1 Biogeochemistry of the Amazonian floodplains. Insights from six end-member mixing models.

Abstract: The influence of Amazonian floodplains on the hydrological, sedimentary and biogeochemical river budget was investigated along the Vargem Grande – Obidos reach, by applying six mixing models based on variable regional and/or variable hydrological sources. By comparing the output of many different models designed for different purposes, the nature and the magnitude of processes linking water and biogeochemical budgets of the Amazonian floodplains were clarified. This study reveals that most of the chemical baseline of the Amazon River basin is acquired before the studied 2000 km Amazonian reach. However, the tight connection between the hydrograph stage of the River and the chemical signals provides insightful information on the dynamics of its floodplains. The chemical expression of biotic and abiotic processes occurring in the Amazonian floodplains can be more particularly perceived during falling waters. It appears delayed in time compared to the maximum extension of submerged area, because the alternating water circulation polarity (filling vs. emptying) between the main channel and the adjacent floodplains determines delayed emptying of floodplains during falling waters. I