Spatio-temporal pattern of mussels (Corbiculidae, Dreissenidae, Sphaeriidae) in the water-system of the Hungarian Danube

Abstract

In 1998 and 2001 collections were carried out at 83 sampling sites in different water bodies of the Hungarian Danube section. In the period of 1996-2003 during the hydrobiological monitoring of the Szigetkoz area 15 sites, which are affected especially by the hydrological changes due to the diversion of the Danube, were studied. Regarding the sampling sites altogether, 16 small-sized mussel species of 3 families were found, which presents 73 % of the nation-wide fauna. Among them two species (Corhicula fluminea, Dreissena polymorpha polymorpha) are invasive mussels and four (Pisidium amnicum, Pisidium milium, Sphaerium rivicola, Sphaerium solidum) are rare species in Hungary. The radical alteration in the hydro-morphology of the Szigetkoz area caused by the diversion of the Danube to the operation channel of Bos/Gabcikovo hydroelectric power plant was followed by the impoverishment of the mussel fauna. The composition of mussel fauna had been considerably changed between 1996 and 2003. In the period before 2000 the small-sized mussel fauna contained 11 species, but in the period after 2000 the fauna increased with 2 fingernail clams (Sphaerium rivicola, Sphaerium solidum) and one pea clam (Pisidium casertanum var. ponderosum). Beside the increase in the number of species a process to uniformity could be detected, since the ratio of species occurring only in one water body gradually decreased. The number of species occurring in more than one water body increased until the beginning of 2000. The previous flood prevention classification of the Szigetkoz area could be found less and less typical in the spatial pattern of the mussel fauna. In the last years of the investigated period the water regime of the active alluvial floodplain followed the natural water fluctuation of the Danube. The water dynamics of the active alluvial floodplain differs both from the abandoned main channel and the protected area due to the changed mode of water supply. The decrease of similarity between the water bodies could represent a tendency towards the situation before altering the hydrological exchange conditions in the wetland.