The role of large gravel bars in nutrient retention may vary according to their physical characteristics and their vegetation. The goal of this study was to estimate nutrient fluxes and retention capacity of four gravel bars of a fifth-order river, the Doubs River (Eastern France), during low river discharge in summer and autumn. Patterns of water chemistry, nutrient concentrations, and hydrological characteristics were investigated at -0.3 m depth, using five sampling points in the downwelling zone and five in the upwelling zone of each bar. The dissolved oxygen (DOC) concentrations strongly decreased inside the bars, reaching values below 1 mg L-1 in the upwelling zones in summer, but remained between 1.1 and 4.4 mg L-1 in autumn. Dissolved organic carbon (DOC) concentrations decreased by 21 % to 44 % inside the bars or remained stable (in two bars in autumn). In the upwelling zones they were significantly positively correlated to shrub cover percentage at the bar surface, suggesting a DOC supply by leaf decomposition and root exudates. Nitrate concentrations decreased by 50 % and phosphate increased by 102 % on average along the interstitial flow path in summer, with similar values in the four upwelling zones. Vegetation cover and hydrological characteristics had no significant effect on nitrogen retention nor on phosphorus production. In summer, retention of nitrate and DOC amounted to 16.5 and 25 g day(-1) per 100 m of gravel bar respectively, with differences among bars linked to the intensity of interstitial discharges. Hence, net retention of DOC and NO3- in the parafluvial zone of the Doubs River was relatively high, while release of SRP was low and limited to the summer period.
