Biogeochemical hot moments upon flow resumption and coupled sediment transport: A comprehensive approach for temporary and perennial stream ecosystems (FlowReSeT)

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The project shall clarify the mechanisms that modulate streambed biogeochemistry (Carbon metabolism and Nitrogen uptake of the microbial community) at various chronologies of flow resumption with and without sediment transport. The resumption of flow after drying is considered as a biogeochemical hot moment, with high rates of C-metabolism and N-uptake influenced by the frequency of previous drying. The mechanisms modulating this hot moment are not weel understood. Research so far focussed mostly on single factor studies in temporary stream and river ecosystems. However, intermittency and resumption of flow occurs more frequently in perennial stream ecosystems and surface flow often implies sediment transport (e.g. migrating ripples, upper stage plane bed) especially in sandy streams. In addition, flow resumption can follow different chronologies such as instant by rain or slow by rising groundwater, and the concentrations of nutrient and carbon leached upon flow resumption can also influence the biogeochemical response. We suggest a new general concept of “intermittent stream habitats” for all areas of a streambed that dry (i.e. lack of surface water) at some point in time despite variable interactions of factors. The here proposed investigation of the mechanisms at various flow resumption chronologies, coupled sediment transport, and cross system comparison (temporary and perennial streams) will show if such general and integrative view can be applied. The interactions of flow resumption, sediment transport and drying frequency will be studied in microcosm experiments with sediment communities of intermittent habitats from temporary and perennial streams. The response variables under attention are: Carbon metabolism, measured by means of changes in oxygen concentration in darkness and in light, net N-uptake by means of the addition of the stable isotope 15N (15NH4Cl) and the structure and architecture (i.e. biofilm) of the microbial community. Further, by using numerical and hydrological modeling, the implications of the obtained results for C and N budgets in streams will be assessed. The results will complete the mechanistic picture of the Cand Ndynamics of lotic ecosystems prone to severe flow oscillations and drying.. Such knowledgeis key for management of Mediterranean and more and more temperate stream ecosystems that experience drying due to increased water abstraction and climate change.