Toward a typology of river functioning: a comprehensive study of the particulate organic matter composition at the multi-river scale

Abstract. In river systems, particulate organic matter (POM) originates from various sources with distinct dynamics related to production, decomposition, transport, and burial, leading to spatiotemporal heterogeneity in the POM pool. This study uses C and N isotope and element ratios, Bayesian models and multivariate analyses to (1) quantify relationships between POM composition and environmental forcings, and (2) propose a typology of river functioning based on POM composition and their seasonal dynamics. Twenty-three temperate rivers, representing a large diversity of temperate conditions, were sampled fortnightly to monthly over one to seven years at their River-Estuary Interface (REI). Phytoplankton and labile terrestrial material were found in all rivers, while sewage and refractory terrestrial material appeared in a few. Across all rivers, phytoplankton dominance correlated with agricultural surfaces, labile terrestrial material with organic-rich leached soil, and refractory terrestrial matter with steep, soil-poor catchments. Seasonal dynamics mainly depended to phytoplankton growth, river discharge, and sediment resuspension. Statistical regionalisation identified four river-dynamics types: (1) POM is dominated by labile terrestrial material year-round; (2) POM is composed of labile and refractory terrestrial material plus phytoplankton with variable seasonality; (3) POM is composed of phytoplankton and labile terrestrial material without pronounced seasonality; and (4) POM is composed of phytoplankton and labile terrestrial material with pronounced seasonality. This research offers a comprehensive understanding of POM composition, dynamics, and drivers at the REI in temperate climates, complementing similar study in coastal systems. Future similar research on estuaries is called to refine knowledge along the Land-Ocean Aquatic Continuum.