In the Netherlands we have participated in a large number of projects concerning the rehabilitation of large Dutch rivers (eg. Rhine and Meuse).
From 1984 onwards we conducted paleo-ecological research in old floodplain deposits of the Rhine and Meuse. Comprising a period of more than 5000 years, we have detailed information on the macro-invertebrate community of these rivers, until 1900. From 1981 we were involved in the monitoring of the Rhine and Meuse and that gave us the information of changes in the last decades of the 20th century. The deterioration process of the Rhine can be described in short as follows:
5000 BP – 1700
Community strongly dependant on snags (wood debris in the main channel). Important inhabitants are filterfeeders (Simuliidae, Hydropsychidae, Psychomyidae), woodminers Elmidae (Potamophilus acuminatus) and Chironomidae (Symposiocladius lignicola and Stenochironomus)).
Symposiocladius lignicola larval submentum
1700 – 1900
Decline in the snag-inhabitatants (see before), diminishing of inhabitants of other suitable habitats and the more sensitive taxa for organic pollution (Plecoptera and most Ephemeroptera). Palingenia longicauda is an example of a clay burying mayfly that became extinct after the enforcement of the banks in the main channel of the Rhine.
Second year larva of Palingenia longicauda from the Tisza (Eastern Hungary)
1900 – 1975
Disappearance of virtually all groups except Oligochaeta, Hirudinea and Chironomidae. In 1975 the water quality was at it worst and Dicrotendipes nervosus was one of a few species encountered on the stone embankment.
Larva of Dicrotendipes nervosus
1975 – 1992
Due to a spectacular rehabilitation of water quality of the Rhine a strong increase in species diversity took place. Many chironomid species returned, but also the burrowing mayfly Ephoron virgo (Ephemeroptera) was swarming again like “Summer Snow” after 50 years of absence.
Head of the larvae of the mayfly Ephoron virgo
“Summer Snow” in august. Swarming of adult Ephoron virgo on the beaches of the Rhine
1992 – present
Delcline in species richness due to the mass invasion of ponto-caspian species which were able to reach the Rhine basis through the newly opened Danube-Main-Rhine canal (Dikerogammarus villosus and Corophium curvispinum).
Dikerogammarus villosus forms a serious threat to the endemic fauna
So despite the fact that the Rhine is becoming one of the cleanest large European rivers, the ecosystem suffers from severe biological stress. Other forms of stress are expressed in Living Rivers
References
Klink, A.G., 1989. 11. The Lower Rhine: Palaeoecological analysis. In: Petts, G., (ed.). Historical changes in large alluvial rivers: Western Europe. Wiley and Sons Ltd. 183-201.
Vaate, A.bij de, 2003. Degradation and recovery of the freshwater fauna in the lower sections of the rivers Rhine and Meuse. Thesis Wageningen University 200pp.