"Hyporheic biotope" is the name I have given to the habitat discovered by Chappuis in 1942. This biotope is formed by the water-saturated interstices of riverbed deposits. The water comes either exclusively from the stream, through what is known as "underflow", or from both the groundwater flowing towards it and the stream. Research started in 1946 within the Work Collective of the Zoological Station of Sinaia, and was pursued by myself from 1950 onwards. The hyporheic waters of more than 70 streams of the Popular Republic of Romania were studied, especially in mountainous areas between 400 and 1000 m. The present work does not contain the definite results of these studies, which will be published later. However, several conclusions can be drawn from this investigation: Based on the analysis of physicochemical and biological parameters, it appears that the hyporheic biotope is an intermediate zone between stream water and groundwater; parameter values are indeed intermediate. This biotope contains subsurface forms fully adapted to the subsurface life, e.g. blind and discoloured amphipods, isopods, ostracods, copepods, syncarids, turbellarians, as well as numerous larvae, and particularly young larvae of stream water insects. The many species of aquatic mites that were recently described in this biotope constitute its characteristic group, and can be considered as hyporheobionts, for they are found neither in groundwater nor in surface water. Subsurface forms collected in the hyporheic biotope ought to be considered, in my opinion, as hypo-rheophile forms. The hyporheic fauna has a mixed nature, as suggested by the fact that aerial hygrophilous forms occupy the interstices as well. Their presence can be explained by the large interface between the aquatic and aerial habitats formed by the upper part of bank deposits. Additionally, some hyporheic water organisms are lentic forms that benefit from the slow current. They cohabit with typical rheophile larvae of surface insects. The cohabitation of eurytherm, stenotherm, palearctic, and tropical forms supports the idea that hyporheic fauna is more of a diversified mix than any other biocenosis. Thus it can be considered as a Fauna Liminaria. The hyporheic biotope can also be seen as an antechamber of the subsurface environment, and it is very likely that, with time, it played an important role in populating this habitat. The presence of species derived from tropical genera indicates that these are relicts from the Tertiary. As our research suggests, the hyporheic biotope, which was formed by the migration of surface animals during freezing periods, was certainly a temporary refuge in which today's hyporheobionts and subsurface species began their adaptation. This migration may have occurred either at the end of the Tertiary, when the climate became more temperate, or during the Tertiary, when the warmer climate caused the drying up of rivers and the emergence of some continental areas. © 2010 E. Schweizerbart'sche Verlagsbuchhandlung, Stuttgart Germany.
