Nutrient dynamics in interstitial habitats of low-order rural streams with different bedrock geology

We Studied water exchange, nutrient dynamics, and microbial activities between the Surface and interstitial habitats of six low-order streams with catchments dominated by agriculture during high and low water periods. In each stream, surface (open) water and interstitial water form downwelling and Upwelling zones were considered. Interstitial habitats of most streams acted as a sink for nitrate, but as a source for ammonium and soluble reactive phosphorus (SRP). The nitrate sink persisted in shallow sediments of downwelling zones, while the source of ammonium and SRP persisted in deeper sediments of upwelling zones, where organic matter was decomposed and microbial uptake was lower than the production of nutrients. Geology (granite versus schist substratum) was the main variable controlling the nutrient contents and the microbial activity within interstitial habitats due to varying sediment grain size and hydraulic properties. Schist streams had the finest sediment grain size and lowest hydrologic connectivity between Surface water and interstitial habitats, and had the highest potential microbial activities, especially for denitrification. Land-use of the catchments (percentage of forested area), channel morphology (river incision), and local characteristics of the benthic sediment (partial clogging) also influenced nutrient concentrations and microbial activities irrespectively of the geology of the catchment.