Geochemical, geological and hydrological influence on ostracod assemblages distribution in the hyporheic zone of two Mediterranean rivers in central Spain

On the catchment spatial scale, rivers can be considered a patchy discontinuity from headwaters to mouth with significant implications on the structure and functioning of the hyporheic zone (HZ) biota. The present study aims to determine the spatial variation of ostracod assemblages from a fluvial HZ of two groundwater-fed rivers of the Jarama basin (Central Spain). We hypothesised that ostracod abundance and composition are subject to changes in water condition, substratum type geology and surface water/groundwater exchanges along the hyporheic flow path. Individual measures of 29 physico-chemical parameters, water discharge, sediment size and sediment organic matter (as loss of ignition) were measured at a depth of 20–40 cm at 20 hyporheic sites of the Henares and Tajuna Rivers. The results show that factors like elevation, carbonate water type, high permeable riverbed substratum and groundwater upwelling at headwaters contribute to shape a diverse, mixed assemblage formed by stygophyle and stygoxene ostracods (14 species). Conversely, low-mineralised waters, siliceous riverbeds with poor permeability, and relatively low surface/ ground water exchanges in the middle and lower sectors, limit species diversity and ostracod abundance to exclusively stygoxene elements (nine species). Ostracod abundance correlates positively with fine sands (0.125–0.25 mm) (Spearman, r¼0.70), and negatively with temperature (r¼�0.40), SO4 2� (r¼�0.45) and Mg2þ (r¼�0.40). Trace metal (Cu, Mn, Ni) accumulation in the intermediate and lower sectors of both rivers appears harmful for Ilyocypris brady (r¼�0.46) and Pryonocypris zenkeri (r¼�0.40), whereas As relates negatively to the abundance of Candona candida (r¼�0.51), Pryonocypris zenkeri (r¼�0.60) and Herpetocypris brevicaudata (r¼�0.72). The results highlight the significance of recent ostracods as a proxy to assess the water conditions in a HZ and to provide forthcoming approaches to depict surface-subsurface hydrological exchanges.