An urbanized phantom tributary subsidizes river-riparian communities of mainstem gravel-bed river

Urbanization transforms natural river channels, and some rivers become invisible over time. How and whether the subsurface domains of the original waterways and aquifers connecting them (a phantom of historical landscape) are functional is not known. This study examined the effects of tributary groundwater (GW) inflow on the response of river-riparian organisms in an alluvial mainstem river in northern Japan, where the tributary disappeared over the course of urban landscape transformation. A 2.8-km long lowland segment of the mainstem gravel-bed river was examined for water properties and the river-riparian food web. In addition, watershed-wide water sampling was conducted to isotopically distinguish several types of groundwater that contributed to the hyporheic water in the study segment. There was a clear effect of altitude on the hydrogen/oxygen stable isotope ratios in the river water collected across the watershed. Groundwater unique both in chemical sand isotopic signatures in several spots occurred within the study segment, and its properties resembled to and its upwelling locations matched groundwater from a tributary river whose surface channel has disappeared 60 years ago. Positive numerical increases in abundance and/or a sign of nitrogen transfer in river riparian communities (algae, invertebrates, and riparian trees) originating from groundwater high in nitrate with elevated nitrogen stable isotope ratios were found. This study demonstrated that tributary groundwater with unique chemical properties manifested by an urban watershed river network continued to have cascading effects on biota across the river-riparian boundary in the mainstem river, even after urbanization transformed the tributary into a historically lost phantom river. We highlighted the legacy effects of landscape transformation in the subsurface domain and the significance of scrutinizing the past landscape and hydrological connectivity at the watershed scale in urban environments.