Northwestern Europe has a country-side criss-crossed by numerous rivers and canals, dotted with numerous lakes and countless smaller bodies of water, permanent or temporary. This abundance of freshwater has however become compromised through human interference in reshaping land and bodies of water to his needs and the dumping of pollutants. To secure future demands on the supply of freshwater, awareness of the ecological consequences of human actions is needed.

Detection of ecological changes in freshwater environments can be effected by repeated surveillance of biological communities, or parts thereof, at specific sites. Most long-term methods of biological monitoring involve bioassessments of communities of macro-invertebrates, less frequently of protozoa, benthic or periphytic algae (esp. diatoms), fish, or aquatic macrophytes.

This program will give an introduction to the families of freshwater macro-invertebrates reported from northwest European waters. Considered as macro-invertebrates are those organisms larger than (0.5-)1.0 mm and for the practice of biological monitoring usually include sponges (Porifera), hydras and jellyfishes (Cnidaria), flatworms (Platyhelminthes), segmented worms and leeches (Annelida), molluscs and bivalves (Mollusca), waterspiders and mites (Chelicerata), crustaceans (Crustacea), and insects (Insecta).

The above list excludes a number of taxa falling in the same category, which are only to be sorted through the use of strong magnification. In most studies therefore, the roundworms (Nematoda) and the crustacean Copepoda and Cladocera are rarely considered. As this practice partly determined our approach to the problem of creating a pictorial key, it explains why for a number of taxa the decision was taken not to progress beyond a certain taxonomic level.

A notable deviation from common practice is the inclusion of a key to the families of rotifers (Rotifera). Rotifers can become the dominant life-form inhabiting our tapwatersystem as a number of species are able to pass filtration-beds.