The stabilizing effect of resting egg banks of the Daphnia longispina species complex for longitudinal taxon heterogeneity in long and narrow reservoirs

We compared the spatial distribution of taxa from the Daphnia longispina complex (D. longispina, D. galeata, D. cucullata, and their hybrids) in the active water column community and in resting egg banks in five long narrow reservoirs in the Czech Republic (Central Europe). In each reservoir, we sampled at both ends of the longitudinal gradient: in the inflow region and at the dam. Ephippia abundance in the sediments significantly increased in the downstream direction, reflecting differences in the sedimentation regime and Daphnia population size. Similarly to the active zooplankton community, in which D. cucullata and D. longispina tended to occur at opposite ends of the reservoirs, Daphnia species and interspecific hybrids in resting eggs revealed a spatially diversified pattern; however, we observed some differences in taxon distributions between sediments and water columns. High relative abundances of hybrid genotypes (up to 16% of resting eggs, and 74% of Daphnia in the water column) confirm that interspecific hybridization is frequent in these reservoirs, and some hybrids are successful in competition with the parental taxa. We assume that the spatial heterogeneity of Daphnia taxonomic composition in reservoirs, being affected by the seasonal selection of taxa within the mixed reservoir species pool, is substantially strengthened by the presence of spatially heterogeneous egg banks.