial substrates of the growing season. Changes in benthic fauna, particularly changes in species composition constitute a good monitoring object. Monitoring of many bioaccumulating substances and compounds which appear in low concentrations requires studies of organisms which are indigenous to the watercourse. Alternatively organism may be incubated in the watercourse under study for a fixed period of time. In many cases, particularly in recipient waters of industrial waste-water, it is necessary to use various biotests as well as toxicity tests.
