Morisaki, N. | bioRxiv Science

bioRxiv2023

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.

ecology↗

bioRxiv2023

Hot spots in a cold river synchronize temporarily separated salmon runs in their offsprings development

The identification of thermal heterogeneity in the environment and its inclusion in adaptive strategies are key to habitat management of cold-water fish, including salmonid species. This study tested the hypothesis that upwelling of groundwater (GW) from a tributary catchment through a relatively deep aquifer (tributary GW) affects salmon redds selection in an urbanized gravel-bed river, and that the spawning preference of such areas depends on the seasonal context. The field study was conducted between 2001 and 2015 in an approximately 6-km long segment of the Toyohira River, Northern Japan. Chum salmon redds distribution data over 15 years (2001-2015) were combined with spatial distribution data of hyporheic water affected by tributary GW in the riverbed to examine seasonally variable redds site selection in relation to the presence of unique GW. Furthermore, models that predicted the hyporheic water thermal regime were coupled with redds count data to estimate the approximate timing of fry emergence from the riverbed. The redds site selection was seasonally variable, with a higher dependence on tributary-GW-affected areas with a decrease in water temperature. The time until fry emergence from spawning was shortened when the tributary-GW area was chosen during the cold winter. Overall, the present study identified hotspots for salmon spawning redds in winter with a disproportionately high level of site selection because of their warmer temperature compared to surface river water in winter. Thermally diverse spawning habitats allow the diversification of spawner strains in synchronized descents to the sea. Signs of tributary-GW pollution was suggested, and thus the conservation of the groundwater pathway and its sources, followed by improvements in quality, can be beneficial to the Chum salmon populations in the Toyohira River.

ecology↗