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In-stream environments, many biogeochemical processes occur in the benthic biolayer, i.e., within sediments at a very shallow depth close to the sediment-water interface (SWI). These processes are important for stream ecology and the overall environment.Here, a 1D diffusive model is used to analyze the vertical exchange of solutes through the SWI and in the benthic biolayer. The model is applied to an extensive set of previously published laboratory experiments of solute exchange with different bed morphologies: flatbeds, dunes, and alternate bars. Although these different bed features induce mixing that is controlled by different physical processes at the SWI, overall mixing within the sediment is well represented by a parsimonious diffusive model, provided that the diffusivity profile declines exponentially with sediment depth.For all morphology types, mixing is better simulated by an exponential diffusivity model than a constant diffusivity approach. Two parameters define the exponential diffusivity model; the effective diffusivity at the SWI, and a depth scale over which the exponential profile decays. Using a combination of classification and regression trees (CART) and multiple linear regression approaches, we demonstrate that a single predictive model captures measured variability in the effective diffusivity coefficient at the SWI across all morphological types. The best predictive model for the decay depth scale, on the other hand, is tailored to each morphological type separately.The predictive framework developed here contributes to our understanding of the physical processes responsible for mixing across the SWI, and therefore the in-bed processes that contribute to the biogeochemical processing of nutrients and other contaminants in streams.

The Impacts of Water Quality Changes on Aquatic Ecosystems: A Case Study of Clariano River, Spain Hamed Vagheei1, Paolo Vezza2, Guillermo Palau-Salvador3, Fulvio Boano4PhD Student, the Polytechnic University of Turin, hamed.vagheei@polito.it Assistant professor, the Polytechnic University of Turin, paolo.vezza@polito.it Associate Professor, the Polytechnic University of Valencia, guipasal@agf.upv.es Associate Professor, the Polytechnic University of Turin, fulvio.boano@polito.it AbstractWater quality degradation resulting from different anthropogenic activities such as agriculture, deforestation and urbanization is a serious worldwide challenge which have negative impacts on aquatic ecology. Unfortunately, it is still difficult to quantitatively determine the impacts of water quality changes on aquatic communities. The objective of the present research activity is to investigate aquatic ecosystem responses to water quality deterioration using a case study of Clariano River, Spain. The Clariano River faces low water quality and the loss of biodiversity in some parts as a result of agricultural, industrial and livestock activities as well as wastewater treatment plants (WWTP) effluents entering the river. The Soil and Water Assessment Tool (SWAT), an eco-hydrological model, is used in the present study for the modelling of discharge, sediment and nutrients. SWAT-CUP is also used to calibrate and validate the SWAT model. We are currently employing the results from the calibrated model to obtain a better understanding of possible relations between water quality and biodiversity. In fact, the present study will focus on macroinvertebrates as biological indicators of stream health, and the model predictions will be coupled with empirical correlations between stream water quality and macroinvertebrates presence in order to assess the impacts of water quality changes on aquatic ecosystem. In addition, different model scenarios will be compared to explore the potential impacts of changes in land use, climate and WWTPs operation on the aquatic ecosystem.Keywords: aquatic ecosystem, Clariano River, eco-hydrological modelling, water quality, water resources management