Many small animals (invertebrates) live in and on the river bottom of the Rhine, such as bryozoa, insects, molluscs, crustaceans, hirudinea, turbellaria and freshwater sponges. They can hardly be made out with the eye and are summarized as macrozoobenthos. Since 1990, the ICPR regularly draws up an inventory of the invertebrates between Lake Constance and the North Sea.
The construction of wastewater treatment plants reduced water pollution and thus led to increased oxygen content of the Rhine, which distinctly increased the number of small animals.
In 2012, more than 500 invertebrate species were detected in the Rhine between the Alps and the North Sea. In 2000, some 300 invertebrate species were found in the Rhine, most of them along the High Rhine and in the southern Upper Rhine. In 1990 and 1995, scientists had only detected about 200 species – the invertebrate fauna is recovering. However, the invertebrates found are undemanding and many insect species which frequently occurred in the Rhine 100 years ago, such as the mayfly Oligoneuriella rhenania are still missing. Some species characteristic of the river and which had vanished from the Rhine tend to spread again. The most recent example is the anisoptera, which larvae have several times been spotted in the Rhine.
Many euryoecious “new species“, such as mussels, snails and copepods from the Black Sea, North America and Asia arriving with vessels are found among the small animals on the bottom of the Rhine.
This biological quality component is discussed in the ICPR report no. 227, “The macrozoobenthos of the Rhine in 2012”. (This report is available in German, French and Dutch)