b'Major controls of base flow soluble reactive phosphorus losses in humid temperate headwater streams Rode M1 and Dupas R21Department of Aquatic Ecosystem Analysis, Helmholtz Centre for Environmental Research-UFZ, Germany2INRA, UMR1069SAS, Rennes, FranceLong term low flow ortho Phosphorus (SRP) monitoring in headwater streams in central Europe revealed a seasonal variability of the SRP concentrations with highest concentrations in summer and lowest in winter. These seasonal concentration amplitudes often reach values above the eutrophication level. It is assumed that temperature dependent biogeochemical processes could lead to P release to streams during the summer low-flow period, where redox processes may be responsible for this increase. Several studies have highlighted the crucial role of reactive zones such as riparian wetlands in controlling solute export regimes. Moreover, and especially in forest headwater streams, in-stream assimilatory uptake shows a distinct seasonal behaviour because of varying shading conditions. These can also lead to seasonal SRP amplitudes. Furthermore sorption and desorption processes are temperature dependent which may alter in-stream SRP release during the year. Often SRP concentrations are higher in the agricultural streams than in more pristineheadwaters. It is not clear how land use (e.g. P status of soils) may impact the baseline SRPconcentrations and which factors control the seasonal change in SRP stream concentration(riparian groundwater heads and redox processes, temperature, in-stream release anduptake processes). Therefore, the objective of this study is to disentangle land use impactsfrom hydrological and biogeochemical controls of low flow SRP losses. A comparative study onseasonal SRP concentration patterns will be presented comprising around 42 long termmonitored headwater catchments in humid temperate climate of northern Europe and the United States. Based on hydrological and SRP headwater signals and catchment properties P release processes are discussed. The results of the study will allow to target SRP mitigation strategies based on knowledge of the dominating control of SRP loss from headwater streams. 8'