Strong gradient of benthic biogeochemical processes along a macrotidal 1 temperate estuary : focus on P and Si cycles 2 3

12 13 This study aims to investigate the role of spatial and temporal physical, biological and 14 biogeochemical gradients on sediment biogeochemistry along a macrotidal and Si-rich estuary. 15 Scanning and biogeochemical analyses were performed in the inner, mid and outer Aulne Estuary 16 (France) at four seasons. The inner estuary shows high diagenetic activity linked to fluid mud 17 dynamics and river loads. The highest authigenic phosphorus (Aut-P) concentrations ever found 18 in the literature are observed in the inner estuary (18 µmol g-1 PS sediment). This is explained by 19 a combination of favorable factors, i.e. the high organic matter and nutrient loads, the reductive 20 conditions, the freshwater properties (low pH, OH-, sulfate and Mg 2+ concentrations), the 21 increase of particle residence time by the upward convergence of particles due to residual 22 currents, and allochthonous riverine Aut-P. We suggest that the high Si(OH) 4 concentrations (> 23 400 µM) may even increase Aut-P precipitation through the increase of Fe-P formation in these 24 low salinity conditions. In the mid estuary, erosion-deposition dynamics dominate in point bars 25 and lead to the succession of poor and rich organic and authigenic phosphorus layers, recording 26 thus the seasonality of matter loads and its seasonal translocation from the inner estuary. In the 27 outer estuary, deposition rates are high and constant and biogeochemical properties are 28 characteristic of marine environments. The precipitation of Aut-P from free phosphate (PO 4 3-) is 29 lower than in the inner estuary and might be limited by higher Mg 2+ concentrations in saline 30 waters. This study highlights that small macrotidal estuaries, and especially their freshwater 31 sediments, may constitute an important phosphorus sink through the precipitation of Aut-P. This 32 precipitation could even be enhanced in fresh or brackish environments, thus increasing long 33