Copper addition by organic matter degradation in the freshwater reaches of a turbid estuary Original Research Article

This study reports on the relationship between copper (Cu) behavior and organic matter (OM) transformation along the turbidity gradient in the freshwater reaches of the Gironde Estuary. During a one-year survey, surface water and suspended particulate matter (SPM) were sampled at least monthly at three sites along the Garonne Branch, representing the main fluvial branch of the Gironde Estuary. Additionally, a longitudinal high resolution profile was sampled along the Garonne Branch, covering the turbidity gradient from the river water endmember to the maximum turbidity zone (MTZ). Seasonal variability and spatial distribution of Cu in both the dissolved phases (< 0.2 μm, Cu0.2 and < 0.02 μm, Cu0.02) and particulate Cu (CuP) clearly suggested Cu0.2 addition during summer, that increased the Cu0.2 concentrations by a factor ~ 2, mainly manifested by an increase in the Cu0.02 fraction. At the annual timescale (2004), this internal Cu reactivity increased Cu0.02 fluxes in the Garonne Branch by ~ 20% (3.6 t year−1), with the equivalent of ~ 2.9 t year−1 derived from the CuP fraction and ~ 0.7 t year−1 from the colloidal (0.02–0.2 μm) fraction, without involving and/or affecting the CuC18 (hydrophobic metal-organic complexes) fraction.