Biological water quality classification (saprobic system) is the oldest existing water quality classification, and a map has been published every five years since 1975 until 2000 by the Working Group of the Federal States on Water Issues (LAWA), the Biological Water Quality Map of Germany. It uses biological indicators (macrozoobenthos = invertebrates dwelling on the riverbed) to describe the oxygen content of watercourses, which is significantly influenced by the degree of pollution with organic constituents that are biodegradable under oxygen depletion. Special signatures were used for the first time in the 1995 quality map to identify river sections for which an assessment using the saprobic classification is impeded or rendered impossible by other impairments (e.g. salt levels, acidification, algal blooms).
Biological water quality classification has the following classes:
Water quality class I (dark blue): unpolluted to very lightly polluted
River sections with pure water that is always more or less saturated with oxygen and low in nutrients; low bacteria content; moderately densely colonised, mainly by algae, mosses, flatworms and insect larvae; if cool in summer, spawning grounds for salmonids.
Water quality class I-II (light blue): lightly polluted
Water sections with low inorganic or organic nutrient supply without any appreciable oxygen depletion; densely populated, usually with wide variety of species; if cool in summer, spawning grounds for salmonids.
Water quality class II (dark green): moderately polluted
River sections with moderate pollution and good oxygen supply; very wide variety of species and high individual density of algae, snails, small crustaceans, insect larvae; water plant populations may cover large areas; wide variety of fish species.
Water quality class II-III (yellow-green): critically polluted
Water sections with critical loads of oxygen-depleting organic substances; fish mortality due to oxygen deficiency; decline in number of species of macroorganisms; tendency to mass development of certain species; filamentous algae often form populations covering large areas.
Water quality class III (yellow): heavily contaminated
Water sections with heavy oxygen-depleting organic pollution and mostly low oxygen content; local sapropel deposits; large-area colonies of filiform wastewater bacteria and sessile ciliates exceed the occurrence of algae and higher plants; only a few macroorganisms resistant to oxygen deficit conditions, such as sponges, leeches, aquatic isopods occasionally occur in large quantities; low fish catches; fish mortality must be expected to occur periodically.
Water quality class III-IV (orange): very heavily contaminated
Water sections with considerably restricted living conditions as a result of very heavy pollution with oxygen-depleting organic substances, often exacerbated by toxic effects; sometimes total lack of oxygen; turbidity due to suspended solids from wastewater; extensive sapropel deposits, densely colonised by red midge larvae or tube worms; decline in filiform wastewater bacteria; fishes are found only intermittently and, if at all, in locally limited areas.
Water quality class IV (red): excessively contaminated
Water sections with excessive pollution as a result of oxygen-depleting organic waste waters; putrefaction processes predominate; for long periods of time oxygen is only available in very low concentrations or is lacking completely; colonised largely by bacteria, flagellates and free-living ciliates; no fish; biological devastation in cases of high toxic stress.
Source: Working Group of the Federal States on Water Issues (LAWA)