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The changes in both physical and chemical properties of interstitial water were investigated within a large gravel bar to determine if the parafluvial zone was a hotspot for nutrient transformations in a lowland eutrophic river, the Doubs (Eastern France). Interstitial water was sampled in 30 piezometers along five transects across the gravel bar, surface water was sampled in main and chute channels. Five campaigns of sampling were performed among summer and winter. In both interstitial and surface samples, water chemistry (electrical conductivity, pH, chloride, dissolved oxygen concentrations—DO) and nutrient concentrations (nitrate—NO3 −, ammonium—NH4 +, soluble reactive phosphorus—SRP, dissolved organic carbon—DOC) were measured. Moreover, temperature, water level, sediment grain size distribution and total bacterial abundance were assessed along a flowpath through a lateral gravel bar of the Doubs River. Measurements of water table elevation in the bar, main and chute channels indicated that the parafluvial flowpath was perpendicular to the main channel. Very low changes in chloride concentration and electrical conductivity showed minor groundwater input along the flowpath. The parafluvial zone was 0.9 m thick under the mean piezometric level, hydraulic gradients along the flowpath were 0.3%, and the discharge of interstitial water through the bar calculated at low flow was 40.6 m3 day−1. Most changes in interstitial nutrients occurred during the warm season, suggesting that biotic mechanisms occurred. Along the flowpath, DO, DOC, and bacterial abundance declined, while phosphate increased. Temporal trends of nitrate were less clear, exhibiting a slight increase during spring (organic matter biodegradation) and a decrease during summer (denitrification). Using the parafluvial discharge and nutrient concentrations of water infiltrating into and seeping from the gravel bar, the retention capacity of the bar was 171 gC day−1 for DOC and 48.3 gN day−1 for nitrate. Phosphate production ranged from 0.65 to 2.3 gP day−1. These values were low compared to the nutrient fluxes in the river Doubs, suggesting a minimal local impact of the parafluvial zone in regulating of river pollution. [ABSTRACT FROM AUTHOR]