Toward a typology of river functioning: a comprehensive study of POM composition at multi-rivers scale 

Abstract. In riverine systems, particulate organic matter (POM) originates from various sources, each having its proper dynamics related to production, decomposition, transport and burial. There is a significant amount of spatiotemporal heterogeneity in the POM pool. The current study, based on C and N elemental and isotopic ratios, applies Bayesian mixing models associated with statistical multivariate analyses to 1) quantify and examine relationships between POM composition and environmental forcings, and 2) draw a typology of river functioning based on POM composition and its seasonal dynamics. Twenty-three rivers of temperate climate accounting for a large diversity of environmental conditions were sampled fortnightly to monthly for one to seven years at the River-Estuary Interface (REI). Phytoplankton and labile terrestrial material were present in all rivers, contrary to sewage and refractory terrestrial material that were present in only a few. At the twenty-three studied rivers scale, POM sources are strongly related to watershed characteristics, phytoplankton being associated with agricultural surfaces and labile terrestrial material to soil organic carbon content and erosion rate. Overall, seasonal variations of phytoplankton, labile and refractory terrestrial material were mainly related to drivers of phytoplankton growth, river flow, and sediment resuspension, respectively. A statistical regionalization defined four river types: (1) systems whose POM is dominated by labile terrestrial material all year long; (2) systems whose POM is composed of labile and refractory terrestrial material, in addition to phytoplankton, with variable seasonality according to rivers; systems whose POM is composed of phytoplankton and labile terrestrial material (3) without and (4) with pronounced seasonality.

This work offers a comprehensive understanding of POM composition, dynamics and drivers at the REI in temperate climates, complementing similar work dedicated to coastal systems. Future work dedicated to estuaries is called to get a comprehensive understanding of POM composition, dynamics and drivers along the Land-Ocean Aquatic Continuum.

This study examines particulate organic matter (POM) composition and dynamics in 23 temperate rivers. Carbon and nitrogen isotope analysis revealed four river types based on dominant POM sources (phytoplankton, terrestrial material). Watershed characteristics influence POM composition while seasonal variations in river flow and sediment resuspension drive POM dynamics. This study improves the understanding of river systems and calls for further studies exploring downstream estuarine functioning.