Spatiotemporal variation in hyporheic nutrient concentrations and interstitial water quality in relation to land use and sediment dynamics

The hyporheic zone at the surface water-groundwater interface is an important compartment of stream ecosystems. It is a hotspot of aquatic biodiversity and key to functional processes, especially in relation to nutrient cycling and retention as well as the self-cleaning ability of streams. The core objective of this study was to understand the complex and heterogeneous role of hyporheic exchange on nutrient cycling and transport over different spatial and temporal scales in relation to anthropogenic land use and sediment dynamics. We assessed sediment dynamics, redox potentials and interstitial habitat quality in conjunction with ion-and nutrient concentrations in the open water and the interstitial zone across a range of silicate stream systems of varying intensities of catchment use and under different discharge conditions. Snow melt events were highly important for the mobilization of fine sediments and stream bed cleaning. In contrast, strong rain events caused high additional fine sediment deposition rates. Fine sediment inputs from small catchment elements like fish ponds strongly depended on pond management. Spatial patterns in hyporheic nutrient concentrations differed from surface water nutrient concentrations, and hyporheic exchange flow varied for different compounds. Intensively and extensively used streams varied strongly in surface water nutrient concentrations while differences in interstitial ion concentrations were much lower. Waterborne denitrification was mostly found in intensively used catchments with elevated fine sediment deposition rates and in fish ponds. Increased fine sediment deposition on the stream bed resulting from excessive erosion input and resulting colmation were regularly observed in intensively used catchments. They can impair the exchange of surface water with the interstitial zone, in turn affecting hyporheic processes. Such knowledge on the potential impact of hyporheic processes on surface water nutrient dynamics along land use gradients is needed to guide future management of catchments and waterbodies to reduce anthropogenic pressures on aquatic ecosystems.