Limnetica 35

The hyporheic zone of stream ecosystems, located at the transition between surface channel and ground waters, exhibits patchy discontinuity on a longitudinal transect and induces changes in the structure and functioning of biotic communities. Nevertheless, the factors that influence biotic communities along spatial and temporal gradients remain poorly understood. This study aimed to characterise the diversity and spatial pattern distribution of Cyclopoida and Ostracoda from the hyporheic zone in relation to the riverbed sediment structure and the quantity of organic matter in two second-order rivers in central Spain, whose alluvium is derived from materials of contrasting geology (siliceous vs. carbonates). Both streams were found to be characterised by marked habitat heterogeneity along the hyporheic flow path, as reflected by the different lithology, riverbed permeability and distinct hyporheic biota assemblages. The results indicate that the alluvium riverbeds dominated by carbonates rocks and associated with high permeable riverbeds had slightly diverse hyporheic assemblages (25 species in all) with species of mixed ecology (stygobites/stygophiles/stygoxenes). Conversely, the siliceous alluvium riverbeds with low mineralised waters, associated with reduced permeability and relatively less active surface/ground water exchanges, displayed slightly lower diversity (23 species) with assemblages exclusively composed of stygophiles/stygoxenes. The results highlight the role of the riverbed substratum for the hyporheic biota and provide forthcoming approaches for depicting surfacesubsurface hydrological exchanges.