Cladoceran assemblages in a karstic lake as indicators of hydrological alterations

The anthropogenic impact on wetlands has increased during the last centuries when infrastructures such as canals or dams, have proliferated. In this article, we have used cladoceran sub-fossils to investigate the effects of a canal on lake El Tobar (Spain). The canal has been transferring water from a reservoir, built in another valley, into this lake since its construction in the mid-1960s. Cladoceran remains were analyzed in two sediment profiles from each of the two sub-basins of the lake. The sedimentary sequences showed that the mentioned human activity provoked a clear shift in the cladoceran community. A PCA was performed with samples from both sub-basins to detect the direction and nature of the changes. Before water transference, the phytophilous chydorid community, represented by Acroperus angustatus in one sub-basin and by both A. angustatus and Graptoleberis testudinaria in the other sub-basin, were well developed. After the construction of the canal and the inflow of additional yet different water from the reservoir, the proportion of chydorids relative to total cladoceran diminished considerably; this was mainly due to the invasion of Bosmina longirostris. These results indicate that the hydrological alteration caused a shift from an oligotrophic, shallower lake with a rich plant-associated cladoceran community to a more eutrophic lake with a predominant planktonic cladoceran community; and that cladoceran remains are a powerful tool to detect hydrological changes and eutrophication.