Hyporheic flow in aquatic Ranunculus habitats in temperate lowland rivers in Central Europe

The study attempted to assess the influence of the filtration stream and hydraulic gradient in the hyporheic zone on the distribution of water crowfoot (Ranunculus sect. Batrachium) in common temperate lowland river type in two European ecoregions: (14) Central Plains and (16) Eastern Plains. A novel filtrometer and a gradient meter were employed for water flow measurements in the hyporheic zone at 24 study sites distributed throughout Central Europe between 2017 and 2019. At the majority of the study sites, hyporheic zones covered with Ranunculus vegetation were found to be dominated by groundwater drainage. Moreover, the intensity of groundwater filtration supplied via the hyporheic zone varied, ranging from 0.0017 m3 d−1·m−2 to 0.4118 m3 d−1·m−2. It was also determined that the average stream of filtration in the non-vegetated parts of the riverbed was 0.6065 m3 d-1·m−2, whereas it was five times lower in the zones overgrown with Ranunculus clumps, amounting to 0.1190 m3 d−1 m−2. The study provides quantification of the interactions between water exchange in the hyporheic zone and the growth and distribution of Ranunculus species, indicating that the exchange of surface water and groundwater is an important environmental factor that stimulates the growth of various Ranunculus taxa. The study provides evidence for a new environmental indicator, specifically the importance of surface water and groundwater exchange in the hyporheic zone of rivers for Ranunculus vegetation conservation and for planning and implementing nature-based river management methods.